composite beam moment capacity
composite beam moment capacity
(OP)
I have a question about the calculation of the nominal moment capacities for steel beams with composite concrete filled deck.
The tables in the 13th edition manual are based on the idea that the entire steel section will yield and reach Fy regardless of where the PNA exists. The question is this:
Doesn't the deformation required for the entire steel beam to reach Fy almost guarantee that the slab will crush before the entire steel section yields?
The failure strain of concrete is 0.003, right?
The tables in the 13th edition manual are based on the idea that the entire steel section will yield and reach Fy regardless of where the PNA exists. The question is this:
Doesn't the deformation required for the entire steel beam to reach Fy almost guarantee that the slab will crush before the entire steel section yields?
The failure strain of concrete is 0.003, right?






RE: composite beam moment capacity
Using Hooke's law the strain (sigma/E)=0.001
RE: composite beam moment capacity
The strain of concrete at failure is accepted to be 0.003, or am I missing something?
RE: composite beam moment capacity
RE: composite beam moment capacity
I appreciate the response. The specific question I am asking, though, is won't the concrete crush BEFORE the steel section is able to fully yield?
Once the concrete crushes, teh section is no longer a composite section, right?
I can see some (probably most) of the beam yielding toward the bottom, but I am having a hard time seeing the steel near the ENA (well, between the ENA and PNA) reaching yield before the concrete crushes.
RE: composite beam moment capacity
Without really digging into the subject, I guess it always works out that way, or perhaps close enough, since 50/29000=0.00172<<0.003.
RE: composite beam moment capacity
Perhaps I am missing you're question. Why do you say that a strain of 0.001 in the concrete means the steel won't yield? Depending on where the PNA falls the steel can most certainly yield.
Judging from you're last post you are referring to yielding at the PNA. In which case for the steel near the PNA to yield, you are correct the strain in the concrete would need to be greater than 0.001.
With that said, you raise an interesting point, I've never really thought about this in terms of strains. This is definitely something you could could run the numbers on and see what the strain in the concrete is when the steel at the PNA yields.
RE: composite beam moment capacity
htt
I think page 3 helps explains it. While we assume that a hair above the PNA is compression, and a hair below is tension... that is never the case... but is just simplified for design...
So you're right you will not yield at the PNA before concrete crushes... you just assume you do...
I guess that's a good reason for phi=0.9
RE: composite beam moment capacity
RE: composite beam moment capacity
RE: composite beam moment capacity
I think you are correct except for the following:
While the concrete may strain beyond it's compressive strength, I think it can still act as a composite section, at least up to the "assumed strain limit" of 0.003. Beyond that, the concrete loses strength. PCA Notes on ACI 318 has stress strain curves for various strength of conretes. If the concrete strains beyond .003, I think you are right, you lose composite section. That may be why AISC has a recommended limit for the location of the PNA.
RE: composite beam moment capacity