Truss Tension Chord Bracing - Really Necessary? 4

[WillisV]

A general question that has long been a topic of disagreement within my design office - do trusses (in particular long span steel trusses) need to be laterally braced at the bottom tension chord in order to provide adequate bracing for the vertical web member "columns" to behave as the pin-pin members (K=1.0) that we typically assume in web member design.

James Fisher wrote an excellent paper on this very topic a while back (Engineering Journal - Third Quarter 1983). His article was, however, followed by many dissenting opinions.

Input anyone?

 

[haynewp]

I talked to a joist mfr. and he said they design using K=1 for the diagonal at the typical location where there is no btm chord bracing, with a 1% overstress allowed.

BTW, my steel professor thought that using K=1 is a bad idea without having bracing. But it seems to be working for the joist mfr's. I would like to see the calculation where this tension chord restraint is justified.

 

[haynewp]

I found on page 57 that they list what they are using

http://www.vulcraft.com/catreq/joistcat.pdf

 

[Tinkering]

Seems to me that if you are looking at a tension element isolated then LTB is no concern. As mentioned before the lateral bracing is due to buckling of compression flange or what have you. Oiler buckling is conributed to compression memebers, where tension members do not behave similarly.

So when a truss is composed of different elements then it no longer is just a compression member or a tension member. Those ideas are idealized. When you look at a truss, some see it as a giant beam, and therefore on a global scale it could be viewed to behave as a deep beam, and thus subjected to LTB. In which case the entire truss needs to be braced for out of plane behavior.

Furthermore, if you agree that a truss behaves as a giant beam then the width is very, very narrow relative to its height and length also suggesting that the out of plane axis is very small in comparison. Again out of plane bracing would be needed.

That's my take. Perhaps I'm off base here?

 

[WillisV]

Note that the original topic was brought up not as a question of the stability of the tension chord as its own isolated tension member but as a question of the ability of the tension chord to provide stability to the vertical compression web members. The premise being that if the tension chord is stiff enough to provide "bracing" to the vertical compression members then no tension chord bracing is needed, but if the tension chord is long/not stiff enough to provide this bracing then adding bracing points to the tension chord could be necessary to increase its stiffness.

 

[GerardJB]

It is always advisable to brace the so-called bottom "tension" chord of a roof truss because of the possibility of a reversal of forces (tension to compression) due to wind loadings on the building. Such bracing also controls the slenderness ratio for the bottom chord and the web members of the truss. In essence, you have to design a roof structure of which the truss is a component

 

[pba]

The problem with providing bracing to a pure tension bottom chord is that there is no means of adequately sizing the bracing. We would be using pure guesswork (or if you like - Engineering Judgement) in both the sizing and spacing.

I can't remember any occasion when I've actually had a lattice girder where there wasn't some load reversal so the bottom chord bracing has always gone in to suit the uplift.

I suspect that there could be some advantage in better defining the actual restraint conditions of the web members. Even a flexible bottom chord must provide some control on the out of plane rotation of the webs members. Likewise the top chord is likely to be more than a simple pin. Is there an argument that what you loose at the tension chord, you re-gain at the compression?

I suspect that the truss guys are on the mark with K=1.0 but I can't prove it. The arguments presented here are good. I'll certainly be conservative and try to include bottom chord bracing in future.

 

[kroe]

I think that there are cases when the bottom chord of a truss will need bracing even if it is in pure tension. Think about it this way: If the top chord of the truss is where the end supports are, and the loads are all applied downward to the bottom chord, then the bottom chord would not need to be braced because any eccentricity between the vert support reactions and the loads will not cause the truss to "tip". However, as an extreme case, if this condition were reversed: supports on the bottom chord and loads on the top chord, bracing the top chord would not prevent the truss from tipping if there is eccentricity between the support reactions and the loading. Since the usual case is a truss where the upward support reaction (at the bottom of the top chord) is below the downward loads (at the top of the top chord), eccentricities will cause a couple that will have to be resisted somehow - either w/ bottom chord bracing or through torsion in the top chord if no bracing is provided.

 

[L775]

In South Florida the debate is, who's responsible for specifying the bracing requirements. Should it be the EOR or the Specialty Engineer?
In my opinion the bracings, whether temporary or permanent, are part of the roof system which is the Specialty Engineer's responsibility.
Since the roof forces are not known at the time of preparation of the EOR’s documents, it makes sense that these bracings are part of the Specialty Engineer’s drawings
Any thoughts on the subject are appreciated.

 

[pba]

In the UK we don't have EOR's as such however it is the Principal engineer's (EOR's) duty to ensure that adequate bracing is provided. That's not the same as designing the bracing. It should be explicit in the sub contract documents as to what bracing is assumed, where it is to be and who is to conduct the design.

I'd recommend this approach regardless of who is actually responsible - We are all meant to be a team working together for the good of the construction project!

 

[jechols]

Bottom chord bracing in my opinion is definitely required. The best analogy I can think of is imagine the bottom chord of the truss is a cable. The web members being stiff and not truly fixed at the top chord, when loaded the truss would look like an Alabama smile. If you have ever walked across a rope bridge you would know exactly what I mean. It is tough to keep your feet beneath you.