Seismic Moment Frames

[LPPE]

I have moment frames for a multi-story building where seismic controls the design, not wind force.

AISC ASD 9th Ed. on page 4-100 states "Many moment connections can be designed to resist wind moments only and, at the same time, to rotate sufficiently to accomodate the simple beam gravity rotation as a Type 2 (simple) connection."

My moment frames are part of a storage mezzanine with a gravity live load of 250 psf. Using composite construction, the beams are OK for simply supported gravity loads. When I combine the seismic loading plus the gravity loading and fix the beams at the columns, my columns and beams are blown out of the water.

AISC allows for designing the moment connection for wind only and the beams as simply supported. Does anyone know of any provision like that for seismic loading?

 

[AlohaBob]

When I read Page 5-25, I see that the additional design restraint at those joints for frame action for wind load, are considered seperately from the design of those joints designed for gravity load simple support. If the rigid joint also carries gravity load, it must be designed for that too. Then the fasteners in the simple joint must be checked for the effects of frame action also.

 

[SacreBleu]

Assuming that the mezzanine beams are not part of the MRF, can you design the beam to column connection as a double angle, and take advantage of its inherent flexibility to reduce the moment at the beam-column?

 

[AlohaBob]

Yes.

That's the intent of the Code. So long as you allow for displacements in the simple connections, you fully design for the lateral system at the fixed joints.