Civil Engineering Reference
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Table 4.1
Recommended minimum thickness of one-way slabs and beams
Minimum thickness, h
Simply supported
One-end continuous
Both-ends continuous
Cantilever
One-way slabs
l
/13
l
/17
l
/22
l
/5.5
Beams
l
/10
l
/12
l
/16
l
/4
4.3 INITIAL MEMBER PROPORTIONING
For steel reinforced concrete flexural members, ACI 318-11 mandates mini-
mum values of member thickness that satisfy given deflection-to-span ratios.
The applicability of this approach for FRP reinforced concrete members is
still under scrutiny. However, ACI 440.1R-06 [4] recommends minimum
thickness values for indirect control of deflections in one-way slabs and
beams, which can be used for initial member proportioning only (Table 4.1).
Deflections in FRP RC members tend to be larger than in their steel RC
counterparts, due to the lower tensile modulus of elasticity of commercially
available FRP reinforcing bars. For this reason, FRP reinforced members are
generally subjected to higher depth requirements for comparable span lengths.
The values in Table 4.1 were derived by Ospina, Alexander, and Cheng
[5] based on the following equation:
l
hK
48
5
η−
ε
1
k
∆
=
(4.10)
l
1
f
max
where ε
f
is the reinforcement tensile strain at midspan,
h
is the height of the
cross section,
k
is the neutral axis depth to reinforcement depth ratio at mid-
span, η is the reinforcement depth to member thickness ratio, and Κ
1
is a
constant that depends on the loading and support conditions. For example, Κ
1
can be taken as 1.0, 0.8, 0.6, and 2.4 for uniformly loaded, simply supported,
one-end continuous, both-ends continuous, and cantilevered spans, respec-
tively. The ratio η may be assumed to range between 0.85 and 0.95.
COMMENTARY
When a linear distribution of the strain over a member's cross section is
assumed, the maximum immediate elastic deflection at midspan,
Δ
max
, of a
one-way flexural member under uniform distributed load can be computed as
2
5
48
Ml
EI
(4 .11)
K
∆=
⋅
max
1
c
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