Reinforced concrete beam - question about theory

[breaking_point]

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.

 

[canwesteng]

The concrete beam will crack, greatly reducing stiffness at the supports, if there is no reinforcing steel. This can be unsightly and reduce durability, but assuming low seismicity and no fatigue loading it shouldn't be an issue. Same with the steel beam. In both cases make sure you have a solid load path to get load from the beam to the column (maybe a quick strut and tie sketch to make sure it works).

 

[JAE]

I'd get a bit "queezy" about allowing a concrete beam to crack open without restraint near the support due to lack of top steel....possibly affecting your shear strength at the column.
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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[Archie264]

It takes rather deliberate effort to make a steel connection a true moment connection. Are you sure that's what you'll have? On the other hand, if you are stiffening a steel connection it could bring about some of the issues JAE mentioned, so that should be taken into consideration.

 

[JAE]

Archie264 is right. Force always follows stiffness so if you have a "stiff" non-moment designed connection it will attract the moment until it either yields or fractures.

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[jike]

I recommend putting top steel in properly anchored to the column and extended past the point of inflection per code. If you want to avoid analysis for the top steel, you could conservatively use wl^2/12 (fixed end moment) and inflection point based upon a fixed end beam. If you like you can put in bottom steel based upon the simple span analysis since it would be conservative.

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.

 

[breaking_point]

Hey everyone. Thanks for the comments. Just to clarify, I'm not actually designing a member, it's just a thought that has been in my head and I wanted to discuss it with you all.

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

 

[breaking_point]

@canwesteng, As you said, the cracked concrete would reduced the stiffness of the connection, so would the beam begin to behave more like a pinned connection? (I understand in reality there is no such thing as a 'pinned' connection, as connections are always some form of semi-rigid).

 

[canwesteng]

The angular deflection at a pinned joint is actually quite small for typical structural applications, so while there is no true pin, the approximation can be fairly close.


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.

 

[JAE]

For a post-installed connection - and the available free rotation of a steel beam attached to that assembly - depends on the configuration of the assembly/connection.

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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[Tomfh]

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).

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...

 

[RahulAtkins]

The beam will crack if there is any moment at the support and you have not provided sufficient rebars inside the beam thorough the columns. You need to develop the rebars too,otherwise the rebars will simply slip from the column as the beam cracks. You can go for the simply supported design if you are only looking to size the beams and getting the reinforcement at the bottom, but for connections and top rebars you should consider the fixity.

 

[KootK]

...crack open without restraint near the support due to lack of top steel....possibly affecting your shear strength at the column.

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.

 

[WARose]

[blue] (breaking_point)[/blue]

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?

I suppose it's possible.....but for reinforced concrete, it's so against practice I would strongly advise against it.

[blue] (KootK) [/blue]

I've seen recommendations indicating that the negative reinforcement should be at least 25-33% of the positive reinforcement no matter what.

Either that or I provide what the FEA model indicates or wl[sup]2[/sup]/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.