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Existing Trusses Failing
2

Existing Trusses Failing

Existing Trusses Failing

(OP)
I was asked by the architects at my firm to measure and analyze some existing trusses for proposed solar panels.

The trusses are 36' clear span , 6pitch roof and every member is a 2x4.

I analyzed them for 40psf snow load (1992 pg - todays is 60psf for the area but existing building code, I think, lets me use 40psf). I found top and bottom chords failing (bottom chord F.O.S. is nearly .5!). Say nothing about additional loads - this roof, as I have analyzed it, is not even strong enough to support its original design loads.

The question is how to get trusses "up to code". By my analysis a 2x8 bottom chord and 2x6 top chords will work with additional bracing (existing bracing is minimal). But that is in theory. Do any of you have experience with "building-up" existing trusses with deeper members? Will simply nailing on needed members (I'd assume it'd have to be a very aggressive nailing pattern) be enough to assume those members are working compositely? - Seems like this could be a dangerous assumption.

There is also some ethical baggage too. I believe the statute of limitations has expired for the owner to take any action against the fabricator or contractor (or even determine who is responsible). Still I'm not sure where the engineers responsibility extends to beyond making any truss affected by additional panel loads up to code.

The caveat is that this is the first truss analysis I've done professionally and it was done with Risa 3-d, a program I am familiar with but am not an expert. I am pretty sure, however, that they are not and have never been up to code.

RE: Existing Trusses Failing

Taking a quick look at your model, I notice that nodes 8 and 13 don't land on the top chord.  Is that right?

RE: Existing Trusses Failing

First, I must a few questions:

Do you have a copy of the truss shop drawings? This will give locations of required bracing as well as the grade and specie of wood used for various members. Often higher grades of wood are used (like MSR, etc.)

Make sure the bracing and member sizes and species are properly modeled.

It is advisible to go to the job site to observe the actual bracing installed (many times this is not installed or incorrectly installed) and note grades and spicies of various wood members and measure truss plates sizes.
 

RE: Existing Trusses Failing

Same for nodes 6 and 12.

RE: Existing Trusses Failing

Next, I would expect the dead and solar loads to be applied in the vertical direction.

RE: Existing Trusses Failing

(OP)
Miecz-  No, that is not right. All members definitely connect at gusset plates. I will try to fix. Your also right about loads, although I think your discovery about the nodes is more related to the results as the snow load really has the biggest impact.

Jike- I don't have a copy of show drawings. I did go to into the attic and measures all the trusses (members and node layout). I modeled the members as No.2 , and from what you have said I should (fix the model per miecz's comment) and reiterate with a higher grade lumber to see what the difference is.

RE: Existing Trusses Failing

VTEIT,
I am helping a buddy build a garage in VT. It happens to be 36' wide with a 6:12 pitch. His truss looks identical to the one you show. He lives where the GSL is 60 psf. I will grab his design info that came with his trusses and let you know. From memory, I think the TC & BC where SYP 2400 Fb. The chords are 2x4's in his truss, also.

RE: Existing Trusses Failing

(OP)
Splitrings- Thanks- that would be great.

Miecz- As I said, I'm pretty new to Risa. I modeled the top chords as continuous members because my understanding is that modeling separate members into each node will complicate bracing (it will assume pin, braced connections at joints I think). I'm not sure how to make an exact intersection on a continuous member, but I think that the way I modeled it Risa thinks they intersect anyway. Maybe I'm wrong though.

See attached photos of members. I'm not really familiar with lumber grades outisde No2,No1, SS and others listed in NDS. I would expect a very special grade to be stamped or labeled, which I did not see.

When I change the material to Select Structural, I still have Euler buckling code check for top chords, but bottom chord is now okay. I guess I really need to track down the lumber grade.

RE: Existing Trusses Failing

(OP)
It just occured to me that SYP is probably southern yellow pine and not a different grade of visually graded lumber.

 

RE: Existing Trusses Failing

VTEIT,
A couple of things I noticed with your model. You aren't using a Cr factor, which you can. (Asssuming a 24" truss spacing, of course). You also have your top chord braced at 5' o.c. in the weak direction. I'd be willing to bet the original truss design required bracing at 2' o.c.

 

RE: Existing Trusses Failing

(OP)
Repetitive member factor...true...Okay will do.

I have the top of the top chord braced at 6"oc for roof sheathing nailing, but you're right, bottom is modeled as 5' o.c.. If you look at the attached picture there really isn't ANY weak-axis bracing for the top chord on the bottom "flange". If anything, 5'oc isn't conservative enough.

You're probably right that 2'oc bracing was required but it was not installed.  

RE: Existing Trusses Failing

VTEIT,
My mistake. I didn't see the decimal in your model for bracing the top of the TC. I saw the 5 and just jumped to 5'. I agree with your bracing assumptions. I typically use continuous bracing on the top when sheathing is applied but based on the nails I see that missed the top chord in your pictures..maybe they are roofing nails? I usually will not show bracing the bottom flange of the TC.

I think the piece we are missing is the species and grade. From the pictures the members look like SPF. I have several truss design here that use SPF 1650F 1.5E. If I change you members to this species and grade, it is adequate for your snow loads. Again, I'll bring my buddies design info with me tomorrow.

RE: Existing Trusses Failing

(OP)
Okay- I updated the model to show all members as No.2's. Top chords and bot chords are SYP and webs are SPF. I had them at No.3.

What I do not understand is the allowable stresses (fb') risa uses. It lists f'b for SYP as 1984 psi. Even with a Cr and load duration for snow (say 1.15 also), I cant figure out how fb' gets so high. This is why I had them at No.3 before.

If I brace top chord at 2' o.c. I get a Bending check of over 2.0 (they are not braced at 2' o.c. but this is easy enough to do, easier than building up chords.).

If I build top chords up to (2)2x4, and brace them adequately, I get no code checks, even with my panel loads.

Thanks for your help everyone. Splitrings- that would be helpful to see your friend's design- thanks.

Attached is updated model of existing truss

RE: Existing Trusses Failing

Fb'=Fb*Cd*Cr
Fb'=(1500psi)*(1.15)*(1.15)
Fb' = 1983.75psi

Fb in NDS '05 is listed as 1500 psi for SYP #2 in table 4B

RE: Existing Trusses Failing

The second photo (SDC1790.JPG) shows no bracing on the compression diagonals each side of the ridge, yet they appear to be pretty long.

I would run hand calculations on this truss as a check against RISA.

BA

RE: Existing Trusses Failing

(OP)
Okay - I'm working out of 2001 NDS so that explains it. I should update our library and suggest a purchase.

BA: You're right, the truss has almost 0 bracing. In the modified model (and in the existing just to get reasonable results) I fudged some in. My design will include bracing on the webs you mentioned as well as on the top chord and everywhere else that has le/d>50 or approaches buckling stresses.

I did change members back to SPF because I cant confirm that it is SYP, which lowers fb' but it still works.

Thanks again for the help - super resource in lieu of in-house mentor.  

RE: Existing Trusses Failing

(OP)
n/m - 01 still lists fb = 1500psi for SYP my fault.

RE: Existing Trusses Failing

And BTW - if the orignal truss was designed by a truss plate mfg - you will probably never be able to duplicate their work.  They have proprietary software that makes extremely good use of all factors available.  

Good luck and I am sure that the truss is at least near its maximum with a 40 psf snow load.  And mayvbe someone jus found some old trusses and put them up - I have seen it done more than once.

Classic case:  A farmer bought some 24' trusses designed for residential use.  He hung his chicken coops from them.  The chickens grew up nice and fat until one day one got just a bit too fat.  KFC had a big sale the next week.

RE: Existing Trusses Failing

I recently had some issues on an existing steel roof truss that I was checking for new loads. The top and bottom chord members were continuous WT sections. I used RISA 3D for the analysis and found that the trusses (1969 era) were failing under the original design loads. Upon further investigation I determined that the chords were failing in a combination of axial load plus bending. Now all the load was applied at panel points so I was wondering where the bending forces came from. It turned out that I modeled the chords as continuous members and not as discrete members that were pin connected at each panel point. In my conversations with the folks at RISA we came to the conclusion that the truss chords were attracting shear forces due to their continuity, and that since this was the reality of the situation we could not discount those forces.

What I'm getting at here is that if you assume that the members are all pin connected but you use a continuous chord then your design no longer matches the assumption.

Perhaps your chords are also failing in a combination of axial force plus bending.

RE: Existing Trusses Failing

I've seen nothing here mentioned as to any snow load reduction taken for roof slope.  At 6:12, there should be some.  

Did you consider that to reduce the snow LL??

Mike McCann
MMC Engineering

RE: Existing Trusses Failing

Can you post the RISA truss diagrams for the nodes, members and applied loads too?

Mike McCann
MMC Engineering

RE: Existing Trusses Failing

(OP)
Mike - Slope reduction for a 6:12, cold, shingled roof is 0. That is Cs = 1. Maybe I'm wrong but I think asphalt shingles arn't allowed to be called "slippery". All the more reason to go with standing seam!
 

Will post diagrams tomorrow with loading shown, assuming you dont have Risa 3d (otherwise I bet you would not ask).  

RE: Existing Trusses Failing

That I do...

Mike McCann
MMC Engineering

RE: Existing Trusses Failing

Hey guys....I think you need to take a closer look at the wording of the code. I remember designing a building in cleveland where the Ground Snow Load (GSL) per ASCE 7 and a Wind Tunnel Study was lower than GSL back calculated from the  the minimum buidling code required snow load.

Is the 60 psf a roof snow load or a GSL?

RE: Existing Trusses Failing

(OP)
its 40psf and it is the gsl. roof snow load is .7*1.1*.9*Pg in this case as no reduction for slope can be taken, it is fully exposed and it is a cold roof.


On another note I went back to the site yesterday and found grade stamps on top chords. It's MSR 2100fb. Good stuff! Trusses are therefore structurally sound and meet original design loads.

RE: Existing Trusses Failing

The 2006 IEBC does not allow additional loads which will increase the stress in any member more than 5%, unless the member complies with current IBC.  Additional loads on trusses are also addressed in IEBC 302.2.4.

Have you checked the plates?  The code report will show the plate values for various limit states.

Look for distress in the plates.  If you see corners pulling out and flexing away from he wood, it is a sure sign of overstress, and if the trusses have been overloaded, this will show before wood failure in most/many cases.  (assuming the design was made by a truss manufacturer - the plate manufacturer frequently requires their design in order to use their plates.)  Obviously, this only happens if the the structure has seen that kind of overload.

The dead load seems light (considering decking and two layers of roofing), and the load on the bottom chord from drywall and insulation is missing.  Attic live load needs to be considered, possibly concurrent with snow, depending on your judgment.

The solar loading makes me think that the solar panels attach flat to the roof, rather than on standoffs.  Point loads can be critical on these trusses.

Also, the bottom chord is obviously spliced (at 36 foot long) and the splices will be the weak link to check.

RE: Existing Trusses Failing

(OP)
See updated model attached:

I had included a small load for r11 bats - there is no sheetrock.

Dead load is up to IBC minimums for shingles, 5/8's sheathing and tar paper.

Attic live loads- there is no floor, just bottom chords to walk on. There may be someone up there on a snowy day, but I'm choosing to neglect that given the difficulty of access and lack of any working surface(you have to be pretty nimble to get up there from the 6' step ladder provided to the hatch- no one that heavy is going to make it!)

Panel loads are 5psf and, because they are warm, they will at least reduce snow load by their own weight and likely more. So I see no increase in the stress at limit state of 5%.

Even still-  I made a large error and modeled boundary condition at the wrong places (did not show 2' roof overhang). This and the high grade top chord lumber make a huge difference and the truss pass with no problem - for design loads or current loads or proposed loads.

Thanks for all the help - i'm putting this to bed.  

RE: Existing Trusses Failing

Hi All, I am new here, but as I having 15+ years experience in the Metal Plate Connected Wood Truss industry as a Truss Engineer, I would like to add 1) That typically the top chord of the truss can reinforced with a add-on, the same size and grade or lower grade and greater size. 2) Less typically, the bottom chord can also work with a add-on unless tension controls the chord which may require a Truss Manufactured frame. 3) The metal plate connectors typically start failing at minimum increases in loads as low as 5%, but plywood gussets and nails can easily fix most joints (though the bottom chord slices will usually require a add-on to one face, the joint plywood to the other face) without the need for a frame. 4) When modeling, each metal plate gusset should be a node point but the chords should be continuous across the joints except where the chord is sliced at the joint.

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