steel special moment frame
steel special moment frame
(OP)
I have a steel special moment frame that I am trying to design according to FEMA 350. The connection type I am considering is the Welded Unreinforced Flange-Welded Web (WUF-W) connection. According to fig. 3-8, the weld of the shear plate to the column flange is symbolically shown as a partial penn. weld with reinforcing fillet all to the near side, but in the notes it says this weld is from the far side. Am I misinterpreting the weld symbol in Fig 3-8, or is there a mistake in Fig 3-8? Welding from the far side does not make sense to me because it seems to interfere with the CJP weld of the beam web to the column flange per note 3.
Any help is greatly appreciated
Any help is greatly appreciated






RE: steel special moment frame
I haven't looked closely at this connection until now. I've got a question. The paragraph on pg. 3-28 says that "web joints for these connections are made with complete joint penetration groove welds of the beam web to the column flange." That quote makes it sound like the beam web is welded directly to the column flange. That's not true, right? I would think that the load path for shear is like a typical connection: through the beam web, into the shear plate, then into the column flange. That's how you see it, right?
Another inconsistency with the quoted sentence is that it says "complete" joint penetration welds connect the web to the column flange, but then the figure shows a partial penetration weld. (the same one that you're not sure which side it's meant to be welded.)
Shemp
RE: steel special moment frame
Thanks for the reply. My interpretation is that it is desired to have a CJP connecting the beam web to the column flange. I think their intent is to fully develope the beam web to the column flange to minimize shear stress in the flange. In any event, this detail is not clear, and when I go to the FEMA webiste there is no guidance there. This is all very fustrating. I wish AISC would give us step by step procedures for these special connections with solved examples like they do for other types of moment joints.
I am a structural engineer from upstate New York, and we have only been required to design for seismic since january of this year. This detail most closely resembles the moment connections that were typically in use, so I was hoping to go with it. This is for a small one story fire house. What experiences have you had with special moment resisting connections?
RE: steel special moment frame
RE: steel special moment frame
Just to be sure, the shear tab and the beam web are welded to the column flange?
I suppose this makes the shear connection extremely rigid in an attempt to take shears away from the flange welds.
I read a little article this morning where the author says that this type of moment connection still doesn't solve the Northridge problems. It's at this link if you're interested:
http://www.muckraker.org/image2/RRFundamental.pdf
So you've got to use the IBC instead of the NYC now, huh? I imagine that the Use Group III pushed you up into Seismic Design Category D, right? I don't have a lot of experience designing special moment frames. A job near the New Madrid fault zone required them so I used the bolted flange plate design. Another job that I'm working on, also in the same area, requires them.
Let me know if you run across any problems. One that I remember happened while checking stresses on the beam's flange for the bolted connections. I was designing the connection for the full moment capacity of the beam. But the beam's flange was too thin. So, I needed a bigger flange, but that gives you a bigger moment capacity. So, I got caught in that circle. Then I ended up with a bigger beam which required a bigger column (because of the strong column weak beam concept. Luckily you won't have to worry about the SC/WB concept b/c you only have a one story building.) Like I said, if you have a problem, post it - I'd like to hear about it.
Shemp
RE: steel special moment frame
Shemp: In upstate (Albany), we never even had to deal with the NYC code, but now that we're IBC based we've been thrown into the deep end. New York State had it's own building code, which was decades out of date but easy to use. I ran into similar problems with the bolted flange plate connection. Next I tried the welded free flange connection, but the web plates and welds for the beam flange were enormous. I've abandoned them and the welded unreinforced flange connection, and am now trying the reduced beam section out. My problem is that for architectural reasons I am stuck with a column (w12x58) that isn't quite as stiff as I would like, so for drift considerations I had to go with a large beam (w24x76). This puts a lot of demand on the connection. I wonder if in the case of one-story buildings where you can form a plastic hinge in the column if it would be ok to just size the beam column connection for the amplified earthquake combination. For my case I don't see the logic in a connection designed to allow for a plastic hinge in the beam when I know that I'm not going to get one there, but in the column instead.
RE: steel special moment frame
Yeah, I did the same thing that you're thinking of doing... sizing the connection for the amplified load instead of the capacity. The force was much bigger than would ever occur - just like you said.
RE: steel special moment frame
I my opinion all of the FEMA 35X series of recommendations are in need of review, recommendations and commentary by more than the SAC committee members. The weld symbols are incorrect for Figure 3-8. (Perhaps comments or suggestions could be incorporated into future errata?)
Don't get me started on FEMA 353...
RE: steel special moment frame
RE: steel special moment frame
Agree or disagree?
RE: steel special moment frame
RE: steel special moment frame
RE: steel special moment frame
Basically, you are "turning the connection around" so that the plastic hinge will form in the column instead of the beam. This column hinge will act as a "fuse" to limit the load that can be transferred to the connection and beam. The connection should then be designed for the expected flexural capacity of the column (including overstrength factors and projected moment magnification) instead of the beam.