aci 351.2 r-94 foundations for static equipment

ACI 351.2R-94

Foundations for Static Equipment (Reapproved 1999)

Reported by ACI Committee 351

Erick N. Larson*Chairman

Hamid Abdoveis*William BabcockJ. Randolph Becker*William L. Bounds*Marvin A. ConesDale H. Curtis*Shraddhakar Harsh*

C. Raymond Hays*A. Harry Karabinis*John C. KingJoseph P. Morawski*Navin Pandya*Ira W. Pearce*Mark Porat*

James P. Lee*Chairman, Subcommittee 351.3

John A. Richards*Robert W. Ross*Philip A. SmithRobert C. Vallance*Alfonzo L. WilsonMatthew W. Wrona*

* Members of Subcommittee 351.3 which prepared this report.The Committee also wishes to extend its appreciation and acknowledgement of two Associate Members who contributed to this report: D. Keith McLean and Alan

Porush.

The committee has developed a discussion document representing the state-of-the-art of static equipment foundation engineering and construction. Itpresents the various design criteria, and methods and procedures of analy-sis. design, and construction currently being applied to static equipmentfoundations by industry practitioners The purpose of the report is to pre-sent the various methods. It is not intended to be a recommended practice,but rather a document which encourages discussion and comparison ofideas.

Keywords: anchorage (structural); anchor bolts: concrete; equipment; forms;formwork (construction): foundation loading; foundations; grout; grouting:pedestals; pile loads; reinforcement; soil pressure: subsurface preparation;tolerances (mechanics).

CONTENTS

Chapter l-Introduction, p. 351.2R-2l.l-Background1.2-Purpose1.3-Scope

ACI Committee Reports, Guides, Standard Practices and Com-mentaries are intended for guidance in designing, planning,executing, or inspecting construction and in preparing specifica-tions. References to these documents shall not be made in theProject Documents. If items found in these documents are de-sired to be part of the Project Documents, they should bephrased in mandatory language and incorporated into the Pro-ject Documents.

The American Concrete Institute takes no position respectingthe validity of any patent rights asserted in connection with anyitem mentioned in this report. Users of this report are expresslyadvised that determination of the validity of any such patentrights, and the risk of infringement of such rights, are entirelytheir own responsibility.

351.2R

Chapter 2-Foundation types, p. 351.2R-22.1-General considerations2.2-Typical foundations

Chapter 3-Design criteria, p. 351.2R-43.1-Loading3.2-Design strength/stresses3.3-Stiffnes/deflections3.4-Stability

Chapter 4-Design methods, p. 351.2R-194.1-Available methods4.2-Anchor bolts and shear devices4.3-Bearing stress4.4-Pedestals4.5-Sail pressures4.6-Pile loads4.7-Foundation design procedures

Chapter 5-Construction considerations, p. 351.2R-245.1-Subsurface preparation and improvement5.2-Foundation placement tolerances5.3-Forms and shores5.4-Sequence of construction and construction joints5.5-Equipment installation and setting5.6-Grouting

ACI 351.2R-94 became effective Feb. 1, 1994.Copyright ~EI 1994, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by

any mans, including the making of copies by any photo process, or by any elec-tronic or mechanical device. printed, written, or oral or recording for sound orvisual reproduction or for use in any knowledge or retrieval system or device,unless permission in writing is obtained from the copyright proprietors.

-1

351.2R-2 ACI COMMlTTEE REPORT

5.7-Materials5.8-Quality control

Chapter 6--References, p. 351.2R-286.1-Recommended references6.2-Cited references

Glossary, p. 351.2R-30

Metric (SI) conversion factors, p. 351.2R-30

CHAPTER l-INTRODUCTION

l.l-BackgroundFoundations for static equipment are used throughout

the world in industrial processing and manufacturing fa-cilities. Many engineers with varying backgrounds areengaged in the analysis, design, and construction of thesefoundations. Quite often they perform their work withvery little guidance from building codes, national stan-dards, owner’s specifications, or other published infor-mation. Because of this lack of consensus standards, mostengineers rely on engineering judgment and experience.However, some engineering firms and individuals havedeveloped their own standards and specifications as aresult of research and development activities, fieldstudies, or many years of successful engineering orconstruction practice. Firms with such standards usuallyfeel that their information is somewhat unique and,therefore, are quite reluctant to distribute it outside theirorganization, let alone publish it. Thus, without opendistribution, review, and discussion, these standardsrepresent only isolated practices. Only by sharing openlyand discussing this information can a truly meaningfulconsensus on engineering and construction requirementsfor static equipment foundations be developed. For thisreason, the committee has developed a discussion docu-ment representing the state-of-the-art of static equipmentfoundation engineering and construction.

As used in this document, state-of-the-art refers tostate-of-the-practice and encompasses the various engi-neering and construction methodology in current use.

l.2-PurposeThe Committee presents, usually without preference,

various design criteria, and methods and procedures ofanalysis, design, and construction currently being appliedto static equipment foundations by industry practitioners.The purpose of this report is to present these variousmethods and thus elicit critical discussion from the indus-try. This report is not intended to be a recommendedpractice, but rather a document that will encouragediscussion and comparison of ideas.

1.3-ScopeThis report is limited in scope to the engineering and

construction of static equipment foundations. The term

“static equipment” as used herein refers to industrialequipment that does not contain moving parts or whoseoperational characteristics are essentially static in nature.Outlined and discussed herein are the various aspects ofthe analysis, design, and construction of foundations forequipment such as vertical vessels, stacks, horizontal ves-sels, heat exchangers, spherical vessels, machine tools,and electrical equipment such as transformers.

Excluded from this report are foundations formachinery such as turbine generators, pumps, blowers,compressors, and presses, which have operational charac-teristics that are essentially dynamic in nature. Alsoexcluded are foundations for vessels and tanks whosebases rest directly on soil, for example, clarifiers,concrete silos, and American Petroleum Institute (API)tanks. Foundations for buildings and other structures thatcontain static equipment are also excluded.

The geotechnical engineering aspects of the analysisand design of static equipment foundations discussedherein are limited to general considerations. The reportis essentially concerned with the structural analysis,design and construction of static equipment foundations.

CHAPTER 2-FOUNDATION TYPES

2.1-General considerationsThe type and configuration of a foundation for equip-

ment may be dependent on the following factors:

1. Equipment base configuration such as legs, saddles,solid base, grillage, or multiple supports locations.

2. Anticipated loads such as the equipment staticweight, and loads developed during erection, operation,and maintenance.

3. Operational and process requirements such as ac-cessibility, settlement constraints, temperature effects,and drainage.

4. Erection and maintenance requirements such aslimitations or constraints imposed by construction ormaintenance equipment, procedures, or techniques.

5. Site conditions such as soil characteristics, topo-graphy, seismicity, climate, and other environmentaleffects.

6. Economic factors such as capital cost, useful oranticipated life, and replacement or repair costs.

7. Regulatory or building code provisions such as tiedpile caps in seismic zones.

8. Construction considerations.9. Environmental requirements such as secondary con-

tainment or special concrete coating requirements.

2.2-Typical foundations2.2.1 Vertical vessel and stack foundations - For tall

vertical vessels and stacks, the size of the foundationrequired to resist gravity loads and lateral wind or seis-mic forces is usually much larger than the support baseof the vessel. Accordingly, the vessel is often anchored to

FOUNDATIONS FOR STATIC EQUIPMENT 351.2R-3

a pedestal with dimensions sufficient to accommodate theanchor bolts and base ring. Operational, maintenance, orother requirements may dictate a larger pedestal. Thepedestal may then be supported on a larger spreadfooting, mat, or pile cap.

For relatively short vertical vessels and guyed stackswith large bases, light vertical loads, and small over-turning moments, the foundation may consist solely of asoil-supported pedestal.

Individual pedestals may be circular, square, hexa-gonal or octagonal. If the vessel has a circular base, acircular, square, or octagonal pedestal is generally pro-vided. Circular pedestals may create construction diffi-culties in forming unless standard prefabricated forms areavailable. Square pedestals facilitate ease in forming, butmay contain much more material than is required byanalysis. Octagonal pedestals are a compromise betweensquare and circular; hence, this type of pedestal is widelyused in supporting vertical vessels and stacks with circularbases (see Fig. 2.2.1).

FOOTING PLAN

ANCHOR BOLTS TYP> ,

Fig. 2.2.l-Octagonal pedestal and footing for verticalvessel

2.2.2 Horizontal vessel and heat exchanger foundations-Horizontal equipment such as heat exchangers and re-actors of various types are typically supported on pedes-tals that rest on spread footings, strap footings, pile caps,or drilled piers. Elevation requirements of piping oftendictate that these vessels be several feet above grade.Consequently, the pedestal is the logical means of sup-port.

The configuration of pedestals varies with the type ofsaddles on the vessels, and with the magnitude and direc-tion of forces to be resisted. Slide plates are also used toreduce the magnitude of thermal horizontal forces be-tween equipment pedestals. The most common pedestalis a prismatic wall type. However, T-shaped (buttressed)pedestals may be required if the horizontal forces arevery high (see Fig. 2.2.2).

2.2.3 Spherical vessel foundations - Large sphericalvessels are sometimes constructed with a skirt and basering, but more often have leg-supports. For leg-supportedspherical vessels, foundations typically consist of pedes-tals under the legs resting on individual spread footings,a continuous mat, or an octagonal, hexagonal or circularannular ring. Concerns about differential settlement be-tween legs and large lateral earthquake loads usuallydictate a continuous foundation system. To economize onfoundation materials, an annular ring-type foundation isoften utilized (see Fig. 2.2.3).

2.2.4 Machine tool foundations - Machine tool equip-ment is typically supported on at-grade mat foundations.These may be soil-bearing or pile-supported dependingupon the bearing capacity of the soil and the settlementlimitations for the machinery (see Fig. 2.2.4). Where a
machine tool produces impact type loads, it is generallyisolated from the neighboring mat to minimize transmis-sion of vibration to other equipment.
2.2.5 Electrical equipment and support structure founda-tions - Electrical equipment typically consists of trans-formers, power circuit breakers, switchgear, motor con-

trol centers. Support structures consist of buses, linetraps, switches, and lightning arrestors.

Foundations for electrical equipment, such as trans-formers, power circuit breakers, and other more massiveenergized equipment, are typically designed for (1) deadloads, (2) seismic loads, (3) erection loads (i.e., jacking),and (4) operating loads. These foundations are typicallyslabs on grade, or slabs on piles. Anchorage is providedby anchor bolts or by welding the equipment base to em-bedded plates.

Foundations of support structures for stiff electricalbuses, switch stands, line traps, and lightning arrestorsare designed to accommodate operating loads, windloads, short circuit loads, and seismic loads. These loadsare usually smaller than those of transmission line sup-port structures; therefore, the supporting foundationscommonly used are drilled piers. If soil bearing condi-tions are unfavorable, however, spread footings or pilesupported footings are generally used.

Support structures for overhead electrical conductors,such as transmission towers, poles, dead-end structures,and flexible bus supports, are designed for tension loadsfrom the conductors along with ice and wind loads.


.._.___.___ -.--..- ..--. -..-..--.-.--.-a-.../ \

i HORIZONTAL VESSEL I

SIDE ELEVATION

FOOTING PLAN

1% REINFDRcEMENT WWN ow THISFIGURE I S INTENDED T O BE ILLUST-R A T I V E ONLY. T I E SPACINGSPLICES AND OTHER SPECIFIC

LAP

DETAILS A R E THE RESPONSIBILITYOF THE DESIGN ENGINEER AS NEED-ED FOR SPECIFIC LOADING REQUIR-EMENTS AND SOIL CONDITIONS

Fig. 2.2.2-Footingswith strap for horizontal vessels

PEDESTALS ARE LOCATED

REINFORCEMENT-T&B

FOOTING PLAN

LUMNS

S E C T ’ “ 3

Fig. 2.2.3-Octagonal footing and pedestals for verticalsphere

Drilled piers are commonly used to support such struc-tures. Spread footings or pile supported footings are alsoused when required by soil conditions.

CHAPTER 3-DESIGN CRITERIA

Criteria used for the design of static equipment foun-dations vary considerably among engineering practition-ers. There may be several reasons for this variability.Most heavy equipment foundations are designed by orfor large organizations, which may include utilities andgovernment agencies. Many of these organizations, withtheir in-house expertise, have developed their own engi-neering practices, including design criteria. Many organi-zations, after investing considerable resources in devel-opment, consider such information proprietary. They findno incentive to share their experience and research withothers. For these reasons, there is limited published in-formation on the criteria used for the design of the typesof static equipment foundations covered by this report.

3.1-Foundation loadingMost practitioners first attempt to use the common

loadings defined by local building codes, or by ACI 318.However, many engineers have difficulty in classifying thelarge number of different loadings into the standard“dead” and “live” categories. There is, therefore, a needto define additional categories of loadings and loadcombinations with appropriate load factors.

3.1.1 Loads3.1.1.1 Dead loads - Dead loads invariably consist

of the weight of the equipment, platforms, piping, fire-proofing, cladding, ducting, and other permanent attach-ments. Some engineers also designate the operating con-tents (liquid, granular material, etc.), of the equipment asdead loads. However, such a combination is inconvenientwhen considering the possible combinations of loads thatmay act concurrently, and when assigning load factors.Equipment may often be empty, and still be subject tovarious other loads. Thus, a distinction between dead andoperating loads is generally maintained.

3.1.1.2 Live loads - Live loads consist of thegravity load produced by personnel, movable equipment,tools, and other items that may be placed on the mainpiece of equipment, but are not permanently attached toit. Live loads also commonly include the lifted loads ofsmall jib cranes, davits, or booms that are attached to themain piece of equipment, or directly to the foundation.


,~.c”~~.~‘~~“~’ _ --.-..-. _-_ .--.-.. - ..--..,

i HORIZONTAL VESSEL I\ I ; ,:

. .-.. A..- ..-_. - ..-..-..-._-..-..- ..2 ..-- -’E

/PEDESTALREINFORCEMENT

FOOTING PLAN

Fig. 2.2.4--Combined footing for horizontal vessel

Live loads, as described above, normally will not occurduring operation of the equipment. Typically, such loadswill be present only during maintenance and shutdownperiods. Most practitioners do not consider operatingloads, such as the weight of the contents during normaloperation, to be live loads.

3.1.1.3 Operating loads - Operating loads includethe weight of the equipment contents during normal op-erating conditions. These are contents that are not per-manently attached to the equipment. Such contents mayinclude liquids, granular or suspended solids, catalystmaterial, or other temporarily supported products ormaterials being processed by the equipment. The oper-ating load may include the effects of contents movementor transfer, such as fluid surge loads in some types ofprocess equipment. However, these latter loads are some-times treated separately and require different loadfactors.

Operating loads also commonly include forces causedby thermal expansion (or contraction) of the equipmentitself, or of its connecting piping. An example of the firsttype would be a horizontal vessel or heat exchanger withtwo saddles, each supported on a separate foundation.Temperature change of the equipment can produce hori-zontal thrusts at the tops of the supporting piers. Tem-perature change of connecting piping can produce up tosix component reactions at the connecting flanges (threeforces and three moments). For large piping, such forcesmay significantly affect the foundation design.

3.1.1.4 Wind loads - When designing outdoorequipment foundations to be constructed in an areaunder the jurisdiction of a local building code, mostengineers will use the relevant provisions in that code fordetermining wind loads on equipment. Most codes, suchas the older editions of the Uniform Building Code (UBC79) specify wind pressures according to geographic area,height above grade, and equipment geometry. Dynamiccharacteristics of the structure or equipment are notrecognized, nor are any types of structures or equipmentspecifically excluded from consideration. The proceduresused are simple even though, as most engineers believe,they are somewhat crude in their representation of theactual effect of wind.

Some practitioners, particularly when designing equip-ment foundations outside the jurisdiction of local build-ing codes, use the more recent and purportedly morerational wind load provisions contained in ASCE Stan-dard 7 (formerly ANSI A58.1). However, these provisionshave the reputation of being significantly more complexthan those in most building codes.

The ASCE 7 wind pressure relationships can, in gen-eral, be represented by the following two equations:

qz= 0.00256Kz(IV)2 (3-l)

Pz= qzGC (3-2)

Where the various parameters are defined as follows:qz

= velocity pressure at height zV = basic wind speed (mph)I = importance factorKz

= height and exposure coefficientPz

= design pressure at height z (psf)G = gust factorC = pressure or drag coefficient

The reputation of complexity and unwieldiness of theASCE 7 wind provisions is unjustified when designingrigid equipment, such as short stubby vertical vessels,horizontal tanks, heat exchangers, machine tools, andelectrical equipment. For these rigid types of equipment,the ASCE 7 wind provisions require only a selection ofa basic wind speed, an “importance factor,” which adjuststhe basic wind speed for mean recurrence interval, anddetermination of a “velocity pressure.” This latter quantityis a function of both “exposure” (topography) and heightabove grade. Design wind pressures are then determinedby multiplying the velocity pressure by a “gust factor” anda pressure (or drag) coefficient. The gust factor adjuststhe mean velocity pressure to a peak value for the givenexposure and height. The pressure or drag coefficientsreflect the geometry and tributary exposed area of theitem being investigated, and its orientation relative to thewind flow.

When designing tall flexible towers, vertical vesselsand stacks, or their foundations, the engineer is facedwith a problem when using the ASCE 7 wind load provi-


sions. This problem occurs in the introductory paragraphto the ASCE 7 wind load provisions, which excludes fromconsideration "structures with. . . structural characteristicswhich would make them susceptible to wind-excited oscilla-tions.” Tall flexible process towers, stacks, and chimneysare indeed susceptible to wind-excited oscillations. Boththe discussion in Chapter 4 of ACI 307 as well as thematerial presented in Chapter 5 of ASME/ANSI STS-l-1986 (steel stacks) are recommended references for thesesolutions.

3.1.1.5 Seismic loads - Determining lateral forcerequirements for equipment is a challenge for practicingengineers. The reason stems primarily from the buildingcodes commonly used to make such determinations.Since the primary focus of building codes is upon “build-ing type” structures, the applicability to equipment andnonbuilding type structures is less than clear, particularlywhen most of the codes use nomenclature applicable tostructures rather than equipment.

These difficulties have been widely recognized, andsteps have been taken to make the equipment require-ment sections of codes more “user-friendly” for thepracticing engineer. Most notably, the 1991 edition of theUniform Building Code (UBC), widely used in the seis-mic zones of the western United States, adopts therefinements and improvements from recommendations ofthe Structural Engineers Association of California(SEAOC). SEAOC’s Subcommittee on NonbuildingStructures, a part of the Seismology Committee, con-tinues its efforts to develop “stand-alone” requirementsthat expand the scope and refine the treatment forseismic loads on equipment.

These efforts and widespread refinements made bySEAOC for structures have made the Uniform BuildingCode the “state-of-the-art” code for lateral loadrequirements, even in many jurisdictions that have notspecifically adopted the UBC. Other codes or standardsthat specify lateral force requirements on buildings orstructures include ASCE 7 (formerly ANSI A58.1), TheBOCA National Building Code, and the Standard Build-ing Code (SBC). The Federal Emergency ManagementAgency’s (FEMA) National Earthquake Hazards Reduc-tion Program (NEHRP) Standard (1991) should also beconsulted for seismic force requirements for equipment.

3.1.1.5.a UBC lateral force requirements for equip-ment - The UBC makes no distinction between “static”and “dynamic” equipment for seismic loads. Rather,whether the equipment is “rigid” or “nonrigid” determinesthe values for the variables used in the formulae forcalculating lateral forces. Therefore, lateral forcerequirements for equipment do not depend upon equip-ment type, but upon rigidity. Equipment with a funda-mental frequency greater than or equal to 16.7 Hertz, ora period less than or equal to 0.06 second, is considered“rigid.”

The performance of many types of vendor-manufac-tured, floor-mounted equipment (both rigid and non-rigid) in past earthquakes has demonstrated a typically

high inherent strength for resisting seismic loads.Whether for operating, manufacturing, or shipping con-siderations, mechanical equipment such as pumps, engineand motor generators, chillers, dryers, air handlers, andmost fans fall into this category, as does most electricalequipment. Note that while these observations are speci-fically for the structural performance of anchored equip-ment, they often are true for their operational perform-ance as well - unless electrical relays are tripped orinstrumentation controls are set to automatically shutdown equipment. Where operational considerations aremore of a concern, as is the case for telecommunicationand computer equipment, engineers often specify muchmore stringent criteria than would be required by anybuilding code.

Operational criteria for equipment are beyond thescope of this document, but the practice of a west coasttelecommunications company in UBC Seismic Zone 4may be instructive. It requires shake table testing oftelecommunications and computer equipment to an inputacceleration of 1g (where g = gravitational acceleration)in both the horizontal and vertical directions. Suchtesting is used by numerous equipment manufacturersand often governs the anchorage requirements for theequipment.

Past earthquake experience has also demonstratedthat static equipment that is properly supported andadequately anchored against normal sliding and over-turning moment (such as small heat exchangers, chillers,pumps, and small shop-fabricated boilers and condensers)may not require an explicit design for seismic forces.Nevertheless, seismic loads are still commonly includedin engineering design criteria.

The UBC requires special seismic provisions for an-choring “life-safety” equipment supported in a structurein the form of a multiplier called the “importance factor”(I). Facilities such as hospitals, fire stations, policestations, emergency communication facilities, and facil-ities housing sufficient quantities of toxic or explosivesubstances that could pose a danger to the general publicif released are considered “Essential Facilities” or“Hazardous Facilities.” Theses facilities require a multi-plier of 1.25 with no reduction if the equipment is self-supported at or below grade. For cases not describedabove, I is to be taken as 1.0.

3.1.1.5.b Equipment supported by structures - TheUBC requires a higher degree of strength for anchoringequipment to structures than is required for the design ofthe structures themselves. This is because equipment sup-ported above ground level typically: (1) has higher abso-lute accelerations than at ground level, (2) can be sub-jected to amplified responses, (3) has little redundancy orenergy absorption properties, and (4) is more susceptibleto attachment failures, thereby becoming a higher riskcomponent.

Rigid equipment not directly supported at or belowgrade would typically be identified by the code as “non-structural components supported by structures.” This in-


cludes most pumps, motors, and skid-mounted compo-nents. For these, the minimum lateral force requirementsare determined by the formula:

Fp = Z

where:

Fp =z =

Ip =Cp =

Wp =

Ip Cp Wp [UBC Formula (36-l)] (3-3)

lateral seismic forceseismic zone factor for effective peak groundacceleration (ranges from 0.075 to 0.40, de-pending upon geographic location)importance factor for componentshorizontal force factor for the specific com-ponent (0.75 in most cases, but 2.0 for stackssupported on or projecting as an unbracedcantilever above the roof more than one-halfthe equipment’s total height)weight of the component

If an importance factor equal to 1.0 is required, theminimum lateral force requirement for Seismic Zone 4is 0.3Wp. Only if the rigid equipment consisted of un-braced cantilevers extending above the roof more thanone-half the equipment’s total height would the re-quirement be greater - 0.8Wp. (see Table 3.1.1.5a). For
nonrigid or flexibly supported equipment the minimumlateral force is determined by the same formula. Theforce factor Cp, however, must consider both the dy-namic properties of the component and the structure thatsupports it. In no case should this be less than Cp forrigid equipment, though it need not exceed 2.0. In lieu ofa detailed analysis to determine the period for nonrigidequipment, the value for Cp for rigid equipment can bedoubled, resulting in a Cp of 1.5. This simplification isgenerally used by practicing engineers. Thus, unless animportance factor greater than 1.0 is required, the min-imum lateral force requirement for Seismic Zone 4would be 0.6Wp for most nonrigid equipment. Only if thenonrigid equipment consists of unbraced cantileversextending above the roof more than one-half the equip-ment’s total height would the requirement be greater -0.8Wp (see Table 3.1.1.5a).
3.1.1.5.c Equipment supported at or below grade -If the rigid or nonrigid equipment is supported at orbelow ground level, the UBC allows two-thirds of thevalue of Cp to be used:

Fp= ZIp(0.67)CpWp

[Adapted from UBC Formula (36-l)](3-4)

as long as the lateral force is not less than that obtainedfor nonbuilding structural systems as given in UBC Sec-tion 2338 (b). These forces are described in the next sec-tion.

3.1.1.5.d Self-supporting structures other than build-ings - Formula (38-l) as given in UBC-91 2338 (b), ap-plies to all rigid nonbuilding structural systems and all

rigid self-supporting structures and equipment other thanbuildings. This would include such equipment as rigidvessels and bins.

V = 0.5ZIW[UBC Formula (38-l)]

(3-5)

If the self-supporting structure is nonrigid (that is, f <16.7 Hertz), as for tall slender vessels, most tanks ongrade, and some elevated tanks and bins, the dynamicproperties must be considered and the UBC prescribesusing the lateral force formula for “other nonbuildingstructures” with some modifications:

V = zzc W- -Rw

[UBC Formula (34-l)]

(3-6)

where:

c =

I =

Rw =

S =

T =V =W =

Z =

1.25 s- Amplificationcoefficient (need not ex-Ty3

ceed 2.75)importance factor (either 1.0 for standard andspecial occupancy structures, or 1.25 for es-sential and hazardous facilities) [See UBCTable 23-L]numerical coefficient for nonbuilding typestructures (either 3, 4, or 5, depending upontype) [See UBC Table 23-Q]site coefficient for soil characteristics (rangesbetween 1.0 and 2.0, depending on site soilconditions) [See UBC Table 23-J]fundamental period of vibration in secondstotal design lateral force or shear at the basetotal seismic dead load (typically the opera-ting weight of equipment)seismic zone factor for effective peak groundacceleration (ranges from 0.075 to 0.40, de-pending upon geographic location) [See UBCTable 23-I]

The modifications or limitations include the following:

1) The ratio C/Rw shall not be less than 0.5.2) The vertical distribution of the seismic forces may

be determined either by static force or dynamic responsemethods, as long as the results are not less than thoseobtained with the static force method. (Note: Dynamicresponse methods are seldom used for equipment).

3) Where an approved national standard covers a par-ticular type of nonbuilding structure, the standard may beused.

Although they would seldom apply to equipment, cer-tain other restrictions as described in UBC 2338(b) forSeismic Zones 3 and 4 apply for Occupancy CategoriesIII and IV (Occupancy Categories in UBC Table No. 23-K). The structure must be less than 50 feet in height, and

TABLE 3.1.1.5a- SUMMARY OF MINIMUM LATERAL FORCE REQUIREMENTS FOR EQUIPMENT (Adapted from the 1991 Uniform BuildingCode)

T TEquipment ornon-building structures

CommentsMinimum values (importance factor = 1.0)

Zone 1 Zone 2A Zone 2B Zone 31

0.06wp 0.11Wp 0.15Wp 0.23Wp

0.11Wp 0.23Wp 0.3Wp 0.45Wp

0.04Wp 0.08Wp 0.1Wp 0.15Wp

UBC formula Typical examplesZone 41

Supported by structures andWp < 0.25W:

Rigid (T 5 0.06 sec)where Cp = 0.75

Nonrigid (T > 0.06 sec)where Cp, = 2 x 0.75

Fp = ZIpCpWpI

Rumps. motors, skidmounted equipment,

0.3Wp

(36-l) I small heat ex-changers

Fp = ZIpCpWpI

Leg-mounted vessels& equipment, stacks,

0.6Wp Minimum values increase 1.33 timesfor unbraced cantilevers, stacks, ortrussed towers where Cp = 2.0

0.2Wp Lateral force cannot be less thanthat from Formula (38-l) in Section

Supported at or below grade:Rigid (T is 0.06 sec)

where Cp = 0.75Nonrigid (T > 0.06 sec)

where CpP= 2 x 0.75 F,-ZI,;C,W,

(from 36-l)

Leg-mounted vessels& equipment, stacks,or slender processcolumns

0.4Wp Lateral force cannot be less thanthat from Formula (38-l) in Section

2338 (b)

0.08Wp 0.15Wp 0.2Wp 0.3Wp

0.04W 0.08W 0.1W 0.15W

0.07W 0.14W 0.18W 0.28W

Self-supporting structuresother than buildings:

Rigid (T s 0.06 sec)Nonrigid (T > 0.06 sec)(or where Wp = 0.25W)

(where C = 2.75and Rw = 3)

V = 0.5 ZIW(38-l)

Rigid vessels andbins

0.2W Based on forces distributed by UBCFormula (34-6)

v-ZICWRw

(34-l)

Tall slender vessels.tanks on grade, andsome elevated tanksand bins

0.37W See Note 2. lo Seismic Zones 3 and4 the code prohibits or restrictsnumerous concrete structural sys-terns, or imposes height limitationson others (see UBC Table 2.3-0)

1) See UBC Section 2334 (j) for vertical force requirements in Seismic Zones 3 and 4, and 2335 and 2336 for all zones.2) Formula (34-l) may govern over (38-l) where Wp > 0.25W because of vertical distribution of forces.


a Rw = 4.0 must be used for design. Additionally, theUBC prohibits or restricts numerous concrete structuralsystems in the higher seismic zones [UBC 2334 (c)3].

Using Formula (3-6) and an importance factor of 1.0,the minimum design lateral force or shear at the base fornonrigid nonbuilding structures would be 0.37W (seeTable 3.1.1.5a).

3.1.1.5e Vertical seismic loads - No verticalearthquake component is required by the UBC for equip-ment supported by structures [UBC 2334 (j)]. Forequipment with horizontal cantilever components inSeismic Zones 3 and 4, however, the UBC specifies a netupward force of 0.2Wp for that component,

If the dynamic lateral force procedure is used, thevertical component is two-thirds of the horizontal accel-eration. However, since the dynamic force procedure haslittle or no application to most equipment, many engi-neers designing structures in Seismic Zones 3 and 4 con-servatively use a vertical component of three-quarters ortwo-thirds of the horizontal component of the static lat-eral force procedure, combining it simultaneously withthe horizontal component.

The UBC also cautions about uplift effects caused byseismic loads. Only 85 percent of the dead load shouldbe considered in resisting such uplift. [UBC 2337 (a)].

3.1.1.6 Test loads- Most process equipment, suchas pressure vessels, must be hydrotested when in place onits foundation. Even when such a test is not initiallyrequired, there is a good possibility that sometime duringthe life of a vessel it will be altered or repaired, and ahydrotest may then be required to meet the requirementsof Section VIII of the ASME Boiler and Pressure VesselCode. Therefore, most engineers consider it necessarythat all vessels, their skirts or other supports, and theirfoundations be designed to withstand test loads. For thefoundation, this consists of the weight of water requiredto fill the vessel.

3.1.1.7 Maintenance and repair loads - For mostheat exchangers, maintenance procedures require thatperiodically an exchanger’s tube bundles be unbolted,pulled from the exchanger shell, and cleaned. The magni-tude of the required pulling force, and the fraction thatis transmitted to the exchanger foundation, can vary overa wide range, depending on several factors. These factorsinclude: (1) the service of the exchanger, including thetype of product, the temperatures, and the corrosivenessof the participating fluids, (2) the frequency of themaintenance procedure, and (3) the pulling or jackingprocedure actually used.

Since the forces transmitted to a foundation frompulling an exchanger bundle are so uncertain and var-iable, the design forces used are often based on pastexperience and rule-of-thumb. Common criteria are todesign for a longitudinal force that is a fraction of thetube bundle weight, ranging from 0.5 to 1.5 times thebundle weight. This force is assumed to act at the cen-terline of an exchanger, and is taken in combination onlywith the exchanger dead (empty) load. For stacked or

“piggyback” exchangers, the bundle pulI is assumed to acton only one exchanger at a time.

3.1.1.8 Fluid surge loads - Many types of processvessels (reactors, catalyst regenerators, etc.) are subjectto “surge” forces. Although the analogy may be less thanperfect, it is often convenient to describe fluid surge asa “coffee-pot” effect. The essential mechanism may besimilar to the boiling of a contained fluid, with theviolent formation and sudden collapse of unstable gasbubbles, currents of merging fluids with fluctuatingdensity, and sloshing of a liquid surface also contributingto the surge forces. These violent forces act erratically,being randomly distributed in both time and space withinthe liquid phase. Obviously, fluid surge is a dynamic load.However, because of the difficulty in defining either themagnitude or the dynamic characteristics of these forces,they are almost always treated statically for foundationdesign.

Surge forces are usually represented as horizontalstatic forces located at the centroid of the containedliquid. The magnitude of this design force is taken as afraction of the liquid below a normal operating liquidlevel. The fraction of liquid weight that is used will varyfrom 0.1 to 0.5 depending on the type of vessel, on theviolence of its contained chemical process, and on thedegree of conservatism desired by the owner-operator inresisting such loads. For most vessels supported directlyon foundations at grade, surge forces are small and areusually neglected.

3.1.1.9 Erection loads - Frequently, constructionprocedures and the erection and setting of equipmentcause load conditions on a foundation that will act at noother time during the life of the equipment. For example,before a piece of equipment is grouted into position onits foundation, local bearing stresses under stacks ofshims or erection wedges should be checked. Anothermore specific example is the case of a vertical vessel orstack that may be erected on its foundation prior to theinstallation of heavy internals or refractory lining. Onceinstalled, these internals are categorized as part of avessel’s permanent dead load. However, many practi-tioners feel it necessary to examine the situation thatcould exist for the interim weeks or even months prior toinstallation of this considerable internal weight. Designof a tall vertical vessel foundation may well be governedby overall stability against overturning, if it is requiredthat the temporary light structure be capable ofwithstanding full design wind.

3.1.1.10 Buoyancy loads - The buoyant effect of ahigh ground water table (water table above bottom offoundation) is sometimes considered as a separate load.That is, some engineers treat it as an upward-acting forcethat may (or may not) act concurrently with other loadsunder all load conditions. Perhaps just as frequently, thebuoyant effects are treated by considering them as a dif-ferent “condition” in which the gravity weight of sub-merged concrete and soil are changed to reflect theirsubmerged or buoyant densities (see Section 3.1.2.).

351.2R-10 ACI COMMITTEE REPORT

3.1.2 Loading conditions -Different steps in the con-struction of equipment, or different phases of its opera-tion/maintenance cycle, can be thought of as representingdistinct environments, or different “conditions” for suchequipment. During each of these conditions, there can beone or perhaps several combinations of loads that can,with reasonable probability, act concurrently on theequipment and its foundation. The following loading con-

ditions are often considered during the life of equipmentand its foundations.

Without addressing the philosophical difference be-tween these two perceptions, the effect is the same. Thebuoyant effect of a high water table may govern not onlythe stability (as outlined in Section 3.5), but may alsocontribute to the critical design forces (moments andshears) used in the design of the foundation.

When it is probable that the elevation of the watertable will fluctuate, most engineers will consider both“dry” (neglecting water table), and “wet ” (including thebuoyancy effects of a high water table) conditions whendesigning foundations.

3.1.1.11 Miscellaneous Loads- Other types of loadsare sometimes defined as separate loadings, and some-times grouped under one of the categories describedabove. Some are fairly specialized in that they are nor-mally applied only to certain types of structures orequipment. They include the following:

1) Thermal loads-Thermal loads are sometimes con-sidered as a separate load category, but were describedearlier in the section on operating loads.

2) Impact loads-Impact loads, such as those due tocranes, hoists, and davits, are sometimes classifiedseparately. Just as often they are classified (as describedabove) under live loads or, depending on the type ofequipment, as operating loads.

3) Blast loads-Explosion and the resulting blast rep-resent extreme upset or accident conditions. Normally,blast pressures are only applied to the design of controlbuildings. Seldom is such a load considered in the designof equipment or foundations, except possibly to set loca-tions so that there is adequate distance between criticalequipment and a potential source of such an explosion.

4) Snow or ice loads-Snow or ice loads may affectthe design of access or operating platforms attached toequipment, including their support members. Seldom dothey affect the design of equipment foundations exceptfor electric power distribution structures. Often, snowload is considered as a live load.

5) Electrical loads-Impact loads caused by thesudden movements within circuit breakers and load breakdisconnects may be greater than the dead weight of theequipment. Furthermore, the direction of the load willvary, depending upon whether the breaker is opening orclosing. In alternating current devices, short circuit loadsare usually internal to the equipment and will have littleor no effect on the foundations. However, in the case ofdirect current transmission lines, in which the earth actsas the reference, a short circuit between the aerial con-ductors and the earth may result in very significant loadsbeing applied to the supporting structures.

3.1.2.1 Erection condition - The erection conditionexists while the equipment or its foundation are stillbeing constructed, and the equipment is being set,aligned, anchored or grouted into position.

3.1.2.2 Empty condition -The empty condition willexist after erection is complete, but prior to charging theequipment with contents or placing it into service. Also,the empty condition will exist at any subsequent timewhen operating fluid or other contents are removed, orthe equipment is removed from service or both. This con-dition usually does not include the direct effect of main-tenance operations.

3.1.2.3 Operating condition - The operating condi-tion exists at any time when the equipment is in service,or is charged with operating fluid or contents and isabout to be placed into service, or is just in the processof being “turned off’ and removed from service. In theoperating condition, the equipment may be subject togravity, thermal, surge, and impact loads, and environ-mental forces such as wind and earthquake.

3.1.2.4 Test condition - The test condition existswhen equipment is being tested, either to verify its struc-tural integrity, or to verify that it will perform adequatelyin service. Although the time period actually required foran equipment test is a few days, the test “condition” maylast for several weeks. Thus, it is often assumed thatduring the test condition, an equipment foundation willbe subjected not only to gravity loads (that is, dead loadplus the weight of test fluids), but also wind or earth-quake. Usually, these loads are taken at reduced inten-sity. Typical intensities vary from one-quarter to one-halfof the wind or earthquake load.

3.1.2.5 Maintenance condition - The maintenancecondition exists at any time that the equipment is beingdrained, cleaned, recharged, repaired, realigned or thecomponents are being removed or replaced. Loads mayresult from maintenance equipment, davits or hoists,jacking (such as when exchanger bundles are pulled), im-pact (such as from the recharging or replacing of catalystor filter beds), as well as from gravity. The gravity loadis usually assumed to be the dead (empty) load.

The duration of a maintenance condition is usuallyquite short, such as a few days. Therefore, environmen-tal loads, such as wind and earthquake, are rarelyassumed to act during the maintenance condition.

3.1.2.6 Upset condition - An upset load conditionexists at any time that an accident, malfunction, operatorerror, rupture, or breakage causes equipment or its foun-dation to be subjected to abnormal or extreme loads.Often it is assumed that equipment subjected to severeupset loads may have to be shut down and repaired.Thus, it is not uncommon for ultimate strength to beused as the acceptance criteria for upset loads.

3.1.3 Load combinations - Codes usually specifywhich of the more common loadings should be assumedto act concurrently for building design. Industrial


TABLE 3.1.3a - REPRESENTATIVE LOAD CONDITIONS AND COMBINATIONS

Load case Condition Load combinations

1 Erection Dead load + erection

2 Erection Dead load + erection + Yz wind

3 Empty Dead load + wind

4 Empty Dead load + seismic

5 Operating Dead load + operating load +live load + thermal expansion +surge + piping forces

6 Operating Dead load + operating load +live load + thermal expansion +surge + piping forces + wind

7 Operating Dead load + operating load +live load + thermal expansion +surge -C piping forces + seismic

8 Test Dead load + hydrotest

9 Test Dead load + hydrotest + Li wind

10 Maintenance Dead load + bundle pull (heatexchanger)

11 Maintenance Dead load + maintenance/service

1 2 Upset Gravity + malfunction loads

Range of load factors*

1.1-1.5

1.2-1.3

1.3-1.5

1.4-1.6

1.6-1.7

1.3-1.5

1.4-1.6

1.1-15

1.2-1.3

1.4-1.6

1.4-1.6

1.0

l Load factors may vary. See Sections 3.1.3 and 3.1.4.

equipment, primarily because of the many possible vari-ations in operating loads, can have a far greater numberof possible load combinations. Often several differentload combinations are possible within a given load con-dition. Judgment, not codes, must be used to decidewhich loads and corresponding load factors can reason-ably be expected to act concurrently. Table 3.1.3a givesa list of twelve representative load combinations. Withsome variations among different practitioners, these com-binations are the ones most commonly used to designindustrial equipment and machinery foundations.

3.1.4 Load factors - Soil pressures and resistance tooverturning are calculated by most practitioners using aseries of load combinations similar to those listed inTable 3.1.3a with the individual combined loads at the“working” or in “service” level (unfactored loads).

When it comes to analysis of a foundation, however,it is not always clear which load factors apply to themany loads and load combinations, particularly those thatinclude “nonstandard” loads peculiar to industrial equip-ment. Most engineers, since they do not have a recog-nized or legal criteria to cite, feel obliged to conform tothe building code. They group the many loads unique toequipment under the common building code categoriesof “dead” and “live,” and directly apply the code’s pre-scribed load factors.

Other engineers contend that there are significant dif-ferences between loads applicable to equipment founda-tions, and those applicable to the design of commercial

or residential buildings. They conclude that these differ-ences warrant departures from a literal application ofcommon building code load factors. Differences includethe relative magnitudes of the different loads, and differ-ences in their durations. These considerations, taken to-gether, lead many engineers to select load factors that,although they may look similar to those in ACI 318, docontain important departures.

The factored loads are applied as follows: (1) Factorthe loads at the top of the pedestal, (2) factor the servicemoments and shears in the footing, and (3) factor thedifferences between multiple analyses. These differentapproaches are further explained in Sections 4.1 and 4.7.
If different load factors are to be used on the individualcontributing loads in a combination, and if compressionover the full width of the footing is not required, thenthese different approaches will give different results. Thisresults from the fact that when the resultant load is out-side the kern, the maximum soil pressure is not a linearfunction of the loads. Therefore, to avoid this possibleconfusion, some engineers apply a single composite loadfactor to all the loads in the entire load combination,rather than a different factor to each individual load.
Table 3.1.3a provides the range of load factors that iscommonly applied to the listed load combinations. Thesemay be single factors used for the entire combination or,where different factors are used for the various con-tributing loads, they may be the average ratio of totalfactored load to total service load.


3.2-Design strength/stressesIn the design of foundations, forces and stresses in the

various elements must be calculated and compared withacceptance criteria. Some types of acceptance criteria areexpressed in terms of allowable stress to which a calcu-lated service load stress is to be compared. Other criteriaare expressed in terms of a design strength to which cal-culated loads are to be compared. For many of the ele-ments of equipment foundations, there is neither apublished standard nor a clear consensus as to whichtype of criteria is appropriate.

Allowable soil pressures, anchor bolt stresses (tension,shear, bond), concrete bearing stress, and the requireddevelopment length of pedestal reinforcement that lapsplices to anchor bolts are some of those for which var-iations in practice are common.

In addition to the variations between the practicesused by different engineers, a second major variance isthat different acceptance criteria are often used foradjacent or interacting elements. This leads to interfaceproblems, and inconsistencies in the logic of the designof the various elements. At the very least, the existenceof different types of acceptance criteria for variouselements presents a tedious bookkeeping problem.

The strength design procedure for proportioning con-crete elements is referred to as Strength Design Method(SDM). The working stress procedure is now called the“Alternate” Design Method (ADM) in the current ACI318, and appears in Appendix A therein.

The following sections describe the individual ele-ments and the state of practice in defining acceptancecriteria for use in their design.

3.2.1 Concrete3.2.1.1 Bending - The flexural (bending) capacity

of concrete elements in a foundation for static equipmentis usually determined using design criteria contained inACI 318. These criteria from ACI 318 appear in the fol-lowing table:

M fdfc’ 4SDM Mu 0.85 0.9A D M Mw 0.45 -

where:f b = extreme fiber stress in compression due to

bendingMu

= factored momentMw = moment

The factor 4 is the strength reduction factor, to takeinto account the probability that an element may be un-able to perform at nominal strength due to inaccuraciesand adverse variations in material strength and workman-ship during construction.

3.2.1.2 Flexural shear - Concrete shearing stressesare of two general types. Where the foundation memberis long relative to its width, or the pedestal dimensions

are a significant fraction of the pad dimensions (say morethan one-third), or both, then the most critical diagonaltension stresses occur at approximately a distance d fromthe support pedestal. The quantity d is the effectivedepth of the concrete foundation pad, measured from theextreme compression fiber to the centroid of the tensionsteel area. In this case, the stress state is termed “widebeam shear,” or simply “beam shear.” As previously indi-cated, the present trend is toward the use of strengthdesign and the use of factored loads (moments) in pro-portioning concrete elements. The normally used stresscriteria prescribed by ACI 318 are as follows:

V v,m4l@~ 4

SDM Vu 2.0 0.85ADM I’, 1.1 -

where

Vw= total working shear on the section through

the foundation padVu

= 4 x Vc is the factored total shearVc

= nominal limiting or allowable shearstrength

b = section width located at a distance d fromthe supporting face.

Although most engineers use the ACI 318 criteriadescribed in the previous paragraphs without modifica-tion, some practitioners choose to use Ferguson andRajagopalan5 These authors point out that the codecriteria for ultimate beam shear stress are significantlynonconservative for low percentages of reinforcement,with reductions in shear capacity approaching 50 percentfor foundations with minimum steel. The authors recom-mend a reduced value for beam shear resistance for flex-ural sections where the tensile reinforcement ratio is lessthan 0.012. The following equation for determining thedesign nominal shear stress vc is suggested.

vc = (0.8 + 100 p)fl < 247

where:

(3-7)

vc= V#bd

P = ratio of nonprestressed tension reinforcement

Most foundations have reinforcement ratios less than1 percent. Many equipment foundations have reinforce-ment ratios less than 0.5 percent. Thus, some engineersuse values for design beam shear stress reduced fromACI 318 criteria in accordance with the recommenda-tions of Ferguson and Rajagopalan.5

3.2.1.3 Punching shear (two-way shear) - When afoundation pad or pile cap is square, or nearly so, or thepedestal dimensions are small relative to the main foun-


dation member (pad or pile cap), or both, then a shear-ing stress state different from the one described in Sec-tion 3.2.1.2 usually becomes critical. This alternativeshearing failure mode occurs when a small pedestal tendsto punch through its supporting foundation pad. ‘Thediagonal tension stress for this shearing stress state isaptly termed “punching shear.” The critical section, bo,for this potential failure mode is taken at a distance d/2from the supporting face. For heavily loaded piles in acluster, consideration for possible misalignment duringpile driving should be included in the calculation.

The normally used stress criteria from ACI 318 are asfollows:

SDM Vu (2 + 4,‘#?c) < 4.0 0.85

a dor Vu J + 2 < 4.0

boA D M Vw (1 + 2/Pc) < 2.0 -

where:

= total working shear force at the critical sec-tion

= total factored shear force at the critical sec-tion

= allowable or limiting shear strength at thecritical section

= ratio of longer to shorter pedestal dimen-sion. /3c = 1.0 for round or octagonal pedes-tals

= 40, but reduced to 30 if the pedestal is off-centered

Although ACI 318 allows some refinements of theserelationships when shear reinforcement is added, suchreinforcement is rarely used in equipment foundations.

The discussions of shear in concrete foundations inthis and the previous section are directed toward indi-vidual footings. ACI 318 is unclear as to the appropriateshear stress criteria for mat foundations. However, mostpractitioners use the punching shear provisions whenchecking shear in such foundations.

3.2.1.4 Tension -ACI 318.1 permits plain concrete(unreinforced) spread footings. ACI 318.1 for plain con-crete limits the use of plain concrete to foundations thatare continuously supported by soil or where arch actionassures compression under all conditions of loading.However, unreinforced concrete spread footings are sel-dom used for equipment foundations, except for verysmall, minor equipment such as for residential air con-ditioner support pads. In the rare cases where unrein-forced foundations are used, the maximum concretetensile stresses permitted by ACI 318.1 are as follows:

SDM Mu 5.0 4 0.65ADM Mw 1.6 -

where ft,= extreme fiber stress in tension.Foundations are often subjected to overturning mo-

ments large enough to produce uplift over a portion oftheir base. Since soil cannot resist uplift by tension, thisresults in a zone of zero pressure, with the resultingtriangular pressure prism shown in Fig. 4.7.3. In the
absence of upward soil pressure, a negative bending mo-ment can be produced in the cantilevered portion of thefooting which must be resisted by tensile forces in the topof the pad. This negative moment is limited to the fullgravity weight of the uplifted part of the footing, plus anyoverburden or surcharge components, regardless of themagnitude of the applied overturning moment.
The tensile capacity of concrete should not be utilizedin a seismic zone, or when a footing is supported by piles(UBC). However, there are differences of opinion andpractice concerning treatment of overturning forcescausing a negative moment in a spread footing in a non-seismic zone.

When the magnitude of this reversed or negativemoment is small, some engineers use the allowable con-crete tensile stresses given by ACI 318.1 for unreinforcedfootings to check the adequacy of the footing. Othersconsider the fact that a reinforced section subjected topositive moment develops cracks through as much as 80percent of its thickness. Relying on such a cracked sec-tion for reversed bending (negative moment) is con-sidered unsafe by many practitioners. Some engineers usetop reinforcement if there is any calculated tension in thetop “fibers” of the footing. Others, although aware of theuncertainty in the section’s capacity, are reluctant toprovide a top mat of reinforcing steel to resist what isoften a very nominal stress level. They may arbitrarily usethe tensile capacity of the uncracked concrete section,but use only a fraction of the tensile stresses permittedby ACI 318.1 for unreinforced footings. The values usedrange from 20 to 50 percent of the nominal code values.Although there is reason to question the validity of thislatter practice, there are no reported failures of footingsdesigned with such an approach.

The above discussion of concrete tensile strength isoften rendered academic by the use of minimum slab re-inforcement in the top of a footing, provided ostensiblyas temperature and shrinkage steel. There is no coderequirement that, in the absence of calculated stresses,such reinforcement be inserted in the top of a founda-tion. However, some practitioners consider it good prac-tice to always have a top mat of steel.

3.2.1.5 Bearing - The allowable bearing stresseson concrete contained in the current ACI 318 reflectrecent studies showing that a triaxial state of stress isproduced in the concrete in the zone beneath the base


(bearing) plate. This effect is considerably more pro-nounced if the equipment or column base plate is cen-trally located so that the loaded zone is surrounded onall sides by concrete.

The allowable and design bearing stresses permittedby ACI 318 are as given in the following table:

f 4

SDM 4 (0.85f,‘) 0.7ADM 0.3f,’ l -

where f = bearing stress.

When A, > A,, the design bearing strength may bemultiplied by Jm 5 2.0.

where:

A1 = area in bearing on concreteA 2 = area of the largest frustum of a right pyramid or

cone contained wholly in the foundation whenthe upper base is area A1 and the side slopesare 1 vertical to 2 horizontal.

When designing base plates and annular base rings forconcrete bearing, many engineers use the strength designconcepts as defined in ACI 318. However, particuarly forequipment foundations such as verticalvessels and stacks,many engineers choose working stress criterion instead.There are two reasons for this departure from the nor-mally accepted ACI approach. First, anchor bolt designis commonly based on a working stress criterion. Thedetermination of required bearing area is an interrelatedfunction of the anchor bolt area provided. Therefore, adesire for consistency leads many engineers to use anallowable working stress for bearing.

The second reason is that design of equipment baseplates and base rings is performed by equipment de-signers. Equipment designers are usually mechanicalengineers with little or no experience in concrete designor in the strength design concepts of ACI 318. The needto simplify communication of design criteria, pointstoward the selection of working stress criteria forconcrete bearing.

When working stress criteria are selected for thedesign of equipment base plates, the allowable stressesspecified in the AISC-ASD specification, Chapter J9, areusually used. This is because equipment manufacturer’sengineers are usually familiar with this specification.

One question that arises in the design of verticalvessels and stacks that are supported on annular baserings is that the bearing area is not centrally located inthe pedestal. Rather, the most heavily loaded area isimmediately adjacent to the edge of the concrete pedes-tal. This fact leads many engineers to neglect the arearatio increases in the allowable stress.

3.2.2 Reinforcement3.2.2.1 Vertical reinforcement - The vertical rein-

forcement in foundation pedestals is, for most types ofequipment, designed as an integral part of the total con-crete section, that is, by treating the pedestal and itsreinforcement as a beam-column. For this approach, ACI318 design criteria are usually employed. For pedestalswith a height-to-lateral dimension ratio of 3 or greater,the required reinforcement should be not less thanminimum reinforement applicable to columns. However,for equipment such as tall vertical vessels, the purpose ofthe vertical pedestal bars is to lap the anchor boltanchorage zone (see Fig. 4.2.lc), and to transfer the
anchor bolt tensile forces from a pedestal into thefooting or pile cap. In this situation, practice for definingthe appropriate acceptance criteria for designing thevertical bars varies widely. Some engineers design thepedestal bars using the total concrete section as de-scribed above. Some use a practice similar to that usedin designing anchor bolts. They proportion the verticalbars either to resist the calculated anchor bolt tensileforces, or to match the design capacity of the anchorbolts, ignoring the concrete.
Still other practitioners replace the yield strength ofthe equipment anchor bolts with an equivalent or greateryield strength in the lapping vertical reinforcement, againignoring the concrete section. This latter practice is usedprimarily in seismically active areas, the rationale beingthat initial yielding should take place in the more visibleanchor bolt before the reinforcement to which the pri-mary anchorage forces must be transferred.8,11

3.2.2.2 Horizontal reinforcement - For small pedes-tals, or where the governing loads are primarily compres-sion, the horizontal reinforcement in pedestals is com-monly sized in accordance with ACI 318 criteria for col-umn ties. However, there are a number of circumstanceswhere other types of criteria are used.

One example occurs in the case of pedestals with alarge area, such as for vertical vessels and stacks. In thiscase, the vertical reinforcement is usually designed toresist tension. The horizontal reinforcement in the pedes-tal faces may be essentially nominal - perhaps just tokeep the vertical bars in place during the concrete place-ment. Sometimes, a minimum reinforcement criterion forbars in faces of mass concrete such as suggested in ACI207.2R is used. Larger size reinforcement and/or lesserspacing than defined by such minimum criterion may beprovided for confinement of the anchor bolts and to pre-clude spalling at the pedestal face.2,3,9

In addition to the main horizontal reinforcementprovided in the face of vertical vessel pedestals, manypractitioners consider it good practice to provide a groupof two to four tie-bars near the top of the pedestal,closely spaced at 3 to 4 in. (see Fig. 2.2.4). This closely

* A one-third increase is permitted for wind and seismic loads.


spaced top set of peripheral reinforcement is to assist inresisting cracking due to edge bearing on the pedestal orto thermal expansion, as well as to provide confinementfor resistance to shear. This practice reduces cracking ofthe concrete near the top of the pedestal due to transferof shear forces through the anchor bolts into the con-crete.

Sometimes, horizontal reinforcement is provided inthe tops of pedestals. For example, reinforcement mayoccasionally be required by stress calculations for rela-tively large, thin, or shallow pedestals (which are essen-tially as large as the pad), where the load is applied atthe edge or periphery of the pedestal. In this situation,the pedestal could tend to dish upwards, and there wouldbe a calculated tension at the top of the pedestal.

A few practitioners provide horizontal reinforcementin the top of pedestals for equipment as a matter of goodpractice, particularly where the equipment operates atelevated temperatures. Reinforcement congestion, how-ever, can lead to construction problems. Engineersshould review the final design to assure that it is abuildable design.

Design of horizontal reinforcement in footings (or pilecaps) uses ACI 318 criteria for flexural reinforcement.The only questions that arise concern minimum amountsof reinforcement, as outlined in Section 4.7.5.

3.2.3 Anchorage -Anchorage of a piece of equipmentto its foundation is often the most critical aspect of afoundation design. This is particularly true for verticalvessel and stack foundations, or for any other equipmentfoundation where consideration of lateral loads dom-inates the design. ACI 355.1R summarizes the mostwidely used types of anchors and provides an overview ofanchor performance and failure modes.

Anchors can be either cast-in-place or retrofit.Retrofit anchors are installed after the concrete hashardened, and can be either undercut, adhesive, grouted,or expansion.l An undercut anchor transfers tensile load to the

concrete by bearing of an expansive device againsta bell-shaped enlargement of the hole at the baseof the anchor.

0 An adhesive anchor consists of a threaded rod in-stalled in a hole with a diameter of about l/16 to l/ein. larger than the diameter of the rod. The hole isfilled with a structural adhesive such as epoxy, vinylester, or polyester. Adhesive anchors transfer ten-sile load to the concrete by bond of the epoxy tothe concrete along the embedded length of theanchor.

l A grouted anchor consists of a headed anchor in-stalled in a hole with a diameter about 1*/z in.larger than the diameter of the anchor. The holeis filled with a non-shrink grout, usually containingportland cement, sand, and various chemicals toreduce shrinkage. Grouted anchors transfer tensileload to the concrete by bearing on the anchorhead, and by bond along the grout/concrete inter-

face.l Expansive anchors transfer tension load to the con-

crete by friction between the anchor and the con-crete. The friction force results from a compressivereaction generated in opposition to the movementof an expansion mechanism at the embedded endof the anchor.

NormalIy, adhesive anchors have higher allowable loadvalues than mechanical anchors. The selection of a retro-fit anchor would depend on its use and type of exposuresuch as temperature, moisture, vibration, and possiblechemical spills. The manufacturer should provide the re-quired information to suit specific needs.

A cast-in-place anchor is cast into the fresh concrete.The tensile load is transferred to the concrete eitherthrough bearing on the head of the embedded anchor, orthrough bond strength between the anchor and the con-crete. The results of the latest research recommend usingheaded anchors rather than the “J” or “L” bolts, whichdepend upon bond.

3.2.3.1 Allowable stresses - Allowable stresses forretrofit anchors are based on the results of tests con-ducted by the manufacturer of the particular anchor. Al-though some manufactured expansion anchors are cap-able of developing the capacity of their bolt stock, mostare designed using allowable loads much lower thanwould be determined by the strength of the bolt metal.Commonly, safety factors of four to five relative to pull-out are used to determine an allowable load for retrofittype anchor bolts.

Cast-in-place anchor bolts are usually designed todevelop applied tensile forces, up to and including thecapacity of the bolt, with appropriate safety factors. Theamount of embedment is dependent on concretestrength, edge distance, and bolt spacing. The designpractices that are used to insure adequate anchorage aredescribed in Section 3.2.3.2. Most commonly, cast-in-
place anchor bolts are sized using the allowable stressesspecified by the AISC-ASD specification. In the AISC-ASD specification, both the allowable stress and, in thepast, the effective area vary with the specific material.For example, anchor bolts fabricated from ASTM A 307material commonly have been designed using the AISCspecified allowable stress of 20 ksi together with thecorroded “tensile-stress” area of the threaded boltstock.2,4 The corroded tensile-stress area A, is usuallydefined as follows:
(3-8)A. = 0.7854 fD - =j2tt n I

where:

D = nominal bolt diameter in in.n = number of threads per in. (the reciprocal of the

thread pitch)

A corrosion allowance may be required and it should


3.2.3.2 Anchorage criteria- In the past, there havebeen wide variations in the criteria used for the design ofthe embedded portion of cast-in-place anchorages whichattach equipment to their foundations. Prior to 1975,many practitioners used the relatively low allowable loadson anchor bolts (both tension and shear) contained inthe Uniform Building Code. The allowables contained inthe UBC cover only headed bolts 1% in. in diameter andsmaller. These allowables were originally based on mini-mal test data on bolts ‘/8 in. in diameter and smaller, and

are appropriate for nominal embedment lengths in unre-inforced concrete sections.

In the absence of more definitive criteria, some en-gineers have extrapolated UBC values. They have calcu-lated bolt capacities using a variety of approaches. Thesehave included using an allowable bond stress on the boltshank (ACI 318), or a code allowable bearing stress onthe anchorage head (usually a plate or washer or both).Configurations used have included either hooks (“L” or“J” type hooks), or a plate or washer at the anchor head.

The lack of accepted definitive criteria for the designof cast-in-place anchorages has been largely a result ofthe absence of reliable test data. However, since 1964,there has been a major increase in the amount of basicresearch in the area of anchorage to con-crete . 1 , 2 , 4 , 6 , 7 , 9 , 1 0 , 1 3 Based on this relatively new data,several guides or suggested practices have been published(ACI 349, PCI Design Handbook, and References 4 and10). The Center for Transportation Research, The Uni-versity of Texas at Austin, has published the following re-search reports: Research Report 1126-1, Research Report1126-2, Research Report 1126-3 and Research Report 1126-4F.

In spite of the new data and newly suggested criteria,industry practice has changed slowly. First, many ques-tions remain unanswered. Thus, a diversity of practicesand opinions exist. Second, perhaps because full consen-sus has not yet been achieved on the appropriate criteria,model codes such as the UBC still have not updatedtheir provisions. The lack of full consensus can also beexplained by reviewing the series of tests referencedabove.

The behavior of anchors depends on a number of vari-ables, including the following:l Loading (axial load, moment, shear)0 Size of the steel attachmentl Size, number, location, and type of anchorsl Coefficient of friction between the base plate and

the concretel Tension/shear interaction for a single anchorl Distribution of shear among the anchorsl Distribution of tension among the anchorsl Flexibility of the base platel Concrete strengthl Base plate configuration (embedded, flush, or on

raised grout pad, important for anchorages subjectto shear forces)

l Reinforcement in the foundation or pierl Embedment lengthl Edge distance and anchor spacingSmooth bolts with hooks ("J” or “L” type bolts) have

been fairly well discredited by the recent research. As aconsequence, their use has declined substantially in re-cent years. The preferred configuration is now either aheaded bolt or a threaded rod with a bearing plate or anut, or both.

A report from the University of Texas (Research Re-port 1126-4F) states that headed anchors should have

be added to the required bolt area. It will vary with bothlocation (seacoast versus inland, etc.) and the possibilityof spills of acids or other chemicals. Such values com-monly range from ‘h6 to ‘%i in.

The AISC design specification permits stresses to becalculated on the nominal body or shank area of boltsand threaded parts (AISC-ASD Specification, Section1.5.2). However, designers of equipment foundations pre-fer greater conservatism in anchor bolt design than thatused in the design of other foundation components. Forexample, when designing anchor bolts, many engineers donot take advantage of the one-third increase in allowablestress that is normally permitted under temporary loadssuch as wind and earthquake. Similarly, many engineers,perhaps acknowledging the possibility of dynamic load-ings, use the bolt tensile area rather than the largershank area, when calculating the effective bolt tensilecapacity.2,4

ACI 349 uses strength design where the service loadsfor the anchors are factored. A strength reduction factorfor the steel and concrete is consistent with AISC-LRFD,and ACI 318.

Occasionally, higher strength bolt materials are usedin the design of anchor bolts for equipment foundations.However, the high material cost of high-strength boltsand the greater complications of attaching them to theequipment being anchored makes their use the exception.For example, if high-strength anchor bolt material isused, a special design of the equipment’s bolt anchor lugsmight have to be performed. Any special design which re-quires a change to an equipment manufacturer’s standardbase detail, may cost more in “extras” than any nominalsavings afforded by a more efficient bolt pattern. Aductile anchor is an anchor sufficiently embedded so thatfailure will occur by yielding and fracture of the steelwhen loaded in direct tension. Higher strength boltswould require more embedment in concrete to reachtheir capacity. Because there is insufficient test data onthe ductility of high strength anchors, ACI 349 re-commends a yield strength fy greater than 120,000 psi notbe used.

Also, particularly for tall vertical vessels and stacks,the lesser ductility of high-strength anchor bolts oftenrules against their use in seismic areas. This is becausethe common assumption that ductile behavior is desirablein large earthquakes, leads to the conclusion that theprimary source of energy absorption will be in yielding ofthe vessel’s anchor bolts.8,11

351.2R-17

dimensions equivalent to a standard bolt head or stan-dard nut. Standard dimensions for bolt heads are givenin ANSI B18.2.1. Standard dimensions for nuts are givenin ANSI B18.2.2. Bearing at the anchor head does notrequire evaluation.

ACI 349 uses the following two methods of sheartransfer:

0 Bearing-In connections where the baseplates aremounted flush or above the concrete surface, thedominant mechanism of shear transfer is bearingon the anchor. Since the holes in the baseplate areusually oversized according to AISC recommenda-tions, there is a question of how the plate goes intobearing against the anchor and how many anchorswill actually transfer the load (see Fig. 3.2.3.2).

1

Figure 3.2.3.2

Some engineers assume only half of the anchorsactually transfer the shear load. Others only use nomore than two bolts for shear transfer.

0 Shear friction-Shear transfer is similar to themechanism described in 11.7 of ACI 318. A frictionforce is generated by a clamping force that acts asa fractured shear plane in the concrete.

Though ACI 349 provides comprehensive proceduresfor anchor bolt design, there remains a considerable dif-ference of opinion and practice in the provision for fullductile embedment for anchor bolts. ACI 318 containsone paragraph concerning ductility (15.8.3.3) that mostengineers consider too vague. ACI 349 specifies that boltembedment be provided for the bolt’s tensile and shearcapacities, regardless of actual loads. This would assurea ductile connection. Some practitioners consider ACI349 to be too conservative and provide anchorage basedon actual strength. Others, using UBC criteria, designanchorages based on factored loads. If the bolt embed-ment is designed for the applied loads, the anchor shouldnot be considered a ductile connection.

The required embedment for a headed bolt (or boltwith a nut) is calculated assuming a frustum of a 45-degpullout cone emanating from the anchor head to the freeconcrete surface. A uniform nominal (tensile) stress of4&r (withf,’ in psi) acting on the projected area of thiscome on the concrete surface is recommended as a de-sign capacity under factored load. Interference or overlapwith concrete edges or cones from adjacent bolts is de-ducted from this effective stress area. A criterion isprovided for computing the edge distances required forresisting both tensile and/or shear forces.

One other aspect of anchorage that merits mention isthat of sleeving of anchor bolts. Here again, practicevaries and some practitioners do not use sleeves in thefoundations for static equipment. Others insist on theirnecessity, but use a variety of types and configurations.

The primary purpose of sleeving an anchor bolt is toease the alignment of the bolt with the home in the base-plate of the equipment. Sleeves may be constructed ofpipe, sheet metal, high-density polyethylene or a holeformed using Styrofoam. After installation of the equip-ment, the sleeves are usually filled with grout. However,

some engineers, particularly those who design a post-ten-sioned type bolt, will specify a grease or mastic type fillerfor the sleeve.

When a bolt is fully embedded in concrete, a bondbreaker to increase ductility of the bolt can be achievedby wrapping the upper portion of the bolt with duct tapeor insulation.

Some engineers specify double nuts for anchors undertension to prevent backoff.

3.2.4 Soil- The procedures for determining allowablesoil pressures or pile capacities are beyond the scope ofthis report. These allowable pressures and capacities areusually established by a geotechnical consultant usingstandard procedures (not unique to equipment founda-tions). However, it is worth noting that besides settle-ment considerations, allowable vertical soil pressures orpile loads are also limited by dividing a nominal capacityby a safety factor that ranges from 2 to 5, depending pri-marily on the soil type and the type of loading (tempor-ary or sustained).

Criteria for the lateral resistance of soil will vary withthe type of foundation as well as the type of soil. Formost shallow spread footings that are excavated, formed,placed, and backfilled, passive soil pressures are neglec-ted. Resistance to lateral loads is usually presumed to bea result of bottom friction alone. This is mainly becauseof uncertainty regarding the quality of the backfill mater-ial and the control of its placement. However, some geo-technical engineers will include the lateral resistance ofpassive pressures to a certain degree, consistent withallowable lateral movement, if a certain depth of backfillfinish grade is ignored in its calculation.

Lateral resistance of pile foundations is often deter-mined using the lateral resistance of the piles only. Inthese instances, the resistance contributed by passive soilpressure acting on the sides of the pile cap is ignored.However, if the lateral displacements of the pile foun-dations become “large” (flexible piles), passive soil resis-tance may be included in the design. Alternatively, ifthere is adequate space available, battered piles may beused to resist lateral loads.

Drilled caissons are often designed using horizontalsoil pressures to resist horizontal shears at the top of thefoundation, as well as overturning moments. The “allow-able” lateral pressure is usually deduced from a permitted


lateral displacement at the top of the foundation. Theprocedure may range from directly assuming a soil pres-sure profile to a complex caisson-soil interaction analysis.

3.3-Stiffness/deflectionsCriteria for stiffness or allowable deflections for

foundations supporting static equipment vary widelydepending on the particular application. For manyapplications, there are no special requirements otherthan engineering judgment. For others, deflections mayneed to be tightly controlled.

Differential settlement or lateral movement betweenadjacent pieces of equipment that are connected bypiping, ducting, chutes, conveyor belts, etc., may have tobe controlled to avoid overstressing the piping or mis-aligning the belts or chutes. Some types of vessels may beserviced by piping that is glass or ceramic lined. Toler-able displacements for such fragile items may be as lowas a few hundredths of an inch. Some equipment may re-quire precise alignment for its proper operation. How-ever, as a rough order of magnitude, long-term settle-ments of I/t in., or short-term lateral movements (such asunder wind load) of 9’4 in. are usually suitable for mostnoncritical static equipment.

For some applications, flexibility rather than stiffness(or rigidity) is the desired result. Foundations that sup-port equipment connected to high-temperature piping, orthat support opposite ends of a horizontal vessel or heatexchanger subject to thermal growth will have substantial-ly reduced forces if they possess even a modest flexibility.

3.4-StabilityIn addition to soil bearing and settlement, stability

must also be checked to determine a minimum founda-tion size. Stability checks must be made, as applicable,for sliding, overturning, and uplift.

Sliding stability may be of concern for foundations onrelatively weak soils supporting equipment subjected tolarge lateral forces. Such situations may include dead-men, retaining walls, or exchangers subject to bundlepull. Sliding stability is usually checked by verifying thatlateral forces are less than allowable base friction or ad-hesion, plus passive pressure.

Overturning stability criteria will frequently control inthe design of foundations with high allowable soil pres-sures, or in the design of foundations for tall equipmentsubjected to high wind or seismic loads. The size of foun-dations for tall vessels and stacks are commonly con-trolled by overturning. A stability ratio is used tocharacterize a foundation’s resistance to overturning. Itis defined as the resisting moment divided by the over-turning moment. Moments are computed at the bottomedge of a spread footing. The resisting moment includesthe permanent weight of the equipment, foundation, andsoil overburden. Working level, or service loads, are usedin the computation.

For foundations supporting an entire piece of equip-ment, such as a vertical vessel on a spread footing or a

heater supported on a combined mat, the stability ratiomay be simplified to the following formula:

PDPStability ratio = - = %M

where:P = vertical load due to weight of concrete and

equipmentM = overturning moment applied to footingD = edge to edge distance of footing in direction of

overturning momente = M/P

Alternative equations in use are based on a requiredfooting area in compression with the soil. A stability ratioof 1.5 equates to half the footing area in compression.

For isolated foundations supporting a piece of equip-ment such as a heater, on separate spread footings, aportion of the overturning moment can be resisted byvertical forces at each footing. This is a situation ofcombined uplift and overturning stability. The stabilityratio is then determined by computing separately thefooting edge moments due to dead weight and wind orseismic.

Generally, a foundation will most probably fail inother modes before overturning. Some practitioners in-clude soil failure considerations in accordance with pub-lished design recommendations of ACI 336.2R and ACI336.3R. Most practitioners, however, use the simplermethods described above as a general indication of thefactor of safety against overturning.

Although the concept of a stability ratio is quitestraightforward, there is a wide range of minimum re-quired values. Some of the more conservative practi-tioners require that the full base of a foundation remainin compression, and thus imply a stability ratio of 3.0 to3.75, depending on the footing geometry. Some engineersrequire that the stability ratio be not less than 2.0, butmany permit a stability ratio of 1.5. A ratio of 1.5 is thelowest value that is commonly accepted and is the mini-mum specified by UBC, SBC, and BOCA for wind loads.None of these building codes specifies a minimum stabil-ity ratio for seismic loads.

For pile foundations, the concept of a stability ratio isstraightforward where the piles are not designed to resistuplift. The center of moments is taken at the most lee-ward pile. However, when the piles have a tension capa-city, the concept becomes ambiguous and is seldom used.

For drilled pier foundations, the procedure for usingthe stability ratio is unclear and many different practicesprevail. For example, since a drilled pier can mobilizelateral passive soil pressure to resist overturning, astability ratio might be defined by either of the followingtwo formulas:

SR1 =PD/2 + Mp

M(3-10)


4.1-Available methodsFoundations for static equipment are generally

designed by either the Strength Design Method or theAlternate Design Method (formerly termed the “WorkingStress” method as defined by ACI 318). While practi-tioners in general have adopted the Strength DesignMethod, there are still many engineers who use theAlternate Design Method. Such usage persists, largelydue to the familiarity gained through many years of use.

A third design method that has yet to achieve formalrecognition or endorsement by an American code-writingbody is Limit Design. Limit Design in reinforced con-crete parallels Plastic Design in structural steel. Its mostsignificant feature is its reduction of complex analysisproblems to relatively simple yield line problems. Thismethod is sometimes utilized in the design of complexfoundations which present complicated elastic analysisproblems. The Limit Design Method is not covered inthis chapter.

SR2 + MpDpMP

(3-11)

Where Mp is the resistance to overturning provided bythe lateral passive soil pressure, and the center of mo-ments is again at the toe of the drilled pier base. Thefirst of the above definitions of stability ratio would bemore meaningful for a drilled pier (particularly a straightshaft) whose diameter is relatively small compared to itsdepth, and that relies predominantly on the lateral soilpressure (pole action) for its resistance to overturning.However, the second definition might be appropriate fora large diameter shallow drilled pier whose major resis-tance to overturning is the size of the bell.

CHAPTER 4-DESIGN METHODS

4.2-Anchor bolts and shear devicesForces produced by wind, seismic, thermal, and other

sources must be transferred through the static equipmentinto the supporting foundation. Typical anchorages con-sist of anchor bolts to transfer tensile forces or a com-bination of tensile and shearing forces. When required,shear lugs may be used to transfer shear forces.

4.2.1 Tension - Anchor bolts are provided primarilyto transfer tensile forces. They consist of several differenttypes and generally fall into one of the categories shownin Fig. 4.2.1a. Types “L” and “J”, which are cast-in-placebolts, rely on bond to develop the capacities of the bolts.Types “P”, “N”, “H”, “PN”, “PH”, and “S”, which are alsocast-in-place bolts, rely on the pullout strength of theconcrete. Types “SD” and “DI” are, respectively, self-drilling and drop-in bolts relying on expansive forces totransfer the tension to the concrete or to the mechanicalanchorage.

The various bolt types are generally carbon steel orlow alloy materials and may be provided with sleeves.

B O L T‘L ’ ‘J ’ B O L T ‘P ’ B O L T ‘N’ BOLT 'PH' BOLT- -

‘PN’ BOLT B O L T'S' ‘S O ’ B O L T 'DI' B O L T 'H' B O L T

Fig. 4.2.la-- Selected anchor bolt types (other anchor bolttypes may be available that are not shown)

Type "S” open-sleeve bolts may be post-tensioned toassure residual bolt stress if desired.

Design of anchor bolts is a multi-step procedure. Thetensile forces in the assumed bolt pattern are computed.Thereafter, the bolt area and embedment are deter-mined, followed by considerations for edge distance andspacing.

Generally, a bolt force formula is used to compute themaximum bolt force, F. Such a formula (below) is easy toapply, and it is always conservative:

F = (W/N) [(4e/d) - 1] or F = (4M/Nd) - W/N (4-l)

where:W = weight of equipmentN = number of bolts

i4= eccentricity of vertical load (M/W)= moment applied to anchorage

d = diameter of bolt circle

For high-profile static equipment (height/diameter >7), typically tall vessels and stacks, the proper determina-tion of anchor bolt forces is a primary requirement of thefoundation design. For such equipment, a more exactmethod is often used. For less critical installations theanchor bolts are assumed to comprise an annular ring ofsteel in a hollow concrete column section (see Fig.4.2.1b). The neutral axis of the section shifts to where
there is a condition of equilibrium between the steel andconcrete. This procedure often results in a considerablyreduced bolt requirement as compared to that computedusing the force formula, but it requires more time andeffort to apply.


NOTE: TIES NOT SHOWN FOR CLARITY.

Figure 4.2.1c

N . A .

Fig. 4.2.1b-Hollow column section anchor bolt design

Having determined the force(s) in the anchor bolts,one must determine the required area, select the type ofbolt, and compute the required embedment. The areamay be determined following the criteria given in Section3.2.3. For the bolt type selected, the embedment is com-puted to satisfy pullout requirements or to transfer thetensile forces to vertical reinforcement from bolts cast inpedestals. When vertical dowels are used to transfer ten-sile forces to foundations, care should he exercised toassure that sufficient development length as shown inFig. 4.2.1c is provided. Chapter 12 of ACI 318 should beused in determining development length for vertical barsin pedestals.

Embedment lengths for “L” and “J” type anchor boltshave been traditionally determined using bond stressprovisions for plain bars taken from Chapter 18 of ACI318-63. Edge distance and bolt spacing are most impor-tant in the design of anchor bolts that rely primarily uponthe pullout strength of concrete (types “P”, “N”, “H”,“PN”, “PH”, and “S”, Fig. 4.2.la).

4.23 Shear - Shear forces may be transferred by avariety of mechanisms: friction resulting from theclamping action provided as a result of tightening thebolt(s), shear lugs, or direct bearing of the anchorageagainst the concrete.

Table 26-E of the Uniform Building Code (UBC-91)can be used to design anchor bolts for shear. Thetabulated values apply to headed anchor bolts (type “N”and “H”) cast in plain concrete.

Designs that rely on the anchor bolts to transfer shearthrough bearing on the sides of a baseplate or throughshear plates welded to the bottom of the baseplateshould be approached with caution. The likelihood thatall bolts in a large group or pattern will participateequally in the transfer of the shear load in bearing isunrealistic. Given the normal practice of using oversizedholes in the baseplate and the small misalignments thatoccur between bolts, only a fraction of the bolts will bearsimultaneously against the baseplate and hence be cap-able of transferring shear load (see Fig. 3.2.3.2).

4.2.3 Tension/shear interaction - When bolt tensionand shear forces are present in an anchorage, the inter-action of the two should be considered. Frequently, inter-action relationships are used such as those specified bythe AISG-ASD Section J3.6 (AISC-1989) for structuralbolts. Recently, Cannon et al., 4 have recommended thatthe areas of steel required for shear and tension beadditive. Current thinking of ACI 349 recommends twomethods for shear and tension on the anchor. For an-chors that transfer shear by bearing, a linear tension-shear interaction is conservative. An elliptical tension-shear interaction is acceptable, but is more difficult toapply. Where shear friction is used, the required strengthof the anchor is a sum of the tensile strength required fordirect tension and the tensile strength required for shearfriction.4.3-Bearing stress

Portions of the foundation in contact with the equip-


ment base plates or mounting rings must be designed tocomply with permissible bearing stresses given in 3.2.1.5.

4.4-PedestalsIn the design of equipment foundations, the piece of

equipment may be located one or more feet above gradefor various functional and operational reasons. The foun-dation pad may be founded several feet below grade.One or more pedestals may be necessary to support theequipment and transfer the design loads to the founda-tion.

By definition, a pedestal is a short column. Theyshould be designed for the critical combination of verticalload and moment. Octagonal pedestals are usually de-signed as circular columns of equivalent area.

Vertical reinforcement is provided to resist the tensilestresses in the pedestal. The controlling loading conditionfor reinforcement is often produced by the maximum mo-ment with minimum vertical loading. The reinforcementis designed by one of four methods: (1) supplying verticalreinforcement with a design capacity equal to or greaterthan that provided by the anchor bolts, (2) designing thepedestal as a column with the vertical reinforcement intension and concrete in compression, (3) applying thecombined stress formula to the reinforcement area alone,or (4) designing the pedestal as a flexural member, neg-lecting axial compression.

4.5- Soil pressure4.5.1 Spread footings - Spread footings may be di-

vided into two general categories: those subject to fullbearing pressure where the resultant vertical force iswithin the kern of the base; and those subject to partialpressure where the resultant force lies outside the kern.For full contact pressure, the gross properties of the basearea may be used to determine the soil pressure distribu-tion. The applicable form of the combined stress formulafor this condition is:

Q = (W\A) -t (Mx/Sx) 2 (My/Sy) (4-2)

where:

Q =W =A =Mx, My =Sx, Sy =

soil pressure at the corners of the footingresultant vertical loadbase area of footingmoments about the x and y centroidal axessection moduli of the base about x and ycentroidal axes

Use of this formula assumes a rigid footing with lineardistribution of strain/stress in the supporting subgrade.

For the partial contact case, the combined stressformula is not applicable, as it would require develop-ment of tensile resistance between the soil and thefooting. If the overturning stability criteria are met, thenthe mathematical assumptions of the following formulaare met.

For partial contact:

Q= 2w3w4r - e)

where:

(4-3)

I3= M/W= width of footing

L = length

4.5.2 Drilled piers - Drilled piers consist of straightshafts with or without belled ends. Drilled piers are gen-erally used in cohesive soils where the sides of the holecan be maintained. In sand, a casing is provided that canbe withdrawn as concrete is placed. Care should be takenduring removal to insure that the concrete will not bedisturbed, pulled apart, or pinched off by earth move-ments (ACI 336.3R, Section 4.3.3). Bells can only bedrilled in cohesive soils with sufficient strength to preventtheir collapse into the base during drilling. Designrecommendations for drilled piers are provided in ACI336.3R.

4.5.2.1 Base pressure and pier capacity - Verticalsoil pressures for a long pier are resisted as skin frictionon the surface of the shaft. Depending on whether or notthe base of the pier rests on rock, the contribution ofbearing pressures against the base (point-bearing) to theoverall capacity of such piers may or may not be signifi-cant. Uplift resistance of long piers is usually a functionof the skin friction and pier dead load.

For a short pier, the vertical force is carried largely bythe base. If lateral loads and moments are small, thepier’s capacity is approximately equal to the base areatimes the bearing capacity of the soil at the base. Iflateral loads and moments are significant, the pier isassumed to resist the applied loads as depicted in Fig.4.5.2. For a straight shaft pier, the vertical pressure onthe base is assumed to be distributed over the leewardhalf of the base. For a belled pier, the base pressure is

V

+M

V

i-M

o. STRAIGHT SHAFT b. BELLED SHAFT

Fig. 4.5.2-Pressure distribution on drilled piers


4.7-Foundation design procedures4.7.1 Factored loads -Foundation base area or the

number of piles or piers is determined from service (un-factored) loads. Use of the Strength Design Method forthe structural design of reinforced concrete foundationelements requires the application of factored loads.Owing to the difficulty of tracking the contribution ofeach type of loading (dead, live, wind, etc.), applicationof a single load factor is often used in the design ofequipment foundations designed using the Strength De-sign Method. Typically, a composite load factor equal to1.6 is used.

4.7.2 Positive moments and flexural shears - Founda-tions for static equipment generally consist of isolatedfootings, mats, or pile caps below grade with one or morepedestals projecting above grade. For square or rectangu-lar foundations, critical sections for moment and shearare as described in Chapter 15 of ACI 318 (see Fig.4.7.2a). An exception to the ACI procedure occurs withdeep/thick pile caps with high capacity piling (refer toCRSI Handbook).

Reinforcing design for octagonal foundations can becumbersome given the mat shape and the reinforcement

computed using the combined stress formula, assumingthat the soil over the windward half of the bell wouldcause the bell to interact with the soil in the manner ofa pile foundation with uplift capacity. Bell and shaftdiameters are selected to keep vertical soil pressureswithin allowable values.

Uplift capacity of belled piers is usually taken as theweight of soil above the bell. One practice is to assumea cone with an angle of 45 to 65 deg with the horizontal.An alternative, more conservative approach considersonly the cylinder of soil above the bell.

4.5.2.2 Lateral pressures - Lateral soil pressures ona long pier, due to lateral loads or moments at the top ofthe pier, are determined by the consideration of load dis-placement characteristics of the soil and the elastic pro-perties of the pier. The pier is treated as a beam on non-linear soil springs. Allowable lateral pressures are basedon limiting the lateral displacement of the pier at theground surface, typical values being % to ‘/z in.

In the case of short piers (those with a shaft length todiameter ratio less than 10), the pier is considered rigid,with the lateral pressures varying in the manner necessaryto satisfy statics (see Fig. 4.5.2a). Where weathered soilsexist at grade, the top two or three feet of soil are oftenignored in determining the resistance to lateral loads.

4.5.2.3 Lateral deflection - Lateral deflections maybe determined by treating the pier as a beam on nonlin-ear soil springs. In order to do so, a coefficient of hori-zontal subgrade reaction is selected based on soil consis-tency and published data, or on an actual field test.

4.5.2.4 Settlement - Drilled piers are generallyfounded on rock, on high bearing capacity granular soils,or on stiff, incompressible clays. For these types of soils,and where due regard for settlement has been accountedfor in the allowable bearing capacities, no appreciablevertical settlements will occur. Consequently, settlementis usually not computed for these soils. However, whenpiers are located in or underlain by weaker clays, a set-tlement analysis is required; in this case, standard con-solidation theory may be applied.

4.5.3 Raft or mat foundations - Bearing pressuresunder raft or mat foundations are dependent on severalfactors. These factors include the type and compressibil-ity of the soil, and the relative rigidity of the mat ascompared to the soil.

Procedures for design of such foundations are pre-sented in ACI 336.2R. A simplifying assumption, whichis conservative from the point of view of flexural design,considers the mat to be rigid and assumes a linear distri-bution of the soil subgrade reaction.

Where equipment layout or mat geometry is complex,some engineers use a finite element analysis in which thesoil is represented as a series of elastic springs.

4.6-PiIe loadsWhen upper soil strata are too weak to support spread

footings, then mat foundations or piles of the end-bearing or friction type are used to support the loads.

Generally, the piles are assumed to be very stiffvertically, and the pile cap is assumed to be flexurallyrigid.

When connectors are not used, the combined stressformula may be applied to determine vertical pile loads.If the piles are subject to uplift, the pile loads areassumed to vary linearly, with the resultant pile forcecoinciding with the location of the applied force at itseccentricity from the neutral axis.

Lateral loads can be assumed to be equally distributedin the pile group with resultant pile shears generally con-sidered to be independent of the vertical forces. Othermethods such Saul’s Procedure,12 could be used foranalysis of groups of piles. For static equipment withlarge surface area subject to wind or with massdistribution resulting in high seismic forces, lateral loadsmay control the number of piles required. Passive earthresistance on the pile cap is sometimes relied on toreduce the shear in the piles.

There are several sophisticated procedures for deter-mining loads in pile groups. Typically, these are usedwhere a combination of vertical piles and batter piles, orall batter piles, are selected. usually the simplistic pro-cedure of setting the batter based on the minimum verti-cal load-maximum shear loading condition is used.

Allowable loads on piles are determined in accordancewith the principles of soil mechanics. For large jobswhere a pile testing program is warranted, selection ofthe most efficient pile type and the maximum permissiblecapacity may be made. Allowable vertical capacity thusdetermined may be subject to reduction for group action.Allowable horizontal capacity is based on limiting lateraldeflection under shear load, and it is not generallyconsidered to be diminished by group action.

351.2R-23

CRITICAL SECTIONMOMENT

P - INOTE:

NEGLECT p+ INOVERBURDEN

DETERMININGNECATIVE MOMENT

Fig. 4.7.3-Negative moment

r- CRITICAL SECTION - MOMENT

It+ CRITICAL SECTION BEAM SHEAR

Fig. 4.7.2a--Critical sections

CIRCLE OF EQUlV AREA

INSCRIBED CIRCLE

Fig. 4.7.2b--Octagonal base options

configuration (Fig. 2.2.1). Therefore, octagonal geomet-ries are often converted to equivalent circular shapes asshown in Fig. 4.7.2b. An equivalent circular shape makesit easier to handle controlling load combinations whichare not oriented on major octagonal axes. The designmoment for an octagonal foundations generally deter-mined in one of two ways (see Fig. 4.7.2c). In method

\TRUE OR EOUIV SOUARE

,-ACTUAL OCTAGON

\ \ CRITICAL SECTION ymAL SECTICN

UNITWIDTH

L INSCRIBED OREOUIV CIRCLEa. b.

Fig. 4.7.2c--Octagonal bases - moment section options

“a,” the moment at the face of the equivalent squarepedestal is based on the area of the footing lying outsidethe critical section and extending the full width of thefooting. In method “b,” known as the “one foot strip”method, a strip of unit width is subjected to the maxi-mum soil pressure distribution; this method provides themost conservative results. Reinforcing steel for the entirefooting is based on the requirements of this strip. Of thetwo methods described above, the full width sectionrequires the least reinforcement.

Beam shear for an octagonal foundation is generallydetermined as shown in Fig. 4.7.2d. In method “a,” the

Fig. 4.7.2d--Beam shear options

shear is computed on the area of the base octagon,bonded by the critical section at a distance d from theequivalent square pedestal, and 90 deg radial lines drawnfrom the center of the base extending through the cor-ners of the equivalent square. Alternatively, in method“b” the shear is computed on the area of the base octa-gon (or circle) lying outside the critical section andencompassing the full width of the footing at the criticalsection.

For a rigid mat foundation supporting multiple pedes-tals, the magnitude of positive and negative momentsmay be determined by elastic one-way or two-way slabtheory. If the mat is designed as a flexible system, acomputer analysis treating the mat as a beam or plate onan elastic foundation is used (refer to ACI 336.2R).

4.7.3 Negative moments - For a spread footing withthe resultant outside the kern, the footing is only partiallysubjected to positive soil pressure on the windward side.The design negative moment is determined by summingthe negative moment components produced by footingweight, overburden, surcharge loading, and any positivecomponent from base pressure. As illustrated in Fig.4.7.3, the positive moment component is often conserva-tively neglected.

In the case of a pile foundation where pile tensioncapacities are developed, the negative moments shouldnot be ignored.

Location and width of critical sections for negativemoments are identical to those for positive moment.Likewise, the computational procedure for octagonal or


4.7.5 Flexural reinforcement - Minimum reinforce-ment requirements of ACI 318 have been variously inter-preted where foundation design is concerned. Some engi-neers specify a minimum reinforcement of 200/fy, unlessone-third more reinforcement than required by analysisis provided. Others specify a minimum temperature orshrinkage reinforcement. Section 10.5.3 of ACI 318Rrecommends a minimum shrinkage and temperature rein-forcement for mats and other slabs that provide verticalsupport. Assuming the category of “mats and other slabs”to include foundations, the provision of temperature andshrinkage reinforcement would appear to meet the code’sintent. On the other hand, the 200/fy provision appliesspecifically to beams that have been oversized for archi-tectural or other reasons. Consequently, most engineersdo not consider the 200/fy provision applicable to foun-dation design.

Code specified criteria should be followed to provideadequate anchorage on each side of the critical section.Particular attention should be given in the case of octa-

gonal footings designed using the procedure of Fig.4.7.2c, method “a.” For octagonal foundations, the flex-ural reinforcement is placed in mats as shown in Fig.2.2.1. Where hexagonal foundations are used, similarconfigurations are typically provided.

4.7.6 Pedestal reinforcement - Large equipment pedes-tals generally encompass a greater area than that re-quired by the loads involved; therefore, only a smallamount of reinforcement is required. Some engineers usethe % percent minimum from Sections 10.8.4 and 1.9.1 ofACI 318. Others question this practice on the basis thatpedestal areas associated with static equipment are gen-erally much larger than those associated with buildingcolumns to which the ACI 318 provisions are primarilyaddressed.

CRITICAL SECTIONPUNCHING SHEAR

;;;;I ER LOADED

Fig. 4.7.4-Punching shear with eccentric Loading

circular foundations is the same as that for positivemoment.

4.7.4 Punching shear (two-way) - The critical sectionfor punching shear is as descried in Section 11.12 ofACI 318. An alternative procedure involves computingthe shear on the heavier loaded half of the criticalsection as shown in Fig. 4.7.4.

For piers subjected to large moments in addition tovertical forces, the punching surface departs from asimple truncated cone or pyramid. The report ACI 426Rshould be consulted in such cases.

CHAPTER 5--CONSTRUCTION CONSIDERATIONS

Foundations for static equipment are similar in config-uration and construction to foundations for structures. Inaddition, foundations must meet any specific require-ments of the equipment manufacturer for maintainingprecise grade and alignment, as welI as for transferringthe loads from the equipment to the supporting struc-tures or soil. For more massive equipment foundations,this may require rigid foundations supported by firm soilsor rock.

Foundation mats may be supported directly by soil orrock, or piles or drilled piers may be used to extend thefoundation to firm soil or rock. The selection of the mostappropriate type of foundation depends upon the geo-technical conditions of the site. The extent of thesubsurface investigation and resulting subsurface prepara-tion, if any, is determined by the engineer and geotech-nical consultant.

5.1- Subsurface preparation and improvement5.1.1 General - The site is prepared in a manner con-

sistent with the design, and with particular attention tothe engineering properties of soils. Compaction or con-solidation of soft soils is commonly used to increasebearing capacity and reduce the potential for foundationsettlement. In many cases unsuitable soils are removedand replaced by sound material that is compacted tomeet the design requirements. Where unsuitable founda-tion soils are encountered, and in situ improvement orreplacement of the soils is not practical, piles or drilledpiers may be used to extend the foundations to suitablebearing soil or rock.

5.12 Specific subsurface preparation and improvements- Specific subsurface preparation and related treatmentmay be required if the geotechnical investigation or exca-vation during construction indicates that the existing soilcharacteristics will not achieve the required foundationperformance. Conditions requiring special preparationand treatment are:

0 Nonuniform conditions that could result in differ-


ential settlement or tilting of the foundationl Soil conditions found to be different than those

assumed for the design0 Unstable slopes0 Loose sands0 Soft compressible soils such as unconsolidated clays

and highly organic soils (e.g., peat)0 Slip planes or faultsl High water table or other saturated conditions

The most common site specific subsurface prepara-tions and treatment for the above conditions are:

a) Unstable slopes of excavation-Unstable slopesmay be stabilized by flattening the slope, benching, de-watering, shoring, freezing, injection with chemicalgrouts, or supporting with dense slurries.

b) Stratification-Excavations with slopes parallel tothe direction of stratification are avoided by flatteningthe slope or by providing adequate shoring.

c) Wet excavation-During construction, ground wateris normally lowered below the bottom level of the exca-vation. One method commonly used to achieve this is byusing deep well pumps or well points. Another methodis to create an impervious barrier around the excavationwith cofferdams or caissons, chemical grout injection,sheet piles, or slurry trenches. A sump pit is typicallyprovided to collect ground water intrusion.

The selection of an appropriate method depends onthe characteristics of the subsurface soils encountered,costs, and the preferences of the constructor.

d) Small surface pockets of loose sand-Loose sandpockets are normally compacted to the degree of speci-fied compaction. Alternatively, if the predominant soil ishard, the loose sand may be removed and replaced withlean concrete.

e) Large deposits of loose sands-- The loose sandsmay be stabilized by vibrofloatation or dynamic consoli-dation, whichever offers an economic advantage.

f) Presence of organic material or unconsolidated softclays-All organic materials and soft clays are normallyremoved and replaced with suitable, well-compacted fillthat provides the characteristics desired for the properperformance of the foundation. Alternatively, piling ordrilled piers may be used to carry foundation loads tosound bearing strata.

g) Fissured rock-The extent of fissures is evaluatedto determine if remedial treatment is needed. Pressuregrouting is a suitable remedy for some types of fissures.In the case of seismic faults, thorough geotechnical andgeological evaluation is required to ascertain the poten-tial hazard. Where significant hazards are found to exist,relocation of the entire facility to avoid the hazard is asuitable remedy.

h) Irregularly weathered rock-The weathered seamsare cleaned and replaced with lean concrete. Alterna-tively, the foundation may be lowered to sound rock.

i) Solution cavities in limestone deposits-The voidsare pumped full of grout, if small, or lean concrete under

a pressure head in the case of large holes.j) Unconsolidated clay-Clays may be preloaded and

related settlements monitored. (Early identification isimportant to gain lead time and avoid slippage in theconstruction schedule.) Alternatively, piling or drilledpiers may be used to carry foundation loads to firmbearing strata.

k) Cold climates-Foundations are not placed on finegrained soils subject to the phenomenon of frost heave.Proper drainage should be provided by placing a freedraining sand or gravel layer under the foundation tomitigate the possibility of frost heave where such hazardexists. As an alternate, the bottom of the footing isplaced below the frost line.

5.2-- Foundation placement tolerances (ACI 117)Foundation placement tolerances depend largely on

the type of equipment being supported. They are speci-fied by the engineer on the drawings or in the specifica-tions. It is good practice to use templates during concreteplacement to support anchor bolts and other embed-ments that must be precisely positioned.

5.3-- Forms and shores5.3.1 Forms and shoring for construction of concrete

foundations should follow the recommendations of ACI347R.

5.3.2 Shoring must support the concrete loads, impactloads, and temporary construction loads. Transverse lon-gitudinal bracing may be required to sustain lateralforces.

Wind loads should be taken into account. It is notusually necessary to consider seismic loads due to thelimited time shoring will be in place. The design of theformwork should be prepared by a registered professionalengineer and submitted to the design engineer for review.

5.3.3 For large equipment foundations, temporaryformwork systems are generally used. Less frequently,permanent systems may be used for special applications.The selection of a temporary support system is normallymade by the constructor. It is influenced by the erectionsequence of the building (if the equipment is enclosed),the equipment installation procedure, and access require-ments at the time of placement of the foundation. Someof the permanent systems may affect the design and costof the foundation. Therefore, the design engineer maywish to consult with building contractors prior to decidingon a permanent formwork system.

Some of the temporary systems used are:

l Standard construction shoring consisting of tem-porary shore legs supported by the foundation matand supporting the soffit forms of a foundationdeck.

l Shoring consisting of structural steel beams suppor-ted on brackets attached to the foundation col-umns. The forms rest on top of the beams. Jackingdevices are used to lower the beams and forms for


removal after the concrete reaches sufficientstrength.

Permanent support systems include:

l Structural steel beams or trusses supported byfoundation columns and carrying the permanentdeck forms. The beams or trusses are part of thedeck design and will also carry operating loads.The deck forms (steel decking) usually are sup-ported on the bottom flanges of the beams ortrusses. Since the steel members are embedded inthe foundation deck, the design engineer has to becareful to avoid interferences with the reinforcingbars and with other embedments (anchor bolts,plates, pipesleeves, and conduits).

l Precast concrete deck forms supported by thefoundation columns.

l Plate steel forms used in the steel industry.

The engineer should review the constructor’s proposedconstruction procedure to assure that the design is notcompromised.

5.4-Sequence of construction and construction joints Many equipment foundations are too large for the

concrete to be placed in one continuous operation. Con-struction joints are used to subdivide large foundationsinto smaller units that can be placed in one continuousoperation.

Subdivision of large structures by construction jointsalso affords a means for reducing stresses due to con-crete shrinkage. To gain maximum benefit, alternate seg-ments should be placed and allowed to cure and shrinkas long as the construction schedule permits before theintervening segments are placed.

The structural integrity of the foundation requires thatjoints be constructed with care in accordance with ac-cepted practices for construction joints in major concretestructures. Project specifications normally require thatthe constructor obtain the approval of the engineer forconstruction joint locations and details.

5.5-Equipment installation and setting5.5.1 Shims, wedges, and bolts - The design engineer’s

choice of the interface system is influenced by the manu-facturer’s recommendations and requirements, the foun-dation construction procedures, the setting and adjust-ment of the equipment, and the final tolerances required.

Shims, which are usually carbon steel or brass stock invarious thicknesses, have both economical and high loadbearing qualities.

Wedges are usually the double-wedge type and are of-fered by several mounting equipment manufacturers. Thedouble wedge mount often has one or more threadedstuds for (1) precise vertical adjustment, and (2) forlocking the sliding wedge into the required position. Alock nut may also be used for locking the main horizontal

stud into the final position.Other types of wedges often utilized by millwrights

include various shaped temporary steel wedges. Tempor-ary wedges are usually tolerance adjustment tools placedprior to grouting, and they are removed after the setting-up of the grout material. Permanent wedge assembliesallow future adjustments on ungrouted equipment bases.

Required bolt diameters are usually given on themanufacturer’s drawings. Bolt lengths, threaded lengths,bolt projections, material, stress levels, and the methodof tightening should be clearly shown on the design draw-ings. When the manufacturer requires a specific preloadfor a bolt, the following equation can be used to selectthe bolt torque:

T = W’CICI, (5-l)

where:

T = tightening torque, in-lb

Wp= initial load @reload) lb

zl= friction factor= nominal bold diameter, in.

The following values of p are typical:

Steel fasteners (as manufactured)Hot dipped galvanized steelLightly oiled steelPlated (cadmium, chromium, etc.)Graphite with mineral oil

Special coatings may require manufacturer’s data.When preload values are not given, a suggested mini-

mum preload value of 15 percent of the yield strength ofthe anchor is often used.

Bolt tightening is specified as being accomplished witheither a post-tensioning jacking procedure, turn-of-the-nut method, or with a calibrated wrench. Post-tensioningjacking is usually used on the deeper anchorages with thenon-bonded shanks. When the shank length is embeddedin concrete, the turn-of-the-nut method or sequential cal-l&rated wrench tightening is specified. Impact wrenchesare not allowed for tightening of a bolt component whenpart of the anchorage is embedded in concrete becauseof the extremely high torques and tensile forces deliveredby such tools.

5.5.3 Embedments - Embedments in the concrete in-clude the anchor bolt assemblies previously described,shear lugs, and shear transferring devices.

Since shear is one of the combined loads transferredto the concrete foundation, steel lugs may be integralparts of the base of the equipment. Such lugs are groutedinto shear key grooves previously cast into the concretebase.

5.6-Grouting5.6.1 Types of grout - There are two basic tvpes of

0.200.140.150.150.10


grout: cement-based grouts and epoxy-based grouts. Ce-ment-based grouts are more commonly used because oftheir availability, ease of use, strong physical propertiesand lower cost. Epoxy grouts are generally used becauseof their high resistance to chemicals, to shock, and to viiratory loads. There are four types of cement-basedgrouts: (1) gas generating; (2) air release; (3) oxidizingaggregate, and (4) expansive cement. In evaluating whichcement grout should be used, one should take into ac-count the placeability of the grout as well as its physicalproperties. The physical properties that are evaluatedare: volume change, compressive strength, working time,consistency, and setting time. In evaluating the propertiesof an epoxy grout, one should look at placeability as wellas the physical properties of volume change, compressivestrength, creep, working time, consistency, and settingtime. The effects of temperature induced volume changeson the epoxy concrete interface should be considered. Inaddition, any specific requirements of the applicationshould be addressed.

5.6.2 Applications - In specifying grout systems, thedesigner should consider the different characteristics ofeach type of grout along with field limitations, and matchthese with the specific requirements of the job. In parti-cular, the designer should review the design of the equip-ment base, the accessibility of the grouting location, theclearances provided for the grout, and the design of theanchor bolts. Most of the grouts on the market are pre-mixed, prepackaged materials, and contain manufac-turer’s instructions on surface preparation, formwork,mixing, placing, and curing procedures.

A detailed discussion on the application of grouts canbe found in ACI 351.1R.

5.7-Materials (ACI 211.1)Large equipment foundations require special attention

to the design and control of the concrete mix (see ACI207.1R and ACI 207.4R).

Many foundation members are massive enough for theheat of hydration of the cement to generate a large ther-mal differential between the inside and the outside andthis may cause unacceptable surface cracking unless stepsare taken to reduce the rate of release of this heat. Also,creep, differential thermal expansion, and shrinkage maycause distortion of the foundation and consequent unac-ceptable changes in equipment alignment. Design of theconcrete mix to minimize creep and shrinkage, and to re-duce the thermal expansion of the hardened concrete istherefore important. Finally, expansive reaction of theconcrete aggregate with alkalies in the cement can beavoided by proper choice of cement and aggregate.

To minimize the rate of release of the heat of hy-dration, and to control shrinkage and creep, the followingsteps are normally followed:

0 The lowest content of cementitious material consis-tent with attaining the required strength and dura-bility used.

l Part of the cement is replaced with fly ash ornon-fly ash pozzolan.

0 The placing temperature of fresh concrete is low-ered by chilling the aggregate and/or using chippedice for mixing water.

l The largest practical size aggregate is used to allowfurther reduction in the amount of cement.

l Moderate heat cement (Type II) is used.l A water reducing agent is used to allow further re-

duction of the cement factor.l Low slump and effective vibration are used.l Concrete placement by pumps, which requires con-

crete mixes having high amounts of cement andsmall aggregate sixes, is avoided.

l Sixes of placements for large foundations are re-duced.

The coefficient of thermal expansion of the hardenedconcrete can be controlled by the choice of aggregatesbecause it depends primarily on the coefficient of ther-mal expansion of the aggregate. When excessive thermalexpansion may be a problem, the coefficient of expansionof available aggregates is measured to determine theirsuitability for the application. (In many regions of thecountry there may be very limited choices in the typesand sources of aggregates.)

Expansion of concrete from alkali-aggregate reactioncan be minimized by using a low alkali cement, by re-placing a portion of the cement with a fly ash or non-flyash pozzolan meeting the requirements of ASTM C 618,and by selecting low reactivity aggregates. The potentialreactivity of aggregates can be evaluated with the pro-cedures and tests described in ASTM C 295, ASTMC 227, ASTM C 289, and ASTM C 586. The evaluationmethods of the potential reactivity of aggregates arecovered by ASTM C 33 and ACI 225R.

The cement content should be low enough to helpmeet heat of hydration requirements, and yet highenough to meet strength, creep, and shrinkage require-ments. (It may not be possible to solve completely theheat problem by reducing the heat of hydration. Cooling,small placements, pozzolan, etc., may also be needed.)

5.8-Quality controlFoundations for equipment should be parts of an

integrated system and are designed as such. Thus, the de-sign requirements should be implemented during con-struction by the imposition of an appropriate qualitycontrol program. The quality control program shouldinclude requirements for control of material quality, theengineer’s approval of critical construction procedures,and on-site verification of compliance with designdrawings and project specifications by qualified fieldengineers responsible to the engineer and/or owner.

Requirements for foundations should be provided tothe constructor and field engineer through the designdrawings and project specifications. The field engineershould maintain close liaison with the design engineer on


any revisions to the design requirements when field conditions differ from those assumed.

The quality control program and inspection and verifi-cation activities should be thoroughly documented. The program should be consistent with those commonly im-plemented for construction projects of similar importance.

CHAPTER 6- REFERENCES

6.1-Recommended references The documents of the various standards producing

organizations referred to in this document are listedbelow with their serial designation.

American Concrete Institute (ACI)

116R117

207.1R

207.4R

211.1

225R

307

318/318R

318.1/318.1R

336.2R

336.3R

347R

349/349R

351.1R

355.1R

426R

Cement and Concrete Terminology Standard Tolerances for ConcreteConstruction and Materials Mass Concrete for Dams and OtherMassive StructuresCooling and Insulating Systems forMass ConcreteStandard Practice for Selecting Pro-portions for Normal, Heavyweight,and Mass ConcreteGuide to the Selection and Use ofHydraulic CementsStandard Practice for the Design andConstruction of Cast-in-Place Rein-forced Concrete ChimneysBuilding Code Requirements forReinforced Concrete and Commen-taryBuilding Code Requirements forStructural Plain Concrete and Com-mentarySuggested Design Procedures forCombined Footings and MatsSuggested Design and ConstructionProcedures for Pier FoundationsRecommended Practice for ConcreteFormworkCode Requirements for NuclearSafety Related Concrete Structuresand CommentaryGrouting for Support of Equipmentand MachineryState-of-the-Art Report on Anchor-age to Concrete Shear Strength of Reinforced Con- crete Members

American Institute of Steel Construction (AISC)

AISC-ASD Manual of Steel Construction -Allowable Stress Design

AISC-LRFD Manual of Steel Construction - Loadand Resistance

American National Standards Institute (ANSI)

ANSI AS8.1 Minimum Design Loads for Buildingsand Other Structures (revised and re-designated as ASCE 7)

ANSI B18.2.1 Square and Hex Bolts and Screws(Inch Series)

ANSI B18.2.2 Square and Hex Nuts (Inch Series)ANSI STS-1 Steel Stacks

American Society of Civil Engineers

ASCE 7 Minimum Design Loads for Buildingsand Other Structures (formerly ANSIA58.1)

American Society of Mechanical Engineers (ASME)

ASME STS-1ASME

ASTM

ASTM A 307

ASTM C 33

ASTM C 227

ASTM C 289

ASTM C 295

ASTM C 586

ASTM C 618

Steel StacksBoiler and Pressure Vessel Code

Standard Specification for CarbonSteel Bolts and StudsStandard Specification for ConcreteAggregatesStandard Test Method for PotentialAlkali Reactivity of Cement-Aggre-gate Combinations (Mortar-BarMethod)Standard Tests Method for PotentialReactivity of Aggregates (ChemicalMethod)Standard Guide for PetrographicExamination of Aggregates for Con-creteStandard Test Method for PotentialAlkali Reactivity of Carbonate Rocksfor Concrete Aggregates (Rock Cylin-der Method)Standard Specification for Fly Ashand Raw or Calcined Natural Pozzo-lan for Use as a Mineral Admixturein Portland Cement Concrete

Building Officials and Code Administrators International,Inc. (BOCA)

The BOCA National Building Code

Center for Transportation Research (University of Texas)

Research Report 1126-l Load-Deflection Behavior


Research Report 1126-2

Research Report 1126-3

Research Report 1126-4F

of Cast-in-Place and Retro-fit Concrete Anchors Sub.jected to Static, Fatigue andImpact Loads; Collins,Klingner, Polyzois, Feb.1989.Adhesive Anchors: Behav-ior and Spacing Require-ments; Doerr, Klingner,Mar. 1989.Behavior and Design ofDuctile Multiple-AnchorSteel to Concrete Connec-tions; Cook, Klingner, Mar.1989.Design Guide for Steel-to-Concrete Connections;Cook, Doerr, Klingner,Mar. 1989.

Concrete Steel Reinforcing Institute (CRSI)

CRSI Handbook

International Conference of Building Officials

Uniform Building Code (UBC)

FEMA, National Earthquake Hazards Reduction Program(NEHRP)

Standard (1991)

Precast-Prestressed Concrete Institute (PCI)

PC1 Design Handbook

Southern Building Code Congress International, Inc.

Standard Building Code (SBC)

The above publications may be obtained from the fol-lowing organizations:

American Concrete InstitutePO Box 9094Farmington Hills, MI 48333

American Institute of Steel Construction400 North Michigan AvenueChicago, IL 60611

American National Standards Institute11 West 42nd StreetNew York, NY 10036

American Society of Civil Engineers345 East 47th StreetNew York, NY 10017-2398

American Society of Mechanical Engineers345 East 47th StreetNew York, NY 10017

ASTM1916 Race StreetPhiladelphia, PA 19103-1187

Building Officials and Code Administrators International,Inc. (BOCA)4051 West Flossmoor RoadCountry Club Hills, IL 60478-5795

Center for Transportation Research (University of Texas)3208 Red River, Suite 200Austin, TX 78705-2650

Concrete Steel Reinforcing Institute933 North Plum Grove RoadSchaumburg, IL 60173-4758

Federal Emergency Management Agency (FEMA)Earthquake Programs500 “C” Street, S.W.Washington, DC 20472

International Conference of Building Officials (ICBO)5360 South Workman Mill RoadWhittier, CA 90601

Precast/Prestressed Concrete Institute175 West Jackson Blvd.Chicago, IL 60604

Southern Building Congress International, Inc. (SBCCI)900 Montclair RoadBirmingham, AL 35213

6.2- Cited references1. Bailey, J.W. and Burdette, E.G., “Edge Effects on

Anchorage to Concrete,” Research Series No. 31, Univer-sity of Tennessee, KnoxviIIe, Aug. 1977.

2. Breen, J.E., “Development Length for AnchorBolts,” Center for Highway Research, Final Report, Uni-versity of Texas at Austin, Apr. 1964.

3. Breen, J.E., “Development Length of Anchor Bolts,”Highway Research Record NO. 147, 1966.

4. Cannon, R.W., Godfrey, D.A., and Moreadith, F.L.,“Guide to Anchor Bolts and Other Steel Embedments,”Concrete International, V. 3, No. 7, July 1981, pp. 28-41.(Also available as a reprint, AB-81, from AmericanConcrete Institute, Detroit)

5. Ferguson, P.M. and Rajagopalan, K.S., “ExploratoryShear Tests Emphasizing Percentage of Longitudinal


SteeI,” ACI JOURNAL, Proceedings V. 65, No. 8, Aug.1968, pp. 634-638.

6. Frank, K.H., “Fatigue of Anchor Bolts,” Center forHighway Research, Report 172-2F, University of Texas atAustin, July 1978.

7. Hasselwander, G.B.; Jirsa, J.O.; Breen, J.E.; and Lo,K., “Strength and Behavior of Anchor bolts EmbeddedNear Edges of Concrete Piers,” Center for Highway Re-search, Report 29.2F, University of Texas at Austin, May1973.

8. Housner, G.W., “Limit Designs of Structures onResist Earthquakes,” Proceedings of the World Conferenceon Earthquake Engineering, Berkeley, June 1956.

9. Lee, D.W. and Breen, J.E., “Factors Affecting An-chor Bolt Development,” Center for Highway Research,Report 8E-lF, University of Texas at Austin, Aug. 1966.

10. McMakin, PJ., Slutter, R.G., and Fisher, J.W.,“Headed Steel Anchors Under Combined Loading,” En-gineering Journal, AISC, Second Quarter, 1973.

11. Scholl, R.E, Czarnecki, R.M.; Kirchner, C.A.;Shah, H.C, and Gerie, J.M., “Seismic Analysis of OilRefinery Structures, Part II - Evaluation of SeismicDesign Criteria,” Technical Report No. 32, John A. BlumeEarthquake Engineering Center, Stanford University,Stanford, Sept. 1978.

12. SauI, W.E., “Static and Dynamic Analysis of PileFoundation,” Journal of the Structural Division, ASCE, V.44, No. ST5, May 1968.

13. Swirsky, R.A.; Dusel, J.P.; Cruzier, W.F.; Stokier,J.R.; and Nordlin, E.F., “Lateral Resistance of AnchorBolts Installed in Concrete,” Final Report, CaliforniaDepartment of Transportation, Sacramento, May 1977.

GLOSSARY

Terms used in this report generally follow ACI Glossary of Terms, ACI 116R. The following terms and their defin-itions, however, are unique to this report and are inaddition to those given in ACI 116R.

Base Ring- A device used to provide a common sur-face between the foundation and the equipment foraligning, leveling, and distributing vertical loads as forvessels or process columns.

Belling-Excavating processused to provide additionalbearing surface at tip of concrete drilled piers.

Bundle load-- load required to break the bond be-tween a tube bundle and exchanger shell.

Davit- A device used to support/swing the covers offopenings of vessels, tanks, etc.

Hydrotest- Filling of equipment (tanks, vessels, etc.)with water to check for leaks and structural integrity.

Operating loads-Loads applied to the equipment orstructure due to nature of operation, liquid loads, inter-nal loads, or pressure, etc.

Pad-Slab-type foundation support for equipment.Shaft-Vertical portion of concrete drilled pier.Sleeve-A device used around anchor bolts to allow

movement of the bolt after casting of concrete.Stack-Cylindrical shaped vertical vent.

METRIC (SI) CONVERSION FACTORS

1 in. = 25.4 millimeters1 in.2 = 645.2 mm2

1 pound = 4.448 Newton1 psi = 0.006895 MPa1 kip = 4.448 kNl ksi = 6.895 MPa

This report was submitted to letter ballot of the committee and approved inaccordance with ACI balloting procedures.