SHEAR STRENGTH OF CONCRETE BEAMS PROVIDED BY REINFORCEMENT

[tbone73]

Fellow structural engineers, I know the code limits the contribution of shear reinforcement to shear strength to 8 SQRT f'c bw d. But I'm interested in hearing your personal maximums, your engineering judgments on the matter. What do you personally prefer to limit the shear contribution to before you consider increasing concrete strength or beam size? Thank you in advance for your time and input.

 

[pittguy12]

I never thought much about increasing from this limit. To me, that was always the maximum.

 

[tbone73]

pittguy12, I didn't intend to suggest going above this limit, rather I was asking for other engineers' personal limit, if they prefer using a lower limit.

 

[frv]

I've never been really close, but I'd have no problem going very close to the maximum.

The reason being that the shear equation tends to be conservative to begin with. Add to it the fact that most engineers are fairly conservative with loading, and the fact that shear strength of concrete is proportional to sqrt of f'c and f'c in the field will usually be quite a bit higher than specified, you'll have extra layers of "feel good".

 

[BAretired]

I don't recall ever getting anywhere near that high a value. Probably half of that, i.e. 4*SQRT(f'c)*bw*d, would be a comfortable limit for most situations.

There could be exceptions, but ordinarily, I would increase the beam dimensions unless forced to do otherwise by architectural or other constraints. In such a case, stirrup spacing could become an issue for placement of concrete.

BA

 

[hokie66]

Agree with BAretired. The sizes of concrete beams in a building are usually rationalized for simplicity, and it would be a rare case where that shear limit is approached. When exceeding half that limit, the ACI Code requires reducing the maximum spacing by half. That provision may affect congestion further, but as I haven't needed to use it, I don't know.

 

[asixth]

What about transfer beams? I had a look at a transfer beam which had an average V/(bw*d) of 4N/mm^2. This was 40MPa concrete was is getting close to Vu.max.

 

[asixth]

I guess to answer your question I wouldn't want to go past 0.1*f'c for typical concrete grades used for slabs in the present day.

 

[pittguy12]

AH..Ok.

BARetired has my thoughts also.

 

[Teguci]

Yup - designed right to the maximum no problems. As I understand it, the limit is imposed because the shear compression strut through the concrete maxes out at around 32 rt f'c for steeply inclined angles in confined concrete(ie closely spaced stirrups near supports). However, without further consideration of the beam confinement, a 10 rt f'c is more appropriate as noted in CRSI's pile cap design guide.

Yeah concrete is cheap and stirrups are expensive, but when you have to transfer a large load with limited space, triple #6 stirrups come in handy. This limit also applies for torsion beams which I've designed a couple of too.

I don't understand why we would limit ourselves to less than the allowed unless we had additional information to contradict the testing and immense amount of research already expended on this issue.

 

[ron9876]

I agree with Teguci.

There are situations where you can't increase the size of a beam for architectural reasons. I have designed max'ed out beams several times. My only real concern is if the people building the structure will get the correct steel in the beams.

 

[tbone73]

Thank you all for your valuable input. Teguci you make a good point.