Question

When the design standard yield strength of the reinforcing bar is 400 MPa,

Why is Minimum allowable strain 0.004 and corresponding rebar ratio 0.714ρb ??

plz prove .

Answer 1

=> The minimum values above are fixed by ACI 318-11 Code . It is done By analysing the behaviour of concrete and steel in their ratios . Hence it is empirical . There is no formula . You need to see or learn the values from the ACI Code.

But the reason for the minimum values are -

Minimum allowable strain 0.004 is because to utilise the steel effectively . As we know steel is a ductile material So, For maximum utilisation , it is fixed by ACI Code that minimum strain to be 0.004 .

and The amount of steel reinforcement in concrete members required to be limited. the placement more reinforcement would not allow the steel to yield before the concrete crushes and there would be a sudden failure.

Hence , the rebar ratio 0.714ρb .

If any doubt kindly comment. Thank you.