Lecture 24  Page 1 of 9
Lecture 24 – Flexural Members (cont.)
Determining the usable moment capacity, M
u
, of a rectangular reinforced concrete
beam is accomplished by using the formula below: (see Lect. 23)
M
u
= 0.9A
s
f
y
d(1 
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
c
y
act
f
f
'
59
.
0
ρ
)
Designing a beam using the equation above is much more difficult.
Assuming the
material properties and dimensions are known, the equation above still has 2
unknown variables – A
s
and
ρ
act
.
Therefore, design of steel reinforcement for a given
beam is largely one of trialanderror.
Beam Design
Design of concrete beam members is often one of trialanderror.
It’s difficult
to directly solve for all the variables in a reinforced concrete beam.
Usually,
material properties are known as well as maximum applied factored moment,
M
max
.
The following Table is useful to get a “trial” beam size:
Minimum Suggested Thickness “h” of Concrete Beams & OneWay Slabs
End Conditions
Member:
Simply
supported
One end
continuous
Both ends
continuous
Cantilever
Solid oneway slab
L/20
L/24
L/28
L/10
Beam
L/16
L/18.5
L/21
L/8
Span length L = inches
Beams are usually rectangular having the width typically narrower than the
height.
The diagram below shows typical beam aspect ratios:
h
≈
1.5b
→
2.5b
b
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 Fall '08
 HULTENIUS
 concrete beam, usable moment

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