Civil Engineering Reference
In-Depth Information
Figure 3.4
Crack patterns at failure of a typical deep beam with circular web openings (under two-
point loading).
diagonal crack in the region of shear between opening and support. The
principle stress trajectories (CIRIA guide, 1977) for the uncracked state
amply support this phenomenon.
3.4
General behaviour in shear failure (under four-point loading)
In earlier stages of loading, up to 30% of the ultimate load, the beam behaves
in a truly elastic manner and the load-deflection relation is linear. Normally,
the diagonal cracks appear first in the vicinity of the opening at about 30-45%
of the ultimate load and extend both ways towards the support and load
bearing points (
Figure 3.5)
.
A diagonal crack may also appear first in the lower
part of the beam and extend up to the mid-depth or join the opening.
The load deflection is in no way appreciably affected at this stage.
Further increase in load may cause the existing cracks to widen and to
extend; simultaneously, new diagonal cracks develop more or less parallel to
the existing ones.
Flexural cracks appear only after the appearance of the diagonal cracks at
loads about 42-90% of the ultimate. The flexural cracks hardly reach the
mid-depth of the beam nor their widths exceed 0.1 mm. The formation of the
diagonal and flexural cracks affects the load-deflection relation. At this
stage, i.e. at 80-90% of the ultimate load, one of the diagonal cracks widens
and extends conspicuously and the final failure of the beam is caused.
