Truss Tension Chord Bracing - Really Necessary?
Truss Tension Chord Bracing - Really Necessary?
(OP)
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?
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?






RE: Truss Tension Chord Bracing - Really Necessary?
Tension must be laterally braced to meet slenderness ratio.
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
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RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
AISC Memberships for 2 to 6 individuals in one firm costs only $160.
If you split it up that comes to $26.67 each.
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Regards to all,
JPJ
RE: Truss Tension Chord Bracing - Really Necessary?
Slenderless limits dont apply to tension members, but do apply to compression members - hence sometimes restraints are necessary to limit the effectve length of the bottom boom which becomes a compression member under wind reversal.
VB
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
Ciao.
RE: Truss Tension Chord Bracing - Really Necessary?
Steel Truss Lateral Bracing
Using this analogy, if the bottom chord of the truss is stiff enough to resist 2% of the compressive load in the vertical web member, then we can assume the pinned-pinned scenario to be applicable.
In relation to bracing the bottom flange of a beam; why would the bottom flange in tension want to move laterally if the top flange is restained? The lateral movement is due to the compression force in the flange resulting in buckling. Only in the unrestrained situation can I understand it being irrelevant which flange is moving.
RE: Truss Tension Chord Bracing - Really Necessary?
You analogy is correct, but only partly so. There are actually two distinct requirements for a member to act as a brace.
1. Strength - commonly referred to as the old 2% rule. The bottom chord does need to be strong enough to resist a certain percentage of the vertical web member load. Note that the 2% rule is out-dated and that the AISC LRFD V.3 has revised bracing strength requirements in chapter C.
2. Stiffness - this is a separate criteria INDEPENDANT of the force in the members that must also be met to be considered a brace. This criteria normally controls over the strength requirement. Again the stiffness criteria are provided in Chapter C of the AISC LRFD spec.
It is the stiffness criteria that is really pushing the tension chord bracing requirements as the stiffness of a long-span bottom chord is normally relatively small out-of-plane.
RE: Truss Tension Chord Bracing - Really Necessary?
I once designed a large bowstring truss (horizontal top chord comprised of jumbo WF shapes and curved bottom chord made up of bridge strand). The truss spanned about 370 feet and was about 50 feet deep at the midspan point. We and another consulting engineer created some models to investigate the possibility that the truss would have a pre-disposition to snap out of plane. The models were somewhat inconclusive, but seemed to show the truss always trying to snap back to its original vertical position after we pre-deflected it sideways.
In any case, we opted to be "safe" and added diagonal struts from the adjacent roof framing to a point about 2/3 down the length of the verticals.
The seminar on stability that Yura (from the Univ of Texas) presents usually shows that shapes need to be either braced at the compression flange against translation, or braced against rotation of the entire section..either working fine. No mention of tension chord/flange bracing was brought up.
RE: Truss Tension Chord Bracing - Really Necessary?
I've never actually run or seen an experiment to prove this, so it's actually just a theory that goes like this:
Given it's druthers, a beam would rather present its weakest axis to the direction of the load. Imagine a beam tilted at 45 degrees about its longitudinal axis from the vertical. The compression flange is continually braced and the tension flange is not. As the load is increased the beam will tend to twist to present it's weakest axis to the load. You can see this with a paper model.
As the initial tilt angle decreases, the tendency to twist decreases, but is still there. For most rolled beams, the stiffness offered by the web and bottom flange is enough to resist the twisting effect. But a truss has very little web and flange stiffness.
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
I have also run numerous models of trusses with imposed out-of-plane deflections and have run into inconclusive results. I think in this case it is best to rest on the side of caution. This same line of thinking, by the way, is one of the reasons while steel joists are typically bridged at the first node from each end. The forces in the vertical web members are highest there and therefore in most need of bracing.
RE: Truss Tension Chord Bracing - Really Necessary?
How come the compression members are assumed to be cantilevers when not restrained at the tension flange?
Why would they be fixed against rotation at the compression flange?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
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.
RE: Truss Tension Chord Bracing - Really Necessary?
http://www.vulcraft.com/catreq/joistcat.pdf
RE: Truss Tension Chord Bracing - Really Necessary?
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?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
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.
RE: Truss Tension Chord Bracing - Really Necessary?
RE: Truss Tension Chord Bracing - Really Necessary?
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.
RE: Truss Tension Chord Bracing - Really Necessary?
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!
RE: Truss Tension Chord Bracing - Really Necessary?