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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

RE: Unbalanced forces in SCBF beam design

(OP)

RE: Unbalanced forces in SCBF beam design

Quote (BAGW)

Irrespective of where the braced frame is located on plan, can I distribute unbalanced forces 50/50 for the mechanism design of the beam?

I wouldn't think so.

Quote (BAGW)

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.

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

Is the frame a special concentric brace frame or buckling restrained braced frame? Are you talking about forces coming into the frame from a floor or loads within the frame?

RE: Unbalanced forces in SCBF beam design

(OP)
Its a SCBF. I am talking about the loads within the frame (due to mechanism)

RE: Unbalanced forces in SCBF beam design

(OP)
if I dont apply the P/2 unbalanced force at two ends of the beam, the forces will not balance out. So I think thats the only way to distribute the unbalanced forces irrespective of the brace location

RE: Unbalanced forces in SCBF beam design

I'm assuming that you have diaphragms delivering the load to the braces at each level and are not designing a multi-tiered frame (otherwise your braces would be the same above and below the beam). A sensible way to distribute the horizontal force is to divide it by the length of the collector that delivers load to the frame. Then design your frame beam for the load it sees as a collector. If your frame is offset all the way to one side of the building, the beam would essentially see 100% of the force. This video describes the method in detail (Link). Note that the collector force is generally controlled by the mechanism analysis that produces the maximum tension and compression in the braces simultaneously, not the analysis that produces the unbalanced force from post-buckling strength loss of the compression brace.

Quote (KootK)

Sounds like a decent argument for keeping the brace sizes the same above and below.

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

Quote (Deker)

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.

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

(OP)
Thanks for the explanation Deker. Even if one has a multi-tiered brace framing, like in a warehouse, the diaphragm and mass is only at the roof level. So all the bracings will be similar size. Still there will be unbalanced forces at the roof right, because there is only one set of bracings on the underside of the roof beams. So do you still calculate the forces in the beams as a collector? With the number of bays of bracing being increased, the unbalanced forces increases. This might result in huge collector forces at the beams connecting at the start of the braced bay frame.

RE: Unbalanced forces in SCBF beam design

It's complicated stuff and I haven't watched Deker's video yet so I may be in conflict with it. However, I think this would be rational for the design of the collector beams:

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

AISC 341-16 was provisions for multi-tiered brace framing, they also have a nice commentary on the design, 9.1-261. It shows all the modes and forces you are required to check. Deker's video will go over the forces in a typical building application. AISC had a continuing education webinar on Video Link if video does not work, it should give you more insight into the design.

RE: Unbalanced forces in SCBF beam design

(OP)
I am concerned about the roof beam design where braces are just one side of the beam. Unbalanced forces are so high as I have 4 bays of bracing. Any way to get around this. Not deigning the roof beams for unbalanced forces?

RE: Unbalanced forces in SCBF beam design

Whatever load you go with at the brace node, it has to get there through some sensible path. In my mind, that sensible path is assuming a delivery scheme consistent with where your seismic masses reside. So no, I don't believe that you can get away from having the frame loaded asymmetrically.

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.

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