A concrete corbel can fail in a few ways based upon the loading mechanism along with other factors. Such failure of a member in its service life may be catastrophic to the integrity of the overall structure.
The failure mechanisms of corbels can be divided in six basic modes: bending, concrete crushing on the strut, shear at the interface between the corbel and the column, loss of anchorage of the main tie reinforcement, horizontal load, and concrete crushing under the bearing pad.
Here are the following in detail:
1. FAILURE DUE TO EXCESSIVE BENDING
In this type of failure, the corbel fails due to the high magnitude of moment upon the corbel member. The lever arm for the moment is the shear span. The member is inadequately designed and hence cannot withstand the moment. The crack initially develops from the top, as shown and then travels down the end of the corbel section. This crack gets bigger and eventually it fails in bending. The use of adequate main reinforcement for the member is vital to resist this main moment.
2. FAILURE BY THE CONCRETE CRUSHING FROM ITS STRUT PORTION
The interface between the column and the corbel is susceptible to this type of failure. This occurs mainly due to the inadequate design of the concrete. Since the concrete cannot bear the large compressive forces, the gradual cracking of the corbel takes place from the bottom as shown. Eventually the entire corbel section crumbles and the member collapses.
3. FAILURE DUE TO SHEAR
This failure occurs at the interface between the column and the corbel member. The shear forces overwhelm the member and the improper merger of the two components creates a failure zone running along the joint. Eventually, the member breaks off from the column and fails.
4.FAILURE VIA BREAKAWAY DUE TO HORIZONTAL LOADS
This type of failure is somewhat similar to the previous failure example. The horizontal forces and the moment thus generated create a vulnerable portion of the corbel member from the point of the action of force. This little part of the corbel gradually develops cracks and ultimately chips off. There is large horizontal force transmitted from the supported beam as a result from the long-term shrinkage and creep deformation. The cracks are usually vertical or inclined pure share cracks.
5. FAILURE DUE TO INSUFFICIENT CORBEL MAIN REINFORCEMENT
Reinforced concrete corbels with a low percentage of longitudinal reinforcement bars typically fail by yielding of the main tension reinforcement bars with a ductile manner.
6. FAILURE DUE TO CRUSHING
The point of force action is subject to direct load which later disperses to the member throughout. This portion must withstand the compressive force incident upon. The improper placement of bearings that causes the concentration of force to a certain end of the bearing block might create a point of excessive force while the other parts just below the bearing might experience a lesser magnitude of loads. In such cases, load impact is likely to cause the crushing of surface concrete is a certain part that crumbles the upper layer of concrete and the corbel fails.
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