ribbed slab design.pdf

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One-Way Joist Floor System l:12 Slope, type Wiclth veries100mm o. rarger . Thetype of slabis also called a ribbed slab. r lt consists of a floor slab, usually 50-100 mmthick, supported by reinforced concrete ribs. c Theribsare usually lapered anduniformly spaced at distances that do notexceed 750mm. J ln some fibbed slabs, the space between ribsmaybe filled with Dermanent fillers to orovide a horizontal slab soffit. Top of Shb One-Way Joist Floor System Advantages: . Longer spans with heavy loads r Reduced dead load due to voids . Electrical, mechanical etc. canbe placed between voids . Good vibration resistance

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Page 1: Ribbed slab design.pdf

One-Way Joist Floor System

l :12 Slope, typeWiclth veries 100 mmo. rarger

. The type of slab is also called a ribbed slab.r lt consists of a floor slab, usually 50-100 mm thick, supported by

reinforced concrete ribs.c The ribs are usually lapered and uniformly spaced at distances

that do not exceed 750 mm.J ln some fibbed slabs, the space between ribs may be filled with

Dermanent fillers to orovide a horizontal slab soffit.

Top of Shb

One-Way Joist Floor System

Advantages:. Longer spans with heavy loadsr Reduced dead load due to voids. Electrical, mechanical etc. can be placed between voids. Good vibration resistance

Page 2: Ribbed slab design.pdf

. ACI Reouirements for Joist Construction(Sec. 8.11, ACI 318-02)o Slabs and ribs must be cast monolithicallv.o Ribs must be spaced consistentlyo Ribs may not be less than 100 mm in widtho Depth of ribs may not be more than 3.5 times the minimum

rib widtho Clear spacing between ribs shall notexceed 750mm... Ribbed slabs not meeting these requirements are designed

as slabs and beams. '*

r Slab Thicknesso (ACl Sec.8.11.6.1)

t >50 mm or > (1/12 distance between ribs)

o Building codes give minimum fire resistance rating:

. 1-hourfire rating: 20mm cover,75-90 mm slab thickness

. 2-hour fire rating: 25mm cover, 1 15 mm slab thickness

Page 3: Ribbed slab design.pdf

Pan Joist Floor Systems

o Slab thickness. Governed by strength, fire rating, available space

o Overall depth and fib thicknessI Governed by deflections and shear

Pan Joist Floor Systems

r Lavinq Out Pan Joist Floors (conf.)

Typically no stirrups are used in joists

Reducing Forming Costs:r Use constant joist depth for entire floorr Use same depth forjoists and beams (not always

possible)

Page 4: Ribbed slab design.pdf

Pan Joist Floor Systems

r Distribution RibsPlaced perpendicular to joists.Spans<6.0m:NoneSpans 6.0 -9.0 m : Provided a mi

o Soans > 9.0 m : Provided at third-oointso At least one continuous 12-mm diameter bar is

orovided at too and bottom of distribution rib..Note: not required by ACI Code, but typically used in

construction

. ACI provides minimum member depth and slabthickness requirements that can be used withouta deflection calculation (Sec. 9.5 ACI 318)

o Useful for selecting preliminary member sizes

Page 5: Ribbed slab design.pdf

Member Depth

ACI 318-02: Table 9.5a

9.5-2 - on€-w.y co.skucrlon (nonpr6l.€$.d)

9.5.2.1 - Minihlm thickless stipolar.d in Tadss 5la) shall apply lor on€ way @nslruction nol sup-poning or atlachod lo parlitims or other bnelruclionlik€ly to b€ dE@g€d by b€e d€ll€cfions, unless @m-pulalion ol deileclion ind €leB a lesse. lhidoe6s @nb€ !s6d wirhour adve60 6fi&ls.

IAAL€ 9.5(EFlllNlMUM THICKNESS OF NONPRE.SIFESSEO BEAMS OF ONE-WAY SLAAS UNIESSOSFLECT|oNS ARE CONPUIEO

ftr ffiruc16n r*.ry 6 b 4tuld bl ralo.

Design Steps

;t 3*

Clrer. . rJr ,JO

Page 6: Ribbed slab design.pdf

l) Flat Slab reinforcement is calculated for bending orminimum reinforcement for shrinkage andtemperature.

(ACl Sec 7.12.2 ) GR40orGR50 00020AqGR 60 0.0018 a.

Jolst Doslgn (continued)

2) Shear Design of Joist Ribs

a) Af fowable V"= 1.10'V"t-

r l -1 1A*1!!h )o

b) Shear strength may be increased using shearreinforcement or by widening the ends of the ribs(not typical) .\ . . { . . ' . -

' . : ,

L- j;a.ris

Page 7: Ribbed slab design.pdf

Jolst Deslgn

3) ACI shear and moment coefficients may bsusod if reouirements in ACI Sec 8.3.3 are met.

4) Ribs are designed as T- Sections. Mainpositive reinforcament includes at lsast 2 bars.

ExampleRIBBED SLAB DESIGN

Page 8: Ribbed slab design.pdf

Design the simply-supporte.t stab shown betow. The slab is Dart ofa floor in a typical residential building.Use f'c = 28 MPa,fv= 420 MPa

0.20 n'

1. The effective span length = Lcr.,,+d or Lcre.!+s=4.5+0,2=1.7m

2. Live load :From minimum design loads for structures LL=2.0 kN/m,

3. Required depth of the ribbed stab by ACt Table 9.Sahnh= L/16 = 4700/'16 = 294 mm

Ribbed Slab Design

5- Suggested Dimensions

Exa m ple

6. Dead Load tor r.o-m stripFlat slab weight = 0.06'1.0t24.0= 1.44 kN/mRibs weight = (0.12+0.16)/2.0.24-24.0/0.52 = t.S5 kN/mOwn weight of blocks = 5.0-0.18/0.52 = 1.73 kN/m

Page 9: Ribbed slab design.pdf

Ribbed Slab Design Example

6. Dead Load for 1.0-m stJip {continue}Own weight ofthe fill = 0,1.'1.0.'12.9 = 1.29 kN/m'zOwn weight of the mortar = 0-025'1.0'22.0 = 0.55 kN/m'Own weight ot the tilss = 0.025'1.0"24.0 = 0.60 kN/m'?Own weight ot plastering = 0.025'1,0*22,0 = 0.55 kN/m'?Own weight of partitions = 1.0 kN/m'z

Total Dead Load = 8.71 kN/m2

Ribbed Slab Design Example

7. Total ultimate load = 1.2DL+{.6 LL= L2(8.7,11 + ,t .6(2.01 = | 3.65 kN/m,

8. Total ultimate load on one rib = 13.65.0.52 = 7.1 kN/m

t. ,t,,a"," roagnt = w,L2l8= 7.1'4.778 = 19.6 kN.m

R M" 19 600') =0.511MPa

" dbd' 0.9(520)t270')

o=os:; : I r

Page 10: Ribbed slab design.pdf

Ribbed Slab Design Example

A, = 0,001385 (520X270) . 19,1 nnz/ ribA.'i' = 0-co3:t(t20x270)

- 107 nt|l'/ rib

A,r,aaa = 2tDl2 mn I dbCh.ck N.A In tho f,ftg. .-225'420r(0.85'28'520). 7.7 nn ok

9. Shoar De3ignUltimaierho on one db V. = t.'1'11,7124.r".27F t,1.06 kN

or. =,.r*o.ryJ4 rt20'210 = 23.s7 w -+orr

Ribbed Slab Design Example10. Upper Plate Do8ignw, = t3.65 kN/m

ConEa tattvely, dslgn tha uppet plata ts a slmpty auppo.ted st bnu = wJ-.n = 13.65.0.4R . 0.273 kr|m

M i i1! ln6\R - - : :+ = "- 1" ' : =0.19.. t /Pa" dbd. 0.90000)(40.)

o _o.lsf: ( | _,f L& l=o.ooo14 | li 0.85r./

A. r 0.00023 {1000X.0} = 9.2 mm,A.'r,,. o.oo18(1000x60) - t08 mm?A@,rdd. to8 mrn / block or uro w€ld.d wlr! noah It avall.bL

Page 11: Ribbed slab design.pdf

Ribbed Slab Design Example

Slmpllfled Bar Cutott Polnt8

l0l rrfi.. rh.l.ro.r ot 4, rnd 4,,

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