Concrete Floor Design:

This design guide is intended to provide guidance for the safe design and economical construction of suspended concrete floor slabs. This design guide and the corresponding calculations are based on the requirements of ACI 318 and strength design method where the capacity of the beam is designed to support factored loads.   Slabs are structural elements whose lengths and widths are large in comparison to their thicknesses.  Unlike beams, shear is generally carried by the concrete without the aid of shear reinforcement.  Longitudinal reinforcement is used to resist bending moments.  The slab thickness is typically governed by deflection criteria or fire rating requirements.

Design of One-way Slabs:
1. Determine the required thickness for deflection control in the slab from the table below:
Minimum One-way Slab Thickness
(unless deflections are calculated)
(Normal Weight Concrete, fy = 60,000 psi (413.7 MPa))


Minimum thickness, h

(fraction of span length)

Simply supported


One end continuous


Both ends continuous




2. Determine the temperature steel required running normal to the flexural steel.  ACI 318 requires that one-way slabs have reinforcement for temperature and shrinkage stresses running normal to the main flexural steel.  The minimum reinforcement ratio, pt, for shrinkage and temperature control (based on gross concrete area) is 0.0014 for grade-40 steel or less, 0.0020 for grade-40 and grade-50 steel, and 0.0018 for grade-60 steel.  For steel exceeding grade-60, the minimum reinforcement ratio, pt, shall be calculated as follows:
[SI] pt = (0.0018)(413.7/fy); units fy is in MPa.
[US] pt = (0.0018)(60,000/fy); units fy is in psi.
The maximum spacing of temperature steel is the smaller of 5 times the slab thickness or 18 in.  Also note while no. 3 bars may be used, for constructability and handling reasons it is common practice to use no. 4 bars.
3. Calculate the flexural steel area required to support the factored moment on the unit width wide section of the floor slab.  Note also for a continuous slab of multible spans it will be analyzed as a multiple span continous beam.  This means that as the flexural reinforcement required will be on the top of the slab and the bottom of the slab corresponding to the positive and negative moments induced on the slab.  To keep the design simple, the floor is sometimes designed with a double mat of steel, both at the top and bottom of the slab to handle the maximum moments induced on the top and bottom of the slab.  The width of the beam is essentially set as the unit width of the strip and the depth to steal is generally taken as the depth or height of the beam, h, minus the steel cover, (minimum of 3/4 in for floors), minus the diameter of steel, d, divided by 2.  An assumption for d is generally used such as no. 4 bars etc.  The procedure then for design of the flexural steel is the same as the iterative design for a known beam section and is as follows.
4. Calculate the effective depth of the slab and calculate the concrete shear strength of the concrete slab.   Check it against the factored shear load on the slab, 0.85Vc > Vu.  If the shear strength is insufficient increase slab thickness until 0.85Vc > Vu.
5. In summary the design procedures for beams and floor slabs are similar with the following differences:
(a)  Minimum steel cover is 3/4 in (1.91 cm).
(b) The minimum steel ratio for flexure is the same as that for temperature and shrinkage given above.
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