6/5 Beam Column Capacity

[EngineerRam]

Hi,

If I have two beams framing into a concrete column for a special moment frame, one beam is part of the special moment frame and the other is a cantilever beam, do both beam capacities need to be considered when computing the requirements for strong column-weak beam, or only the special moment frame beam?

Thanks.

 

[MrHershey]

Both, at least in my opinion. You also need to count all slab rebar (even throwaway steel) within the effective T-beam width. And any post-tensioning you may have as well.

It's about protecting the column, making sure it's not the weak link in the system. So everything has to fail before the column, including non-lateral elements like slab rebar and PT.

 

[JAE]

Wouldn't the cantilever beam just "go along for the ride" and not affect which element fails first - the other beam or column?



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[MrHershey]

I apologize. I totally missed the word 'cantilever'. Changes things quite a bit.

For cantilever I would say no, with a caveat. If the cantilever reinforcement is heavier or in addition to the special moment frame beam reinforcement and developed past the column then I would make sure to count that in your beam capacity that the column has to be designed for.

 

[KootK]

In keeping with the capacity design philosophy, I think that it rationally ought to be:

Column strength > 6/5 x (SMF Beam Strength + Cantilever Gravity Moment Present @ EQ)

That cantilever moment will be present and drawing on the column capacity when the SMF beam hinges.

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.

 

[JoshPlumSE]

I'm with KootK here. I might make a slight caveat that there is some engineering judgment as to what the "cantilever gravity moment" is during the seismic event.

Is it 1.2*DL + 1.6*LL

or, is it (1.2+0.2*Sds)*DL + 1.6*LL?

Or, some reduced factor for LL (1.0 or lower)

 

[EngineerRam]

Thanks all. In conclusion, I analyzed the joint for 6/5 * (capacity of the moment frame beam + the worst case seismic load combo moment present at the connection of the cantilever beam).