Concrete Design Rules of Thumb

Concrete Design Rules of Thumb Dimensions of structural members in concrete construction Beams A convenient formula for approximating the dimensions of a concrete beam is the following based on the cross-sectional dimensions 'b' and 'd':  where: 'b' = beam width (inches); 'd' = beam depth (inches); Mu = Max moment in beam (ft-kips)

The required area of tension steel reinforcing can be approximated by the following formula:

 where: As = area of steel (square inches); Mu = max moment in beam (ft-kips); 'd' = beam depth (inches)

Under normal building loading conditions, the minimum depth of beams using Grade 60 reinforcing should be the following: 

Simply Supported: D = L/16



One End Continuous: D = L/18



Both Ends Continuous: D = L/20



Cantilever: D = L/8

Moment of Inertia[1] The moment of inertia of a 'T' or 'L' shaped beam can be approximated by the following: 

For an 'L' beam



For an 'T' beam

where

= the moment of interia of the rectangular web

Columns a. Minimum dimension is 8” b. Cross sectional area in square inches = load (in psi)/1000

Slabs One way spans: (assumed to support structurally in one direction, or having joist ribs in one direction)

Slabs without joist ribs Under normal building loading conditions, the thickness of solid one-way slabs with Grade 60 reinforcing steel can be : 

Simply Supported: Thk = L/20



One End Continuous: Thk = L/24



Both Ends Continuous: Thk = L/28



Cantilever: Thk = L/10

Joist slabs Here are some guidelines for joist slabs. 

good for spans from 12’ to 30’



good for oblong bays



overall depth of slab plus joist ribs = span/20 + 2 ½”



common spacing of 20” and 30” between ribs



joists commonly have a width of 5” to 9”

Two-way slabs: Without drop panels The minimum thickness of two-way slabs can be based on the following: 

Exterior panels without edge beams: Thk = Ln/30

Rules of thumb for concrete structural floor layout Two-way spans (assumed to support structurally in two directions or having perpendicular joist ribs).

Slabs without joist ribs

Here are some guidelines for two-way slabs. 

Bays of two-way slabs (without joist ribs) may be up to 24’x24’ (i.e. 576 sq. ft.)



Slab thickness of the slab = span/30

Joist slabs Here are some guidelines for two-way joist slabs (Waffle Slabs). 

Waffle slabs are used for 24’ to 50’ in the large dimension (maximum of 2500 sq. ft.)



Overall thickness of slab and joist rib = span/30 + 2 ½”

Concrete Walls 

The thickness of bearing walls should be at least 1/25 the supported height or length of the wall.



The thickness of nonbearing walls should be at least 1/30 of the least distance between lateral supporting members.

References 1. ↑ W.F. Chen and Charles Scawthorn. Earthquake Engineering Handbook. 2003. CRC Press LLC. Other References: 1. Structural Engineer Magazine (2001). Time Saving Design Aids for Reinforced Concrete: Concrete Beam Design. 2. http://www.arch.mcgill.ca/prof/friedman/arch240/winter1998/lecture8/lecture8.html