Thickness Below which Deflections Must be Computed

[ajk1]

In CSA A23.3 Design of Concrete Structures, there is a Table for determining minimum thickness of beams and slabs, if deflections are not computed. There is a footnote saying that "(b) for fy other than 400 MPa, the values shall be multiplied by (0.4 + fy/ 670)". Units are metric. Similarly in ACI I believe.

Question

If I want to use 750 mm deep reinforced concrete Tee beam to span 16.3 m clear single simply supported beam, can I manipulate the expression to solve for fy as shown below?

Span/16 x (0.4+ fy/ 670) = 750


16300 / 16 x (0.4 + fy /670) = 750

Solve for fy = 225 MPa or 32,600 psi

Then calculate steel area required based on 225 MPa rather than based on the actual fy (400 MPa) of the rebar.

This gives me a required steel area of 7200 mm² or 11.2 in² giving a c/d ratio of 0.066 which is less than the 0.636 c/d Code limit, so OK.

Is that legitimate? Any comment?

I will of course also calculate deflection.

 

[KootK]

I didn't check your math but I believe that is precisely how that provision is intended to be used..

 

[BAretired]

I agree that using an assumed yield lower than the actual yield is a legitimate tool to use to satisfy the code.

BA