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Web Elements as a Tortional Restrain

Web Elements as a Tortional Restrain

(OP)
Hello guys,

I am wondering whether do you consider web elements as a tortional restrain. The following picture is from a project that is made in my design office. When I ask Lx,Ly and Lt values of upper chord to my supervisor he told me that both lx and lt are 1.7m and ly has a 5.5 m unbraced length. As you can see in the picture, there are bridging trusses between my trusses. Additionally, I must say that I do not assume my purlins (CYAN) restrain my trusses.





I do agree that lx is restrained by web elements. And I agree that unbraced length of ly equals to 5.5m. But I and my supervisor have a conflict over this - lt - issue. Therefore, I thought I must ask this to you guys.

Thank you for in advance.

RE: Web Elements as a Tortional Restrain

I personally side with your supervisor. The chord members cannot torsionally twist at the connections of the web members provided there is an adequate connection between the two members.

RE: Web Elements as a Tortional Restrain

(OP)
jayrod12

Thank you for your reply. I think you are right but don't we think that the upper chord is able to twist between bridging trusses with itself and its web members as a whole?

Thank you.

RE: Web Elements as a Tortional Restrain

The purlins absolutely restrain your trusses, don't ignore their contribution.

RE: Web Elements as a Tortional Restrain

(OP)
MotorCity

Thank you for your answer.

You are right but if I don't ignore the purlins, I should consider that some of them are bearing axial force too. I don't know whether I am thinking logical or not.

Thank you.

RE: Web Elements as a Tortional Restrain

You are correct, they will have some axial load from bracing the truss but its probably very small. As a rough estimate, assume your top chord has 100k compression load. Using the 2% rule of thumb for a bracing force, that means that your purlins will only see a 2k axial load.

RE: Web Elements as a Tortional Restrain

Agree with MotorCity, and in fact, I would expect the demand of the purlins to resist torsion for the top chord would be significantly less than 2 kips axial load.

RE: Web Elements as a Tortional Restrain

(OP)
Thank you guys,

Sorry for my late reply. I think I will consider that it is braced against torsional buckling every 1.7 m from now on.

Thank you guys.

RE: Web Elements as a Tortional Restrain

Quote (OP)

I think you are right but don't we think that the upper chord is able to twist between bridging trusses with itself and its web members as a whole?

I think that this is the crux of your confusion here. And, frankly, you're on to something. Consider that there are two distinct mechanisms of torsional buckling at play:

1) Torsional buckling of the chords themselves about their own longitudinal axes. For this mechanism, jayrod is absolutely correct. For most common web members and connection configurations, the webs can be relied upon to brace the chords torsionally. While it's not technically "proof", just imagine the havoc that would ensue were this not the case. Pretty much every steel truss ever would have it's compression chord torsionally unbraced for the entire truss length. For trusses comprised of torsionally weak chord members such as wide flanges and double angles, this would be a deal breaker.

2) Torsional buckling of the truss as a whole, involving both chords rotating about some point in space aligned along the vertical axis of the truss. Essentially, this is the truss version of the lateral torsional buckling that we investigate when designing wide flange beams etc. Thing is, we almost never check this explicitly. Rather, we simply brace the compression chord(s) laterally and assume that does the trick. And, usually, it does do the trick. Not always though as described here: Link. While tension chord buckling rarely causes anybody any problems, it is still very much a real thing. I believe that, fundamentally, it the reason why open webbed steel joists generally must have bridging near the bearings. In heavy trusses like yours, a careful analysis will often yield the result that the self weight of the truss itself is more than enough to stabilize it against "truss LTB". At least that's how it pans out for the ubiquitous case of a truss constrained to LTB about an axis at the elevation of the top/compression chord.

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.

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