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Fixed-Pinned Beams in Steel Special Moment Frame

Fixed-Pinned Beams in Steel Special Moment Frame

Fixed-Pinned Beams in Steel Special Moment Frame

I've got a building in a high seismic area where I am using steel special moment frames. The building layout is somewhat forcing me to have my columns oriented differently in each bay. Can I have a fixed connection at one end and the other end not fixed? This would make the 'moment frame' span a bay or two until the next rigid connection.

Next question is can the 'non-moment connection' be a simple shear tab or would it need to be a CJP weld through the column web to make the beam continuous? I haven't found anything in AISC 341/358 that specifically allows or prohibits either.

I doubt I could use anything but hand calcs for the design. I know Risa won't run the SMF checks for a pinned-fixed beam. Also, I don't think the basic design/analysis formulas from AISC 341/358 would apply so I would have to be somewhat creative. Have any of you ever came across this scenario or have any suggestions on the design? Any references you know come to mind?

RE: Fixed-Pinned Beams in Steel Special Moment Frame

I have seen one or more adjacent bays with full moment frames with a fixed-pinned bay at one or both ends. I don’t recall any prohibitions in the code for SMFs. The pinned connection can be a shear tab provided it can handle the loads.

RE: Fixed-Pinned Beams in Steel Special Moment Frame

As wannabeSE said, i don’t know of any prohibition for what you mentioned in your question. There are another option you could try, using cruciform columns providing moment frames in both directions.

The idea of having a shear plate at the other end, this is a good idea but I have some concerns. With this method, I’m not sure if the design checks for bearing and tearout and rupture would satisfy if you have large forces in axial and shear.

I’m not sure, only the analysis would say.

RE: Fixed-Pinned Beams in Steel Special Moment Frame

I'm not in the States, but I see this from time to time when people have a two way system with common columns and they are trying to minimise the minor axes column bending moments. Still have to deal with the axial components though and inherently lower stiffness in this type of system.

One thing that does make it tricky with a two way system though is that there are often stiffeners and the like within the orientated columns depth, this means that the shear tab may need to extend out past the column face, which of course increases the eccentricity and the moments induced in the columns (but still probably better than having the column continuous with the beams.

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