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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