Civil Engineering Reference
In-Depth Information
For the beam of Example 6.2 with its two continuous ends, the effective moment of
inertia would be
Avg
I
e
(0.70)(34,412)
(0.15)(24,257
24,257)
31,365 in.
4
6.9
TYPES OF CRACKS
This section presents a few introductory comments concerning some of the several types
of cracks that occur in reinforced concrete beams. The remainder of this chapter is con-
cerned with the estimated widths of flexural cracks and recommended maximum spacings
of flexural bars to control cracks.
Flexural cracks
are vertical cracks that extend from the tension sides of beams up to
the region of their neutral axes. They are illustrated in Figure 6.12(a). Should beams have
very deep webs (more than 3 or 4 ft), the cracks will be very closely spaced, with some of
them coming together above the reinforcing and some disappearing there. These cracks
may be wider up in the middle of the beam than at the bottom.
Inclined cracks due to shear can develop in the webs of reinforced concrete beams ei-
ther as independent cracks or as extensions of flexural cracks. Occasionally, inclined
cracks will develop independently in a beam, even though no flexural cracks are in that
locality. These cracks, which are called
web-shear cracks
and which are illustrated in Fig-
ure 6.12(b), sometimes occur in the webs of prestressed sections, particularly those with
large flanges and thin webs.
The usual type of inclined shear cracks are the
flexure-shear cracks
, which are illus-
trated in Figure 6.12(c). They commonly develop in both prestressed and nonprestressed
beams.
Figure 6.12
Some types of cracks in concrete members.
