Civil Engineering Reference
In-Depth Information
critical shear transfer section for circular interior columns can be found in Reference 4.2. Note that in the case
of flat slabs, two different critical sections need to be considered in punching shear calculations as shown in
Fig. 4-16. Tables 4-7 through 4-10 can be used in both cases. Also, Fig. 4-17 can be used to determine
γ
v
and
γ
f
given b
1
and b
2
.
Critical
sections
d
1
d
2
/2
d
1
/2
Figure 4-16 Critical Shear-Transfer Sections for Flat Slabs
Unbalanced moment transfer between slab and an edge column (without spandrel beams) requires special
consideration when slabs are analyzed by the Direct Design Method for gravity loads. To assure adequate shear
strength when using the approximate end-span moment coefficient, the moment 0.30 M
o
must be used in
determining the fraction of unbalanced moment transferred by eccentricity of shear (
γ
v
M
u
=
γ
v
0.30M
o
)
according to ACI 13.6.3.6. For end spans without spandrel beams, the column strip is proportioned to resist the
total exterior negative factored moment (Table 4-2). The above requirement is illustrated in Fig. 4-18. The total
reinforcement provided in the column strip includes the additional reinforcement concentrated over the column
to resist the fraction of unbalanced moment transferred by flexure
γ
f
M
u
=
γ
f
(0.26M
o
), where the moment
coefficient (0.26) is from Table 4-2. Application of this special design requirement is illustrated in Section 4.7.
4.5
COLUMN MOMENTS DUE TO GRAVITY LOADS
Supporting columns (and walls) must resist any negative moments transferred from the slab system.
For interior columns, the approximate ACI Eq. (13-7) may be used for unbalanced moment transfer due to
gravity loading, unless an analysis is made considering the effects of pattern loading and unequal adjacent
spans. The transfer moment is computed directly as a function of the span length and gravity loading. For the
more usual case with equal transverse and longitudinal spans, ACI Eq. (13-7) simplifies to:
M
u
= 0.07(0.05 q
Lu
˜
2
˜
n
2
) = 0.035 q
Lu
˜
2
˜
n
2
where q
Lu
= factored live load, psf
˜
2
= span length transverse to
˜
n
= clear span length in direction M
u
is being determined
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