Load-Bearing? Wall Under Hemlock Wood Truss - Existing 1

[designres]

Hi,
I am working on a project, a residential remodel, where the clients would like to remove an interior wall which is located directly under the middle of the existing roof trusses. This mid-point is about as far from a panel point as it can be. The bottom chord appears to be 3 equal spans. The bottom chord is 2x4. During the home inspection I noticed the trusses were stamped with the word "Hemlock." However, I did not see a grade indication.

Now I am attempting to perform the analysis in RISA and Hemlock is not one of the available choices where the 2x4 or 2x6 sections can be applied in the program, nor can I use a custom rectangular section with Hemlock in RISA. I posted the question in the RISA analysis forum too.

Also, I was not able to find Western Hemlock in the 2015 NDS Supplement. I can select Hem-Fir, in RISA, which I think should have similar mechanical properties. Is that correct? How do the mechanical properties of Western Hemlock compare to those for Hem-Fir? Secondly, using Hem-Fir grade 1, the existing trusses appear to be overstressed at the bottom chords by about 2%. When the support is removed, the overstress is about 12%. But, when I use structural select grade, there is no overstress in either scenario. Is it safe to assume structural select was used originally as grade 1 looks overstressed? Or is grade 1 more likely? Have wood design rules become less demanding or more demanding over the years? The house was built in the late 60's. Any assistance you could provide would be greatly appreciated.

Can I simply deduce that since the wall/improbable, but possible support location is so far away from a panel point, the wall is not load-bearing and can therefore be removed?

Thank you,

 

[XR250]

Can I simply deduce that since the wall/improbable, but possible support location is so far away from a panel point, the wall is not load-bearing and can therefore be removed?

Yes

 

[phamENG]

thread1419-494694

 

[RontheRedneck]

In trusses there is virtually always a web at a bearing wall. Otherwise the bearing point would not do much of anything.

 

[ChorasDen]

The only section I'm aware of in the NDS that contains mechanical properties for a species labeled "Western Hemlock" is table 4E of the supplement, which is visually graded decking, which is not material I would normally expect to go into wood trusses.

What is the overstress failure caused by? Tension? Tension properties have changed since the 60's, because they were 'incorrectly' determined based on bending tension values. Are you certain your model is correct, what are the stresses at each joint?

You can make any assumption in your model that you want, but it's best to ensure that it matches reality. Even if this wall wasn't considered a bearing point, if the truss chord has deflected sufficiently to rest overtop the wall, then it is now a bearing point, even if the relative wall support stiffness is low, some load will be going through this support.

 

[designres]

Chorasden, the overstress condition appears to be mostly from flexural stress in the bottom chord. In the load combo in question, the chord is in axial tension. Since it’s a rectangular section, the controlling stress would have to be tension from bending plus the axial tension, I believe. My feeling is that the bottom chord was not originally designed for the 10 psf live load for uninhabitable attic space required by ASCE 7 - the same 10 psf LL I included in the model. Was the uninhabitable attic Live Load required in the 60’s and 70’s?