Unbalanced forces in SCBF beam design
Unbalanced forces in SCBF beam design
(OP)
Hi
If I have a one bay chevron brace to resist lateral forces and if the brace sizes are different above and below the beam, there will be unbalanced forces which needs to be accounted while designing beam. Irrespective of where the braced frame is located on plan, can I distribute unbalanced forces 50/50 for the mechanism design of the beam?
Thanks
If I have a one bay chevron brace to resist lateral forces and if the brace sizes are different above and below the beam, there will be unbalanced forces which needs to be accounted while designing beam. Irrespective of where the braced frame is located on plan, can I distribute unbalanced forces 50/50 for the mechanism design of the beam?
Thanks






RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
I wouldn't think so.
Sounds like a decent argument for keeping the brace sizes the same above and below.
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.
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
I'd like to elaborate on this a bit more for BAGW. I believe KootK is suggesting to use the same brace every two levels (above and below where they intersect at midspan of the beam) as opposed to using the same brace all the way up the building. One of the reasons you don't want to use the same brace all the way up the building is that it exacerbates the potential to form soft/weak story mechanisms in the lower levels by providing too much overstrength in the upper level braces.
Not to get too far off topic, but one of the more innovative strategies I have seen for mitigating this is a hybrid braced frame that incorporates a strong-back truss, wall, etc. The strong-back is designed to remain elastic and serves to distribute drifts more uniformly up the building. Some additional info in the links below.
Link #1
Link #2 (Page 525)
Link #3 (Page 297)
RE: Unbalanced forces in SCBF beam design
Indeed. Thanks for the elaboration. In retrospect, my response was a bit spartan.
The strong back truss is a neat concept. Going all the way back to Paulay and Priestley, that was touted as one of the benefits of shear walls used on combination with moment frames and other systems. Most of the way up the building, you get pretty consistent drift over groups of floors.
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.
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
1) Design them for the normal collector loads calculated without giving consideration to the brace mechanism formation.
2) Design them again for a collector load consistent with the mechanism imbalance and distributed as the collector loads were in #1.
I believe that I've seen some stuff in the past indicating that #2 was optional and not code mandated.
It's a sometimes frustrating feature of this kind of work that you are often not working with a set of forces that is consistent throughout the structure. Rather, certainl load/design cases are really set up to tax things locally in some instances. Here, I believe that the brace mechanism imbalance was really set up to tax the connections and frame beam vertically, in a suitably conservative way given non-simultaneous load plateauing in the T&C braces. That case may well be too punitive for the collectors outside of the braced frame.
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.
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
RE: Unbalanced forces in SCBF beam design
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.