Concrete Floor Systems

8/17/2019 Concrete Floor Systems

1/36Book ContentsPublication List


2/36

About tie author%

August W. Domel, Jr. is Senior StmctumJ Engineer, Engineered

Stmckues md Cmfes, Portland Cement Association.

S. K. Ghosh is Director, Engineered Stmctures and Codes, Pofi-

kmdCement Association.

This publication is based on the facts, tests, and authorities stated

herein, It is intended for the use of professional personnel

competent to ev.l”ate the significmce and Iimitatiom of the

reported findings a“d who will accept respmsibility for the

application of the material it contains, Obviously, the Pmtkmd

Cement Association disclaims any and all responsibility for

application of the stated principles or for the accuracy of any of

the sources o her than work performed or information developed

by the Association

0 Portland Cement Association 1990

Book ContentsPublication List


3/36

Contents

lWodution .......................................................................................2

Flat Plate Floor System .....................................................................6

Flat Slab Floor System ...................................................................lO

One-Way Joist Fioor System ..........................................................l4

Two-Way Joist Floor System ...........................................................2O

Wam.SuppoRed Slab System ........................................................26

Overview of Floor Systems ............................................................3O

Publication List Book Contents


4/36

The main objectives of this publication are trx

.

.

Asaiat in the selection of the most econom-

ical cast-in-place concrete floor system for

a given plan layout and a given set of Ioady

Provide a preliminary estimate of material

quantities for the floor system; and

Discuss the effect of different variables in

the selection process.

Five different floor systems are considered in

this publication. These are the flat plate, the flat

slab, the one-way joist, the two-way joist or

waffle, and the slab supported on beams on all

four sides. Material quantity estimates are

given for each floor system for various bay

sizes.

Pricing Trends

The total cost to construct a building depends on

the use for which the structure is designed, the

availability of qualified contractors, and the part

of 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 and

Table 1 were obtained from Means Concrete

Cost Data, 1990.) ‘Ilte average price per square

foot is considerably greater for office buildings

than for apartment buildings. Part of the higher

Figure 1- Price Comparieorra for Different

Building Typea

.

cost ia because ofi-kc buildings are designed

with more open spaces which in structural terms

means costlier, longer clear spans.

Table 1 gives cost indices for many major

cities in the United States and Cartada. The cost

index includes both labor and materials, with the

value of 100 representing the average cost for

30 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 as

60% higher than that of a similar building in

Charleston, South Carolina (80.2), Jackson,

Mksissippi (81) or Sioux Falls, South Dakota

(82.2). Figure 2 shows the relative change in

costs in current dollars of material and labor

over 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 coat

Comperiaona

The majority of the structural cost of a build-

ing typically is the cost of the floor system. This

is particularly true of low-rise buildings and

buildings in low seismic zones. Therefore, it is

imperative to select the most economical floor

system.

In this publication, estimated quantities are.

provided for concrete, reinforcing steel and

formwork for the tive floor systems discussed

in the following sections. Prices for labor and

material for these items over the past several

years are shown in Figores 3 through 5.

L



5/36

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_

84.0

127.9

91.9

84.5

112.0

126.0

83.5

lW.1

1CCI.3

95.4

89.9

89.7

111.1

83.3

101,8

97.6

eu.7

88.8

88.3

88.6

89.8

98.1

115.6

108.9

S9.4

61,0

101.6

%?.1

88.6

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 (NEWARQ

NEW 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 (PITTSBURGHI

RHODE ISIAND (PROVIDENCE)

SOUTH CAROUNA (CHARLES1ON)

SOUTH DAKOTA (SIOUX FALLS)

TENNESSEE (MEMPHIS)

TEXAS (DAUAS)

UTAH (SALT LAKE CITY)

VERMONT (BURLINGTON)

VIRGINIA (NORFOLKI

WASHINGTON (SEATTLE)

WEST VIRGINIA (CHARLESTON)

WISCONSIN (MILWAUKEE)

WYOMING (CHEYENNE)

CANADA (EDMONTON)

CANADA (MONTREAL)

CANADA (QUEBEC)

CANADA (TORONTO)

CANADA (VANCOUVER)

CANADA (WINNIPEG)

‘“ ~—

ao

aQ

I

30.3

104.9

91.5

126.9

84.5

80.8

107.3

95.3

89.4

101,0

107.2

1D3.6

100.8

80.2

82.2

87.6

87.8

91.7

8a. 1

83.3

101,6

97.4

97.3

87.4

100.2

100,0

99.0

109.8

105.5

101.5

m

30

20

10

1

a84 1985 1888

1987 1988 1988 1890

Figura 4- Coat of Ready-Mxed Concrete

3



6/36

f lat skb

2

t

1984

1985

198.S

1987 1988 1989 1990

Figure 5- Cost of Formwork

Presentation of Results

The following pages provide discussion and

quantity estimates for the five floor systems.

These results were obtained using a five bay by

five bay structure. Bay sizes are measured from

centerline 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 formwork

quantities are presented for each of the floor

systems. An overview of the floor systems is

provided, following the discussion of the floor

systems,

Included with each floor system is a discus-

sion of the factors that may affect the estimated

quantities. The factors discussed are column

dimensions, live loads, and aspect ratios. A cost

breakdown is also given in each case. Following

the discussion for each individual floor system

are several tables and graphs. The graphs show

the variation in costs for increased bay size and

higher concrete strength. The tables give quan-

tities for various bay sizes.

Fire Resistance of Concrete

Floor Systems

Fire resistance rated construction will often be

required by the governing building code, or the

owner may desire a highly fire resistant structure

in order to qualify for the lowest fire insurance

rates,

Concrete floor systems offer inherent tire re-

sistance. Therefore, when the floor system is

completed, no additional protective measures

are necessary in order to achieve code required

tire resistance ratings.

On the other hand, for steel floor systems for

instance, additional protection must be provided

by special acoustical ceilings, or fireproofing

sprayed on the underside of the steel deck and/or

beams. In addition, when an acoustical ceiling

is an integral part of a rated floor/ceiling assem-

bly, special ceiling suspension systems, and spe-

cial protective devices at penetrations for light

fixtures and HVAC diffusers are required.

These additional costs associated with pro-

tecting the structural framing members must be

added to the cost of the structural frame to

produce an accurate cost estimate. If this is not

done, the actual cost of the competing floor

system is understated, makkrg a valid compari-

son with a concrete floor system difficult, if not

impossible.

Fire resistance rating requirements vary from

zero to four hours, with two hours typically

being required for high rise buildings. Before

selecting the floor system, the designer should

determine the fire resistance rating required by

the applicable building code. Except for one-

way and two-way joist systems, the minimum

slab thickness necessary to satisfy structural

requirements (usually 5 in.) will normally pro-

vide a floor system that has at least a two hour

fire resistance rating.

Table 2 shows minimum slab thicknesses

necessary to provide fire resistance ratings from

one to four hours, for different types of aggre-

gate. If the thickness necessary to satisfy fire

resistance requirements exceeds that required

for structural purposes, consideration should be

given to using a different type of aggregate that

provides higher fire resistance for the same

thickness. For example, a one-way joist system

may require a 3 in. thick slab to satisfy structural

requirements. However, if a two hour fire resis-

tance rating is desired, a 5 in. thick slab will be

required if siliceous aggregate normal weight

concrete is used. By using lightweight aggre-

4



7/36

gate concrete, the slab thickness can be reduced

to 3.6 in. This 28% reduction in thickness

translates into approximately a 45% reduction

in dead load.

Table 2—Minimum Slab Thickness for

Fire Resistance Rating

Floor

Construction

Material

r

iliceous Aggregate

Concrete

Carbonate Aggregate

Concrete

Sand-lightweight

Concrete

Lightweight Concrete

Mhimum slab

thickness (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 keep

reinfor~ing steel temperat~res within cods

prescribed limits. The amount of cover depends

on the element considered (i.e., slab, joist or

beam), and whether the element is restrained

against thermal expansion. All elements of cast-

in-place concrete framing systems are

considered to be restrained.

For positive moment reinforcement in beams

spaced at 4 ft or less on center, and in joists and

slabs, regardless of the type of concrete aggre-

gate used, the minimum cover required by ACI

318 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 given

in Table 3.

Table 3—Cover Thickness for Fire

Resistance 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 and

the fire-exposed surface of the structural mem-

ber. When more than one bar i:]used, the cover

is assumed to be the average of the minimum

cover to each bar, where the cover for comer

bars used in the calculation is one-half the actual

value. The actual cover for an individual bar

must be not less than one-half the value shown

in Table 3, nor less than 3/4 in. IForbeam widths

between tabulated values, use direct interpola-

tion to determine minimum cover.

The foregoing is intended to give a brief

overview of the subject of fire resistance of

concrete floor systems. While the information

cited is consistent with the three model building

codes in use in the United States, the legally

adopted building code governing the specific

project should be consulted.

7

1

3/4

1

3/4

1

3/4

1

3/4

210

3/4 3/4

3/4 3/4

I

5



8/36

DATA

SPAN LENGTH:

PracticalRange

= 15 ftt030ft

Wmomicul Range = 15 ft to 25 fl

ADVANTAGES:

.

.

.

Sirrrplecorrstmction and fomrwork

Architectural finish can be applied directly to the

underside of slab

Absence of beams allows lower story heights

Flesibtity of paflition location

Required fire resistance rating Maimed without addi-

tio&l corrcrtte thickness or o~herpmtcctive mea-sums

DISCUSSION

SkdJ Thickness

Floor slab thickness for flat plates under normal

loading conditions (live loada of 50 paf or less) is

usually corrtrokd by deflection considerations. The

Building Code Requirements for Reinforced Cmr-

crete (ACI 318-89) Table 9.5 (c)requires that the slab

thickness for flat plate floors without edge beams he

greater than one-tldrtieth of the span length (for

Grade 60 reinforcing steel) and no less than 5 in.

Becauae deflection controls the slab thickness, the

reinforcing steel required for bending moments will

be about the minimum prescribed by Code. An

increase in slab thickness beyond the minimum re-

quired is not economical. Athickerslab will increaac

the concrete quantity and not reduce the steel rein-

DIMENSIONS:

“ Slab thickness 5 in. to 10 in.

DISAOVANZAGES:

.

l+onomicdh’ viable onlv for short and medium scram

and for mod&’atelive Io;ds

forcing quantity. Also, the minimum code-pre-

scribed slab thickness is independent of the concrete

strength f& A higher strength concrete will increase

the cuat of the concrete without any allowable reduc-

tion in qusntity. Therefore, for normal loading con-

ditions (live loads of 50 paf or less), the most

economical flat plate floor will he the one with the

minimum allowed tfrickneas and an f: of 4000 psi.

When heavier loads are encountered (live loads of

100 psf or more), the deflection criteria may not

control. In that situation the slab th]ckrress is con-

trolled by shear forces at the column face and by

bending moments in the slab. Au increase in the slab

thickness will result in a decrease in the steel rein-

forcing quantity. The reduction in steel cost, how-

6



9/36

ever, will not offset the increase in concrete costs.

Using a concrete strength of 5000 or6000 psi may

result in a decrease in the slab thickness without a

significant change in the steel reinforcing rm@re-

merrts. Cast analyses show that the decrease in

concrete quantities in conjunction with the increa~

in price for higher strength concrete results in

roughly the same cost as that of tire flate plate with

an & . WOO pai. Therefore, floor cost should be

estimated using a minimum slab thiclmcss with an

~ = 4000 pi, when live

loads are 100 paf or lCSS.

Column Dimension E ects

The height and cross-sectional dimensions of the

columns above and below the slab will affect the

resulting floor slab shears and bending moments.

Column heights can range from 10 ft to 30 ft, but

typi~lly are between 10 ft and 13 ft. A column

height of 12 ft was used in the calculations for this

publication.

Column stiffness is a function of the column

height. Stiffness is determined as EI/L, where E is

the modulus of elasticit y of the column material, I is

the moment of inertia of the column cross-section

and L is the column height. Since the stiffness is

inversely proportional to the column height, it fol-

lows that a longer column is more flexible. A flexi-

ble column will allow greater rotation at the

slab-column joint and larger bending momenta in the

slab. Analyses were performed for a variety of bay

widths and column dimensions, with column heights

ranging between 10 ft and 15 ft. Increasing the floor

height from 10 ft to 15 ft resulted in an increase in

the slab momentx of less than 4~o. This small in-

crease will have minimal effect, if any at all, on the

material quantities. Thus, the floor quantities are

independent of the flnor height under normal loading

conditions.

Column cross-sectional dimensiom will deter-

mine the clear span between the column faces. The

bending momenta are determined using this clear

span length. The shear resisting properties are also

related to the column cross-sectional dimensiofrs. A

larger column width or depth will result in a larger

shear carrying capacity of the slab. In the analyses

of this publication, the column cross-sectional di-

mensions for the different bay widths were chosen

to represent the column sizes used in 10- to 20-story

buildings. If a structure has a column width and

depth considerably different from those used in the

tables, adjustments should be made. The required

adjustment is made by using the same forrnwork and

concrete quantities, with an incrcmsc or decrease in

the reinforcing steel quantity. The steel reinforcing

quantities should be increased by 1% for each 2 in.

decrease in square column dimensions, or decreased

by 1% for the same increase in square column di-

mensions.

Live Load Effects

Gravity loads consist of the floor slab weight, super-

impmed dead loada, and live loads. Typical live

loads rmrge from 40 psf to 50 psf and constitute 20%

to 40% of the total gravity load.

If heavy live

loads

are used (1OOpsf), the resulting stresses and bending

momen~ are increased by 3070 to 4070. Since flat

plate flours have minimum thickness requirements

baaed on deflection considerations, the costs associ-

ated with this incresse is not proportional. Flat plate

floom with live loads of 100 psf are typically onfy

5% to 10% more expensive than those carrying 50

psf of live loads.

Aspect R@”o

The

aspect ratio is defined as the larger dimension of

the slab panel divided by the smaller dimension of

the slab panel. As previously discussed, the flat plate

slab thickness is controlled by the span length. For

a building with square bays (aspect ratio = 1.0), the

slab thickness requirement is the ;same for both di-

rections. A slab with an aspect ratio other than 1.0

will have a different thickness requirement in each

direction. Obviously, the larger of the two is used,

resulting in a loss of economy. Fnr example, a bay

with 625 aq ft of floor area and an a.s~ct ratio of 2.0

will cost 20% more than a square bay with the same

floor area. Unless column layout is dictated by

functional requirements, square ba:ysshould be use~

since they will provide the most economical layouts.

Cost Breakdown

The

formwork costs for flat plates represent approx-

imately 50% of the floor system cost. Concrete

material, placing and finishing account for 30% of

the cost. The remaining 20% is the material and

placing cost of the reinforcing steel.

7



10/36

.ive Load = 50 psf

superimposed Dead

Load = 20 psf

cost

Index

0.5

0.45

0 .4

0.35

0 .3

1

15

r -c = 4000

psi

,n -

20 25

Squere Bay Size

n

30

Bay Slab

Square

QUANTITIES

Size Thickneea

Column

ft

Concrete Reinforcement

in. Size (in.)

Forme

(ft3/f?)

(psq (f?/f+)

15X15

6.0

14 0.50

2.20

1.0

15x20

7.5 18

0.63

1.95

1,0

15x25 9.5

m 0.79

2.51

1,0

15x30 12.0

22 1.Cxl 3,CQ 1.0

20X20

7.5 20

0.63 2.12

1.0

20x25 9.5

22 0.79 2.55

1.0

20.30

12.0

24 1SW 3,18

1.0

25 X 25

9.5 26

0.79 2.76

1.0

25x30

12.0 30 1CO

3.22

1,0

30XXJ 12.0

32 1.CiJ 3.50

1,0

s



11/36

Live Load .100 psf

Superimposed Dead Load =20 psi

0.5

I

0.45

f ’c . 5000

psl

cost

Index

0 .4

0.35

0 .3

15

20

25

30

Squ are B ay Size

n

Bay

Slab

Size Thickness

it

in.

‘:” km %%

ize (in,)

15X15 7.0

14

0.58

2.24 1,0

15X20 8.5 18

0.71

2.44

1.0

15x25

10,0 22

0.85

2.89

1.0

15X30

12,5 24

1.04

3.52

1,0

2Qxm 9.5 22 0.79 2.48

1.0

20.25

11.0 24

0.92

3.01 1,0

2Qx3a

13.5 26

1.13

3.63 1.0

25 X 25

11,0

28 0.92 3.22

1.0

25x3CI 13.5 32

1.13

3.70

1,0

3QX2KJ

14,0

34

1.17

4.cKl 1,0



12/36

nl

rn

DATA

SPAN LENGTH:

Pmcticd Raoge

= 15tlt030ft

Economical Range = 18 R to 30 ft

ADVANTAGES:

.

Simple construction and fomrwork

Architectural firriah can be applied direzily to the

underside of slab

Abscncc of beams rdlows lower

story

heighta

Reauircd fire reaistancc rating obtained without rrddi-

tioiiaf concrete thickness or O-&r protective measures

DISCUSSION

GenerrrlDiscusa”on

A flat slab floor system is similar tn a flat plate floor

system,

except that the former has drop panels. Drop

panefa are formed by thickening the bottom of the

slab around the columns. This thickerring provides

the slab with increased shear carrying capacity at

locations where the shear is the largest. The discus-

sion on

the flat plate floor system in the preceding

section stated that under normal loading conditions

the slab thickness ia

controlled by deflection con-

straint. Thickening of the slab at the cohrrnn does

little to decreaae the deflections in the span. The

main nae for a drop panel is where the slab has the

proper thickrreas for deflection control, but lacks

sufficient shear capacity at the column. Under nor-

DIMENSIONS:

Slab

thickness 5 in. to 10 in.

Depth of Drop Panels 2~4 in. to 8 in.

DLSADVANZAGES:

.

E.amomicslly viable orrlyforshott and medium, heav.

ily loaded spares

mal loading conditions, ths nccura in spans over 25

ft. For span Iengtha larger than 25 ft, the flat slab can

be more economical than the flat plate. When spana

exceed 35 f~ other systems become more economi-

cal than the flat slab or the flat plate floor system.

Flat slabs are typically economical for heavily

loaded short and rrrcdirrm spans, or possibly when a

relatively flat ceifirrg is required for medium to long

spmrs.

Slab and Drop Panel Dimensions

The

minimum tbickrreas permitted for a slab with

drop

panels

and without beams ia equal to the clear

10



13/36

span length divided by 33 (ACI 318-89 Table 9.5(c)),

but not less that 4 in. This gives a minimum thick-

ness 10% less than that required for flat plates on

similar spans. This reduction in the required thick-

ness accounts for the decrease in deflection from the

addition of dmp panels around the eolmrrns.

Minimum dimensions for drop panels are given

in Section 13.4.7of the ACI Building Code. The first

restriction requires that the drop panels extend in

each direction from the centerline of the supprt a

distance not less than one-sixth of the span length.

The second restriction is that the projection of the

drop panel below the slab shall beat least one-quarter

the slab thickness.

Drop dlmensiom are also controlled by formwork

considerations. Standard lumber dimensions should

be used when choosing drop depths and should be

limited to either 2.25 in., 4.25 in., 6.25 in., or 8 in.

Any other depth will unnecessarily increase form-

work costs.

A design is begun by choosing a slab thickness

based on the minimum slab thickness requirements

of the ACI Building Code. Drop panel plan dimen-

sions are then chosen on the basis of the spmr lengths.

These drop psnel dimensions are usually adeqnate,

since the shear stress will be critical at the column

face. Analysis should be performed with the mini-

mum drop depth of 2.25 in. If this proves to be

inadequate, the next larger suggested drop depth

should be considered.

Column Dimension Effects

The floor quantities are independent of the floor

height mrder normal loading conditions. If the struc-

ture has columns widths and depths different fmm

those shown in the tables, adjustments should be

made by increasing the steel reinforcing quantities

by three-quarters of 1% for each 2 in. demeuse in

square column dimensions. The qrrarrtities should be

decreased by the same amount for each 2 in. increase

in square column dimensions.

Live LoodEffects

The material quantities required for a flat slab are

typically controlled by deflections. Therefore, an

increase in live loads will not cmrxe a proportional

increase in costs. Alive load of ltXl psf increases the

total cost of a flat slab fleer system by an average of

10% over that of the same system carrying a live load

of 50 pf.

Aspect Rotio

Square bays (aspect ratio = 1.0) represent the most

economical floor layout, since minimum thickness

based on deflection requirements can be exactly met

in both directions. A rectangular bay with an aspect

ratio of 1.5 is 870 more expensive than a square bay

with the same fleer area.

Cost Breokdinvn

The formwork costs for the flat slab are approxi-

mately 51% of the floor system costs. Concrete

material, placing and finishing account for 3070 of

the cost. The remaining 19% is the material and

placing cost of the steel reinforcing.

11



14/36

Live Load =50 psf

Superimposed Dead Load =20 psf

0 .5

0.45

cost

Index

0 .4

0.35

20

rc = 6000psi

f’c=

5000ps i

fc = 4000p s i

25

Square Bay Size

n

30

3s

Bay

Slab

Drop Size

Square

Size

QUANTITIES

Thi::ess Dimpions Thii;kneea

Column c~ncrat~ ReinfOr~ement

l?

Forms

Size (in.)

(ft3/f )

(I@

(f?/f?)

20.233

7.0

7x7

2.25

m 0,61

2.04 1.01

2Qx25

8,5

81/9 ~

7

2.25

22 0 .73

2 .35 1 01

20X34) 10.5 10X7

4.25 24 0.92

2.78 1.02

20.35 12.0

12x7 4.25 30 1.04 3.29 1.02

25X 25

8 ,5

814 ~ 81/2

2.25 26 0.73

2.54

1.01

25x30 10.0

8172X1(J

4.25

30

0.87 2.78

1.02

25x35

12.0

81, X12

4.25

32

1.04

3.36 1.02

30X30 10.0 10X1O 4.25 32 0,87 3.02

1.02

30X85 12.0 lox 12 4,25 36 1.04 3.51

1.02

35x35

12.0 12X 12

4,25

38 1.04 3.82

1.02

12



15/36

Live Load = 100 P(

Superimposed Dead Load =20 p:

0.55

I

0.5

fc . Sooo p si

t ’c = 5 00 0 ps i

cost o.

Index

t-c .4000

psi

0.4

0.35

I

I

20

25

30 35

Square B ay Size

n

~~

Thi taas Dimpions Thi:kneaa Column c~ncr~e ReinfOr~emeX

20X30 I

10.5 10x7 I 4.25 I 26 I 0.92 / 3.37

20X35 12,0 12x7 4.25

32

1.04 3.86

—

25 X 25 8.5

f31z x

J31~

4.25

28 0.75

3.02

Forms

(f?/f?)

1.01

1.02

1 02

1,02

1 02

25.30 I

10.0 I 8VZX 10 I

4.25 I 32 I 0.87 I 3.41 I 1.02

25x35 I 12.0 I 8V2X12 4.25 I 34 I 1 ,04 I 4 ,WI I 1 ,02

1 I

1 1 1

1

34)X30 10.0 lox 10

4.25

34 0.87

386 =

30X35

12.0 10X12

6.25

38

1 06

4.02 “~

35x35 I 12.0

12x12 I 6.25 I 40 / 1.0+5 I 4.50 t 1.02

13



16/36

DATA

SPAN LENGTH:

Practical Range = 15 ft to 40 i?

Emnomical Range .25 ft to @ ft

ADVANTAGES:

Economical for long spans

9 Pan voids rcducc dead loads

Attmctive ceiling

Electrical fixtures can be placed between joiata

DISCUSSION

General

A one-way joist floor system consists of evenly

spaced concrete joista spaming in one direction. A

reinforced concrete slab iscast integral

with the joists

to

form a

monolithic floor system. Reinforcing bars

are located at the top or bottom of the joista, depend-

ing on the sense of the bending moment. The slab

has reinforcement at mid-depth in a direction perpen-

dicular to the direction of the joista. This steel allows

the slab to span bctwcerr the joists, though the amount

of steel required for temperature and shrinkage typ-

ically controls. The one-way joists frame into beams

that span between the cohrmaa, perpendicular to the

joists.

The results presented in this section are for 3 ft

DIMENSIONS:

Slab thicknessvaries

befwccn 3 in. and 5 in. based on

either fire rcsiatancc requirements or structural consid-

erations

Joista extend from 8 in. to 20 in. below the slab, with

web width ranging tlom 5 in. to 7 in.

DISADVANTAGES:

Not economical for short spans

Higher formwork cnsta than for other slab systems.

Deeper members result in greater fkmr heighta

and

5

ft joist spacings. The ACI Building Code

Requirements for Reinforced Concrete (ACI 318-

89) section 8.11.3 restricts the clear diafance between

joista to a maximum of 30 in. (which correspmds to

a 3 ft joist spacing). If the clear spacing exceeds this

value, the floor system mrrat be designed aa a beam-

strpported slab system, rather than as a joist system.

Both systems have the same design requirements,

except for a smaller reinforcing cover and a 10%

increase in the allowable shear stress permitted for

the joist system.

Floor System

The

members that form a complete one-way joist

system are the slab, joista, interior beams and span-

drel beams.

14



17/36

The slab tldckrreas is controlled by either stnrc-

tural or fire resistance considerations. A 5 in. thick

slab was used in the design of the one-way joists of

th~ section. Thii thickness gives a two hour

fire

rating and is sufficient to span between the joists.

This publication considers only normal weight

concrete for the flcmr systems. But it shordd be noted

that since the slab thickness may be controlled by

tire-resistance, a lightweight concrete may have

aume advantages, bccarrae a two hour

fire

resistance

rating is met by a considerably thkrrrer slab. This will

also result in a sizable dead load reduction.

The dirnensionx of the joists depend on both de-

flection and strcsa considerations. The minimum

depth of the slab plus joist to aatiafy deflection

constraints is gNen in Table 9.5(a) of ACf 318-89.

This table prescribes a minimum slab thickrrcax plus

joist depth of at least the span length divided by 18.5.

The span length for members not built irrtegrully with

supports is defined in section 8.7 of ACI 318-89.

The span length is defined as the lexser of the clear

span plus the depth of the joist or the distance be-

tween suppurt centerlines. The maximum span

lengths for a 5 in. thick slab in combination with

various joist depths arc listed below. Thcxe span

lengths are as defined in .S@ion 8.7 of ACI 318-89

and are not the clear spans. After satisfying deflec-

tion criteria, a joist width is chosen (5 in., 6 in., or 7

in.). The joists are then designed for bending mo-

ments and shear forces.

Joist

Depth

(in.)

8

10

12

14

16

20

Maximum Span

Length

(ft)

20

23

26

29

32

39

Spandrel and interior beam depths are dictated by

the thickness of the slab plrrsthejoist depth, toreduce

formwork costs. The requirement of a level soffit

results

in wide, shallow beams referred to as joist

band beams. Formwork costs are also reduced by

using joist band beams with widths no less than the

column width. Using beams narrower than the col-

mnrr width results in costly formwork details.

In the tables of qrrarrtitiea for this section, the

percentage of pan formwork is shown in the last

column. This represents the percentage of the floor

area that wilf require pans for forrnwork.

Joist OrieW”on

Joists should preferably span in the shorter direction,

and the supporting beams in the longer direction in

rectangular bays, to achieve maximum economy.

This is not crucial for bays with aspct ratios less

than 1.5, since the cost differential is typically leas

than 1%. For baya with aspi?ct ratios between 1.5

and 2.0, orientation of the joists in the short direction

can result in cost suvings of as much as 5%

Column Dimerus”on Effects

Aa

mentioned, the supporting beama should be at

least as wide as the columns they frame into for

reasona of economy of forrrrwork. Other than this

requirement, the width and height of the column

membcm have almost no effect cm the cost of the

one-way joist floor system.

Live LoodEffects

Material quantities are to a large extent controlled by

deflection constraints. An increase in live loads does

not have a proportionate impact on cost. Alive load

of 100 psf increases the total cost by less than 570

over the cost of a one-way joist system designed for

a live load of 50 psf.

Aspect ROtJ-O

The

aspect rutio has a

minimal effect on the qrmrti-

ties for the one-way joist system for aspect ratios less

than 1.5.

Cod Breukdown

The

formwork costs for one-way joist systems are

approximately 58% of the fkmr system costa. Cmr-

crete

material, placing and finishing account for 25’%

of the cost. The remaining 1770 is for material and

placing costs of the reinforcing steel.

15



18/36

3 ft Module

Live Load =50 psf


Slab Thickness = 5 in.

0“5 ~

fC = 6000 pSi

0.45

rc = 5000

psi

cost

Index

0 .4

0.3s

I

I

(

I

1

20

25

30

35

40

Square Bay Size

n

Bay

Rib Beam

Square

Size Depth

QUANTrrlES

Rib Width

Width Column


ft

in,

Pan Forms

in,

in. Size (in.)

(ft’/fP)

psf

%

20X2U

8

5 25 20 0.59 1,45 89

2Qx25

8 5

39

22

0.62 1.67

84

2UX30 8

5 59 24 0.85 1.77 79

20X35 10

5 58 30 0.71 1.91

79

2QX40 12

5

60

32 0,78 1.93

79

25X 25

10 5 34 26 0.64 1,89 87

25x30 10

5 49 30 0.67 1.98

83

25x35 12

5 53 32 0.73 2.02

83

25X 40 14 5 42 34 0,76 1.42

86

30X30 14

5 35

32

0.73

2.03

68

343 X3.5 14 5 49 36 0.76 2.23 85

30X40 14

5 66 38 0.80

2.46 82

35x35 16 6 46

38

0.82

2.48 87

35.40 20 6 45

40 0.92 2.52 88

40x 40 2U 6 50

42 0.92 2 .83 88

I

16



19/36

3 ft Module

Live Load = 100 psf



0 .5

0.4s

cost

Index

0 .4

0.35

f ’C = 6 0 00

pSf

fc =

5000 psi

rc .4000

psi

I

I

I

20

25

30

35

40

Squa re Ba y Size

n

Bay

Rib

Beam

Square

Size Depth

QUANTITIES

Rib Width

kWdth

Column

R

Concrete

Reinforcamenl

in.

Pan Form

in. in. Size (in.)

(ft’if?)

paf

%

2QX20 8

6

34

22 0.62

1,86 65

20x25 8

6

53

24 0.66 2.03

79

20.30 10 5 56 26 0.71 2.09 79

233X35 12 5 57

32 0.77 2,13 79

20X40 14 5 61 34

0.65

2.16 79

25X 25 12

5 33

26 0 .69 2 .08

87

25x30 12 5 49 32

0.73

2 .29 83

25x35 14

5

54

34

0 .79 2 .36

82

25x40 16

5 58 36

0 .87 2 .53

81

30 X2J3 14

6 44 34

0 .77 2 .48

85

30X35 2U 5 38

36 0 .89 2 .36 88

30.40 .20

5 40 40

0 .90 2 .52

86

35x35 20

3 40 40

0 .68 2 .79

88

35X40 m

5

42

42

0 .92 3 .CO 86

40x 40 .233

6

44

44

0 .95 3 .38 65

17



20/36

II Module

~e Load = 50 psf

Jperimposed Dead Load = 20 psf

ab Thickness = 5 in.

0.4s

cost

Index 0“4

0.35

f ’C = 6000 pSi

r’c .5000 psi

rc .4000 psi

20 25

30

35

40

Square B ay Size

n

Bay Rib

Beam Square QUANTITIES

Size Depth

Rib Width

Width Column

Concrete

fr

Reinforcement

Pan Forms

in. in.

in. Size (in.)

(ft’/f?)

Ff

“k

20 X.2CI 16

7

m 20 0.70 1.25

92

.23x25 16 7 22 22 0.71 1.38 91

2QX30 16 7

24

24 0.72 1.43 w

X3X35 16 7 30 WI 0.75

1.58

88

20.40 16

7

37 32

0.77

1.77

85

25X 25 16

7

26 26 0.70 1 55

91

25x3JJ 16

7

30 30

0.72 1,71 80

25x35 16

7

33

32 0.73 1.87 88

25X

40 16 7 44 34 0 .77 2 .00 86

30X30 16

7 32 32

0.71

2.00

90

30X35 16

7

38

36

0.73

2.20

88

30X40 16

7 51 36

0 .77 2 .33

85

35x35 20

7

38

38

0 .80 2 .30

93

35X40 m

7 41

40 0.81 2 .50

69

40X40 m

7 44 42

0,82 2 .80

89

18



21/36

5 fi Module

Live Load = 100 psf



0.45

cost ~

Index “

0.35

fC = 6000 pSi

fc = 5000 psi

m

,

.

m

I

20

25

30

Square B ay Size

ft

Bay Rib Beam

Square

QUANTITIES

Size Depth Rib Width

Wkfth

Column

ft


Pan Forms

in.

in. in.

Size (in.)

(ft3/ft’)

m

%

2QX2U

16 7 22

22

0.71

1.41

91

20x25

16 7 24

24

0.72

1.55 93

2QX30

16 7 26

26

0.73

1.73 89

20X25

16

7 3-5

32 0.77 1.86 66

2QX40

16

7 47

34

0.62

2.06 83

25X 25

16 7 28 28 0,72 1.91 %3

25x30

23

7 32

32 0.81 1,84 89

25x3.5

m

7 34

34

0.82

2.03

88

25x40

Xl

7 39

36

0.s5

2.30

87

30X20

20

7 34

34

0.s0

2,23

89

30X35

20

7

3a

88

0.82 2.45 88

20X40

20

7 45

40

0.84

2,56 86

19



22/36

DATA

SPANLENGTH:

Practical Range

= 15 ftt040fr

Fxonomical Range .35 ft to 40 ft

ADVANTAGES:

Economical for long, heavily loaded spans

Dome voida reduce dead loads

Attractive ceiling

Electrical fixtures can hc ptacedin the voida

Discussion

General

A two-way joist system consists of evenly spaced

reinforced concrete joists spanning in both direc-

tions. A reinforced concrete slab is cast integral with

the joists to form a monolithic floor system. Rein-

forcing bars are located at the top

or

lmttom of the

joista, depcndirrg on the sense of the bmrding mo-

ment. The slab has reinforcing bar’s at mid-depth to

allow the slab to span between the joists, though the

amount of steel required for temperature and shrin-

kagestrcsaes typically controls.

The perpendicular orientation of the joists results

in evenly spaced square voida on the underside of the

slab (which is the reason why the system is often

referred to ss a waffle slab). These voida, which

DIMENSIONS:

Slab thickness varies from 3 in. to 5 in. baaed on either

fire resistance requirements or structural conaidera-

tiona

. Joists extend frnm 8 in. to 24 in. below the sJab, with

web widths of 6 in. or 8 in.

DISADVANTAGES:

.

.

.

Not ccmromical for short spans or for light to medium

supcrimpascd loads

Higher fonnwork casts than for other slab systems.

Dccpcr members result in greater story heights

allow a considerable reduction iu weigh~ are forrued

by placing steel or tiberglsas domes on top of flat

fonrrwork. The rcxrdting voids from the domes have

aqusre dimensions between 2 ft and 5 ft in even foot

increments. The domes are omitted in the areas

around the cohrmrra to provide a deep sIab with a

high shear csrrying capacity. The solid portion typ-

ically extendx one-sixth of the span length in all four

directions from the column.

The results presented in this wction are for the 3

ft and 5 ft wide domes. ACI code section 8.11.3

restricts the clear distance between joists to a maxi-

mum of 30 in. (which corresponds to a 3 ft dome).

If the clear spacing exceeds this value, the floor

system must be. designed as a beam-supported slab

system. rather than as a joist system. Buth systems

20-’



23/36

have the same design requirements, with the excep-

tion of a artraller reinforcing cover and a 10% in-

crease in the allowable shear stress permitted for

joists. The results for the 5 ft domes are shown in

tfria section, although this system is not considered

by ACf to be a joist system.

Floor System

,

described previously, the slab thickness ia cmr-

trolled by either structural or fire resistance conaid-

erationx. A 5 in. thick slab was used in the dexign of

the two-way joista. This thickrress was chosen tu

provide a two hour fire rating, and more than met the

structural requirements.

The thickness should be

adjuated as required, to obtain the applicable fire

resistance rating.

The standard joist widths for two-way joist sys-

tems rraing 3 ft and 5 ft domes are 5 in. and 8 in.,

respectively. With the slab thickness controlled by

fire resistance requirements and the joist width con-

trolled by indnatry starrdarda, the only geometric

variable to be determined ia the joist depth.

This publication considered only normal weight

concrete for the floor systems. It should be noted that

since the slab thickness may be controlled by fire

resistance requiremerrta, a lightweight concrete may

have some advantages, because a two hour fire rating

is met by a considerably thinner slab. This will alau

result in a sizable dead load reduction.

The depth of the floor system ia controlled by

deflection constraints for the loads used in this pub-

lication. Minimum thickrresa requirements sycified

in Table 9.5(c) of the ACI Code are for solid two-way

slabs. To determine the deflection control rcqrrire-

menta for the two-way joista, the cmaa-section of the

flcor system must be transformed into an equivalent

section of uniform thickness. This is accomplished

by determining a slab thickrre.sa that provides the

same moment of inertia aa the two-way joist section.

Listed below are the maximum clear span lengths for

the two-way joist system baaed on this approxim-

ation.

3 it Modufe

Joist

Joist

Maximum Clear

Depth

ThiCkncss Span Length

(in.)

(in.)

(ft)

8 6 27

10

6 30

12

6 34

14 6 38

16

6

41

5 fl Mndufe

JOixt

Joist Maximum Clear

Depth Thickncm Span Length

(in.) (in.) (ft)

14 8 34

16 8

37

20 8 43

24 8 52

In the tables of quantities for this section, the

pmcentsge of dome formwork is shown in the last

column. ‘Ms reprexenta the percentage of the floor

area which will require domes for forrrrwork.


The

material quantities are independent of the floor

height under normal loading conditions. Column

dimensiorra rracd in the analyacs of this publication

were chmen to repreaerrt the column sizes used in

10- to 20-story buildings. If a structure has a column

width and depth different from those used in the

tables, adjustments should be made by increasing

steel reinforcing quantities by l% for each 2 in.

decrease in square column dimensions. The quanti-

ties should be decreased by II% for each 2 in. increase

in square column dimensions.

Live Lood Effects

Since the material qrrantities required for a two-way

slab system are typically controlled by deflection

constraints, an increase in live loada dces not have a

proportionate impact on costs. Alive load of 100 psf

increasea the total cost by leas than 5% over the cost

of a two-way joist system designed for a live load of

50 pf.

Aspect Ratio


economical floor layorr~ since deflection control re-

quirements are exactly met in both directions. A

rectangrdarbay with an aspect ratio of 1.5 is 5% more

expensive than a square bay with the same floor area.

Cost Breukabwn

The

formwork costx for two-way joist systems are

approximately 54% of the floor system coats. Con.

crete material, placing and finishing account for 28%

of the cost. The remaining 18% is for material and


21



24/36

) fl Module

.ive Load = 50 psf

hperimposed Dead Load = 20 psf

lab Thickness = 5 in.

0.55

r

cost

Index

0.5

0.4s

rC = 6000 /)S/

fk . 5000

psi

r%=

4000

psi

20

25

30

35

40

Square Ba y size

n

Bay

Rib

Solid Head Square

QUANTITIES

Size

Depth Size Column

Concrete

n

Reinforcement Dome Forma

in.

rt Size (in,)

(ft’/ )

@

“/.

20X20 8

81A x 81/9

xl 0.73

2.28

82

20.25 8

81/ 9~

101A

22 0.73

2.58

82

2UX30 10

81,4 ~ 121A

24 0.81 2.85 82

23X35 14

81Ax 141,4

m 0.97 3.19 82

20X40 16

81,4 ~ 161,+

32 1,06 3.58 82

25 X25

8

101/2x 1ol~ 26

0.73 2.82

83

25x30 10

lol~ x 1214 30

0.81

3,03

63

25x35

14

10W x 14V2 32

0.97 3.22

83

25X 40 16

lol~ x 161A 34

1.06 3.52

83

30X30

10

121,+x 121A 32

0.61 2.96

83

30X35 14

121,4 x 141,+ 36

0.97 3.25 83

30X40

16

121,+~ 161A 39

1,06

3.58

83

35x35 14

1414 x 1414 38

0.97

3.56

83

35X40

16

141~ . 161A 40

1.26 3.97 83

40x 40 16

161A x 161+ 42

1,06

4.18

S3

i

22



25/36

3 ft Module

Live Load = 100 psf


Slab Thickness = 5

in.

0.55

tb = 6 00 0psi

fc = 5 0U0 ps i

cost

0 .5

Index

20

25

30

35

40

Square B ay Size

n

my

Rib

Solid Heed

Square

Size

Depth

QUANTITIES

Size

Column

Concrete

Reinforcement

Dome Forme

rt

in,

n

Siza on.)

(ft3@

@

“/.

2U X20

8

81A ~ 81/4

22

0.73 2.48 82

20x25 8

81A x lol~

24 0.73 3.15 82

20X30

10

81/9 ~ 121/9

26

0.81 3,50 82

2Q X35

14

81A ~ 141/42

32 0.97

3.72

82

20X40

18

81/9 ~ 161,4

34

1.06

4.10

82

25 X 25

8

lly~ ~ lol~

28

0.73

3.42

83

25x30

10

1ol~ ~ 121,+

32

0.81 3.81 83

25x35 14

lol~ ~ 141/9

~

0.97 3.78 83

25X 40 16

lol~ ~ 1614

36

1.C6

4.24 83

30X30 10

121A ~ 121A

34

0.81

4.00

83

30.35 14

1,314 ~ 141/9

38

0.97

3.93 83

30X40

16

121/9 ~ 161/

40

1.CKr

4.29 83

35x35

14

141A -q141A

40

0.97 4.26 83

35X40

16

ll~x161~

42 1.06

4.53 83

40X40 16

161,4 ~ 161/9

44

1.06

4.98 83

23



26/36

j ft Module

.ive Load = 50 psf

Superimposed Dead Load = 20 psf

SlabThickness = 5 in.

cost

Index

0.55

I

0.45

I

I

I

I

20 25 30

35 40

.

—

. .

Square-Eay S/ze

ft

Bay Rib Solid Head Square

QUANTITIES

Size

Depth Size Column

Concrete

Reinforcement

n

in.

Dome Forms

ft Size (in.)

(ft’/f?)

psf Y.

2QX2U 14

101,+ ~ ~ol~ 2fJ

0. 32

2.74 72

20x25 14

lol~ ~

101,+ 22

0.s0 2 .70 77

Z) X30 14

lol~

lol~ 24

0,90

2.77 81

2QX35

16

lol~ ~ 151/9 30

0.98

3.01 76

.23 X40 20

lol~ )( 151,+ 32

1.12

3.40 79

25X 25

14

lol~ ~ lol~ 26

0.30

2.40 82

25x30 14

lol~ ~ 101A ~

0.90

2.74 65

25x35 16

fol~ ~ 151A 32

0.9s

3.13 S1

25X 40 m

101~ ~ 151,4

34 1.12

3.29 83

30X30 14

lolfi ~ lol~

32 0.93

2.94 87

30X35

16

11)1~ y. 1514 36

0.9s

3.26 84

30.40 2U

101+ ~ 1514

3s 1.12

3.54 84

35x35 16

1514 ~ 151,4 38

0.98

323

m

35X40 m

151/9 ~ 151,’9 4(3

1.12 3.43 82

40x 40 m

151/ )(


27/36

5 ft Module

Live Load = 100 psf



0.6

0.55

cost

Index

I

0.5

20

25

30

35

40

Squ are B ay Size

n

Bay Rib

‘~

::”)WF

Solid Head

Size Depth

ft in.

20X35

16

lol~ )( 151A

32

0.98

3.51 76

20.40 20

11)1~ ~

151A

34

1.12

3.70 79

25 X 25 14

lIJIA ~ lol~

28

0.90

2.76 82

25x3CI 14

101A ~ lol~

32

0.90

3,04 65

25x35 16

101,+ ~ 1514

34

0.96

3.53

61

25X 40 a

lol~ ~ 151/9

36

1,12

3.91 83

30X30 14

101,+ ~ 101+

34

0.943

3.34 87

30X35

16

lol+ ~ 151A

38

0,98

3.51 84

30X40 m

lol~ )( 151,4

40

1.12

3.S73 84

35 X3.5 16

151zX 151,4

40

0.98

3.84 80

35X40 20

151fix 151A

42 1.12

3.96

82

40X40 m

151/9 ~ 151+

44

1.12

4.26

65

2S



28/36

DATA

SPANLENGTH:

Practical Rarrge = 15 ftt040ft

Ecmromical Range .25 ft to 40 tl

ADVANTAGES:

Economical for longer spans

DISCUSSION

Floor System

The slab thickness for a beam-arrppmttxf slab system

is controlled by deflection constraints. These con-

straint are given by eqnxtimrs 9-11, 9-12, and 9-13

of the Building Code Requirements for Reinforced

Concrete (ACI 318-89). These equations are

/, (0.8+ A)

h=

(9-11)

36+5fl [am- 0.12(1 + ]

but not lexs than

0.8 + A)

h-t.

36+9p

(9-12)

DIMENSIONS:

Slab thickness between 5 in. and 10in.

DISADVANTAGES:

Preserrcc of beams may require greater story height

Flnnr layout maybe dictated by beam lncations

and need not be more than

0.8 + A)

h-t.

36

(9-13)

length of clear span in long direction of two-

way construction, meaaured face-to-face of

columns in slaba without beams and face-to-

face of beams or other supports in other cases.

ratio of clear spans in long to short dkction

of two-way slab.

ratio of flexrrral stiffrresa of beam section to

tlexural stiffness of a width of slab bounded

laterally by centerlines of adjacent panels (if

any) on either side of beam. Values of a can

be obtained from the following two grapha.

25



29/36

urn = average value of c? for all four edges of a slab

panel.

In no case shall the slab thickness be less than 5

in. for am> 2.0 and 3% in. for a ~s 2.0.

. .

@

2.7

~ ‘+’=;” w H

2.6

2.5

2.4

2.3

2.2

2.1

2.0

1.9

I ,8

1.7

1.2

I .5

1.4

I ,3

I.2

.1

.

“

1

I,5 2345678910

. Ill

2.0

[.9

1e

17

,6

[5

t.4

1,3

1,2

1.1

1.0

13

234567890

Equations 9-11, 9-12, 9-13, are graphically

illktmted below”

L

9.5.3.3- .Wh (fY= 60 ksi)

70 EqQ-11,..= 4

// -

&w <

—2

h

_———

—1

30

——— ~

S13

1.0

M

20

B

The beam depth is also typicully governed by

defktion constraints. Table 9.5(a) of ACI 318-89

requires that the beam shall have a minimum depth

equal to the length of the span as defined in Section

8-7, divided by 18.5. Whh this given beam depth,

the width of the beam is sized to meet loading re-

quirements. Beam widths that are smaller than the

column width should be avoided to reduce formwork

costs.


The

supprting beams should be at least as wide as

the columns they frame into, for reasons of formwork

simplicity. Other than this rexpriremen~ the width

and height of the column members have minimal

effects on the cost of this flcux system.

Live Load Effects

Material quantities are, for the most par~ controlled

by deflection constraints, and sn increase in live

loads does not have a proportionate impact on costs.

Alive load of 100 psf increases the total cost by less

than 5% over tbe cost of a beam-suppted slab

system d=igrted for a five load of 50 psf.

Aspect Ratio


economical flour layout, since deflection rwprire-

ments can be exactly met in both directions.

A

rectangular bay with an aspsct ratio of 1.5 is on an

average 470 more expensive than a square bay with

the

same tlcmr area.

Cost Breokdown

The

formwork costs for beam-supported slabs are

af2p?OXi2S?t2tdylvo

of the ffnm system cmts. Ccm-

crete material, placing and finishing account for 21 %

of the cost. The remaining

2570

is for material and


oh

27



30/36

ive Load =50 psf


0.5s

fC = 6WOjJSl

4

0.5

cost

Index

fc =

5000psi

0.45

0.4

I

I

15 20 25

30

Square Bsy Size

n

Bay Slab

Beam Squara

Siza

QUANTITIES

Thickness

Depth

Column

Concrata Reinforcement Forms

ft

in.

in. Size (in.)

(ft3/f?)

w

(f?m?

15X 15

5.0 10

14 0.46

3.50 1.15

15X2U 5.5 14

1s 0.55

3.29 1.23

15x25

6.0

16

m 0.88

3.41 1,17

15x31J

7.0 20 22 0.68

3.71 1.16

20 X.ZO 6.0

14

23 0,57 3.54

1.17

20x25 7.0

is

22 0.72 3.59 1.17

.20.30 9.0

20

24 0.83 4.23 1.16

25X 25 70

18

26

0.67

3.86

1.18

25x30 9.5 2U

30 0.89 4.70

1.17

30X30 9.0 20

32 0.s5 5.07 1.17

1

2s



31/36

Live Load = 100 ps

Superimposed Dead Load =20 psi

0.6

0.s5

cost o ~

Index “

0.45

0.4

15

20

25 30

Squ are Ba y Size

ft

Bay

Slab Beam

Sqluare QUANTITIES

Size

Thickness Depth

Column


Forms

ft in.

in.

size (in,)

(ft’if?)

Paf

( /ft’)

15X15 5.0 10

14

0.46

4,15

1.15

15X20 5.5 14

118

0. 55

3. 63 1. 23

15x25 6. 0

18 22

0 60 3. 91 1. 22

15X30 7. 0

20

: 14

0. 66

4. 33 1. 16

20x20 6. 0

14

22

0. 57

4. 35

1 16

20x25 7. 0 18

: 14

0. 69

4. 08 1. 17

20X30 9. 0 m

26 0. 83

5. 23

1. 18

25 X 25

7. 0 18

: 8 0. 68

4 78

1. 19

25x30 9. 5

m

32

0. 89

5. 51

1 17

30 X2J I

9. 0 20

34 0. 67 6. 12 1. 19

29



32/36

General Discussion

This section

provides overall comparisons of the

economics of the various fkmr systems discussed in

this publication. It provides a summary of the factozs

that may influence the costs of cast-in-place concrete

floor 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 figures

clearly show that the optirrrality of the slab system

depends on two major factors: the span in the long

direction, and the intensity afsrrperimposed dead and

live loads. For a given set of loads, the slab system

that is optimal for short spans, is not necessarily

optimal for longer spans. For a given span, the slab

system that is optimal for light superimposed loads,

is not necessarily optimal for heavier loads. The foor

figures should facilitate the section of a strnctrrral

floor system most appropriate for a certain applica-

tion.

Column Dimenm”ons

Analysis shows that the height between floors has

very little influence on the material qrrantitiea for the

floor system. Column cross-sectional properties de-

termine the clear span length and the shear capacity

of the slab. The column cross-sectional d~mensions

used in this publication were representative of 10- to

20-story buildings Increasing or decreasing the col-

umn dimensions by 2 in. did not affect the concrete

quantities 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 onfy

resulted in a 4~o to 10% increase in the floor system

cost.

Aspect Ratr”o

Sqrrare bays usually represent the most economical

floor layou~ since deflection control requirements

can be exactly met in both directions. A rectangular

bay with an aspect ratio of 1.5 ranges between 4% to

10% more in cost than a bay with an aspect ratio of

1.0 and the same floor area. This, however, is not

the case for one-way joist systerna. Tfds type of floor

system shordd have the joista span in the short direc-

tion, and is almost unaffected by aspect ratios of UP

to

1.5.

Concrete Strengths

Concrete strengths of 4000 psi, 50@ psi, and @OO

psi were used in this publication. Cost analysis

shows that for gravity loads, 4000 pi concrete is

more economical than higher concrete strengths.

Cost Breakdown

The formwork for the floor systems will absorb from

50% to 58% of the costs. Concrete material, placing

and finishing account for 21’% to 3090. The material

and placing costs of the reinforcing steel amount to

between 17% and 25% of the cost.

Repetition

A cost efficient design utifizes repstitiorr. Changes

should be minimized from floor to floor. Changing

column locations, joist spacing, or the type of floor

system increases the cost of forrnwork, time of corr-

strrrction and the chance of field mistakes, and there-

fore should be avoided.

Column-Beam Intersections

Thebearrra

Orat

frame into columrrashould be at least

aa wide aa the columns. If the beams are narrower

than the columns, the beam forms will require eostl y

field labor to Pas the formwork around the columns.

Stindd Dimenn”ons

Standard available sizea should be used for structural

fornring. For instance, joist fomrwork pans are

available in various web depths of 20 in. and from 8

in. to 16 in. in 2 in. increments. Specifying a depth

different from these sizes will require the fabrication

of costly special forrnwork. When detailing drop

panefs or other changes in the floor system depti

actrral lumber dimensions should be taken into ac-

Corrrrt.

Depth of the Ceibrg Sandwich

This publication haa addrxd the economy of the

structural slab system only. However, the structural

engineer usually has to look beyond. The structural

slab system ia part of the so-called ceiling sandwich

which also includes the mechanical system f,HVAC

ducts), the lighting fixtures, and the ceiling itself.

3tl



33/36

The flnor-to-floor height of a building ia the total

depth of the ceiling sandwich plus the clear tlnor-to-

cciling height. Any variation in the depth of the

ceiling sandwich will have an impact on the total

height of the shearwalls and cohsmna, the mccban-

ical, electrical and plumbing riaezs, the staim and

interior architectural finiahcs, and the exterior clad-

ding. It will also have an impact on the total heating

cooling and ventilation volume. To minimize the

depth of the ceiling sandwich is very often the goal

of the structural engineer. This becnmes particularly

impmiant in cities like Washington, D.C. that impose

a height limit on buildings Optimization of the

ceiling sandwich depth may tranalate intu an extra

story or two accommodated within the prescribed

height limit.

“’’”-l

TT

—

A number of details have been attempted in the

paat to accomplish a reduced depth of the ceiling

sandwich. The HVAC ducts can paas through the

webs of joiata or beams. llrii will reduce the floer-

to-floor height, but will increase formwork and field

labor costs Another alternative is to cut notches at

the bottom of the joist or beam to allow paxsage of

the 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 in

the use of the floor space. Such flexibility is becom-

ing more important in view of the shifting emphasis

towardx corracionaly designing buildings for a long

service life.

31



34/36

Ne Load = 50 psf

superimposed Dead Load = 20 psf

~= 4000 psi

0.9

0.8

0.7

cost 0.6

Index

0.5 ,

0.3

I

1

1 1

1

[

15 20

25 30

35 40 45

50

Square Bay Si ze

n

Owway

joist

(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 Size

rt

32



35/36

Live Load. 100 ps

Superimposed Dead Load =20 ps

fc’= 4000

ps

0. 8

t

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 o st

———

[wide m.adub]

15

30

25

30 35 40

45

50

Square Bay Size

ft

33



36/36

Concrete Floor Systems

Documents

Transcript of Concrete Floor Systems