# AISC Seismic design manual - SCBF



## tws (Oct 12, 2011)

on p. 3-48, they divide the unbalanced brace force on the beam by 2 saying it is shared equally. (SEAOC does not do this) What do they mean? Is it that 1/2 of beam is in compression and 1/2 in tension? Wouldn't the effective length of the beam-column Lx then be 1/2 L?

Also they do not use the Chevron braces to brace the beam-column for the effective length, Lx. Is this not permitted or not typically done? (SEAOCC example uses braces to brace beam)

thanks in advance


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## Ble_PE (Oct 13, 2011)

The reasoning behind dividing the compression force by 2 is it is assumed that half of that force will be carried by the beam and the other half will go directly into the pin supports at the base. In other words, the brace will transfer half of it's load to each support, which in this case is BM-1 and the base.

As for your question about the effective length of the beam, Section 13.4a.1 of the Seismic Provisions states that the required strength of the beam shall be determined assuming no support from the braces for dead and live load. Is this what you're talking about?


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## tws (Oct 13, 2011)

Thanks for your response.

That makes sense, but then wouldn't you also have to consider the horizontal force from the braces from the floor above the beam? (2nd floor braces - 1/2 goes to 3rd floor beam and 1/2 to 2nd floor beam)

This brings up another point:

What if the first floor braces were a "V" configuation (as opposed to an inverted "V")? Then the second floor beam is not intersected by braces and thus section 13.4 would not apply to that beam. So the beam wouln't be designed for the unbalanced force unless you're splitting braces forces as described above.

Also similarily, if you have a single story "V" braced frame the beam would not have to be designed for the unbalanced horizontal load per AISC 341 (although it seems it should?).

My second question about the effective length of the beam: SEAOCC design example uses 1/2 length of beam for Lx (effective length for axial force in beam). See p. 53. It seems they are using the chevron braces as a lateral support point for the unbalanced horizontal load. This seems incorrect; 1 brace is buckled, 1 is in tension.


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## Ble_PE (Oct 14, 2011)

tws said:


> Thanks for your response.
> 
> That makes sense, but then wouldn't you also have to consider the horizontal force from the braces from the floor above the beam? (2nd floor braces - 1/2 goes to 3rd floor beam and 1/2 to 2nd floor beam)
> 
> ...


The load from the braces that frame into the beam-column connection will go directly into the column, so the beam will not receive the axial load from that brace (see cxn design sketch on pg 3-66). I believe if you were to have a one story V-braced frame then the beam would not need to be designed for an unbalanced load. It's my understanding that you design the beam for the unbalanced load when there is a brace framing into the middle of the beam. Someone correct me if I'm wrong.

I don't have the SEAOCC book, so I can't see the example problem, but I don't think you can consider the brace as bracing the beam for the reasons that you stated.


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## McEngr (Oct 17, 2011)

Just a sidenote: of you look at page 3-51 of the AISC Seismic Design Manual, you'll find that even doubling the axial load on the beam results in a negligible difference with second order effects. I'm not sure if I understand or agree with the half load, but I think it also is a difference of being a floor-level beam vs. a roof-level beam in the SEAOC manual.


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## McEngr (Oct 17, 2011)

I, for one, think there are much bigger errors such as why they chose to use B1 with Mlt in lieu of B2 with Mlt on page 3-51. Am I missing something? This seems like an obvious error.


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## tws (Oct 17, 2011)

Ble_PE said:


> tws said:
> 
> 
> > Thanks for your response.
> ...


What is "cxn"? The column can not resist the load. The load needs to travel from braces above through beam to braces below. Let me know if I am missing something.



McEngr said:


> I, for one, think there are much bigger errors such as why they chose to use B1 with Mlt in lieu of B2 with Mlt on page 3-51. Am I missing something? This seems like an obvious error.


B2 = 1 per p. 3-50. On p. 3-51 they say Mr ~ B1Mu. But for an exact answer, yes it should be Mnt*B1.


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## Ble_PE (Oct 17, 2011)

tws said:


> What is "cxn"? The column can not resist the load. The load needs to travel from braces above through beam to braces below. Let me know if I am missing something.


Ok, ignore my previous answer as it was wrong (my excuse is that it was Friday and I was thinking about having a beer!). I've done some reading and I think I've figured it out. First of all, cxn is simply an abbreviation for connection. I still stand by my reasoning as to why half of the axial load is added to the beam since the other half goes into the beam or foundation below. The reason you don't see the axial load from the brace above is assuming you have the same size brace, the loads would cancel out (I think). Take a look at Figure 3-8 on pg 3-37. If you resolve the forces from the braces you should see that they all cancel. Does that make sense? My only experience with SCBF's comes from studying for the SE exam in April, so take my opinions with a grain of salt. Just trying to help out anyway I can.


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## McEngr (Oct 17, 2011)

page 25 of SEOAC Vol 3 also shows a different case. I agree with BLE - the forces cancel each other out above and below.


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## tws (Oct 17, 2011)

The forces are opposite in the SEAOCC example. They are not in AISC example with stacked chevron braces. See p. 3-37 AISC.


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## McEngr (Oct 18, 2011)

SEAOC also has a good discussion about this on page 25 as well as others. The discussion rests on whether designing to the failure of the brace on both levels actually makes sense. SEAOC does the 4th floor level (the top floor just under the roof beam) with 1/2 the upper and all of the lower. All of these seem like approximations and I'm not convinced that the AISC 341 is really a good specification for seismic design. There seem to be lots of holes of interpretation, which in my opinion is difficult to justify to a client.


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## McEngr (Oct 18, 2011)

After reading page 25 again, I think that the point of the yield mechanism is based on the dynamic analysis of the 2nd or 3rd mode of vibration, which is beyond the text of the SEAOC Vol III. I wish I had a better understanding of vibration analysis, which is probably necessary to truly understand the full aspect of seismic design on high-rise structures. All my knowledge is bachelor's level structural and about 9 hours worth of additional steel, concrete, prestress in grad school. On the west coast especially, I think a vibration analysis is probably necessary to truly understand the "why" not just the "how".


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## McEngr (Oct 19, 2011)

Does anyone know of a good resource that discusses failure modes and how to identify the loads to various columns/beams in the SEAOC volume iii steel lateral force resisting systems? I don't quite understand the 1/2 load to each member in all cases - it seems that all braces would have to fail at the same time in order for the 1/2 load to work (as BLE stated).

Thanks to anyone who can help!


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## tws (Oct 21, 2011)

McEngr said:


> SEAOC also has a good discussion about this on page 25 as well as others. The discussion rests on whether designing to the failure of the brace on both levels actually makes sense. SEAOC does the 4th floor level (the top floor just under the roof beam) with 1/2 the upper and all of the lower. All of these seem like approximations and I'm not convinced that the AISC 341 is really a good specification for seismic design. There seem to be lots of holes of interpretation, which in my opinion is difficult to justify to a client.



This makes sense, AISC is assuming the braces above the second floor do not buckle.


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