OMF Seismic Connection - WF to HSS

[mnovicki]

My question is in regard to the design of a steel OMF connection. For architectural reasons, we have provided beams and columns that are much larger than required. The plastic hinge will form in the column. My understanding of AISC 341 is that I must design using the force delivered to the connection of 1.1*Ry*Mpcol. This results in forces exceeding the anticipated seismic forces on a scale of at least ten times, probably way more. In my case I have a wide flange framing into an HSS. Do I really need to use these elevated forces for AISC 360 Chapter K checks on the HSS, considering my members sizes are oversized? Then there is a blurb in 11.2a(4)of 341 that says a lesser value of Va is permitted if justified by analysis. "The required shear strength need not exceed the shear resulting from the application of appropriate load combinations in the applicable building code using amplified seismic load". Is this true for the tension and compression forces from the flange as well?

 

[dgkhan]

I believe you are aware that this over design is for connection only and not the member. Member might be 0.9 times same parameters. Code wants to make sure that connection will not fail and member itself will fail.

 

[kslee1000]

I think your delimma (being punished for over sized members) is caused by design method used. Since you are using strength method that require the member to fail by yielding, then code requires higher connection strength to be consistent with the design assumptions. Can you justify the design by ASD?

 

[cjw99]

I am confused as to why you would be using AISC 341 for the design of an ordinary steel moment frame. Since you are using OMF's you must be in seismic design category C or lower. (They are not permitted in Category D or higher). If you use an R=3 instead of R=3.5, you can avoid using AISC 341 provisions altogether. Your story shear forces will increase slightly, but avoiding the AISC 341 provisions will always end up being cheaper and easier. Your seismic force resisting system would technically be "Steel Systems Not Specifically Detailed for Seismic Resistance" instead of OMF's. (See part H of Table 12.2-1 if ASCE7-05).

 

[mnovicki]

thanks cjw99. That's the answer I was looking for. I have somehow missed Part H all this time... obviously, that makes a huge difference. Thanks!!!