Timber roof trusses 4

[DCStructures]

I'm doing some work on an existing building with timber roof trusses. I have never seen a truss with this configuration or these types of connections before. (See attached photos.) Has anyone seen these before?

 

[DCStructures]

Another photo

 

[DCStructures]

Another photo

 

[KootK]

For a metal plate connected wood truss, that is one bizarre configuration. Does anything that you've seen suggest that it might have been shipped in two or more pieces and subsequently sit assembled? Quality control on the plates looks questionable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DCStructures]

Yes pieces do seem to possibly the assembled on site. Have you ever seen metal connection plates like that before? They seem to have numerous daggers built in to them that then were hammered or otherwise driven into the wood members. The metal connection plates occur on both sides of each truss connection. I hate to turn down work but I am wary to get involved in anything related to this roof, especially if there is concurrence on this site that this has not been seen before.

 

[TehMightyEngineer]

And I thought I've seen some weird trusses. This looks like someone modified a cambered fan truss by adding on another bottom section. What's the goal of the project?

I'd run away like crazy, the amount of conservative assumptions you would have to make to put a number on those to cover your rear is probably going to make these trusses unusable to the client. Then, to spend the time to find the exact capacity would likely take too much time on your end.

The only way I'd personally tackle this is if I could reinforce each of those interior "hip" joints with custom metal plates and through bolts (stupid expensive I'm sure).

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[SLTA]

I wonder if it's designed as a tilt-up as some sort... What are the dimensions on it?

Please remember: we're not all guys!

 

[jayrod12]

Are you doing anything with the trusses? i.e. does any of the proposed work actually require you to check the existing?

 

[DCStructures]

Client wants to add solar panels to the roof (very lightweight ~3PSF) but I still need to verify the capacity of the existing roof trusses.

 

[JAE]

The capacity of the trusses would very much be dependent on the capacity of the truss member connections ... those poorly fabricated gang-nail plates.

Since I'm not aware of the methodology of determining a plate capacity for that type of connection - and since the condition is so poor - I'd either not take the challenge or propose that the only solution to me signing off on them would be to physically add my own gusset plate connections (plywood/glue/screws perhaps?) to fix the truss capacity.

All of that sounds like a lot of work (and money) for a solar panel.



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[MikeHalloran]

The shadows make it look like the gang-nail plates are not even fully seated in the wood.

Mike Halloran
Pembroke Pines, FL, USA

 

[TehMightyEngineer]

Why not add your own framing directly supporting the solar panels? Skip the trusses entirely and probably reinforce the roof at the same time. How crazy would it be getting lumber into the attic?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

The configuration of the trusses is a bit unusual, but that is not a problem provided they are adequate to carry the load to which they will be subjected.

Inspect the existing trusses carefully. If they are satisfactory, the small additional load of solar panels is not likely to be a problem. If the existing trusses are not deemed sound, fix or replace as necessary.

BA

 

[boo1]

Photo 2
Truss plate not engaged in the 1 o'clock position web on first 2 trusses
Are truss plate installed in pairs in the 180-degree position on opposite faces of sheathed truss chord members?

only can see one​

Photo 3
Plate not centered on webs

Evaluation issues:
Truss plate gauge, and strength
no bracing visible
wood grade and species

attached is a document about truss evalauation

 

[boo1]

one more doc

 

[ThomasH]

Hi

I have seen that type of connections before. But I think that the plates are usually pressed into the wood from both sides of the connection. That means two steel plates for each connection. I can't see in the pictures if that is the case for the example. And the examples I have seen were factory built.

As for the layout of the truss, that was odd. Perhaps somebody wanted to have a walkway at the center.

Thomas

 

[BS2]

I have designed several fixes for trusses of this type (not this configuration). The capacity of the steel plates is rated in lb/sq. in. of contact area. You can find these values on the manufacturer's ESR reports. HOWEVER, the plates must be tight to the lumber. If not, you lose a significant amount of capacity. For instance, I believe if you can slide a dime between the wood and steel then you lose around 50% of your capacity. If the additional weight of the solar panels exceeds the dead load allowances, you will need to analyze the truss and likely add plywood gussets (fun!).