Diagonal Stiffeners in Special Moment Resisting Frames

[JIMEY]

I have been designing OMRF's for years and I have always used diagonal stiffeners in the panel zone when needed if the shear was to high. Now my office is starting to get into SMRF's and so I am reading FEMA 350 to learn how to design them properly. I can't help but notice that FEMA 350 requires doubler plates when the shear in the panel zone is too high. So I am wondering why FEMA 350 makes no mention of diagonal stiffeners as an alternative? Wouldn't they be just as effective as doubler plates? It just seems odd to me because in my office we pretty much use diagonal stiffeners 99% of the time just for the hell of it even if they aren't needed. Perhaps it has something to do with the way it affects the ductility of the SMRF. I guess the main question that I want answered is whether or not I am now forced to switch to doubler plates, or if it is still acceptable to stick with diagonal stiffeners. Thanks to anybody who can provide any insight on this issue.

 

[jdgengineer]

I've never used a diagonal stiffener in a moment frame before. If I've needed to reinforce for shear I've always used a doubler plate.

With that said, if it's possible, it's my understanding that the doubler plates are very expensive to install. Therefore, if you can increase the weight of the column 30-40# a foot and avoid the doubler plate I've heard it's cheaper to do it that way. But maybe I'm wrong on that...

 

[Whiffle]

You'll have to look at AISC 341. That governs the design of Special Moment Frames. I don't think it mentions diagonal stiffeners, so you might be stuck with doubler plates if your web is not thick enough.

 

[JoshPlumSE]

I'm guessing it has to do with the testing that's been done. And, that either they didn't test diagonal stiffeners (thinking that they were too costly) or that they did not perform well.

The behavior of a panel zone with a doubler plate seems to me to be very similar the behavior of a column with a thicker web.

But, the behavior of a moment connection with a diagonal stiffener does not seem all that similar in that you could get significantly different behavior for moment reversal (stiffener in compression vs tension).

 

[JIMEY]

Thanks for all your responses. So I guess I will just use doubler plates whenever I need to increase my panel zone shear capacity for special moment frames. Just to clarify, does everyone think I am still OK to use diagonal stiffeners for ordinary moment frames?