Continuous wood sheathing in bracing top chord of wood truss for axial load? 1

[abusementpark]

How are you supposed consider continuous wood sheathing in bracing the top chord of a wood truss for axial load? It seems like you could argue that it prevents euler buckling out-of-plane, but wouldn't there be a torsional buckling mode that could then govern?

Is there standard assumption that is made? I imagine this would come up a lot in the design of pre-engineered wood roof trusses.

 

[msquared48]

With plywood roof sheathing, I always consider the top chord of wood trusses as laterally braced.

Mike McCann
MMC Engineering

 

[abusementpark]

Laterally braced for both bending and axial load?

 

[msquared48]

For bending, yes. Particularly if the plywood is glued to the trusses too.

Regarding the axial load, I guess the situation amounts to asking if sheathing on only one side of a 2X stud wall constitutes continuous lateral bracing of the weak axis. Personally, I think sheetrock is crap for lateral bracing, particularly after a seismic event, but have I no problem with plywood sheathing for lateral restraint, even on one side. Remember, you still have the minimal contributions of the node points and any blocking in the roof diaphragm if present. Plus, some trusses have periodic transverse 2X members framing between the top chords of the trusses. Unless there is specific testing to show otherwise, I would consider it fully braced.

Mike McCann
MMC Engineering

 

[abusementpark]

For bending, yes. Particularly if the plywood is glued to the trusses too.

But wouldn't the plywood only brace the chord for positive bending (i.e. compression in the top of the member) and not negative bending? If the chord is continuous, negative bending would be induced.

Regarding the axial load, I guess the situation amounts to asking if sheathing on only one side of a 2X stud wall constitutes continuous lateral bracing of the weak axis. Personally, I think sheetrock is crap for lateral bracing, particularly after a seismic event, but have I no problem with plywood sheathing for lateral restraint, even on one side. Remember, you still have the minimal contributions of the node points and any blocking in the roof diaphragm if present. Plus, some trusses have periodic transverse 2X members framing between the top chords of the trusses. Unless there is specific testing to show otherwise, I would consider it fully braced.

Well, in this case, there is no 2x blocking or periodic 2x transverse members.

 

[msquared48]

Regarding the negative bending of the top chord over the panel points, two things:

1. The axial compression load will be many times greater than the negative moment forces induced by the short spans between the panel points.

2. The press plate connections do offer a very small degree of fixity of the members in the weak direction, although un-quantified, and probably untested.

Personally, I have never had a problem with this scenario. Are you looking into a failure condition?



Mike McCann
MMC Engineering

 

[woodman88]

In the 2001 NDS section 4.4 covers the subject. Do not have the more current NDS's here to check them.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[abusementpark]

Does anyone know what standard assumptions are made in the pre-engineered truss industry?

 

[nac521]

For most plate connected wood roof trusses I've come across, the truss designer assumes a rigid roof and rigid ceiling to brace the top and bottom chords in the weak axis unless you otherwise tell them not to.

@ M^2: How do you brace your load bearing wall studs in the weak axis direction if you do not like sheetrock for lateral bracing? All plywood sheathed load bearing walls?

 

[jayrod12]

nac,

Here if we are considering the walls for shear walls then yes, otherwise no but we design them for only being supported in the weak axis at our blocking specifications.

 

[Splitrings]

The BCSI Section B3 addresses permanent restraint/bracing. They list two methods of bracing the top chord; structural sheathing or purlins. They do go on to say purlins need to braced to prevent the top chord from buckling.

I would agree that a deep top chord would not be lateral braced against negative bending in its weak direction. NDS Section 4.4.1 requires full depth blocking for d/b ratios between 5 and 6, ratios between 6 and 7 require both top and bottom edges be supported throughout their entire length.

 

[Splitrings]

ANSI/TPI 1-2007 Section 7.3.5 states that members that are continuously braced throughout their entire length by continuous sheathing only need to be checked for buckling "within the plane of the truss".