ARCH 631 Note Set 8.2 F2008abn

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The main objectives of this publication are trx

.

.

●

Asaiat in the selection of the most econom-ical cast-in-place concrete floor system fora given plan layout and a given set of Ioady

Provide a preliminary estimate of materialquantities for the floor system; and

Discuss the effect of different variables inthe selection process.

Five different floor systems are considered inthis publication. These are the flat plate, the flatslab, the one-way joist, the two-way joist orwaffle, and the slab supported on beams on allfour sides. Material quantity estimates aregiven for each floor system for various baysizes.

Pricing TrendsThe total cost to construct a building depends onthe use for which the structure is designed, theavailability of qualified contractors, and the partof the country in wh]ch the structure is built.Figure 1 gives cost comparisons for two differ-ent types of uses over the past several years.(The data presented in Figures 1 through 5 andTable 1 were obtained from Means ConcreteCost Data, 1990.) ‘Ilte average price per squarefoot is considerably greater for office buildingsthan for apartment buildings. Part of the higher

Figure 1- Price Comparieorra for DifferentBuilding Typea

.

cost ia because ofi-kc buildings are designedwith more open spaces which in structural termsmeans costlier, longer clear spans.

Table 1 gives cost indices for many majorcities in the United States and Cartada. The costindex includes both labor and materials, with thevalue of 100 representing the average cost for30 major cities. The table shows the wide vari-ation in costs depending on the locale. In An-chorage, Alaska (127.9) or New York City(126.9) the cost of a building can be as much as60% higher than that of a similar building inCharleston, South Carolina (80.2), Jackson,Mksissippi (81) or Sioux Falls, South Dakota(82.2). Figure 2 shows the relative change incosts in current dollars of material and laborover the past 40 years.

too

Sa

Sa

70

m

cost ~Indsx

4a

30

20

‘TJ_L_—iwo ?960 tern ieao two

Figure 2- Annual Construction coatComperiaona

The majority of the structural cost of a build-ing typically is the cost of the floor system. Thisis particularly true of low-rise buildings andbuildings in low seismic zones. Therefore, it isimperative to select the most economical floorsystem.

In this publication, estimated quantities are.provided for concrete, reinforcing steel andformwork for the tive floor systems discussedin the following sections. Prices for labor andmaterial for these items over the past severalyears are shown in Figores 3 through 5.

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ARCH 631 Note Set 8.2 F2008abn

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Table l—Relative Construction Costs for Reinforced Concrete

ALABAMA (BIRMINGHAM)ALASKA (ANCHORAGE)ARIZONA (PHOENIX)ARKANSAS (LlllLE ROCK)CALIFORNIA (LOS ANGELES)CALIFORNIA (SAN FRANCISCO)COLORADO (DENVER)CONNECTICUT (HARTFORD)DELAWARE (WILMINGTON)WASHINGTON, D.C.FLORIDA (MIAMI)GEORGIA (A~NTA)HAWAll (HONOLULU)IDAHO (BOISE)ILLINOIS (CHICAGO)INDIANA (INDIANAPOUS)lOWA (DES MOINES)KANSAS (WICHITA)KENTUCKY (LOUISVILLE)LOUISIANA (NEW ORLEANS)MAINE (PORTU4ND)MARYLAND (BALTIMORE)MASSACHUSElT8 (BOSTON)MICHIGAN (DETROIT)MINNESOTA (MINNEAPOUS)MISSISSIPPI (JACKSON)MISSOURI (ST. LOUIS)MONTANA (BILUNGS)NEBRASKA (OMAHA)NEVADA (MS VEGAS)

0.6

0.4

W/b

0.2

0_

84.0127.9

91.984.5

112.0126.0

83.5lW.11CCI.3

95.489.989.7

111.183.3

101,897.6eu.788.888.388.689.898.1

115.6108.9S9.461,0

101.6%?.188.6

104.6

t 1 1 I 1

fw Ieaa la fM7 Im# 1969 fw

Figure 3- Cost of Reinforcing Bars in Place

NEW HAMPSHIRE (MANCHESTER)NEW JERSEY (NEWARQNEW MEXICO (ALBUQUERQUE)NEW YORK (NEW YOR~NEW YORK (ALBANY)NORTH CAROUNA (CHARLOTIE)OHIO (CLEVELAND)OHIO (CINCINNATl)OKIA-IOMA (OKIAHOMA CITY)OREGON (PORWND)PENNSYLVANIA (PHILADELPHIA)PENNSYLVANIA (PITTSBURGHIRHODE ISIAND (PROVIDENCE)SOUTH CAROUNA (CHARLES1ON)SOUTH DAKOTA (SIOUX FALLS)TENNESSEE (MEMPHIS)TEXAS (DAUAS)UTAH (SALT LAKE CITY)VERMONT (BURLINGTON)VIRGINIA (NORFOLKIWASHINGTON (SEATTLE)WEST VIRGINIA (CHARLESTON)WISCONSIN (MILWAUKEE)WYOMING (CHEYENNE)CANADA (EDMONTON)CANADA (MONTREAL)CANADA (QUEBEC)CANADA (TORONTO)CANADA (VANCOUVER)CANADA (WINNIPEG)

‘“ ~—ao

aQ I

!30.3104.9

91.5126.9

84.580.8

107.395.389.4

101,0107.21D3.6100.8

80.282.287.687.891.78a. 183.3

101,697.497.387.4

100.2100,0

99.0109.8105.5101.5

m

30

20

101la84 1985 1888 1987 1988 1988 1890

Figura 4- Coat of Ready-Mxed Concrete

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ARCH 631 Note Set 8.2 F2008abn

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I flat skb

2

t

1984 1985 198.S 1987 1988 1989 1990

Figure 5- Cost of Formwork

Presentation of ResultsThe following pages provide discussion andquantity estimates for the five floor systems.These results were obtained using a five bay byfive bay structure. Bay sizes are measured fromcenterline of column to centerline of column.Floors were designed using ACI 318-89 Build-ing Code Requirements for Reinforced Con-crete. Concrete, reinforcing steel and formworkquantities are presented for each of the floorsystems. An overview of the floor systems isprovided, following the discussion of the floorsystems,

Included with each floor system is a discus-sion of the factors that may affect the estimatedquantities. The factors discussed are columndimensions, live loads, and aspect ratios. A costbreakdown is also given in each case. Followingthe discussion for each individual floor systemare several tables and graphs. The graphs showthe variation in costs for increased bay size andhigher concrete strength. The tables give quan-tities for various bay sizes.

Fire Resistance of ConcreteFloor SystemsFire resistance rated construction will often berequired by the governing building code, or theowner may desire a highly fire resistant structure

in order to qualify for the lowest fire insurancerates,

Concrete floor systems offer inherent tire re-sistance. Therefore, when the floor system iscompleted, no additional protective measuresare necessary in order to achieve code requiredtire resistance ratings.

On the other hand, for steel floor systems forinstance, additional protection must be providedby special acoustical ceilings, or fireproofingsprayed on the underside of the steel deck and/orbeams. In addition, when an acoustical ceilingis an integral part of a rated floor/ceiling assem-bly, special ceiling suspension systems, and spe-cial protective devices at penetrations for lightfixtures and HVAC diffusers are required.

These additional costs associated with pro-tecting the structural framing members must beadded to the cost of the structural frame toproduce an accurate cost estimate. If this is notdone, the actual cost of the competing floorsystem is understated, makkrg a valid compari-son with a concrete floor system difficult, if notimpossible.

Fire resistance rating requirements vary fromzero to four hours, with two hours typicallybeing required for high rise buildings. Beforeselecting the floor system, the designer shoulddetermine the fire resistance rating required bythe applicable building code. Except for one-way and two-way joist systems, the minimumslab thickness necessary to satisfy structuralrequirements (usually 5 in.) will normally pro-vide a floor system that has at least a two hourfire resistance rating.

Table 2 shows minimum slab thicknessesnecessary to provide fire resistance ratings fromone to four hours, for different types of aggre-gate. If the thickness necessary to satisfy fireresistance requirements exceeds that requiredfor structural purposes, consideration should begiven to using a different type of aggregate thatprovides higher fire resistance for the samethickness. For example, a one-way joist systemmay require a 3 in. thick slab to satisfy structuralrequirements. However, if a two hour fire resis-tance rating is desired, a 5 in. thick slab will berequired if siliceous aggregate normal weightconcrete is used. By using lightweight aggre-

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ARCH 631 Note Set 8.2 F2008abn

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gate concrete, the slab thickness can be reducedto 3.6 in. This 28% reduction in thicknesstranslates into approximately a 45% reductionin dead load.

Table 2—Minimum Slab Thickness forFire Resistance Rating

FloorConstruction

Material

rSiliceous AggregateConcrete

Carbonate AggregateConcrete

Sand-lightweight

Concrete

Lightweight Concrete

Mhimum slabthickness (in.)

or fire-resistance rating

1 hr

3.5

3.2

2.7

2.5

2 hr

5.0

4.6

3.8

3.6

3 hr

6.2

5.7

4.6

4.4

7.0

6.6

5.4

5.1

Adearrate cover must be provided to keepreinfor~ing steel temperat~res within codsprescribed limits. The amount of cover dependson the element considered (i.e., slab, joist orbeam), and whether the element is restrainedagainst thermal expansion. All elements of cast-in-place concrete framing systems areconsidered to be restrained.

For positive moment reinforcement in beamsspaced at 4 ft or less on center, and in joists andslabs, regardless of the type of concrete aggre-gate used, the minimum cover required by ACI318 is adequate for ratings of up to four hours.For beams spaced at more than 4 ft on center,the cover must not be less than the values givenin Table 3.

Table 3—Cover Thickness for FireResistance Rating for Beams Spaced

More than 4 ft on Center

=

The cover for an individual bar is the mini-mum cover between the surface of the bar andthe fire-exposed surface of the structural mem-ber. When more than one bar i:]used, the coveris assumed to be the average of the minimumcover to each bar, where the cover for comerbars used in the calculation is one-half the actualvalue. The actual cover for an individual barmust be not less than one-half the value shownin Table 3, nor less than 3/4 in. IForbeam widthsbetween tabulated values, use direct interpola-tion to determine minimum cover.

The foregoing is intended to give a briefoverview of the subject of fire resistance ofconcrete floor systems. While the informationcited is consistent with the three model buildingcodes in use in the United States, the legallyadopted building code governing the specificproject should be consulted.

I 7

1

3/4

1

3/4

1

3/4

1

3/4210

3/4 3/4 3/4 3/4 I

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General Discussion

This section provides overall comparisons of theeconomics of the various fkmr systems discussed inthis publication. It provides a summary of the factozsthat may influence the costs of cast-in-place concretefloor systems. These factors include column dimen-sions, live loads, aspect ratios and proper detailing.A few other aspects that have an influence on econ-omy are also discussed.

Overall Comparisons

Four figures that compare the economics of the dif-ferent structural floor systems considered are prn-vided at the end of this publication. The figuresclearly show that the optirrrality of the slab systemdepends on two major factors: the span in the longdirection, and the intensity afsrrperimposed dead andlive loads. For a given set of loads, the slab systemthat is optimal for short spans, is not necessarilyoptimal for longer spans. For a given span, the slabsystem that is optimal for light superimposed loads,is not necessarily optimal for heavier loads. The foorfigures should facilitate the section of a strnctrrralfloor system most appropriate for a certain applica-tion.

Column Dimenm”ons

Analysis shows that the height between floors hasvery little influence on the material qrrantitiea for thefloor system. Column cross-sectional properties de-termine the clear span length and the shear capacityof the slab. The column cross-sectional d~mensionsused in this publication were representative of 10- to20-story buildings Increasing or decreasing the col-umn dimensions by 2 in. did not affect the concretequantities and charrgcd the steel reinforcing quanti-ties by lCSSthan 1%.

Live Loads

The material quantities for the floor system are typ-icall y controlled by deflections rather then stresses.Irrcreusing the live load from 50 psf te ltM paf onfyresulted in a 4~o to 10% increase in the floor systemcost.

Aspect Ratr”o

Sqrrare bays usually represent the most economicalfloor layou~ since deflection control requirementscan be exactly met in both directions. A rectangularbay with an aspect ratio of 1.5 ranges between 4% to10% more in cost than a bay with an aspect ratio of1.0 and the same floor area. This, however, is not

the case for one-way joist systerna. Tfds type of floorsystem shordd have the joista span in the short direc-tion, and is almost unaffected by aspect ratios of UPto 1.5.

Concrete Strengths

Concrete strengths of 4000 psi, 50@ psi, and @OOpsi were used in this publication. Cost analysisshows that for gravity loads, 4000 pi concrete ismore economical than higher concrete strengths.

Cost Breakdown

The formwork for the floor systems will absorb from50% to 58% of the costs. Concrete material, placingand finishing account for 21’% to 3090. The materialand placing costs of the reinforcing steel amount tobetween 17% and 25% of the cost.

Repetition

A cost efficient design utifizes repstitiorr. Changesshould be minimized from floor to floor. Changingcolumn locations, joist spacing, or the type of floorsystem increases the cost of forrnwork, time of corr-strrrction and the chance of field mistakes, and there-fore should be avoided.

Column-Beam Intersections

The bearrraOratframe into columrrashould be at leastaa wide aa the columns. If the beams are narrowerthan the columns, the beam forms will require eostl yfield labor to Pas the formwork around the columns.

Stindd Dimenn”ons

Standard available sizea should be used for structuralfornring. For instance, joist fomrwork pans areavailable in various web depths of 20 in. and from 8in. to 16 in. in 2 in. increments. Specifying a depthdifferent from these sizes will require the fabricationof costly special forrnwork. When detailing droppanefs or other changes in the floor system deptiactrral lumber dimensions should be taken into ac-Corrrrt.

Depth of the Ceibrg Sandwich

This publication haa addrxd the economy of thestructural slab system only. However, the structuralengineer usually has to look beyond. The structuralslab system ia part of the so-called ceiling sandwichwhich also includes the mechanical system f,HVACducts), the lighting fixtures, and the ceiling itself.

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The flnor-to-floor height of a building ia the totaldepth of the ceiling sandwich plus the clear tlnor-to-cciling height. Any variation in the depth of theceiling sandwich will have an impact on the totalheight of the shearwalls and cohsmna, the mccban-ical, electrical and plumbing riaezs, the staim andinterior architectural finiahcs, and the exterior clad-ding. It will also have an impact on the total heatingcooling and ventilation volume. To minimize thedepth of the ceiling sandwich is very often the goalof the structural engineer. This becnmes particularlyimpmiant in cities like Washington, D.C. that imposea height limit on buildings Optimization of theceiling sandwich depth may tranalate intu an extrastory or two accommodated within the prescribedheight limit.

“’’”-l

TT—

A number of details have been attempted in thepaat to accomplish a reduced depth of the ceilingsandwich. The HVAC ducts can paas through thewebs of joiata or beams. llrii will reduce the floer-to-floor height, but will increase formwork and fieldlabor costs Another alternative is to cut notches atthe bottom of the joist or beam to allow paxsage ofthe up~r portions of the HVAC ductx. This altern-ativealan requires additional forming cmta. Further,special detailing would be needed to meet the .WUC.

tural integrity requirements of the ACI 318-89 Cnde.More importantly, however, such practices take flex-ibility away from accommodating futrrre changes inthe use of the floor space. Such flexibility is becom-ing more important in view of the shifting emphasistowardx corracionaly designing buildings for a longservice life.

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Ne Load = 50 psf

superimposed Dead Load = 20 psf

~ = 4000 psi

0.9

0.8

0.7

cost 0.6Index

0.5 ,

0.3 I 1 1 1 ! 1 [

15 20 25 30 35 40 45 50

Square Bay Sizen

Owwayjoist(wi& m-d”k)

. . . . S!abandkm

- , , - Flat P1.te

— . — Flat dab

---, Twc-wy joist

Two-nay joist———(wi& mod”k?)

15 20 25 w 35 40 45 50

Square Bay Sizert

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ARCH 631 Note Set 8.2 F2008abn

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Live Load. 100 ps

Superimposed Dead Load =20 ps

fc’= 4000ps

0.8tI

4.’

/

15 20 25 30 35 40 45 50

Square Say Size

n

—. ‘me way pi.,

. . . . S!ab and ban

- u , -. FM pl.te

— . — FM ,kb

m m _ Tw..wq joist

Tw.w.y joist———[wide m.adub]

I

15 30 25 30 35 40 45 50

Square Bay Sizeft

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