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Truss Tension Chord Bracing - Really Necessary?
4

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?

RE: Truss Tension Chord Bracing - Really Necessary?


Tension must be laterally braced to meet slenderness ratio.

RE: Truss Tension Chord Bracing - Really Necessary?

Is there a link that you know of where we could read that article?  That sounds like a good read to me.

RE: Truss Tension Chord Bracing - Really Necessary?

Thanks, now if only I were a member or knew someone who is.

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.

You get free downloads of all 19 of their Design Guides valued at $60 each.

When the new 2005(?)Unified Design Manual comes out you will get half price on the manual.

You get free downloads of all the recent Structural Engineering articles and many of the articles from years gone by.
 
This price is a pretty good deal.

In this electronic age, membership is getting to be a necessity for staying up to date with the industry.

I feel that our employers should to pay the membership dues (of course you employers out there may disagree). The benefit of a better-informed employee does accrue to the employer; however if your employer won't pay the dues, try banding together with other professionals to split the cost.

Regards to all,

JPJ


RE: Truss Tension Chord Bracing - Really Necessary?

I don't think there is a requirement to provide lateral bracing to the tension members of trusses.  In BS 5950, the tensile capcity of a tie is a factor of the material yield stress x (Gross area of section - area of holes for fixings).

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?

(OP)
The argument in Fisher's paper is that the vertical web members of a typical truss are in compression.  These web members are typically designed as column elements with an effective length factor of 1 (their actual length) - i.e. pin-pinned.  For this assumption to be true the bottom tension chord must provide adequate out of plane stiffness to cause the vertical web members to act as if "pinned" at the tension chord rather than "free."  Otherwise the vertical web members would need to be designed as upside down cantilever columns with an effective length factor of 2.  In order for the tension chord to provide this out of plane stiffness it sometimes becomes necessary to brace the tension chord out of plane at certain points.  

RE: Truss Tension Chord Bracing - Really Necessary?

Read the article and it makes sense to me.  Many years ago, a brilliant engineer told me that beams (in this case, trusses) need to be braced at the tension flange as well as the compression flange.  When I challenged his assertion, he convinced me with this argument:  When you load a beam in the plane of its major axis, it wants to relax, that is, turn its minor axis to the load.  It doesn't matter if the compression chord moves laterally to the tension flange, or the tension flange moves laterally to the compression flange.

RE: Truss Tension Chord Bracing - Really Necessary?

Like a beam, truss is also susceptible to Lateral Torsional Buckling. It is a good idea to brace the bottom chord when the members are slender or there is a possibility of misalignment in the plane of truss.

Ciao.

RE: Truss Tension Chord Bracing - Really Necessary?

In my experience it is unusual to laterally brace the bottom boom of a truss, unless stress reversal is a problem. In a previous thread entitled "Steel Truss Lateral Bracing", several posts mention how little stiffness is required to provide lateral bracing.

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?

(OP)
Pappyirl,

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?

No one here has mentioned the fact that tension members always tend to snap back to a straight line, like a string on a guitar.  Now this only applies to truss tension chords that are fixed at the ends of the truss (also like the guitar string).  Flanges of beams, for instance, are in tension due to a continuous build-up of tension through the cumulation of horizontal shear along the length.  The ends of a wide flange beam arent always fixed axially at the ends.

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?

pappyirl,

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?

for whoever mentioned that the tension flange of a beam needs to be braced as well as the compression flange, i completely disagree.  LTB causes the compression flange to go out of plane, not the tension flange.  As far as a truss and the tension bottom chord being stiff enough to act as a brace for the vertical members, thats another story.

RE: Truss Tension Chord Bracing - Really Necessary?

(OP)
JAE,

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?

Willis,

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?

Consider a related situation:  A vertical steel bar welded to, and supporting, thin steel plate (this is a tank roof).  How deep can the bar be?  The failure mode I can visualize is the bar just folding up flat against the plate (which is what happens on a paper model).

RE: Truss Tension Chord Bracing - Really Necessary?

Thank you JStephen.  Good to see that someone else makes paper models.  How deep the bar can be depends on the thickness of the bar and the thickness of the roof plate.  This is similar to a rectangular gas duct under positive internal pressure with thin bar stiffeners.  Even a stocky bar will flop over if the roof (or duct)plate is very thin.  I believe that even a wide flange will flop if the roof plate is very thin, even though the plate gives contiuous lateral support to the compression flange.

RE: Truss Tension Chord Bracing - Really Necessary?

I would think that the top chord of an OWSJ would fail before the bottom chord ever did.  Therefore, if you design the top chord as the limiting factor, the bottom chord would never fail, hence no need for the lateral support.

RE: Truss Tension Chord Bracing - Really Necessary?

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.

RE: Truss Tension Chord Bracing - Really Necessary?

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?

RE: Truss Tension Chord Bracing - Really Necessary?

(OP)
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.  

RE: Truss Tension Chord Bracing - Really Necessary?

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

RE: Truss Tension Chord Bracing - Really Necessary?

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.

RE: Truss Tension Chord Bracing - Really Necessary?

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.



RE: Truss Tension Chord Bracing - Really Necessary?

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.

RE: Truss Tension Chord Bracing - Really Necessary?

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!

RE: Truss Tension Chord Bracing - Really Necessary?

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.

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