SCBF bracings gravity loads AISC-341-10

[Daniel_8888]

Good Day to everyone!
I'm new on this forum. I'm a structural engineer from Poland and I use AISC-341-10 to design a structure for high seismic region.
Due to the reason of huge seismic base shear force I need to design my structure as a Special Concentrically Braced (SCBF).

AISC 341-10 states:
"Braces shall be determined to be in compression or tension neglecting the effects of gravity loads."

I kindly ask for help me to unerstand. The braces are not designed for any gravity loads? Only for horizontal load from seismic?
Is it means that my braces can be overloaded during the state of gravity loads (without any seismic or wind) and I'm sure if I use slederness requirements they will.

Thank you for answers and discussion in advance!

 

[sandman21]

No, the gravity loads are transferred to the surrounding elements (beams, columns, etc.) at low drift levels. Effectively the braces will not see the loading from the gravity. This is not to say that it cant happen at transfer columns, girder with large live loads etc..

 

[ARKeng]

I take that to mean that when you're "determining" (deciding) if a brace is in tension or compression, you ignore the gravity loads and only consider the seismic. That's mostly just deciding it in your head I believe for deciding which provisions apply... For example, if the brace is in tension, you're likely to have to design the connection for the full tensile capacity of the brace.

When it comes to the design equations, you have to consider all the load combinations including the gravity loads. I think the quoted statement is included so you can't say "my braces aren't ever in tension because they have big compression loads on them from gravity" and get out of the provisions for braces in tension.

As sandman21 seems to be referencing, I do believe there is a provision also that says you also have to design your beams and columns for the gravity load combinations assuming the braces aren't there.

 

[JLNJ]

Are you asking about a chevron (inverted "V") type arrangement? These are a special case.

 

[sandman21]

No, the braces are sized without the consideration of the gravity loading. The section specifically wants you to ignore the effects of gravity. If you include the gravity loads you are effectively over-sizing the braces. Page 5 of this PDF its on BRBF but the principals are the same. Both AISC Seismic manuals design the brace this way it does not appear that the manual examples for SCBF have been updated.

 

[Daniel_8888]

Thank you all for your answers!

@JLNJ, @ARKeng
Yes, I'm asking about a chevron inverted-V braces. In my opinion such braces will transmit gravity forces also during the earthquake. The columns between will shorten and forces will occur.
When I have inverted V-braces and high level of dead load, the problem with dissipative elements like braces is that I can not design them too strong. If I do this, the effect of seismic energy dissipation will be very low.

I'm interested how do you calculate SCBF in practice (in the software design). Do you make two caclulation models, one for gravity and one for lateral forces? I use Autodesk Robot and I see it is not a perfect software for seismic structural systems, but I have no other way.