analysis of beams 6

[aladdin76]

After analyzing a concrete roof slab , some of what appear as simply supported beams ( single span beam supported at ends by main beams ) the negative moments were much higher than the mid span moment ) , That happen because all connections between beams were assumed to be rigid , shall I design the beams with the high negative moments or release torsion at the connections
Furthermore , in BS EN design code it is clearly stated that the negative moments can be redistributed up to 30% , is there any thing similar in the ACI

 

[Enhineyero]

aladdin76 - have you analyzed it by FEM (computer program) or by hand calculations? Is this really a single span beam or a continuous beam? it is highly unlikely for sub-beams (supported by girders/beams) to have a high end moments. unless the supporting beam is very large and rigid.

for fem program like staad,etabs,risa, etc. I would usually assign an end moment release on the sub-beam and modify (decrease) the torsional stiffness of the main beam/girder (if the sub-beam is continuous).

 

[BAretired]

Provide a framing plan. Otherwise we are groping in the dark.

BA

 

[aladdin76]

I am using staad for the analysis and it is a single span beam , I thought of releasing torsion at the ends, but in reality the beams are cast together.

 

[MiketheEngineer]

Check it both ways - use worst case..

 

[msquared48]

A negative end moment is different than torsion. Which do you have? Or do you have both?

Frankly, I do not understand how you could have torsion from what you describe.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

In re-thinking this...

If there is sufficient reinforcing and embedment in the simple beams to develop the negative momentat the girder, and the girder is large enough to develop it (I guess this is where the torsion comes into play here) then, yes, design for the negative moment and torsional forces.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[aladdin76]

It is clear that torsion is different from negative moment , what I meant is that the negative moment in the secondary beam caused by the torsion in the main beams , any way thank you all for your help.

 

[BAretired]

I believe some conservatism is warranted here. I would be inclined to provide enough positive reinforcement for a simple span but enough negative reinforcement for, say fifty percent of fixed end moment. For a uniformly distributed load, that would be wl²/8 and wl²/24 for positive and negative moment respectively.

BA

 

[JAE]

I've done similar to what BAretired suggests. We assume that the end of the interior, orthogonal joists/beams are simple ends and this kicks up the positive bending moment as well as subsequent moments (negative and positive) further into the building. Then we would add a supplemental amount of top reinforcement on the exterior end span to keep cracking down.

The problem with assuming the end is fully fixed is that it is never fully fixed - the exterior spandrel beam will rotate to some extent and if the torsional restraint is compromised by cracking, you would approach a simple end anyway.

ACI actually deals with this in the design of perimeter beams for torsion as they allow a reduced torsional design if the interior beams/joists are designed as though they were simple ends. (see 11.6.2.2 in ACI 318-05 or 11.5.2.2 in ACI 318-11).

 

[hokie66]

Good advice from BAretired and JAE. This is a case where the computer output should be ignored, and engineering judgment applied.

 

[TLHS]

It depends how you design the rest of it and where/if it's okay to crack. If you design the beam as fixed then it's the strong point and if your girder isn't designed for the torsion you might get unacceptable rotation or some cracking. If you design the beam as simply supported then the weak point may be the beam to girder connection and you might get cracking there to release the moment. If you design everything for everything then you'll get less movement or cracking but it's also more expensive!

I don't do a lot of concrete, so I may not be the best one to ask, but this seems like a situation where most answers are reasonable as long as you're aware of the consequences and are okay with them. Your structure will likely end up acting to match whichever assumption you designed to.

 

[msquared48]

I would con a well known phrase here - "If you build it, they will come."

You can tell the structure how you want it to behave by the reinforcing you provide. Just hope that it does not have a mind of its own...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com