Reinforced concrete beam - question about theory
Reinforced concrete beam - question about theory
(OP)
Hi,
If I choose to design a reinforced concrete beam as simply supported (generally conservative), but it is in reality connected to a column in a more rigid connection, do I need to theoretically need to provide reinforcement on the top side of the beam?
I know that the beam will want to go into bending on the top side in a rigid supported beam, but if there is no steel to resist it, will the beam actually begin to behave like a simply supported beam and still remain safe?
The question I am asking also applies to steel. If I design a steel beam as simply supported, and then it is decided that they will make the connection more rigid, do I need to design the connection for moment? Or will the connection just 'shed' in a manner which the beam can take it (in a simply supported manner).
I hope this is clear, I found it difficult to explain my thought process.
If I choose to design a reinforced concrete beam as simply supported (generally conservative), but it is in reality connected to a column in a more rigid connection, do I need to theoretically need to provide reinforcement on the top side of the beam?
I know that the beam will want to go into bending on the top side in a rigid supported beam, but if there is no steel to resist it, will the beam actually begin to behave like a simply supported beam and still remain safe?
The question I am asking also applies to steel. If I design a steel beam as simply supported, and then it is decided that they will make the connection more rigid, do I need to design the connection for moment? Or will the connection just 'shed' in a manner which the beam can take it (in a simply supported manner).
I hope this is clear, I found it difficult to explain my thought process.






RE: Reinforced concrete beam - question about theory
RE: Reinforced concrete beam - question about theory
If you design for simple span you can and should still put top bars in there. In the US, under ACI 318, Chapter 7, there is still need for integrity reinforcement on top at the columns.
For steel beams your question's answer "depends" on a lot of things - bracing of the beam, connection capacity vs. actual moment induced at the connection, etc.
The idea of a connecting "shedding" a moment assumes that it goes plastic, which needs to be dealt with in terms of bracing and possible local buckling checks. Not a simple yes or no answer.
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RE: Reinforced concrete beam - question about theory
RE: Reinforced concrete beam - question about theory
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RE: Reinforced concrete beam - question about theory
Reinforced concrete does not act exactly the same as steel construction when it yields.
Don't forget to check shear and provide stirrups per code.
I hope this helps.
RE: Reinforced concrete beam - question about theory
To put my point question another way... If I was designing a steel beam that was to be post-anchored between two RC walls using some adhesive anchors, and I designed the beam as simply supported (i.e. specified the anchors based on the assumption they would not need to resist moment), would the anchors be expected to fail completely, or at the point of failure would it redistribute load and remain stable (as the beam has been designed to take full mid-span moment).
I understand there may not be a clear cut answer to this, but I enjoy hearing your opinions.
Cheers
RE: Reinforced concrete beam - question about theory
RE: Reinforced concrete beam - question about theory
In the case of the beam end connection, I would never recommend neglecting moment on the anchors, since concrete failure in these members is brittle. You should either have a fuse in the connection that limits moment, or detail the connection such that fixity can't be achieved.
RE: Reinforced concrete beam - question about theory
For a single plate shear connection, there may be more than adequate rotational freedom due to short slotted holes, etc.
An end plate connection bolted to a concrete wall...not so much.
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RE: Reinforced concrete beam - question about theory
Depends how you design it. If you designed it such that the anchors required for shear pull out in tension then you're in trouble. Most times in practice you have multiple anchors, and the top anchors might pull out a bit and release the moment leaving the bottom anchors to carry the shear.
I've seen a lot of quite rigid 4-anchor end plates connected to panels and concrete walls. Not a very good detail, yet one of those things people seem to get away with...
RE: Reinforced concrete beam - question about theory
RE: Reinforced concrete beam - question about theory
I consider this to be hugely important. Without meaningful top steel, I think that you'd have to consider your shear capacity as being based on the depth of cover to the bottom steel. So d ~ 2.5 in.
I've seen recommendations indicating that the negative reinforcement should be at least 25-33% of the positive reinforcement no matter what. And I think that's a great idea. I also believe that European codes actually address the amount of negative steel required. I can't recall the details however.
The issues are similar for both steel and concrete. The beam design itself will usually be fine assuming pin endedness and redistribution. It's the vertical supporting elements and the connections to those elements that will be vulnerable under the influence of unintended moments.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Reinforced concrete beam - question about theory
I suppose it's possible.....but for reinforced concrete, it's so against practice I would strongly advise against it.
Either that or I provide what the FEA model indicates or wl2/12. (Whichever of the three is the greatest....just considering vertical loads.) The column stiffness is hard to predict.....and that will control the moments developed.....so you don't want to underestimate it.