Intermediate Moment Frame Connection

[RDMESH]

Intermediate Moment connection requirements.
- I have project where the intermediate moment frame is not braced at the bottom flange as per the AISC requirement near the support. The top of beam is braced by the concrete planks. My question is if the bottom flange of beam is not braced is there other design requirements i could verify to see if the connection design is still acceptable. Any suggestions would be appreciated.

Thanks,

 

[Deker]

Could you tell us more about why you can't provide bracing? Many of the prequalified connections in AISC 358 allow the plastic hinge bracing to be ommitted where the beam supports a concrete structural slab that is connected to the beam with welded shear connectors at 12" o.c. You may be able to detail the plank to beam connection in a way that provides a convincing torsional stabilizing effect.

 

[RDMESH]

I received the steel shop drawings after contractor started erecting the steel on the site. These location has high ceiling and i could not add angle brace below the beam. Can the concrete plank be treated as structural slab? I was thinking of adding angle braces on both sides and beam to conform the torsional requirement but this will be adding lot of angles and still not convinced with if this will be effective.

 

[Deker]

Can the concrete plank be treated as structural slab?

Whether you can rely on the planks as a structural slab that torsionally braces the beam depends on how they are connected to the beam. If the beam is detailed for composite behavior with shear studs spaced at 12" o.c., then possibly. If not, you will need to add discrete bracing.

These location has high ceiling and i could not add angle brace below the beam.

I was thinking of adding angle braces on both sides and beam to conform the torsional requirement but this will be adding lot of angles and still not convinced with if this will be effective.

So the bottom flange extends below the ceiling and the bracing needs to be kept above the ceiling, is that correct? You could add bracing beams above the ceiling connected to full-depth stiffeners in the frame beam. The stiffeners would need to be designed as cantilevers to carry the bottom flange brace force up to the bracing beam connection. Design examples are discussed here: Link.

I'm confused by your comment about adding a lot of angles. Was the bracing omitted only at the plastic hinge locations or along the entire length of the beam? The slab is only effective at bracing the plastic hinge location near the column. Discrete bracing would be required between the hinges.