Existing Trusses Failing 2

[HeavyCivil]

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.

 

[msquared48]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[HeavyCivil]

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).

 

[msquared48]

That I do...

Mike McCann
MMC Engineering

 

[InDepth]

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?

 

[HeavyCivil]

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.

 

[TXStructural]

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.

 

[HeavyCivil]

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.

 

[Splitrings]

VTEIT,
Sorry it took so long but here is the design for a very similar truss. You might note that only the upper portion of the TC is SPF 2100F. The rest of the truss is all SPF #2.

 

[woodman88]

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.