economical concrete construction30

8/13/2019 Economical Concrete Construction30

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INTRODUCTION

The cost of concrete construction can bereduced, sometimes dramatically, by following afew simple rules in the preliminary layout anddesign of the project. Summarized below aresuggestions on formwork, reinforcement, andconcrete. The more nearly these suggestions arefollowed, the more likely a least cost frame willresult. Occasional exceptions for specialconditions will be evident to experiencedengineers.

FORMWORK

Select one framing scheme and stick with itthroughout the project. Two framing schemes canonly be justified on a large project and then onlyfor special reasons such as different occupanciesin two parts of the building. Each framing schemecosts something for mobilization and formworkmaterial as well as a learning curve for the con-tractor's personnel.

Arrange framing member sizes and spacing sothat the capacity of minimum-sized members isfully used. For example, slabs of minimumthickness controlled by fire ratings should span atleast as far as the minimum reinforcement willpermit. Also, concrete walls of minimum thicknessand reinforcement may carry column loads, act asgrade beams or transfer girders, provideresistance to lateral loads, and serve as partitionsor exterior walls all simultaneously.

Use architecturally exposed concrete framing.The extra cost of more careful formwork, details,steel and concrete placement may be less thanother options such as stone.

Orient all framing in one direction for one-waysystems, such as beams and joists. There will beless time-wasting confusion on the job and fewer expensive problems in the areas where theframing changes direction.

Use "flying forms" to form large areas of walls or floors where the forms can be moved in largesections and reused many times (say, 10 or 20times or more) on one project. More intricateshapes can be justified without unduly increasingthe forming cost shapes that may reduce con-crete or reinforcing steel, add architectural

interest, or have other advantages.

Space columns uniformly. Uniform sizes for col-umns, joists, and beams will result and formworkwill be simpler, hence cheaper. The contractor canreduce costs in a repetitive production line setting.

Make all columns the same size, vertically inone stack as well as horizontally in one story. Varythe amount of reinforcement and concretestrength to achieve size-uniformity. Column formswill be cheaper, fewer expensive variations to slab

or beam forms will be required, and steelfabrication can be standardized.

Use shallow floor framing, limited primarily bydeflection considerations. Architects and ownerswant to minimize floor-to-floor heights because itreduces total project costs. The cost of walls,mechanical risers, stairs, and the structuralframing itself is increased by greater story heights.

ECONOMICAL CONCRETECONSTRUCTION

CRSIENGINEERINGDATA REPORT

NUMBER 30 A SERVICE OF THE CONCRETE REINFORCING STEEL INSTITUTE

933 N. Plum Grove Rd., Schaumburg, Illinois 60173-4758

Copyright 1988 by the Concrete Reinforcing Steel Institute


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Make all beams and joists the same depth. Thesavings in formwork and shoring will exceed anycost increase in concrete and steel. Furthermore,reducing or eliminating structural interferenceswith mechanical ducts and piping reduces projectcosts. A level structural ceiling simplifies the taskof avoiding structural-mechanical interference.Heavily loaded beams on long spans can bemade as wide as necessary, up to four or fivetimes their depth, to carry the load.

Keep the same beam concrete outlines eventhough loads and spans are not the same, andvary the amount of reinforcement. Only if themaximum moment on a beam is more than 100percent higher than that on typical beams, shouldtwo sizes be considered.

Use one joist pan size in a project because allsizes cost about the same. The cost of shippingand handling two sizes of pans on a job can rarelybe overcome by other savings. Furthermore, thefloor structural depth is established by thedeepest pan or deepest beam so that no spacesaving is realized by using a shallower pan.

Use standard form sizes for one-way joists andtwo-way domes because nonstandard sizes arespecially fabricated and the entire cost may becharged to the one project rather than amortizedover several projects. Contact potential suppliersfor pan size availability in the area of the project.

Use prismatic members. Avoid tapered haunches.Make the beam wider than the column on eachside by at least 2 inches so that bars in thecorner of the beam and bars in the corner of thecolumn can pass unobstructed. Furthermore, it ischeaper and easier to cut a hole in the bottom of a beam form for a column penetration than it is tocut holes in the side of a column form for a beampenetration.

Keep floor-to-floor heights constant. if changesare necessary, reduce the height in the upper

stories. It is cheaper and easier to cut off acolumn form than it is to stretch it.

Use a flat plate for spans up to about 25 feetbecause it is the cheapest, fastest, and shallowestframing method available.

Consider using steel shear heads to avoid col-umn capitals and drop panels in flat plate con-struction if the slab is at least 8 inches thick and if the shear head will not interfere with either slab or column bars.

Use small drop panels around columns in flatplates rather than tapered column capitals if shear strength must be increased without using steel.See Fig. 1.

Fig. 1 Column Capitals in Flat Slabs

Make the height of drop panels fit standardlumber dimensions. See Fig. 2.

Fig. 2 Drop Panel Height for Economy

Specify when forms may be stripped. Use a timecriterion for columns and walls (e.g., 12 hours) and astrength criterion for beams and slabs (e.g., 60percent to 75 percent of design strength) butrequire reshoring until design strength has beenreached to prevent excessive deflection. Allowstripping at lower strength if adequate shoring re-mains to support the green concrete at all times.Unnecessarily conservative specifications maydouble or triple the quantity of formwork requiredfor a project and increase the cost of form ma-terial proportionally.

Use high early strength concrete to permit earlyform stripping if the time saved avoids winter con-creting, reduces total construction time or has other advantages.

Allow reasonable tolerances. Some tolerancesspecified by ACI 117 can be relaxed by 50 per-

cent or more in many cases. Specifying tight tol-erances only where they are needed, such as loca-tion of bars in beams and columns, allows thecontractor to concentrate his effort where it ismost important. Tight tolerances require more fieldlabor, hence more cost.

Locate construction joints where the contractor wants them with as little restriction by the en-gineer as possible. That allows the contractor toselect the most efficient sequence of pours for theconstruction method used.


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Use mechanical bar splices or lap splice con-nector systems to eliminate dowel penetration of reusable formwork. Holes for dowels may ruin apiece of plywood.

Eliminate recesses, protrusions , offsets, haunches,brackets, corbels, pilasters, bulbs, nodes, bumps,stubs, curves, wiggles, knuckles, knees, knobs, andanything else that will botch, bungle, butcher, blight,

bugger up, screw up, louse up, damage, demolish,degrade, destroy, fudge, frazzle, fray, mar, abrade,spoil, ruin, maim or wreck the formwork. Formworkrepresents about half the cost of a concrete frameand deserves tender loving care.

REINFORCEMENT

Use Grade 60 bars. Grade 40 requires 50 percentmore steel than Grade 60. Grade 75 is available inbar sizes #11, #14, and #18 at competitive pricesonly on mill orders in lots ranging from 20 to 75tons per bar size. Smaller quantities can probablybe obtained at higher prices from warehouses.

Availability from warehouses for quantities less thanfull heat lots should be checked prior to and includ-ing Grade 75 in the design of a structure. If aminimum mill order of one bar size can be used for column vertical bars in a tall building, Grade 75may lower the cost of material.

Use the largest bar size that will meet designrequirements. Large bars reduce the quantity andtherefore the placing costs. In most cases, it takesas much time to place one small bar as it does onelarge bar.

Use #14 and #18 bars for column verticals inheavily loaded columns if the project requires atleast five or ten tons of one size. The large barsreduce congestion of steel and reduce the cost of splices. However, because a crane is required toplace heavy bars, their use might increase thecost of construction if only a few heavy bars arerequired or if a crane is not available on the jobsite for other reasons.

Use tied columns instead of spiral columns.The weight of spirals is two or three times as muchas the weight of ties in a comparable column and

the cost of spiral steel is about twice the cost of tiesteel. Furthermore, bars and machinery suitable for main spirals are not found in every fabricatingshop, therefore, delivery of spirals in small quan-tities may be delayed. Only rarely are these penaltiesovercome by the 13.8 percent increase in columncapacity for spiral columns. [From ACI 318-83 (Re-vised 1986) Building Code, Sections 9.3.2.2 and10.3.5, the ratio of the strength of a spiral columnto that of a tied column is (0.85 x 0.75)/(0.80 x0.70).]

Eliminate bent bars where possible, because bend-ing increases fabrication costs.

Use ACI standard bending details because spe-cial bends disrupt shop routine and cost more tofabricate.

Keep bars in one plane because multi-plane barsare more difficult and expensive to fabricate.

Make beams wide enough to avoid minimum bar spacing because deformed beam bars that rubagainst each other take longer to place. Bar con-tact hampers movement, so ironworkers get tiredand slow down their work. Of course, bars thatdon't fit in the space allotted require time-con-suming solutions.

Repeat bar sizes and lengths. Many bars can bea few inches shorter or longer while still meetingdesign requirements. Combining two or more sizesand lengths into one group makes job site storageand sorting operations more efficient.

Permit the use of stock length of bars that canbe cut and spliced in the field for special trape-zoidal or irregular-shaped walls and slabs. SeeFig. 3. The savings in fabrication costs, storage andsorting in the field of many variable length barsmay be more than the cost of field cutting and lapsplicing. Of course, the splices should be staggered.

Fig. 3 Using Stock Length of Bars Cutand Spliced in Field

Use lap splices whenever possible. The cost of additional length of bar is usually less than thecost of material and labor for mechanical splices.

Use mechanical splices in projects with a largenumber of heavily loaded columns especially if the

splices result in less bar congestion. (Minimumspacing must be maintained between bar laps.) Inlarge quantities, the unit cost of material andplacing labor for mechanical splices is lower andless bar congestion reduces bar placing costs.

Use the least expensive type of mechanical splice,preferably one with the lowest field labor installa-tion cost. Use sleeved, square-cut bars in columnverticals with no tension in preference to splicesthat develop fifty percent or more of the tensilestrength.


4/4H N H 988/16M Printed in U.S.A.

CONCRETE REINFORCING STEEL INSTITUTE933 N. Plum Grove Road, Schaumburg, Illinois 60173-4758 312/517-1200WESTERN REGION OFFICE1110 East Alosta Avenue, Gendora, California 91740 818/335-5292

This publication is intended for the use of professionals competent to evaluate the significance and limitations of its contents and whowill accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoingmaterial as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for theaccuracy of the sources other than material developed by the Institute.

CONCRETE

Use low strength concrete (typically 3 ksi) in thefloor system unless there is a clear advantage tousing higher strength concrete such as, for exam-ple, to limit the strength of column concrete to 1.4times that of floor concrete, as required by ACI318-83 (Revised 1986).

Use high strength concrete in column s if thehigh strength reduces the amount of reinforcementrequired or reduces the size of columns or the useof high strength concrete permits one size of column to be used. If high strength concrete isused, use the same strength concrete in all columnsin each story because fewer mistakes will be madein placing the correct grade of concrete in eachcolumn. The engineer will sleep easier and the con-teactor will make fewer costly repairs.

Use 6 inch slump and pea-gravel aggregatewhere reinforcement is congested. Fewer expen-

sive repairs of honeycombs will be required. (Aspecial mix design may be required to maintainstrength, durability, and prevent aggregate seg-regation.)

Specify few mix designs. Frequently only twoclasses of concrete are necessary, such as, 4 ksi

air-entrained concrete for concrete exposed to theweather and 3 ksi non-air-entrained concrete for everything else.

Provide a 4 to 6 inch gap between closely spacedtop bars (if the grid of bars is several feet wide) toallow placement of concrete below the bars.

Bundle some bars if necessary to provide the gap.Concrete with 4 inch slump and inch aggregatewill not flow easily through a 2 inch space betweenbars. Furthermore, a 2 to 3 inch vibrator head maybe required to consolidate the concrete properlyand the contractor may not be able to insert thevibrator or it may become entangled in the steeland cannot be withdrawn. An expensive vibrator head may become an additional direct job cost.

Arrange column ties to facilitate concrete place-ment by minimizing internal ties. The contractor must get concrete to the bottom of the column to

have a viable structural member and columns of reliable strength allow the engineer to design lessconservatively.

Limit coarse aggregate size to inch if theminimum clear bar spacing will be one inch, as itusually is.

Contributed by Russell S. Fling

ENGINEERING DATA REPORT was formerly called Product Service & Information (PSI).

Complete set of reports is available; Series include:

Serviceability Requirements with Grade 60 Bars-ACI 318-71 (Bulletin 7603A) Splicing Reinforcing Bars-Welding and Splice Devices (Bulletin7604A) Reinforcing Bars Required-Minimum vs. Maximum (Grade 60) (Bulletin 7701A) Saving Steel incolumns (Bulletin 7702A) New MaximumColumn Capacities-1977 ACI Building Code (Bulletin 7703A) Implication of Recent Tests Upon "Standard" Details, Part 1 of 2 (Bulletin 7801A) Part2 of 2 (Bulletin 7802A) Combined Strength-Slenderness One-Step Design for Columns in Ordinary Structures (Bulletin 7803A) Grade 60 Bars inSanitary Structures (Bulletin 7804A) Selection of (Open) Stirrups/closed) Ties in Plexural Members for Economy (Bulletin 7901A) Update on ASTMSpecifications for Reinforcing Bars (Bulletin 7902A) Evaluation of Reinforcing Steel in Old Reinforced Concrete Structures (EDR Number 11) FieldCorrections to Rebars Partially Embedded in Concrete (EDR Number 12) Preliminary Design for Tied Columns (EDR Number 13) Epoxy-CoatedReinforcing Bars (EDR Number 14) Orientation of Bars in Round Columns (EDR Number 15) Limitations Upon Use of Lap Splices in Columns (EDRNumber 16) Lap Splice Lengths for Rebars (EDR Number 17) Radius Bent Reinforcing Bars (EDR Number 18) Suggested Project SpecificationsProvisions For Epoxy-Coated Reinforcing Bars (EDR Number 19) Placing Drawings for Reinforcing Steel-Obligations/Responsibilities. (EDR Number 20) Rebar Design, Detailing and Economy (EDR Number 21) Direct Solutions for Minimum Steel in FlexuralTension-ACI 318-83 (EDR Number 22)

Suggested Specifications-Reinforcement (EDR Number 23) Suggested Specifications-Reinforcement Including Provisions for CoatedReinforcing Bars (EDR Number 24) Common Applications of Wide-Module Joist Systems (EDR Number 25) ASTM Restores Grade 75 Rebar-ByPopular Demand For High-Rise Construction (EDR Number 26) Proper Load Tests Protect the Public (EDR Number 27) Reinforced ConcreteDesign Includes Approval of Details (EDR Number 28) Two Failure Modes for Column Footings (EDR Number 29) Economical Concrete

Construction (EDR Number 30)

economical concrete construction30

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