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Re: Ordinary Truss Moment Frame

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It's fairly common in mid rise buildings (20-30 stories) to place a full story height truss "outrigger" which is essentially a drift reducer for the building. On buildings in this range, it's usually at the top story. On very tall buildings, they have these every 15-20 stories. Perhaps there is some guidance there, in that for very large buildings it's an effective way to reduce drift. Since your situation doesn't go full ht, it's not exactly the same, but similar enough to say it's been used effectively in the past.

You may also want to treat it as a K-Brace type of configuration. I would think the major flaw in the system is forcing plastic hinge locations in the column.

Again, personally, if it meets Omega level forces and designed and detailed properly, I'd be comfortable with it.


On 1/17/07, garyg(--nospam--at) <garyg(--nospam--at) > wrote:
Quoting "Gerard Madden, SE" < gmse4603(--nospam--at)>:

> So are they doing this to reduce drift or keep the depth of the truss down?

I'd say it's to reduce drift.  It has a deeper (3'+-) section than
you'd likely use with a conventional moment frame.  That's what I've
read about the Special Truss Moment Frames is that they are stiffer
than a SMRF so I'd expect the same about an Ordinary Truss Frame.
They're laid out same as if he was using typical moment frames;
2-stories, 3-bays, each side of the building.  From what I've read and
what we've discussed in-house, we're leaning towards allowing this.

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