Beams in RC buildings have two sets of steel reinforcement, namely longitudinal bars placed along the length and stirrups placed vertically at regular intervals along its full length. Longitudinal bars resist bending moment while vertical stirrups resist shear forces.
Beam sustain two basic type of failures, namely :
(a) Flexural failure
(b) Shear failure
FLEXURAL FAILURE :
As the sags under the increased loading, it can fail in two possible ways. If relatively more steel is present on the tension face, concrete cruse in compression, is a brittle failure and is therefore undesirable. If relatively less steel is present on the tension face, the steel yields first(it keeps elongating but does not snap) and redistribution occurs in the beam until eventually the concrete crushes in compression, this is a ductile failure and hence is desirable. Thus more steel on tension face is not necessarily desirable ! The ductile failure is characterized with many cracks starting from the stretched beam face and going towards its mid-depth.
SHEAR FAILURE :
A beam may also fail due to shearing action. A shear crack is inclined at 45 degree to the horizontal. It develops at mid depth near the support and grows towards the top and bottom faces. Closed loop stirrups are provided to avoid such shearing actions. shear damage occurs when the are of stirrups is insufficient.
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