Civil Engineering Reference
In-Depth Information
16.12
TRANSFER OF MOMENTS AND SHEARS BETWEEN SLABS
AND COLUMNS
On many occasions the maximum load that a two-way slab can support is dependent upon
the strength of the joint between the column and the slab. Not only is the load transferred
by shear from the slab to the column along an area around the column, but also there may
be moments that have to be transferred as well. The moment situation is usually most crit-
ical at the exterior columns.
If there are moments to be transferred, they will cause shear stresses of their own in
the slabs, as will be described in this section. Furthermore, shear forces resulting from
moment transfer must be considered in the design of the lateral column reinforcement
(that is, ties and spirals), as stated in Section 11.11.1 of the Code.
When columns are supporting slabs without beams (that is, flat plates or flat slabs),
the load transfer situation between the slabs and columns is extremely
critical
. Perhaps if
we don't have the flexural reinforcing designed just right throughout the slab as to quanti-
ties and positions, we can get by with it; however, if we handle the shear strength situa-
tion incorrectly, the results may very well be disastrous.
The serious nature of this problem is shown in Figure 16.20, where it can be seen that
if there is no spandrel beam, all of the total exterior slab moment has to be transferred to
the column. The transfer is made by both flexure and eccentric shear, the latter being lo-
cated at a distance of about
d
/2 from the column face.
Section 13.6.3.6 of the Code states that for moment transfer between the slab and
edge column, the gravity load moment to be transferred shall be 0.3
M
o
(where
M
o
is the
factored statical moment).
When gravity loads, wind or earthquake loads, or other lateral forces cause a transfer
of an unbalanced moment between a slab and a column, a part of the moment equal to
f
M
u
shall be transferred by flexure, according to ACI Section 13.5.3.2. Based on both
tests and experience, this transfer is to be considered to be made within an effective slab
Figure 16.20
