Steel percentage of reinforced concrete beam

[gotlboys]

Hi everyone!

I am designing a reinforced concrete beam where the required steel percentage is greater than the allowable max steel percentage.
The beam size has been predetermined by the architectural demand which carries a larger moment causing the steel required to go over max allowable steel. I remember my professor once said that if this case happens, then use the max allowable steel to compute for steel area to be furnished. The idea is to prevent over-designing in the member for this will cause unnoticeable or sudden collapse. But my doubt is that the beam may not be able to withstand the applied moment because the steel area required has been lower.

Any input and reference to help enlighten this issue.

 

[hokie66]

It sounds like you are on the right track, but you should get some help with this from an experienced engineer.

Too much steel is not good for the reason you described. Too little steel is not good because the strength is too low. So that leaves making the beam larger. You can't always satisfy architects.

 

[KootK]

Your professor's suggested approach sounds terrible to me. You simply wouldn't have enough capacity. If you use ACI 318, you can over reinforce and accept a lower phi value. Otherwise you can supply compression steel to bring the beam back to an under reinforced condition.

You probably do need a larger beam and I imagine that your deflection estimates will bear that out. Are you making use of T-beam action? That can help.

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.

 

[spats]

Agree with KootK... use compression steel to reduce the concrete compressive stress, thus allowing more tension steel to be used. But as KootK says, you probably will have deflection problems.

 

[sponton]

Agree 100% with KootK

 

[JAE]

Yes - use compression steel.

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[gotlboys]

Thanks a lot for giving your valuable inputs. I forgot to mention that he also suggested to follow doubly reinforced procedure which is exactly what everyone has recommended. I went through my professor's handout, it recommends using rho max or 0.9rho max for As1 to get the trial capacity of the section M1, then compute trial As2 = As' (assume still yields; check it later on). If both steel in compression and tension yield, so total steel in tension is As = As1+As'. The use of rho max in this handout is similar to the recommendation given in reinforced concrete design by Jack C. McCormac, however, I have other books which suggest rho = 0.85 f'c B 0.003 / fy (0.005+0.003) for As1, this is to keep phi = 0.9 (0.005 is in the tension-controlled zone) in computing for the capacity. Jack also explains that compression reinforcement can reduce deflection.
Based on your respective experience, which rho value would you like to use to compute for As1?

T-beam is another option to overcome this issue, but personally I am not really convinced to accept the reality that T-beam is safe in the long run for moment resisting frame of a bldg, instead I prefer doubly reinforced rectangular section.

 

[ahmedhegazi]

Well Normally when you use compression reinforcement to increase tension , you have to make sure many other factors are satisfied,such as

long term deflection.
clear spacing between bars.
congestion of reinforcement at column


better to have a look on possibility of making the beam wider

 

[podosci]

The architect Primary Objective is to make the Building Aesthetically Pleasing, Your Job is to make the building structurally sound....In Limit State design Structural Soundness supercedes Aesthetics. Make the building sound even if it means tweaking the architectural plan a bit in consultation with the architect