Civil Engineering Reference
In-Depth Information
4.
Section 12.3.3 says required
d
can be multiplied by
A
s
reqd
/
A
s
furn
. This only applies to the
first bars cut off (“bottom” bars here).
4.24
4.71
Reduced
d
(53.4)
48
5.
In Figure 14.36 these values are applied to the bars in question and the results are given.
The structural designer is seldom involved with a fixed-moment diagram—the loads
move and the moment diagram changes. Therefore, the Code (12.10.3) says that reinforc-
ing bars should be continued for a distance of 12 bar diameters or the effective depth of
the member, whichever is greater (except at the supports of simple spans and the free ends
of cantilevers), beyond their theoretical cutoff points.
As previously mentioned, the bars must be embedded a distance
d
from their point
of maximum stress.
Next, the Code (12.11.1) says that at least one-third of the positive steel in simple
spans and one-fourth of the positive steel in continuous spans must be continued along the
same face of the beam at least 6 in. into the support.
Somewhat similar rules are provided by the Code (12.12.3) for negative steel. At
least one-third of the negative steel provided at a support must be extended beyond its
point of inflection a distance equal to one-sixteenth of the clear span or 12 bar diameters
or the effective depth of the member, whichever is greatest. Other negative bars must be
extended beyond their theoretical point of cutoff by the effective depth, 12 bar diameters
and at least
d
from the face of the support.
Trying to go through these various calculations for cutoff or bend points for all of the
bars in even a modest-sized structure can be a very large job. Therefore, the average de-
signer or perhaps the structural draftsperson will cut off or bend bars by certain rules of
thumb, which have been developed to meet the Code rules described here. In Figure 14.37
a sample set of such rules is given for continuous beams. In the
CRSI Handbook
,
10
such
Figure 14.37
Recommended bar details for continuous beams.
10
CRSI Handbook
, 9th ed., Concrete Reinforcing Steel Institute, 2002 (Chicago), p. 12-1.
