Lecture GoalsLecture Goals

• Slab design reinforcement

Introduction

• Except for very heavily loaded slabs, slab thickness is governed by deflection criterion.

• One way slab is assumed if ratio of side panels is greater than 2. Loads are assumed to transfer in the short direction.

• Near the girders, which are parallel to the one-way slab strips, the floor load is supported by two-way slab action. This is ignored in design of one-way slab strips but accounted for in ACI 8.12.5.

One-Way Slab DesignOne-Way Slab Design

Design of one slabs is like design of parallel 1m beams.

Thickness of One-Way Slabs

Minimum thickness for solid one-way slabs not supporting or attached to partitions, etc. Likely to be damaged by large deflections:

ACI Table 9.5(a)

One-Way Slab DesignOne-Way Slab DesignThickness of One-Way Slabs

The table calculates the minimum thickness t ( l = span length in meters)

(normal weight concrete, fy = 420MPa see code for modification factors)

One-Way Slab DesignOne-Way Slab DesignThickness of One-Way Slabs Table A-9

tmin, when damage to non-structural components may occur

One-Way Slab DesignOne-Way Slab DesignThickness of One-Way Slabs

Fire Rating

This is equal to the number of hours for unexposed surface to rise a set amount usually 140o C

9cm 1 hour

12.5cm 2 hours

16cm 3 hours

Cover for Slab ReinforcementCover for Slab Reinforcement

ACI Sec. 7.71 (min. cover for corrosion protection)

( 1.) Concrete exposed to earth or weather.

( 2.) Concrete not exposed to earth or weather.

φ16 and smaller = 40mm

φ19 and larger = 50mm

φ36 and smaller = 20mm

Min. covers for fire ratings should also be considered (20mm for 1.25hr, 25mm for 1.5hr and 40mm for 3hrs)

One-Way Slab DesignOne-Way Slab DesignReinforcement

Typical Reinforcement details in a one-way slab:

One-Way Slab DesignOne-Way Slab DesignCutoffs

If requirements for use of ACI Moment Coefficients:

Figure A-5 one-way slab

Flexural Reinforcement in SlabsFlexural Reinforcement in Slabs

For a 1 m strip of slab is designed like a beam As (req’d) is in units of (cm2/m)

or

s b

1000/

bar spacing in mm, s

mmA m A

bs

1000 mm

/

mms A

A required m

cmBar diameter (mm)

6 8 10 12 14 16 18

5 5.65 10.05 15.70 22.61 30.77 40.19 50.8

6 4.71 8.37 13.08 18.84 25.64 33.49 42.3

7 4.04 7.18 11.21 16.15 21.98 28.71 36.3

8 3.53 6.28 9.81 14.13 19.23 25.12 31.7

9 3.14 5.58 8.72 12.56 17.10 22.33 28.2

10 2.83 5.02 7.85 11.30 15.39 20.10 25.4

11 2.57 4.57 7.14 10.28 13.99 18.27 23.1

12 2.36 4.19 6.54 9.42 12.82 16.75 21.2

13 2.17 3.86 6.04 8.70 11.84 15.46 19.5

14 2.02 3.59 5.61 8.07 10.99 14.35 18.1

15 1.88 3.35 5.23 7.54 10.26 13.40 16.9

16 1.77 4.91 7.07 9.62 12.56 15.9

17 1.66 2.96 4.62 6.65 9.05 11.82 14.9

18 1.57 2.79 4.36 6.28 8.55 11.16 14.1

19 1.49 2.64 4.13 5.95 8.10 10.58 13.3

20 1.41 2.51 3.93 5.65 7.69 10.05 12.7

21 1.35 2.39 3.74 5.38 7.33 9.57 12.1

22 1.28 2.28 3.57 5.14 6.99 9.13 11.5

23 1.23 2.18 3.41 4.91 6.69 8.74 11.0

24 1.18 2.09 3.27 4.71 6.41 8.37 10.6

25 1.13 2.01 3.14 4.52 6.15 8.04 10.1

26 1.09 1.93 3.02 4.35 5.92 7.73 9.78

27 1.05 1.86 2.91 4.19 5.70 7.44 9.42

28 1.01 1.79 2.80 4.04 5.50 7.18 9.08

29 0.97 1.73 2.71 3.90 5.31 6.93 8.77

30 0.94 1.67 2.62 3.77 5.13 6.70 8.48

Areas of bars in slabs (cm2 per meter)

Flexural Reinforcement in SlabsFlexural Reinforcement in SlabsThe maximum spacing of the bars for flexure is given as:

max

3t slab thicknesssmaller of ACI Sec. 7.6.5

450S

mm

Flexural Reinforcement in SlabsFlexural Reinforcement in Slabs

• Thin slabs shrink more rapidly than deeper beams.

• Temperature & shrinkage (T&S) steel is provided perpendicular to restrain cracks parallel to span. (Flexural steel restrains cracks perpendicular to span)

Maximum & Minimum reinforcement requirements

Flexural Reinforcement in SlabsFlexural Reinforcement in SlabsMaximum & Minimum reinforcement requirements

T&S Reinforcement (perpendicular to span) ACI Sec 7.12

y s min

y

y y

0.0020* 1000 * 280 or 350 MPa

0.0018* 1000 * 420 MPa

4200.0018* * 1000 * 420 MPa

0.0014* 1000 *

A t f

t f

t ff

t

Flexural Reinforcement in SlabsFlexural Reinforcement in Slabs

T&S Reinforcement (perpendicular to span) ACI Sec 7.12

Note: shrinkage cracks could be wide even when this much of reinforcement is provided. This is especially true when shear walls, large columns or stiff elements restrain the shrinkage and temperature movements.

max

5t smaller of

450mmS

Flexural Reinforcement in SlabsFlexural Reinforcement in SlabsFlexural Reinforcement (parallel to span) ACI Sec 10.5.4

s min s min &T SA A

The maximum spacing of the bars is given as:

max

3t slab thicknesssmaller of ACI Sec. 7.6.5

45S

cm

Also, check crack control ACI Sec. 10.6.4

Flexural Reinforcement in SlabsFlexural Reinforcement in Slabs

(Smax from flexural reinforced spacing).

Use 12mm or larger bars for flexural reinforcement in slabs , because smaller bars or wires tend to be bent out of position by workers walking on the reinforcement during construction.

One-Way Slab DesignOne-Way Slab DesignNeed to confirm thickness is adequate for one-way shear. Difficult to place shear reinforcement in a slab. Minimum area of shear reinforcement required in slabs

if

u c

cc w

6

V V

fV b d

ACI Sec. 11.5.51

ACI Eqn. 11-3Usual use

Note: See Example 5-7m p243 design of one-way slabs