Western Hemlock Roof Truss - Existing 2

[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. They are 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. 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.
Thank you,

 

[phamENG]

This is probably more of a general structural engineering forum topic, but I'll give it a go here.

Look up "Hem-Fir" species group. Things like Hem-Fir, Spruce-Pine-Fir, Southern Pine...they're all species groups. Southern pine has something like 5 different individual species of pine that fall under it. I think that will answer you question about the appropriateness of using Hem-Fir values in your analysis.

As for whether or not current values are appropriate, you'll need to do a little bit of research. (I don't know - I'm on the East Coast and we don't have Hem-Fir so I don't know much about it.) For Southern Pine, there was a big downgrade of visually graded lumber about 9 years ago. So if I were analyzing a truss made of southern pine from before 2013, I would not use current values. The 60s could be tricky though. I remember from some time ago (though I can't find my references for it, so if I'm imagining things hopefully somebody will tell me) that it was around the 50s or 60s that somebody realized using flexural tension values for pure tension in wood is unconservative. So it was around that time that a lot of lumber suddenly 'lost' a lot of tension capacity.

I wouldn't decide on a lumber species based on what makes the model work. That's a really bad habit to get into. The model should reflect reality. You need to look at the lumber and, if necessary, see if you can determine the grade of the lumber. You can either hire a grader to come out and inspect it, or you can pull some references (ASTM D245 is a good place to start) and try your hand at grading it yourself. (Be careful here, though...like all engineering undertakings, you should be qualified to do it or under the supervision of somebody who is.)

Generally speaking the design of wood hasn't changed that much, but the required loads have.

Since this is the RISA forum, why don't you post some screenshots of the model with members and boundary conditions, maybe some information on applied loads, etc.?

 

[JoshPlumSE]

Also, keep in mind that the NDS may vary from release to release. With some species available in some years for all sizes of members and some years not available for all those sizes. So, you may want to look for an NDS from the year the building was constructed.

 

[jayrod12]

Lastly, if the wall being removed is, as you say, far away from any bottom chord panel point, then I can't imagine it being load-bearing. I have never seen a metal plate connected truss use the bottom chord in bending to act as a support intentionally. It's much easier to just move a panel point over to the bearing location.

 

[designres]

Thank you guys!

Pham, I appreciate your thorough response. I will post the question in the general forum. I am attaching some model graphics as well. Spacing is 24" O.C.. combo 4 has 10psf Roof DL, 5psf Ceiling DL, and 10 psf Ceiling LL. All wood is specified Hem-Fir #1, in these models.

JoshPlumSE, I have contacted NDS with the same question. No response yet. Hopefully, they provide some info.

jayrod12, that was my first inclination. No way the wall could be load-bearing that far away from the panel points, right? The bottom chord would be worse-off, I thought. After seeing the overstress increase from removing the wall, though, it mad me want to dig deeper.

As-built:

Wall Removed:

 

[phamENG]

Hmm...did you model those as individual members between nodes? What about end releases? It looks like all your web members are fixed?

These trusses aren't the nice, simple things from a statics textbook. The Metal Plate Connected Truss industry takes advantage of all sorts of stuff to eek out every last bit of economy that they can from these things.

- Heel Joints are modeled with at least partial fixity
- Top and Bottom Chord members are typically continuous across joints with web members which allows them to contribute flexural stiffness to the joints
- Some other joints, depending on demand and size of plate, can be considered to have partial fixity as well

So make sure, at the very least, that you have placed the joints in your chords in the right places.

A few other questions:
What is the full span of the truss?
Did you check to see if there's a gap of any kind between the truss and the wall to be removed?
Are there any foundation elements directly below the wall to be removed? (If the answer is no, it doesn't mean the trusses aren't relying on it, but it does invalidate the infinite stiffness vertical support in your model.)

 

[JoshPlumSE]

What about end releases? It looks like all your web members are fixed?

Good point, you might want to look at the moment diagrams. You don't want to see moments at the ends of any of your chords or web members. Internal moment along a chord, that's okay, of course. At least that's my assumption for any truss that gets my stamp.

Granted, the big time truss manufacturers have some assumptions about how those connection plates affect rotational connectivity. But, I don't generally play with that. If I'm re-working a truss like this that doesn't work as is, then it's not usually all that difficult to double up a chord that is failing.

 

[phamENG]

Josh - when you do that, do you make any effort to account for the eccentricity, do you ignore it, or do you do symmetrical augmentations to prevent it?

 

[JoshPlumSE]

I'll account for an eccentricity if it's significant. But, it's an engineering judgment call. I usually start my models off as simple as possible and only add complexity when necessary.

Most of the time, I suppose, I'll leave out the eccentricity. Out of laziness and the "keep it simple" principle. But, if it's something truly critical and the code check is pretty high (but not over unity) then I suppose I'd include it.

Now, if I've got 50 of those trusses that are all similar then I might investigate the assumptions of the original truss manufacturer. But, I haven't had a project like that.... at least not yet.

 

[phamENG]

Thanks. My superpower seems to be overthinking things, so it's nice to hear (or read) where others I respect are willing to let something go and why.

 

[designres]

Thanks again, Pham and Josh!

Pham,

The chords are continuous across the joints, including at the node I placed for the support in the middle of the bottom chord. The webs are pinned at the ends. I did not consider any rotational stiffness from the webs. I could possibly take a look at that.
The full span is 31.5'. 10.5' between panel points at the bottom chord.
I didn't check for a gap. I assumed the top plate of the wall was nailed to the bottom chord of the joists. It has drywall on al sides right now.
The foundation is a slab. So, no indication from that either.

Josh,
If I were to reinforce, I would likely run another web member from the lower top chord panel points down to half-way along the outer bottom chord panels. This would eliminate the eccentricity from doubling up a chord. I'm not sure if you considered that already, but I was thinking it would save some design work and some framing lumber.

Another couple thoughts to consider -

does the full ceiling live load apply in the smaller side openings of the truss? It seems like it would be really difficult for a person to walk there, in that space, and anything stored would be smaller.

It looks like overall, the joist would not collapse. The top chords are well below unity for all load combinations. The only thing I would worry about, after seeing this, is if somebody were to apply a big load on a single bottom chord out there where it shows overstress. Like by jumping on it. But, the wall would not help in that case anyways as it supports the middle span.

A couple general questions. And most of my experience is in the steel joist industry. I work for a steel joist manufacturer and I have never seen a steel joist designed to bear on a support that was not directly at a panel point. Not saying that it's never happened. Just that I have not seen it.

Are wood trusses ever designed to be supported at significant distances from the panel points?
In general, if it is determined that a wall is not load-bearing, would it be necessary to evaluate any members/components/structures to which it is attached? I can see maybe a compression situation, if the wall is bracing the members to which it's attached. But the bottom chord is in tension in this case.
Is wood below grade 1 or 2 ever used for manufacturing wood trusses? In the 60's or 70's?

 

[phamENG]

Yes, the 10psf is over the whole thing. I've crammed myself into the eaves of a 4/12 truss before. I'm not small. It hurt. But I got in there. It is a non-concurrent live load, so you don't need to include it with other live loads as you chase them through a building. But for your truss...what kind of access does the attic have? What happens if you apply a 20psf live load where you have 42" of headroom with no other live load? (the 10psf is non-concurrent). Make sure you read the fine print in ASCE 7 chapter 4 on the live load table. It goes into some detail on how to apply live loads to truss bottom chords.

What happens to the truss if you release the moment in the bottom chord at that central point? (I can just about guarantee you they didn't have 31.5' long 2x4s.)

As for bearing far from a web member....pretty much never. It can be done, but I don't think I've actually seen one 'in the wild'.

Not sure about then, but I see #3 SP for webs all the time.

And for what it's worth, 32' is pretty reasonable for a typical truss like this.

 

[designres]

Again, thank you so much for the help! Your honest answers have given me much needed insight.