Civil Engineering Reference
In-Depth Information
quite a bit of moment resistance, but deflections are often large. As a consequence, their
depths are very carefully controlled by the ACI Code so as to limit these deflections.
This is accomplished by requiring the designer to either (a) compute deflections and
make sure they are within certain limitations or (b) use certain minimum thicknesses as
specified in Section 9.5.3 of the Code. Deflection computations for two-way slabs are
rather complicated, so the average designer usually uses the minimum ACI thickness
values, presented in the next few paragraphs of this chapter.
Slabs without Interior Beams
For a slab without interior beams spanning between its supports and with a ratio of its
long span to short span not greater than 2.0, the minimum thickness can be taken from
Table 16.1 of this chapter [Table 9.5(c) in the Code]. The values selected from the table,
however, must not be less than the following values (ACI 9.5.3.2):
1.
Slabs without drop panels 5 in.
2.
Thickness of those slabs with drop panels outside the panels
4 in.
In Table 16.1 some of the values are given for slabs with drop panels. To be classified
as a drop panel, according to Sections 13.3.7.1 and 13.3.7.2 of the Code, a panel must
(a) extend horizontally in each direction from the centerline of the support no less than
one-sixth the distance, center to center, of supports in that direction and (b) project verti-
cally below the slab a distance no less than one-fourth the thickness of the slab away from
the drop panel. In this table,
n
is the length of the clear span in the long direction of two-
way construction, measured face to face of the supports in slabs without beams and face
to face of beams or other supports in other cases.
Very often slabs are built without interior beams between the columns but with edge
beams running around the perimeter of the building. These beams are very helpful in stiff-
ening the slabs and reducing the deflections in the exterior slab panels. The stiffness of
slabs with edge beams is expressed as a function of
, which follows.
is used to represent the ratio of the flexural stiff-
ness (
E
cb
I
b
) of a beam section to the flexural stiffness of the slab (
E
cs
I
s
) whose width
equals the distance between the centerlines of the panels on each side of the beam. If no
beams are used, as for the flat plate,
Throughout this chapter the letter
will equal 0. For slabs with beams between
columns along exterior edges,
for the edge beams may not be
0.8 as specified in a
footnote to Table 16.1.
E
cb
I
b
E
cs
I
s
where
E
cb
the modulus of elasticity of the beam concrete
E
cs
the modulus of elasticity of the column concrete
I
b
the gross moment of inertia about the centroidal axis of a section made up of
the beam and the slab on each side of the beam extending a distance equal to
the projection of the beam above or below the slab (whichever is greater) but
not exceeding four times the slab thickness (ACI 13.2.4)
