Existing Wood Residential Truss

[atrizzy]

I'm analyzing an existing wood truss that I'm certain is meant to span from the back to the front of the house continuously. My issue is that no matter how tight I get on my loading, the top chords fail by about 20% in compression + bending.

I can just BARELY make it work if I consider the plywood as contributing to the strength of the member (aka increasing the member depth by 1/2"). I'm not utilizing the entire 'T' section with webs or anything, only the additional 1.5"x1/2" section stacked on top of my 2x chord. Shear flow in the connectors doesn't seem to be a problem.

So, does anyone see a reason why I can't consider this some kind of crude composite section?

The owner wants to remove some interior walls. The house was probably built about 20 years ago and I don't expect it to meet current snow load.

Thanks in advance!

 

[phamENG]

I wouldn't use it as a composite section...maybe if it was glued, but even then I wouldn't want to do that. You do have the top chord continuously braced in compression for the weak axis, correct? I would assume the nails to do that...or at least use an unbraced length of 12".

Are you accurately accounting for splice points in the chords, or are you analyzing it as a pure truss with pins at joints? The continuity of the chords through panel points that exists in real life prefab wood trusses goes a long way.

 

[atrizzy]

pham, the thing is being analyzed exactly as built.

Continuous members where continuous, pinned where not, braced in weak direction. I'm even reducing snow load as much as I can, accounting for roof slope/vertical projection, ignoring attic live load. The analysis is about as tight as I can get it as far as I can tell.

The reason I'm asking is that I wouldn't want to do it either if I didn't have to. I'm more curious about... if there's really something wrong with the approach.

 

[phamENG]

I don't like it because it doesn't seem all that reliable to me. You're going to depend on a non-continuous element (sheets of plywood that may have joints every 4ft) to provide a continuous improvement to the flexural capacity of the top chord. Slip at the fasteners is a real possibility. These are small things, but by the time the top chord has deflected enough for all the plywood to be in contact in positive moment regions and all the nail slip to be gone, it may not matter. And what about negative moment? What if your plywood joint is over (or near) a panel point where you need continuity? I don't think it would be appropriate to take the composite help within 6" to a foot of each panel point. Maybe more unless you were able to increase the number of fasteners which is probably a non-starter with the roof already existing.

 

[Ron247]

Do you think it met the original code from 20 years ago but now it does not because the loading increased?

I would not rely on that 1.5" strip of plywood. So many flaws in the logic as pointed out by others. You still have a break in the strength at every plywood joint. What happens at that specific point? It still probably fails. Also any slight gap in the sheets causes a problem with compression since you say it fails in compress and bending. Can't transfer compression across a gap.

How many trusses do you need to correct? Nailing a 1x4 or 1x6 on the side may work.

 

[dik]

Does the original code require that UDL flexure be included with the compression forces in the top chord. Some codes excluded this.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[atrizzy]

Yes, these are all good points. Ron, I have no idea why it fails by so much now, but snow load increase is just one idea. Maybe the top chord happens to be select structural, who knows... But this is quite odd and it's unnerving me a bit. I've never had an existing truss be so far out, and since the interior walls are all over the place I very much doubt they're providing any support. In fact, the identical house next door had these walls removed previously so SOMEBODY seems to think it's ok. Just not me!

But I digress: the point is that I'm just trying to take advantage of all of this redundancy these wood structures are famous for! But I, of course, see the issue with my previous suggestion.

Another idea:

My critical section for negative moment is at a node where the top chord is continuous, mid truss. The top chord is then continuous over the next adjacent nodes. If I can justify using a k value for compression of 0.5 for the member, it works. I realize this isn't recommended, but the due to the continuity the deflected shape indicates triple curvature. It's not crazy to think that the effective length would be half of the span. And the plywood must add SOME stiffness in the strong axis direction...

 

[atrizzy]

dik's point may well be why it's failing... I just don't know for sure.

 

[phamENG]

K=0.5 is the ideal condition where the the curvature of the member leaving the support is 0. 1) no case is ever ideal, and 2) your case doesn't approach the ideal scenario anyway. You have curvature through each pin, so your K is still 1 but with L is the distance between panel points for the strong axis.

 

[dik]

What was the lumber strength at the time of construction?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[atrizzy]

dik, I wish I knew.
I don't even know for sure what the time of construction was... owner and builder aren't appearing too helpful.
As to your previous point, was there a time when it was acceptable to just lump the load in at the nodes and ignore flexure in the chords?
I can't imagine that would be a justifiable reason for me to leave them be...

 

[phamENG]

atrizzy - you're in Canada and I'm not sure how things have gone up there with regard to lumber grading and allowable stresses (pretty sure you guys don't use allowable stress for wood design anymore anyway), but down here our Southern Pine got a big downgrade 8 years ago. Everything before that was pretty consistent...I've even found that most historic buildings can be checked to pre-2013 values if I use No.1 or, if it's a really nice piece, Select Structural. Though I tend to stick to No.2 because somebody always has a knot in a joist somewhere. I digress...

Have you guys gone through anything like that? If so, check it with lumber from the previous round of grading assessments. What about property records with the local jurisdiction? I run into issues when trying to date a house built in the 1700s or 1800s, but most buildings from the 20th and 21st centuries have accurate construction dates listed on publicly accessible online property databases.

 

[dik]

It may have been code compliant at the time of construction... the code and/or allowable stresses likely made the simplified analysis 'work'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jayrod12]

We just built a cottage for my parents. The roof trusses had MSR top chords. So it's not unthinkable that they used select structural chords.

I assume that there's a panel point directly over the wall you're interested in taking down?

 

[Ron]

Why are you looking at bending in the top chord? That should be irrelevant.

 

[dauwerda]

Do the webs of the truss actually land on top of the existing interior walls? If not, I would think it very safe to state that the interior walls are not designed as load bearing. Therefore, removing the partition walls would not be changing the structural design anyway.

 

[atrizzy]

Thanks all, to clarify, no the trusses do not appear to be load bearing in any way. I'd just like to see some numbers working in my favor regarding this.
Ron, the bending in the top chord comes from considering the UDL snow/dead load on the top chords. Not sure what you mean.

 

[Ron247]

If there is an isolated location that is overstressed, why not just reinforce that area?