Question

When using a cantilevered reinforced concrete slab the equation to solve the coefficient of resistance is phi 0.9 or 0.75 and why?

Answer 1

ANSWER:- phi = 0.9.

There are different criteria's are used for calculating strength reduction factors. In my view, this is used totconsider imperfections in material design and also on Construction site. Strength reduction factors also accounts for unwanted behaviour of element during failure. That is how they fail, when they are loaded with Ultmate design load.

When element fails in compression then concrete fails and there would be no such sign before failure, which is dangerous. If element fails in tension, then it gives sign before failure. For that reason, we use low strength reduction factors for compression controlled elements.

According to ACI 318-14, the strength reduction factors for moments is lies between 0.65 to 0.90.

Cantilever slab is a tension controlled element so that we can take phi = 0.9 .