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Bottom Chord Bracing
2

Bottom Chord Bracing

Bottom Chord Bracing

(OP)
I am investigating some existing wood roof trusses for the owner to verify that they are structurally sound. The trusses are about 30 years old, comprised of 5-1/8"
wide glulam members with bolted side plates at the panel points, about 65 feet long and spaced at 20 feet on center. They are a simple type truss with a constant sloping
top chord and a horizontal bottom chord, (and web members between). The top chord is braced by the wood roof framing, and there appears to be no visual distress in the truss members or connections.

My question to you is: Is there any requirement, (code wise or standard of the industry),that the bottom chord needs to be braced? The bottom chord will always be in tension,
even when acted on by earthquake or wind forces, and there is no diaphragm action requirted of the bottom chord.

Thanks in advance for your response.

Larry Hauer, Structural Engineer

RE: Bottom Chord Bracing

You will get a lot of different opinions regarding bottom chord bracing of glulam truss members that are always in tension.

The engineers that mentored me generally would not brace a bc that was always in tension.  A rule of thumb I was taught was when in doubt don't.  This rule of course is the opposite of the typical structural enginnering rule of when in doubt make it stout.

I was taught not to add bracing to the bottom chord unless I had a reason to do so.  Common reasons to brace the bottom chord would be would be stress reversals and the need to transfer a lateral force through the system.

In Appendix A of the NDS there are some non-mandatory provisions for bracing trusses.  Based on experince those I work with don't follow those provisions.

With your existing trusses, since you are not seeing signs of distress I would not woory about the lack of bottom chord bracing.  One key things to look for is excessive truss deflection.

Also if you do a search on this web site you should be able to find a past thread that also discuss bottom chord truss bracing.

RE: Bottom Chord Bracing

One reason to provide bottom chord bracing is not so much to brace the tension member, but to provide lateral support to web members that may see large compression forces.  I believe the thought is that if the bottom chord isn't stiff, it won't adequately provide lateral support out of plane of the truss for the web member in compression, which is often assumed "pinned" in truss design.

RE: Bottom Chord Bracing

(OP)
ctcray's message advises about providing out of plane bending for the web members in compression, (by providing bracing of the bottom chord), and I thought about this before posting the thread- and it does seem like they would require bracing of the bottom ends- but where do they go if they are unbraced?? I can't picture a failure with the truss bending over out-of-plane becuase of this non-condition. Also, some of the web members have 20 kip compression force in them, but have been stable for the life of the trusses, (30 years).

Any other thoughts on this, (bracing the ends of the compression web members)?

Thanks in advance,

Larry Hauer SE

RE: Bottom Chord Bracing

ctcray comments are a valid concern.  I don't know of an analytical way to evaluate if buckling out of plane will occur because of the compression webs.

When I discussed this with one of the retired company engineers, I worked with in the past, he idicated in over 30 years of practice he had never seen a problem with a heavy timber truss.  He also stated that he had a hard time visualizing the bottom tension chord being pushed out of plane by the compression in the webs.  He felt that the tension in the bottom chord would hold the truss in line.

In the glulam trusses I deal with generally the tension in the bottom chord is very high in comparsion to the forces in the webs.  Also the webs are often oversized just based on the width of the top and bottom chord and the depth that is practical to make a good connection.

In the case of metal press plate trusses every member is being optimized so I would think questions about bottom chord bracing would be more imporrtant.

As I stated before there are a lot of different opinions on this topic.  What a particular engineer does may depend as much on the design practices of the engineers he or she  worked with as any thing else.

The company I work for has several people with over thirty years of experience in designing with wood.  Yet there are somethings those people don't even agree with each other on.

RE: Bottom Chord Bracing

Have you checked the case of lateral wind load on the bottom chord(from the adjacent wall) and maximum wind uplift on the roof?

I would be surprised if this does not result in some compression in the bottom chord.

Regarding the bottom tension chord restraining itself from tension. It is not that difficult to check, take 1% of chord forces as a lateral load on the bottom chord and treat like a cable with tension equal to that from bending. See what the deflection needs to be to resist this lateral load and compare to what is acceptable.

Alternatively, you could widen the spacing and call up double trusses with a 2x6 on flat under to provide lateral restraint ( have done this in Australia for a few 40 or 50' trusses in schools).

RE: Bottom Chord Bracing

(OP)
I appreciate everyones input re. whether truss bottom chord bracing is required. "csd72" questioned whether the bottom chord could go into compression due to wind uplift- not in this case, it is an enclosed structure with a relatively heavy roof, (20 psf), which would neutralize any wind uplift. Also, no wind lateral since an enclosed structure.

In this particular case, (the building is about 30 years old), I guess the "test of time" has shown that bracing is not required.

Thanks again,

Larry Hauer

RE: Bottom Chord Bracing

WTCA has limitations on slenderness ratios for tension members as well as compression members.  L/d for tension members is limited to 80.

RE: Bottom Chord Bracing

I'm sorry, it is TPI, not WTCA that specifies that.  

RE: Bottom Chord Bracing

(OP)
I've seen the L/d ratio of 80 for tension members somewhere before, too. But I would assume for trusses the length would be between panel points, not the entire lenght of the bottom chord, (for my situation with a 63 foot long truss that woud mean a 9-1/2" wide member)

RE: Bottom Chord Bracing

I am thinking that given the bottom chord is always in tension, it cannot buckle regardless of what the forces in the webs are.  The only question is if the bottom chord can go into compression, in which case bracing may be required.

RE: Bottom Chord Bracing

Per Ansi TPI all truss design software is required to provide maximum tension and compression in chords and webs.  If you look at the wind load cases, you will discover compression in the bottom chords.  They either assume that a rigid ceiling is applied or it will tell you how far apart the bottom chord purlins must be.

woodengineer

RE: Bottom Chord Bracing

I would agree that lightly loaded 2x roof trusses should have bottom chord bracing because compression will usually always occur due to wind loading.  As far as I know, the AITC Timber Construction Manual does not require bracing if the bottom chord does not go into compression.  Doesn't TPI only govern the design of 2x trusses connected connected with gang nail steel plates?

The point I was making is that it is impossible for the bottom chord to buckel if it is always in tension, regardless of how large the compressive forces are in the webs.  Does anyone disagree with this statement?

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