reinforcing steel beams

[bjb]

I'm looking for a reality check on my procedure for deisgning reinforcement for in-place steel beams. My method was recently questioned, and I would like to know what others think.

First of all, for a roof beam I specify that the reinforcing be done when there is no snow on the roof, so I calculate the existing dead load stress in the unreinforced section. I then calculate the live load stress (or snow load stress) based on the transformed composite section. I then check to make sure the sum of these two stresses are equal to or less than the allowables in the ASD manual. In equation format, f=(Md/Ss)+(Ml/Sreinforced)<=Fb, where Ss is the section modulus of the unreinforced steel beam. I think that this is probably conservative because you could probabaly get some economy by using an LRFD approach.

Your comments are greatly appreciated.

 

[ishvaaag]

I think is OK. It is conservative not only in maybe LRFD as a code giving maybe a lesser size, but in that giving the plasticity of the shapes very likely you may count on the full plastic moment of the combined section (if properly braced and local buckling is enough prevented). The elastic way you add the stresses prevents the economies of the whole section becoming plastic.

 

[dik]

Treating it elastically as you have done is conservative. At limiting conditions, the added steel (assuming no instability) will be stressed to yield and not some value less than fy...

 

[JAE]

Your method is exactly what I use (when using ASD - which is rare).

Be sure that your Fb is based on the new, strengthened cross section and not the original section.

 

[bjb]

JAE, I typically do not usr LRFD, but I can see the writing on the wall. When designing this reinforcement using LRFD, do you make sure that the nominal plastic moment capacity is greater than the applied factored moment? This would imply that the ultimate strength is independent of the dead load stress. What procedure do you use for doing this in LRFD? Which method, LRFD or ASD is more labor intensive for the design engineer?

 

[JAE]

For LRFD, you are dealing with an ultimate condition so there is no consideration of dead load stress on the section prior to strengthening. I simply calculate my section properties of the improved beam and apply LRFD rules to the design check.

This is similar to the situation in composite beams. The basis of composite design is ultimate strength (even when you are using ASD as the ASD composite specs are based on ultimate). So your shoring condition does not alter the final strength of the composite section. Similarly in LRFD with beams strengthened with plates, the ultimate strength is the same regardless of prior stress on the section.

As far as labor comparisons between LRFD and ASD - that's a can of worms that I'd rather not open. I &quot;grew up&quot; on ASD but then made the decision a few years ago to switch...not because I preferred LRFD, but because my staff were all trained in LRFD and I wanted to be able to teach, monitor, check, etc. my dept's. work. Yes, LRFD is a bit more work, but that is due to the more up-to-date specs for LRFD that required more.

Recent reports are that ASD would also be a lot more effort if it had been brought up to date with LRFD specs. In fact, the next AISC spec will be a combined ASD/LRFD spec where the designer can choose. And the word is that the ASD provisions will be much more in line with the time that LRFD takes due to updating the ASD provisions.