Concrete/Steel Composite Beam Question

[NS4U]

In composite construction, when you have a concrete slab on a steel beam, say that the PNA is at the bottom of slab/top of steel:

To calc Mn- one assumes that the extreme fiber of the concrete has reached a strain of -0.003 (Negative for compression) and the concrete has developed the whitney stress block over its entire section. One also assumes that the steel section had fully yieled and developed a strain at the very top of 0.002 (for 50 ksi steel).

Now- if the strain at the top of the steel is at 0.002, that means the strain at the bottom of the steel is much larger. But the strain at the bottom steel is never checked to see if it exceeds the rupture strain. Why is that?

 

[msquared48]

You set up the problem of the NA at the concrete/steel interface but state that "One also assumes that the steel section had fully yieled and developed a strain at the very top of 0.002".

This cannot be the case. The strain is zero by definition at the NA.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[NS4U]

I see what you mean but this is a very common assumption... regardless...

assume that the top of the steel has a strain of 0.002 and the strain in the concrete is 0.003... why is the strain in the bottom not checked for rupture.

 

[Lion06]

I think you're looking at it wrong. AISC gives two options - strain analysis (in your case strain at PNA = 0), or just assume complete yielding of the steel.

I had a similar question a while ago that directly relates to this - specifically, if the PNA is at the top/steel or btm/slab how do you count on the whole slab in compression. In my mind that is analogous to using c to determine the compression block in a RC beam instead of using a=Beta1*c.