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I've just started to study reinforced concrete design and I came across this

in the "Design of Concrete Structures" by Dr.Nilson. and I was wondering why do we increase the minimum thickness if we are using stell grade more than 60 ksi instead of decreasing it?

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The principle behind that table is that you will be doing a strength calculation in any case and need the table for a guideline of whether deflections or bending strength will be the design driver. High strength reinforcement does not contribute any more to the stiffness than low strength reinforcement, as the elasticity modulus of steel is the same for reinforcement of different strength grades. (If you used a more accurate approach and actually calculated the deflection, the yield strength of the reinforcement would not change the deflection.)

But if you are using low strength reinforcement you will need relatively more or larger rebars to satisfy the strength requirement, resulting in a stiffer beam or slab, which in turn will make it less likely that deflections become the design driver.

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