Braced STMF
Braced STMF
(OP)
The company I work at sometimes acts as the specialty engineer for jobs using Special Truss Moment Frame lateral systems. We just recieved push back on a job because we proposed bracing our STMF to reduce column sizes (See below image). The EOR said that it wasn't codified and there's been no testing on it. Testing has only been done on non-braced STMF.
According to the senior engineer I work with, while there's been no testing on the braces, their only purpose is to keep the inelastic behavior in the special segment zone (the double x's), and so testing is not needed. Bascially we can do whatever we want to keep it elastic outside of the special segment.
The company has already done several other jobs with braced STMF without pushback, and most likely will continue to do so on future projects.
I'm a recent college grad and still learning a lot, but there's only 2 other engineers here besides me. I'd like to hear other professional engineers input. Should this not be practiced until testing is done on this exact set up, or are the current codes and testing we already have sufficient to allow for the braced STMF design?
According to the senior engineer I work with, while there's been no testing on the braces, their only purpose is to keep the inelastic behavior in the special segment zone (the double x's), and so testing is not needed. Bascially we can do whatever we want to keep it elastic outside of the special segment.
The company has already done several other jobs with braced STMF without pushback, and most likely will continue to do so on future projects.
I'm a recent college grad and still learning a lot, but there's only 2 other engineers here besides me. I'd like to hear other professional engineers input. Should this not be practiced until testing is done on this exact set up, or are the current codes and testing we already have sufficient to allow for the braced STMF design?






RE: Braced STMF
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Braced STMF
Loads - ASCE 7-10
RE: Braced STMF
- In my mind, the system more closely resembles EBF than it does STMF
- You'll need serious lateral bracing in the trusses where the braces tie in. I'm not sure that we're allowed to work out the demand for that on our own in special seismic systems.
That said, I've never done an STMF myself. Your team is likely far more up to speed on the system than I am.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Braced STMF
RE: Braced STMF
Basically a cross over between a EBF and an STMF. STEBF?
While you mention it, however, an STMF (vertical braces or not) behaves more like an EBF than a traditional moment frame. An STMF is designed to stay elastic except for the special segment similar to the "link" in the EBF. On the contrary, the middle of the beam in a Moment Frame stays elastic while the nonelastic behavior is pushed towards the ends of the beam. I believe this is the premise for justifying the vertical braces: the inelastic behavior is still confined in the special segment.
That's one of the main things I've been wondering about. Lateral bracing is already required at those locations (6% of the chord's axial capacity). One would think more lateral bracing capacity would be required by throwing in large vertical braces. When our senior engineer gets back in town I'll ask him if he increases the lateral bracing demand...
The axial demands in the braces are actually relatively low compared to the moments that would develop in the columns and axial demands in the truss chords should the braces be removed!
RE: Braced STMF
If the architect will allow the braces, I don’t see any benefit to keeping the truss. Might as well use a wide-flange beam and design it as an EBF. The higher R value could benefit the diaphragms and foundations.
RE: Braced STMF
Thankfully you don't have to worry about this yet but it's definitely something to keep in the back of your mind. In my EIT years (which wasn't too long ago) my boss definitely let me create some interesting designs that I would be much more hesitant to perform now that I'm stamping my own designs.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Braced STMF
Certainly, I agree with the logic. My only concern is that which sandman has highlighted: the absence of test results of a sufficient similitude to guide detailing. More pointedly, does this adaptation resemble tested configurations well enough to satisfy AHJ? I'm skeptical of that.
Consider this difference between conventional STMF and Braced STMF:
1) In an STMF, the entire span contributes to the story displacement ductility. More at the part that goes plastic and less at the elastic remainder but, still, the entire span contributes. And the columns to a degree.
2) In a braced STMF, it is pretty much just the portion of the span between braces that contributes to story displacement ductility (like an EBF).
This difference implies that the curvature demand in the portion of the truss permitted to yield will be much greater for the braced STMF than the un-braced STMF. And I would submit that is a rather important difference. It's very much like the the curvature demands that arise on shear wall coupling beams. The shorter the beam, the greater the demand.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Braced STMF
RE: Braced STMF
RE: Braced STMF