Lecture 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 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 Design Thickness 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 Design Thickness of One-Way Slabs Table A-9 tmin, when damage to non-structural components may occur

One-Way Slab Design Thickness 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 Reinforcement ACI Sec. 7.71 (min. cover for corrosion protection) ( 1.) Concrete exposed to earth or weather. φ16 and smaller = 40mm φ19 and larger = 50mm ( 2.) Concrete not exposed to earth or weather. 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 Design Reinforcement Typical Reinforcement details in a one-way slab:

One-Way Slab Design Cutoffs If requirements for use of ACI Moment Coefficients: Figure A-5 one-way slab

Flexural 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

Areas of bars in slabs (cm2 per meter) Bar diameter (mm) 6 8 10 12 14 16 18 5 5.65 10.05 15.70 22.61 30.77 40.19 50.8 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 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 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 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 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 1.77 4.91 7.07 9.62 15.9 17 1.66 2.96 4.62 6.65 9.05 11.82 14.9 1.57 2.79 4.36 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 7.69 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 6.69 8.74 11.0 24 1.18 2.09 3.27 6.41 10.6 25 1.13 2.01 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 5.70 7.44 28 1.01 1.79 2.80 5.50 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 Slabs The maximum spacing of the bars for flexure is given as:

Flexural Reinforcement in Slabs Maximum & Minimum reinforcement requirements 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)

Flexural Reinforcement in Slabs Maximum & Minimum reinforcement requirements T&S Reinforcement (perpendicular to span) ACI Sec 7.12

Flexural 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.

Flexural Reinforcement in Slabs Flexural Reinforcement (parallel to span) ACI Sec 10.5.4 The maximum spacing of the bars is given as: Also, check crack control ACI Sec. 10.6.4

Flexural 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 Design Need 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 ACI Sec. 11.5.51 ACI Eqn. 11-3 Usual use Note: See Example 5-7m p243 design of one-way slabs