Truss bearing point at interior? 3

[EngStuff]

I want some opinions on this truss. It's an existing residential cathedral truss that spans only 30 feet. I am 99% sure that the reactions are on the outsides only, especially 30 feet isn't that long for a truss to span to both of its ends. The part I am unsure of is because of a vertical web that runs straight to an interior wall. That being said, just because it has a vertical web on top of a wall, doesn't mean it is a bearing point. I have designed trusses that have similar configurations with 2 bearing points on its ends only, but I have also designed similar configurations with 3 bearing points with one in the interior. Not saying I am a pro when it comes to designing trusses, but enough times to have an idea of what can or can't be done, but not enough times to not get stumped on this situation. Though I still think it's not load bearing. There is no gap, but I've seen many times that contractors don't leave gaps between truss bottom chords and top of plate members for partition walls. Also, maybe they did leave a gap, but as years went by, it sagged to the point that it's bearing on the wall, but not enough to cause any structural or serviceability issues. I did not see any "crushing" on the top plate (didn't check all locations).

What I unfortunately can't check is if the wall is sitting on a beam or joists below it. If it's on joists that are 2+ feet away from the support, then I would ultimately assume not load bearing. If the wall was on a beam, i would still be unsure at that point. Until we get a chance to go into the crawl space (have to create a hole somewhere which adds more time and cost, and would like to try figuring it out before we go that route)

 

[RontheRedneck]

EngStuff - You asked: "The program you guys use. so, they pin the webs and have continuous top and bottom chords? I assume in this specific truss; you also have to pin the bottom chord at the cathedral?"

It's not nearly as simple as a pin joint model. I believe they call it a "matrix analysis". Chords are continuous through joints, each joint is considered semi-rigid, etc. The days of designing trusses by hand with a calculator are gone.

If you design a beam up in the truss space, remember to consider the bracing of the top flange. That detail is often forgotten.


stanleyshum1997 said: "When you have an interior support inside the bottom of a trusses, the trusses is not no longer consider as a trusses. The trusses becomes a frame and you no longer have the triangular stability a trusses promised. In other words, the trusses is unstable?"

I have no idea what you're getting at. Adding an interior bearing does not change the fact that a truss is triangulated.

 

[le99]

Can you identify all elements that are in touch with the bottom chord? You say the wall is 32" away from the truss, is it parallel to the truss? If it is, the wall has nothing to do with the truss.

 

[BAretired]

It's not nearly as simple as a pin joint model. I believe they call it a "matrix analysis". Chords are continuous through joints, each joint is considered semi-rigid, etc. The days of designing trusses by hand with a calculator are gone.

The behavior of semi-rigid joints is only as good as the assumptions made. I have seen examples where the assumptions were clearly wrong for the conditions prevailing on site. Excessive moisture, omission of bracing, misplacing of truss plates and damage during delivery are some of the potential problem areas with prefabricated trusses.

The pin joint model, while not theoretically exact, is conservative and, unless codes have changed since I retired, is still permissible as a design method.



BA

 

[phamENG]

BA - permissible, yes. Cost competitive, no. I have a copy of a paper that discusses rigidity of the connections. I'll see if I can find it tomorrow. TPI-1 also specifically covers a simplified approach to accounting for rigidity in the heel connection.

 

[XR250]

I suppose there is nothing conservative about the design of pre-engineered wood trusses. My suspicion that we don't see more failures is that they rarely see their design loading.

 

[RontheRedneck]

XR250 - There's a 3X safety factor in truss design, both in the plates and in the lumber.

 

[ALK2415]

@ EngStuff
you did caused a lot of anticipations ?
did you got the job done ? Any pictures of the designated truss ?

 

[EngStuff]

did you got the job done ? Any pictures of the designated truss ?

The owner is still determining how they want to precede. I will keep you guys posted once I hear from them and hopefully an end result with the wall removed/beam design.

Luckly, we did a good due diligence on this project instead of assuming non-load bearing wall, definitely an eye opener. I do want to thank you all for the help!

 

[ALK2415]

@ EngStuff
- did add a good selection of wooden structure textbooks design and analysis could be useful to you
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