Beams in SCBF

[BAGW]

Hi,

I have a question regarding multi-tier SCBF. I am designing a storage building 200’ in SDC D region. The building is regular in plan and area inside the building is all open. The framing I have is just perimeter framing and roof framing. The roof is a simple truss system. There are no other floor levels except the roof. The braces are on the perimeter in both directions. 200’ building is divided into five tiers so that I will have beams at every 40’ along the entire perimeter which helps in reducing the column unbraced length and I can have multi-tier X brace. Brace frames into the beam column intersection at every tier.

My question is do I have to design the beams based on ‘mechanism Analysis’? These are not really beams, they are more like struts. They don’t support any gravity loads. I am planning to design these members for over-strength combination rather than ‘Mechanism Analysis’. This ensures that the weak point is still in the brace.

Does anyone see any issue with this approach?

Thanks

 

[Deker]

Mechanism analysis is required (AISC 341-16 F2.3) along with a few other checks that are unique to multi-tiered systems (AISC 341-16 F2.4e).

 

[BAGW]

@Deker

There is an exception for column for not performing Mechanism Analysis in Section F2. Exception (2)a. Can I apply the same concept to struts as well?

Does the code specifically mention Mechanism Analysis is required for struts? Just to point out again, the brace intersects struts at strut column intersection.

 

[BAGW]

Section F2. Exception (2)a as per AISC 341-10. Required strength of columns does not exceed the forces determined using load combinations stipulated by the applicable building code including the amplified seismic load applied to the building frame model in which all compression braces have been removed.

 

[Deker]

The exception you refer to has been removed from AISC 341-16 in favor of explicitly performing a mechanism analysis. See the commentary for F2.3 for additional information. I recommend using 341-16 rather than 341-10 since multi-tiered braced frames are not addressed in the latter. Mechanism analysis is required to design columns, beams, struts and connections per F2.3.