Wood Truss Chord Members Effective Length

[jay156]

I have a project where I'm checking some garage attic trusses that were modified by a contractor to fit in some stairs.

My question though is about the top chords of the trusses and their effective lengths. For out of plane buckling, is it correct to assume that the top chord is braced by the roof sheathing, giving a K value less than one?

 

[jayrod12]

Yes for weak-axis buckling I count on the roof sheathing to provide full restraint. Strong axis buckling is length between webs.

 

[lexpatrie]

If it's actually a wood truss there is a CT factor in the NDS that may have been used by the original manufacturer.

My impression is that the truss top chord is typically considered braced by the sheathing, weak axis for sure, and probably strong axis as well, (rather presumes an appropriate bracing system). I'd suggest you review the NDS on the subject as the language can be a bit murky.

If you can see where they indicate braced length in this calc, you're doing better than I am. I see forces and a combined stress index (CSI) but the actual guts of the allowable stresses I don't see.

Again, if they are actually trusses and you can identify the manufacturer, I'd send the as-built to them and see if they are willing to modify/redesign. These are proprietary and the design can be difficult to model without their software, especially if you are trying to do it efficiently.

Regards,
Brian

 

[jayrod12]

It specifically indicates that at a minimum there needs to be purlins attached to the top chord at 24" o/c. To me that means they're considering it braced at 24" o/c for top chord design.

 

[lexpatrie]

It might be a default in their software, as there is roof sheathing on the top chord in this particular case. They did some strange stuff elsewhere, substituted these goofy 6' long "trusses" to replace the 2x10s in the corridor. Quality of design fluctuates. There's no purlins that I know of on that roof. This is probably not the best reference drawing but it's what I had on hand.

Let's not discuss what they did with the parapets.

 

[RontheRedneck]

The software considers the TC as continually braced if plywood is attached. But only in the weak axis.

I don't mind telling you what I know if you want to email some drawings:

rsitruss at yahoo dot com

 

[KootK]

Yeah, braced weak axis for sure. It would be a rare, common truss chord that could survive without that. As an EOR checking something like this, I'd be inclined to use the distance between panel points as the unbraced length so long as as any splices are located fairly near to panel points.