Braced STMF

[cal91]

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?

 

[TehMightyEngineer]

My gut feeling is I don't like it but I'd definitely want to dig into the code to see if there was anything specifically prohibiting it. What local codes are you designing to?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[cal91]

STMF - AISC 341 Seismic Manual E4
Loads - ASCE 7-10

 

[KootK]

I'd be inclined to agree with the EOR here.

- 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.

 

[sandman21]

You have created an EBF as Kootk points out. To answer your question code equations and behavior of systems are based on testing to determine failure modes and detailing requirements. To keep the truss and column elastic the forces in the braces and connections must be huge.

 

[cal91]

Thanks for your responses, it's appreciated.

- In my mind, the system more closely resembles EBF than it does 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.

- 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'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...

- To keep the truss and column elastic the forces in the braces and connections must be huge.

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!

 

[Deker]

This is an interesting question. Adding the braces (turning it into an EBF) doesn’t really change the target yield mechanism, which is shear yielding in the special segment / link. However, it is clear that the STMF code provisions are only meant to apply to configurations that have previously been tested (see commentary to E4.2). By adding the braces I believe you are violating the intent of the 6 ft. depth limitation in section E4.2.

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.

 

[TehMightyEngineer]

I think the biggest question to ask yourself, as a new engineer, is could you sleep at night not knowing if the next big one were to hit tomorrow? Will the building be standing or will you be reading about a 3 story building collapse? Even if it wasn't your fault and the ultimate failure was a construction error, would you want to be dragged into a court room to defend your radical STMF design and be able to prove it didn't contribute to the failure? Sounds like the EOR doesn't want to and I wouldn't either, even if I found no clause in AISC 341 that prohibited it.

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

http://www.americanconcrete.com

 

[KootK]

I believe this is the premise for justifying the vertical braces: the inelastic behavior is still confined in the special segment.

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.

 

[sandman21]

You are only looking at the static load in the brace, post-elastic behavior is when loads get high. EBF braces are fully restrained connections which uses the shear strength of the beam to determine overstregnth factors. Range from 2.5-3.5. Only recently based on further testing was the requirement for the connection reduced it used to be based on overstrength of the brace.

 

[daranguiz]

Under seismic loads, this system act as an excentric braced beam, with a plastification zone between the braces. In a I beam, you will have a plastic hinge there that will dissipate energy trough rotation. I don't know how much rotation you will have in that system because you will need to have a ductile resistance and not a buckling (elastic) resistance.

 

[sandman21]

Based on your responses I am assuming that you are using an R of 7 and not OMF R 3.5. If that is true the brace also interferes with the protected zone as its at the ends of the special segment, 2*d from web connection. If the system is designed as an OMF than I would say you have a valid system. Without testing to demonstrate that the system provides the desired response, I agree with the EOR the configuration is not permitted.