Beams
A convenient formula for approximating the
dimensions of a concrete beam is the following
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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
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 non bearing walls should be at least 1/30 of the least distance between
lateral supporting members.
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