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Re: Steel Moment Frame Connections[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Steel Moment Frame Connections
- From: <Parkerres(--nospam--at)aol.com>
- Date: Tue, 25 Aug 1998 02:00:14 EDT
In a message dated 98-08-21 21:41:04 EDT, you write: << On another front, I have noticed as recent as yesterday, observing residential projects under construction where steel moment frames are going in with no apparent post northridge detailing. Same old full pen welds with backer bars and rats nest. There are wood framed, type V projects all over the bay area where I see this going on. Is FEMA 267 considered the current standard of care? I am sure some engineers have never heard of it. It would be very helpful if some of the testing of moment connections would address the concerns of us little guys using small frames for residential applications such as W8 through W14 sections. >> We do lots of large residential construction, and almost all of our projects have steel moment frames. We do not rely on the fancy connections developed for large building frames because I'm not convinced that the small frames (W8, W10, W12) have the same weld problems with heat-effected zones, through- thickness, etc. Instead, we use the dumb and dumber approach. We design all of our steel frames as Ordinary Frames. Since the majority of our buildings already have shear walls, the use of an Rw=6 is not a penalty for the frame. In order to satisfy the LA City requirement that the OMRF connections resist a load based on Rw=2.67, we design the frame members for stresses based on a multiplier of 2.25 (Rw=6 / Rw=2.67) in all load combinations. If the frame members satisfy this high load demand, then we conclude that we can use standard pre-Northridge full-pen butt-welded flange and bolted shear tab moment connections without cover plates, dogbones or the like. We do require that the backup bars be removed, even though the City doesn't because we feel this is good practice. It should be noted that we check the drift requirements for the frame based on Rw=6 (the Code loads) not the increased loads. We have found that the large (2.25) increase in frame forces does not effect the frame sizes too drastically since drift usually governs at Code load levels. Additionally, by using the higher forces we get stiffer frames which are more compatible with the rest of the shear wall building and help minimize nonstructural damage. Lastly, the use of "typical" connections is much easier to install for the smaller steel fabricators that we are using at the residential level. That's my 3 cents worth. Bruce Resnick, SE Parker Resnick Str. Eng.
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