OMF w/HSS members

[bookowski]

SDC D, small single story structure. This meets the exception in 341 to allow for an OMF to be used. The provisions do not prohibit the use of HSS, there are no particular requirements for members. I cannot do an r=3 here, not allowed in sdc d.

The client wants all HSS, beams and columns. 341 requires the connection to be designed for 1.1 x Ry x Mp. To do this would require a cjp. I can't find in 341 if a cjp with an HSS moment connection would require backing bars removed. I don't have aws d1.8, but I'd suspect it says yes or doesn't address it specifically in which case I should assume yes. I did find in 341 a statement that hss columns requiring a cjp to base plates can leave backing bars in place, so perhaps that is a close analogy. A backing bar cannot be removed in an hss, so does this requirement short circuit the possibility of an all hss OMF?

The only workaround I can think of is to create haunches at the beam end, making a deeper section and thereby reducing the connection forces to avoid a cjp. Not a great solution though and goes against most of the reason the client wants hss to begin with.

Anyone done an all hss moment frame or seen an example? I've found a few people asking about this online but no good answers. I found one ppt presentation from seaoc with a slide that said this would likely fall under 'alternate' systems and may be acceptable but no details.

 

[JoshPlumSE]

All HSS, or just HSS columns. If it's just HSS columns, there is a connection in AISC 358 that would work.

 

[driftLimiter]

The 1.1 Ry Mp requirement is one of 2 options, you can also use 6b, subpart (b) and design for the limiting load of a ductile element in the load path. The Commentary suggests you can use the overstrength level load as a limiting load.

So you could use sections that are much larger than required to get the weld demand down...

If your column workable face is much larger and very stout, you might be able to get a fillet welded connection developed to resist this moment. Otherwise you could attempt to reinforce the HSS column with through plates. And Use PJP and/or Flare Bevel welds.

A CJP on an hss beam is not a great option in any scenario let alone a seismic connection. If you are trying to get an equal width column and beam this is going to be very difficult. My goal would be to size the beam so a PJP weld can work against the overstrength loads. It means way heaver sections but relatively simple connection design.

 

[JoshPlumSE]

The 1.1 Ry Mp requirement is one of 2 options, you can also use 6b, subpart (b) and design for the limiting load of a ductile element in the load path. The Commentary suggests you can use the overstrength level load as a limiting load.

That's right. I forgot about that. It's in the commentary to section E1.6b(b) where they specifically talk about the overstrength seismic load being allowed. It's very important to realize this is ONLY for OMF's.

 

[Deker]

Have done several moment frames with all HSS, but typically only for small ancillary structures. I usually design the whole frame and connections for R=1 due to limited ductility. As mentioned by others, the commentary discusses capping the connections demand at R=1 or overstrength. You might also look into designing as a cantilevered column system if you have enough columns.

 

[bookowski]

@driftlimiter - thanks, that should do it.