One Story Biaxial Ordinary Moment Frames Options Per AISC 341

[engjg]

What are good options for shapes & FR moment connections (if any) for a one story biaxial Ordinary Moment Frame designed in accordance with AISC 341? I am assuming the structure size does not warrant the price tag of utilizing proprietary connections. For the sake of discussion consider a one story frame with columns in a square arrangement (15'x 15'x 15'), structure is in a SDC D and by height exception an OMF would be permissible per ASCE 7-10.

1. HSS column & HSS Beam; my understanding these are permissible shapes for an OMF per AISC 341 (although not really addressed in the specification)...what about connections that would satisfy 341's OMF requirements? Is a simple fully welded connection if strength checks per 341 permissible? What about continuity plates requirements?

2. HSS column & WF Beam; what about connections? AISC 360 & Design guide 24 shows some options for FR connections...I assume these would be permissible for an OMF 341?

3. Built up WF & WT cruciform; is this permissible to treat as classic WF moment frame in both axis and use typical WF prequalified connections?

 

[RPMG]

I believe that the connection strength has to be greater than the expected beam strength with 1.1Ry. So, that effectively eliminates direct weld moment connections, and leaves you with HSS columns & WF beams with flange plates. That will also stiffen the HSS and provide continuity. The HSS should have widths greater than the beam flanges as a practical consideration.

As a disclaimer, I'm probably wrong about at least one of those statements.

 

[JLNJ]

When I did one of these recently, I gave up trying to make it biaxial and went to a OBF in the weak axis of the columns and a bolted endplate moment frame in the strong axis.

 

[JoshPlumSE]

Take a look at AISC 358 for ideas of moment connections. You don't have to follow all the provisions of that spec since it only applies to IMF and SMF, but it should give you an idea of your options.

For biaxial, your options are limited. There is a proprietary HSS column connection that's biaxial and described in 358 (or one of its supplements). But, if you want to stick with a wide flange, then you would probably have to Cruciform it (i.e. weld WT's of the same section to each side) and go with one of the 358 connections. I don't see any other way to develop the probable beam strength in the weak axis connection otherwise.

Alternatively, I suppose you could say the hinging occurs in the column instead. There is an example (I believe) in the Seismic design manual where they do this at the roof level of a building. So, it is allowed....