Composite Beam Design

[quest4more]

On page 5-57 of ASD Manual, for composite beam design, the calculation of transformed section properties for stress check is always based on normal weight concrete, but for deflection checks we are asked to take into account whether the concrete is normal weight or light weight. Why is it so?

 

[14159]

You could ask the AISC Solutions Center. They're super-fast and thorough.

Why not use LRFD composite design? The difference in results are extreme and it's also really easy to understand, unlike Allowable Stress Dinosaur's composite design, as illustrated by this thread.

14159

 

[quest4more]

I have already emailed them .... no response yet. I cannot switch to LRFD I have to deal with the equations of ASD !

 

[JAE]

quest4more -
The modulus of elasticity (E) for normal weight and light weight concrete is different. If you check out ACI - you'll see that for concrete - Ec = wc^1.5 x 33 x sqrt(f'c); where wc is the unit weight of concrete.

Thus, E is affected by density of the concrete. So to get accurate deflections, you must use an accurate E which is based on the unit weight.