Corbels (aka brackets.) are reinforced structural members or short structural projections used to transfer vertical and horizontal forces from beams to walls or columns. They are extensively used in the design of reinforced concrete structures, bridge structures and precast structures.
A corbel is typically characterized by a shear span-to-depth ratio lower than unity. A span to depth ration greater than 1 indicates the member is a bending member. The short hunched geometry of the corbel makes them behave in two-dimensional Manner.
In the the arrangement of a corbel, we can observe that it is casted overhanging from a column. The fixity to the column allows the transfer of the load from the member to the pier/column.
In the context of bridge design, the corbel is placed with a bearing pad upon which the force acts perpendicularly and horizontally. This neoprene interface arrests the rupture of the concrete and the likelihood of unequal action of load on the member. The distance from the support end of the corbel to the point of force application is known as the ‘Shear Span’.
The interface between the corbel and the column is the plane along which most failures take place.
GEOMETRY
The geometry of the corbel in its section is gradually increasing towards the column. This is to counter the increasing moment towards the end of the corbel. This geometrical variation can also be made uniform if need be.
A double corbel is a case where two protrusions are placed opposite to each other. Both the corbels may have different dimensions based upon the load intensities.
There are different types of corbels as shown in the illustration.
Single corbels can have either rectangular or trapezoidal in section.
Double corbels can be designed with different depths and shapes at the same time.
REINFORCEMENT
The Reinforcement of a single corbel is usually provided in the following way.
Reinforcement 1 2 3 4 are placed in conjunction with the reinforcement of the column. Reinforcement number 2 is also the primary reinforcing bar.
There may be some slight variations in the shape of main bars to allow for sufficient development length but the generic reinforcement shown here remains the same.
Reinforcement for double corbels follows the same pattern as a single corbel except for the primary reinforcement indicated as reinforcement number 4. It is imperative that sufficient development length has been achieved for the bars embedded inot the column from the corbel so as to prevent the failure due to a lack of fixity.
In bridge engineering, many structures may behave as corbels-including, station piers, concourse arms, platform arms, piercap, seismic arrestors, bearing pedestals, etcetera.
Sometimes, an I-section with an overhang cantilever is placed upon the corbel. The overhang is incident upon the corbel upon the following points. The I section is fixed to the corbel at the points at the corbel and the column end as shown.
A detailed explanation for steel corbels and its design will be dealt in a separate article.
VIDEO LINK
Refer to the video below for further light on the topic
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