is it a nonstructural component or a nonbuilding structure

8/18/2019 Is It a Nonstructural Component or a Nonbuilding Structure

http://slidepdf.com/reader/full/is-it-a-nonstructural-component-or-a-nonbuilding-structure 1/4STRUCTURE magazine July 2008

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Is It a Nonstructural Component or a Nonbuilding Structure ? By Robert Bachman, S.E., and Susan Dowty, S.E.

This article provides guidance to code users on how todetermine whether or not an item should be treated as anonstructural component or a nonbuilding structure forpurposes of seismic design. In order to explain the differencesbetween nonstructural components and nonbuilding struc-tures, many generalizations need to be made so that the

discussion can be simplified as much as possible. Commonsense also needs to be exercised, as these generalizationsdon’t always apply. Also, it should be noted, that the guid-ance provided herein represents only the collective opinionof the authors at this point in time. It is hoped that as thenonstructural component and nonbuilding structure codeprovisions evolve, some of these guidelines will become ob-solete as the code will directly address these issues.

system similar to build-ings and the other type

has a structural systemthat is not similar tobuildings (Figures 1 and

2 ). The focus of thisarticle is on nonbuild-ing structures which arenot similar to build-ings, and the determination of whenthe design rules of ASCE 7-05 Chapter15 apply and when the design rules of

ASCE 7-05 Chapter 13 apply.Figure 3 provides a visual illustration

that shows:

1) those items which shouldbe treated as nonstructuralcomponents;

2) those items which should be treatedas nonbuilding structures; and

3) those items, such as tanks, thatappear to be ambiguous sincethey are permitted to be treatedeither way.

It should be noted that most, if notall, nonbuilding structures support at-tached nonstructural components ofsome type. One item not shown on thelist that is of interest is skid mountedequipment modules.

Differentiating BetweenNonstructural Componentsand Nonbuilding StructuresThere are a number of obvious differ-

ences between nonstructural componentsand nonbuilding structures. First is size.To say it simply, nonstructural compo-nents are typically small, and nonbuildingstructures are typically big. “Small” and

“big” now need to be described in a mordefinitive way. Mechanical and electricaequipment components are typicallsmall enough to fit within a buildingsomething on the order of less than 1feet tall. There are, of course, exception

to the 10 feet, such as signs attached tbuildings and very large generators anturbines. The 10-foot height is obviouslan arbitrary height. The basis for selecting 10 feet is twofold:

1) it represents a story height which will allow installation within abuilding, and

2) the weight of the component islikely small relative to the weightof the supporting structure.

Another obvious difference betweenonstructural components and nonbuild

ing structures is whether they are factorassembled or field erected. Nonstructuracomponents are typically factory assembled in that they are transported inone piece on a truck and require no assembling at the jobsite. Obviously therare many nonstructural components thaare assembled on the site in pieces suchas ceilings, partition walls, exterior curtai

walls, exterior siding, masonry veneecable trays, piping systems, ductworklarge roof mounted roof handlers, elevators etc. Fortunately, there is no questio

Figure 1: Nonbuilding Structure Similar to Building.

Figure 2: Nonbuilding Structure Not Similar to Building.

Nonstructural ComponentsTraditionally, nonstructural components

have been identified as architectural, me-

chanical and electrical components foundin buildings. Prior to adoption of theInternational Building Code (IBC), thefocus had been on the design of anchor-age and bracing of these componentsand not the components themselves. Thecomponents themselves had been treatedas a “black box” and whatever was insidethe “black box” was not required to bedesigned for seismic loads. Exceptionsto this “black box” approach were itemssuch as towers on tops of buildings (radioand cell towers), heavy cladding, battery

racks, billboards and signage attached tobuildings. Current IBC seismic code re-quirements for anchorage and bracingrequirements for nonstructural compo-nents are found in ASCE 7-05 Chapter13. It should be noted that, starting withthe 2000 IBC, when the component isdetermined to have an importance factorof 1.5, the “black box” itself now generallyneeds to be designed, or qualified, to resistthe effects of seismic forces.

Nonbuilding Structures

Self-supporting nonbuilding structuresare structures other than buildings thatcarry gravity loads and resist the effectsof earthquakes. The concept of provid-ing specific seismic design requirementsfor nonbuilding structures was first in-troduced into code requirements in the1988 Uniform Building Code (UBC). Themost current seismic design requirementsfor nonbuilding structures are found in

ASCE 7-05 Chapter 15.There are two types of nonbuilding

structures. One type has a structural



Nonstructural Components:

F p =0.4a pS DS W p ( )z

h1 + 2

( )R p

I p

F p shall not be taken as less than: F p = 0.3S DS I pW pF p is not required to be taken as greater than: F p = 1.6S DS I pW p

Nonbuilding StructuresNot Similar to Buildings: C s = for T≤ Ts

S DS

( )R

I

C s = for TL≥ T > TsS D 1

( )R

I T

C s = for T > TLS D 1T L

( )R

I T 2

For nonbuilding systems that have an R value provided inTable 15.4-2, C s shall not be taken as less than:

C s = 0.03

For nonbuilding structures located where S 1 ≥ 0.6 g , C s shallnot be taken as less than: C s =

0.8S 1

( )R

I

that these all should be treated as nonstructuralcomponents and the design criteria in ASCE

7-05 Chapter 13 apply. Nonbuilding struc-tures are most likely field erected, as they areconstructed at the jobsite. Exceptions to thisrule would be large vertical vessels, hoppersand bins which can be shipped on trucks orrailcars to the site. Essentially everyother type of nonbuilding structureis constructed at the jobsite. Another major difference betweena nonstructural component and anonbuilding structure is the basisfor its design and construction.Nonstructural components are typ-

ically constructed to function forarchitectural, mechanical or elec-trical purposes. The primary func-tion of nonbuilding structures is tomaintain structural stability, andthey are typically constructed toresist gravity and lateral loads andthe structural system is typicallyvery carefully designed for seismicloads. For example, a communica-tions tower is designed to maintainstructural stability whereas a motoris primarily designed for mechanical

functionality and seismic designmay be inherent and therefore notrigorously evaluated (other thanfor anchorage).

Designer’s Option When in doubt, the designeralways has the option of calculatingthe force demand in accordance

with the nonstructural componentequation in ASCE 7-05 Chapter13 and the nonbuilding structure

equation in ASCE 7-05 Chapter 15, andusing the most conservative design.

The design force equation for nonstructuralcomponents is indirectly dependent on the pe-riod of the item (is it flexible or rigid?) whereasthe design force equation for non-buildingstructures is directly dependent on the period

Escalator components

EvaporatorsFurnaces

Generators

Heat exchangersHVAC

InvertersLighting fixtures

Manufacturing equipmentMotor control centers

Motors

Panel boards

ParapetsPenthouse (except whereframed by an extension of

the building frame)Piping

PlumbingProcess equipment

PumpsWall panel

Walls

Water heatersVibration isolated components

and systems

Access floors

Air conditioning unitsAir distribution boxes

Air handlersAir separators

Battery racksBoilers

Cabinet heatersCabinets

Cable traysCeilingsChillers

Communication equipmentCompressorsComputersDuctwork

Electrical conduitElevator

Engines

Switch gear Transformers

Tubing Turbines

Veneer

Billboards & Signs

Bins

Chimneys

Conveyors

Cooling Towers

Stacks

Tanks

Towers

Vessels

Amusement structuresHoppers (elevated)

MonumentsSilos (cast-in-placeconcrete having wallsto foundation)

Nonstructural Components

Chapter 13

Nonbuilding Structures

Not Similar to Buildings

Chapter 15

Figure 3: Intersection of Nonstructural Components and Nonbuilding Structures.

of the structure. For nonstructural componethe period of the item affects the determtion of the a p factor. Because of differencethe form of the two sets of equations and defined parameters, as shown in the box belthey may yield quite different results.

Penthouses: Are They Really Nonstructural Component?

There is one item listed as a nonstructu

component in Figure 3 that may come asurprise: penthouses (except where framby an extension of the building frame)penthouse is typically a structure on a roofhousing elevator machinery, equipment, watanks, etc. It can easily be more than 10 fin height, and it is most likely constructedthe jobsite. Because penthouses are typiclocated at the roof level, are of a small mand footprint size compared to the structas a whole thus not affecting the dynamof the entire structure, they are classifiednonstructural components provided they

not part of the building frame. One coargue that their framing system is similarthat of a building; however, they also canviewed as a small appendage to a structsimilar to a tower. If the penthouse is fram

by an extension of the building frathen it needs to be designed as parthe building as a whole.

Nonbuilding StructureSupported Within a

Building or

Other StructuresSometimes a nonbuilding struct

needs to have its seismic design fodetermined using the same equatiused to determine forces as specififor a nonstructural component. Thappens when, rather than being sported on the ground, the nonbuildstructure is supported within a buildor by another nonbuilding structu

ASCE 7-05 Section 15.3 provides cinstructions for this condition. Where the weight of the suppor

nonbuilding structure is “relativsmall” compared to the weight of supporting structure, the assumptiothat the supported nonbuilding strture will have a relatively small efon the overall nonlinear earthquresponse of the primary structure ding design level ground motionsthese cases, the code allows such strtures to be considered as nonstructcomponents and they can therefutilize the requirements for Chap



13 for their design. The code equates “rela-tively small” to weighing less than 25 percentof the combined weight of the nonbuildingstructure and supporting structure. Where the weight of the supported structureis large as compared to the weight of thesupporting structure (greater than or equalto 25 percent of the combined weight ofthe nonbuilding structure and supportingstructure), it is expected that the combined

response could be significantly affected bythe combined response of the supported andsupporting nonbuilding structure. There aretwo analysis options that are permitted to beused for this situation, and they are set forthin ASCE 7-05 Section 15.3.2.

Equipment Modules(Equipment Skids)

Equipment modules are currently not ex-plicitly (but implicitly) covered by ASCE7-05, and therefore a suggested seismic design

approach is provided in this article. Some-times these modules have exterior coveringsand one could consider them as very large“black boxes”. There’s a certain inherent rug-gedness of equipment modules because thesepreassembled modular units are typicallytransported on a truck and therefore musthave a certain level of inherent ruggedness

because of transportation loadings. It is the

authors’ opinion that transportation designloads are usually sufficient to assure adequateseismic performance in lower areas of seis-micity (structures assigned to Seismic DesignCategories A, B or C) if the module is notstiffened or braced in anyway for transporta-tion. In higher areas of seismicity (structuresassigned to Seismic Design Categories D, Eor F), seismic design of the primary structuralframe of the modular unit should be demon-strated. Obviously if a modular unit doesn’t

weigh much, for example 400 pounds, it doesnot make sense to require a design. On the

other hand, if a modular unit weighs 2000pounds, it is prudent to design for seismicloads. In such a case, the primary supportmembers should be designed in accordance

with ASCE 7-05 Chapter 13, and a p and R p values should be the same as that for the en-try from ASCE 7-05 Table 13.6-1 shown inFigure 4 .

Design of Items Which Can

Be Either a NonstructuralComponent or a

Nonbuilding StructureNow that the differences between nonstruc

tural components and nonbuilding structurehave been described, it is appropriate to taka look at the apparently ambiguous items (thoverlap items in Figure 3, page 19 ). Table provides discussion on these items.

Conclusion

The foregoing article provides guidance ohow the authors believe items that could bidentified as either nonstructural componentor nonbuilding structures can be treated froma seismic design perspective. Because of th

wide variety of items, it is extremely difficulto write all-inclusive definition rules withithe code. It is hoped that the guidance provided along with the application of a goodose of common sense will lead users to thappropriate treatment.▪

Bob Bachman, S.E., is a licensed StructuralEngineer with offices in Laguna Niguel,California. He chaired the AmericanSociety of Civil Engineers ASCE 7 SeismicTask Committee during the developmentof ASCE 7-02 and ASCE 7-05. He is amember of the NEHRP RecommendedProvisions Update Committee and chairedthe NEHRP Recommended Provisions TS-8 subcommittee.

Ms. Dowty, S.E., a registered civiland structural engineer in the State ofCalifornia, is a project manager with S.K.

Ghosh Associates, Inc. She has over twenty years experience in the administration anddevelopment of building code provisions. Inrecent years she has played an active role inthe development of structural provisions forthe International Building Code, serving asthe Structural Committee Secretariat. Sheserved as the consultant for the reformatof the ASCE 7-05 seismic provisions andis currently serving on the CaliforniaBuilding Standards Commission.Seattle, WA • Tacoma, WA • Portland, OR • Sacramento, CA • San Francisco, CA • Los Angeles, CA

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Mechanical and Electrical Components a p R p

Wet-side HVAC, boilers, furnaces, atmospheric tanks and bins, chillers, water heaters, heat exchangers, evaporators, air separators, manufacturingor process equipment, and other mechanical components constructed ofhigh deformability materials.

1.0 2.5

Figure 4: From ASCE 7-05 Table 13.6-1



ItemCh 13

R p-valueCh 15

R-valueDiscussion

Billboardsand Signs

2.5 3.5If the item is attached to a structure, then it is considered a nonstructural component andChapter 13 is used. If the item is attached and supported by another structure, it may be ofsuch a size and weight that it needs to be treated as a nonbuilding structures supported byother structures with the seismic design forces determined in accordance with Section 15.3

which refers back to Chapter 13. If the item is assembled and is a free standing, ground-

supported structure, then it should be treated as a nonbuilding structure and Chapter 15should be used.

Bins 1.0 2 or 3

Chimneys varies 3

There are many types of chimneys ranging from residential to industrial chimneys. Achimney attached to a structure may be of such a size and significance that it can beconsidered an architectural nonstructural component and designed in accordance withChapter 13. Industrial, institutional and commercial chimneys that are attached andsupported by other structures may be of such a size and weight that they need to beconsidered nonbuilding structures supported by other structures with the seismic design forcesdetermined in accordance with Section 15.3 which refers back to Chapter 13. Freestandingindustrial, institutional, and commercial chimneys, on the other hand, are typically of a sizethat they are considered nonbuilding structures and designed in accordance with Chapter 15.

Conveyors 2.5

They are usuallyconsidered“nonbuildingstructures similarto buildings”except for themechanical parts.

The support structures for conveyors which are used in large industrial complexes such assurface mining facilities typically qualify as nonbuilding structures similar to buildings. Thereare, however, smaller manufacturing, processing and material handling conveyors which canbe transported in a truck in one piece and would qualify as a nonstructural component. Inthe case where conveyors are designed as nonbuilding structures in accordance with Chapter15, the bracing and anchorage for the actual mechanical workings should be designed inaccordance with Chapter 13.

Cooling Tower 2.5 or 1.0 3.5

If the cooling tower is shop fabricated and transported in a truck, it is considered anonstructural component and designed in accordance with Chapter 13. If it is self-supporting and field erected, then it is considered a nonbuilding structure and designed inaccordance with Chapter 15.

Stacks 2.5 or 1.0 3

Stacks that are supported by or attached to a structure are considered nonstructuralcomponents and are designed in accordance with Chapter 13. Stacks that are free standing orguyed-supported at grade are considered nonbuilding structures and designed in accordance

with Chapter 15.

Tanks 1.0 varies

It is recommended that tanks which are small (5 feet or less in diameter), shop fabricated andtransported to the site in one piece be considered nonstructural components and designed inaccordance with Chapter 13. It is recommended that tanks which are larger in size (greaterthan 5 feet in diameter) and constructed at the jobsite be considered nonbuilding structuresand designed in accordance with Chapter 15. Should such a larger tank be installed withina building or on the roof, then design forces may need to be determined in accordance withSection 15.3 which refers back to Chapter 13.

Towers 2.5 or 1.0 varies

A tower is typically shop fabricated and shipped to a site in pieces to be field erected. Assuch, they are considered nonbuilding structures. But there are times when a tower is smallenough and transported in one piece that it qualifies as a nonstructural component. If thetower is attached and supported by another structure, it may be of such a size and weight thatdesign forces may need to be determined in accordance with Section 15.3 which refers back to

Chapter 13.

Vessels 2.5 or 1.0 varies

If the vessel is less than 10 feet tall, it can typically fit within a building and is considereda nonstructural component. If the vessel is more than 10 feet tall, it most likely shouldbe considered a nonbuilding structure. Vessels that are power boilers typically qualify as anonbuilding structure. Free standing vessels should typically be considered nonbuildingstructures. If the vessel is attached and supported by another structure, it may be of such asize and weight that design forces may need to be determined in accordance with Section 15.3

which refers back to ASCE 7-05 Chapter 13.

Table 1: Discussion of Items Which Can Be Either Nonstructural Components or Nonbuilding Structures.

Reprinted with permission from ICC Building Safety Journal , April/May 2008.

is it a nonstructural component or a nonbuilding structure

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