SCBF bracings

[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 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 in advance!

 

[KootK]

I'll look into this if you save me some time and provide the clause number for your quote.

 

[Toby43]

I've not designed a SCBF, but as I understand it that requirement is to because the braces would lose their ability to support gravity loads post-elastic. Thus you must ensure that the gravity loads are appropriately distributed to the columns, something which may not happen if using typical software program that runs an elastic analysis to combines gravity and lateral loads. You would need to perform a separate plastic mechanism analysis aswell, to appropriately capture the overstrength demand etc...
Search the internet for NEHRP Seismic design technical brief No. 8 "Seismic design of steel special concentrically braced frame systems" published by NIST. Short document that should point you in the right direction.

Toby

 

[Daniel_8888]

Thank's Toby43 for your advice. Of course I will check the NEHRP provisions. If I good understand the braces will work elastic until the plastic hinge occur. Braces are loaded gravity and lateral (there is no possible to divide those loads in real structure), that is a question from which type of load the plastic hinge will occur first...
I use Robot Structural Analysis Software (RSA) and you have right that it is not possible to calculate it correctly in elastic phase. In the RSA I can do a pushover analysis to check the level of plastic strength of structure, but I still can not properly design members.

 

[structSU10]

As I understand it, that provision is for the design check at expected brace strength levels. You still perform a design check for the typical building code loading, including the effects of gravity on the brace, but when you check the system at full expected strength, the effects of gravity are neglected.

 

[sandman21]

Its section AISC F2.3

Copying my response from the other thread:

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..