Why should steel fail(yield) first before the concrete in RC beam? 2

[jnlbctln]

1. Could someone kindly explain why steel should yield first before concrete fails?
2. Let's say on the tension side of a beam, how would the steel fails first before the concrete knowing that the steel is basically the one that support the tensile stress? Isn't it that the concrete would crack/crash first before the tension rebars yield?

 

[JStephen]

I understood the intent was that the beam didn't go "Kapow" and fail suddenly and unexpectly.

 

[asixth]

Because the steel will yeild and deform a fair bit before rupture, I think the reo strain will go to 10% or something like that. You will see large deformation of the beam prior to failure.

 

[hokie66]

JStephen and asixth are correct, but the steel doesn't yield before the concrete cracks.

 

[jnlbctln]

@hokie66, if steel does not yield before concrete cracks/crushes (in other words: steel yields after concrete cracks/crushes) then it follows that concrete fails first ? hmmm? ( P.S. Did I used the words crack and crush correctly? )

 

[kingnero]

I always thought concrete cracking != failing, but I'm not the expert here. Surely someone will elaborate on the different stages before actual failing... ?

 

[IDS]

The point is that the steel should yield before the concrete fails in compression (or if it doesn't you should use a smaller capacity reduction factor).

The steel should not yield before the concrete cracks in tension; if it does there is a possibility that the beam will develop sufficient momentum to cause immediate failure.

So the sequence is:
- Concrete in tension cracks; transfer of stress to the steel with large increase in steel strain, but still within the elastic range.
- Steel starts to yield, large increase in deformations, loads are transferred elsewhere.
- Either steel rupture or concrete crushing; sudden failure.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[civeng80]

This was absolutely pounded in us at school and I still remember after 30 years.
Because you can get a ductile failure (desired) as opposed to a brittle failure (not desired).

 

[hokie66]

Doug is correct. Concrete is strong in compression, weak in tension. So we generally don't depend on concrete in tension, and cracking is not considered 'failing'. Crushing, on the other hand, does amount to failure. In bending elements, we want the ultimate failure mechanism to be yielding of steel rather than crushing of concrete, as that is a much more ductile failure mode.

 

[jnlbctln]

Thanks IDS! you verified the thoughts running in my head right now! :clap:

 

[Enhineyero]

Doug - is there an instance where steel would yield 1st before the concrete cracks in tension? the only instance i can think of this to happen is if the steel is weaker than concrete (fy < fc'), still the lowest grade of steel around 200+ mPa is stronger than the strongest concrete grade I have encountered which is around 50mPa. I also believe steel will have a very small stress if concrete in uncracked.

 

[TXStructural]

It is not a function of strength, it is function of strain, and the existence of microscopic cracks formed at early age. Concrete will crack under tension at a strain much lower than the strain in the steel when it yields. ASTM A615 steel generally elongates about 0.3-0.5% before yielding begins. In an under-reinforced member, the steel may yield and quickly rupture if there is not sufficient steel crossing any cracks as they develop suddenly.

I hope the OP is a student, because a practicing structural engineer who does not understand the basis for designing for yield of reinforcement has no business designing concrete. This would be like designing steel without understanding LTB. The entire code is developed around this concept. Phi factors for compression failure are significantly lower to prevent the design of members subject to compression failure without substantial margins.

 

[IDS]

Doug - is there an instance where steel would yield 1st before the concrete cracks in tension? the only instance i can think of this to happen is if the steel is weaker than concrete (fy < fc'), still the lowest grade of steel around 200+ mPa is stronger than the strongest concrete grade I have encountered which is around 50mPa. I also believe steel will have a very small stress if concrete in uncracked.

What I said was misleading. At the point where the concrete cracks the strain in the steel will be much less than the yield strain, but after cracking the steel strain increases until the moment provide by the steel tension is equal to the moment previously supplied by the now cracked concrete. What I meant to say was that the moment capacity of the section ignoring concrete in tension should be greater than the cracking moment, so that after the concrete has cracked the steel is still well below its yield stress.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/