Why is there a maximum limit on the percentage of main reinforcement in RC columns, while in all cases compression failure is expected?
RCC construction comprises reinforcement (steel) and concrete. While concrete takes care of compressive loading, steel takes care of tensile loading and it provides enough ductility to prevent failure due to earthquake. Any RCC structure experiences both compressive and tensile loading, it has to be designed as per these forces acting on the structure. This is the primary reason to limit use of reinforcement in a RCC structure. On the other hand, a minimum percentage of reinforcement has to be used to provide requisite ductility in the structure which is otherwise missing in pure concrete structure.
Yes, we discuss here the problem of so-called overreinforced and underreinforced concrete. Following to answer of Vinod Kumar, one should be take into account the number of rods in rows, the distance between rods as well as their diameters (including grooves), especially in joints of contacting rods. These factors influence on percentage of reinforcement.
In long columns the failure is always in bending, so, even where lateral loads do not exist, the failure must be ductile. In most real structures there is some lateral load component (wind, horizontal live loads, differential temperature, creep and earthquake) which all result in bending of the columns. Hence, columns must be designed to be ductile. Therefore, limits on reinforcement apply.
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