Civil Engineering Reference
In-Depth Information
range of to 2. For longer spans, better economy is usually obtained if deep, narrow sec-
tions are used. The depths may be as large as three or four times the widths. However,
today's reinforced concrete designer is often confronted with the need to keep members
rather shallow so as to reduce floor heights. As a result, wider and shallower beams are
used more frequently than in the past. You will notice that the overall beam dimensions
are selected to whole inches. This is done for simplicity in constructing forms or for the
rental of forms, which are usually available in 1- or 2-in. increments. Furthermore, beam
widths are often selected in multiples of 2 or 3 in.
1
2
2.
Deflections.
Considerable space is devoted in Chapter 6 to the topic of deflections
in reinforced concrete members subjected to bending. Should deflections not be computed
and proved thereby to be acceptable, the ACI in Section 9.5.2 provides minimum permis-
sible beam and slab depths. These values are shown in Table 4.1. The purpose of such
limitations is to prevent deflections of such magnitudes as would interfere with the use of
or cause injury to the structure. If deflections are computed for members of lesser thick-
nesses than those listed in the table and are found to be satisfactory, it is not necessary to
abide by the thickness rules. For simply supported slabs, normal-weight concrete, and
grade 60 steel, the minimum depth given when deflections are not computed equals
/20,
where
is the span length of the slab.
For concretes of other weights and for steels of dif-
ferent yield strengths, the minimum depths required by the ACI Code are somewhat re-
vised as indicated in the footnotes to Table 4.1.
The ACI does not specify changes in the
table for concretes weighing between 120 and 145 lb/ft because substitution into the cor-
rection expression given yields correction factors almost exactly equal to 1.0.
The minimum thicknesses provided apply only to members that are not supporting or
attached to partitions or other construction likely to be damaged by large deflections.
3.
Estimated beam weight.
The weight of the beam to be selected must be included
in the calculation of the bending moment to be resisted, because the beam must support it-
Table 4.1
Minimum Thickness of Nonprestressed Beams or One-Way Slabs
Unless Deflections Are Computed
1,2
Minimum thickness,
h
Simply
One end
Both ends
supported
continuous
continuous
Cantilever
Members not supporting or attached to partitions or other
Member
construction likely to be damaged by large deflections
/20
/24
/28
/10
Solid one-way slabs
Beams or ribbed
one-way slabs
/16
/18.5
/21
/8
1
Span length
is in inches.
2
Values given shall be used directly for members with normal-weight concrete (
w
c
145 pcf)
and Grade 60 reinforcement. For other conditions, the values shall be modified as follows:
(a) For structural lightweight concrete having unit weights in the range 90-120 lb/ft
3
, the
values shall be multiplied by (1.65
0.005
w
c
) but not less than 1.09, where
w
c
is the unit
weight in lb/ft
3
.
(b) For
f
y
other than 60,000 psi, the values shall be multiplied by (0.4
f
y
/100,000).
