Analyzing an existing wood truss (metal plate)
Analyzing an existing wood truss (metal plate)
(OP)
Analyzing an existing wood truss (metal plate)
Hello All,
I am trying to determine the best method to analyze an existing metal connected wood truss to determine if there is any additional capacity left for rooftop installations. Are there any publications out there outline this analysis?
In most cases I won’t have any truss manufacturer specs or drawings.
I was considering just modeling the truss, applying DL’s and anticipated live loads (as per local code), determining reactions, and member forces. Once I have that I can determine stress in the material, but I know that there are other factors to consider other than just member stress,
Any ideas would be greatly appreciated!
Thanks
Hello All,
I am trying to determine the best method to analyze an existing metal connected wood truss to determine if there is any additional capacity left for rooftop installations. Are there any publications out there outline this analysis?
In most cases I won’t have any truss manufacturer specs or drawings.
I was considering just modeling the truss, applying DL’s and anticipated live loads (as per local code), determining reactions, and member forces. Once I have that I can determine stress in the material, but I know that there are other factors to consider other than just member stress,
Any ideas would be greatly appreciated!
Thanks






RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
In Australia, there are truss manufacturers like Mitek and Pryda that have published capacities for metal side plates so I would have a look at those.
This sounds like a high liability risk application that you are working on. As soon as you rate it for a higher load than the original intended use than you are essentially taking on the responsible for the truss design.
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
When you say to design another connection, what exactly are you thinking by connection?
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
here we generally just use a square(ish) plywood gusset both sides with clinched nails. Especially for the ~300 lbs total that the truss is going to see, that's like sweet bugger all. 1/2" plywood with a couple nails into each member would cover that.
If it is only a small number of trusses that need reinforcing then this method is what we found to be easiest and cheapest.
RE: Analyzing an existing wood truss (metal plate)
One other source you may want to look at is the IEBC. I believe they allow an increase of 5% in stress before you need to take dramatic members. This may help you eliminate the need to reinforce all of the members/joints in the truss.
RE: Analyzing an existing wood truss (metal plate)
My 2001 NDS (I do not have the later ones here at home)section 10.1.4 states "Methods of analysis and test data for establishing design values for connections made with more than one type of fastener have not been developed. Design values for mixed fastener connections shall be based on tests or other analysis (see 1.1.1.3)"
Usually the critical slip of one type on connector is greater or less than the other so you have the existing or the new connection taking a larger amount of the load than calculated.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Analyzing an existing wood truss (metal plate)
Plan A: See if the original Truss Design Drawings are available from the owner or building department.
Plan B: Take photos of the plates, sketch the truss, and see if the Mitek or Alpine engineer's will run an analysis for you. They will recognize their own plates and have been helpful in the past in similar situations.
Plan C: Establish a relationship with a local truss manufacturer and truss technician who can run an analysis for you based on your directions (plate sizes, loads, grade, etc). This will not be an engineered analysis since it will not go back to Mitek HQ for seals but it is still better than running it yourself. Insist on paying them a flat amount or hourly for the analysis time so they don't get tired of you. Join the local WTCA/SBC/TPI chapter and make some friends at the local industry meetings if you will be doing this regularly.
Plan D: Do a 2D analysis of the truss for member forces. Then do a shear check on the plates. This can be reliable for single span trusses. The member forces for multispan trusses are highly dependent on stiffness matrices and loading patterns. I'm not as confident in the results for those conditions.
Plan E: Share the load between nearby trusses with strongbacks or structural curbs until you are satisfied that the stress from the new superimposed load is below a reasonable threshold (5% is pretty conservative).
Plan F: Reinforce the truss with gussets, sistering, etc.
Good Luck
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
I would have to ahve to sit and analyze this stuff by hand.
Thanks again for all of the good info.
RE: Analyzing an existing wood truss (metal plate)
http://www.finesoftware.eu/download/file/demo/trus...
RE: Analyzing an existing wood truss (metal plate)
I am looking at the demo for Truss2D (Engisssol) and it is about $100 for the full program. It is a simple frame/truss analysis app.
RE: Analyzing an existing wood truss (metal plate)
Any direction on how to go about designing the plate thickness and fastner arrangement?
Right now I am trying to analyze a metal connected truss, but I also come across wood gusset truss connections.
Thanks all
RE: Analyzing an existing wood truss (metal plate)
If you have a good contractor, as we did, you can support the truss, peel old plates and install new ones using a field press. On floor trusses, this was an excellent solution to undersized plates. On larger attic trusses, we used square/rectangular plywood gussets (3/4") and bolts. This was required because our experience told us that we would have no luck getting enough nails into the right places without splitting and splintering the members.
RE: Analyzing an existing wood truss (metal plate)
Need to determine the wood thickness, then number of nails, and then pattern of nailing (or screwing)
Any Ideas how you designed the plates and nailing pattern (that you didn't use)
RE: Analyzing an existing wood truss (metal plate)
RE: Analyzing an existing wood truss (metal plate)
1. Could you provide some further info on the field press?
2. Why couldn't you use it for the attic trusses?
Thanks!
EIT
www.HowToEngineer.com
RE: Analyzing an existing wood truss (metal plate)
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Analyzing an existing wood truss (metal plate)
I get how he is designing the plywwod gussett, but I can't seem to figure out where he is getting his panel design values of 467 lb/in/pair, and 208 lb/in/pair?
Maybe I'm looking at it wrong somehow? I am stumped? Feel really dumb right now! ha
RE: Analyzing an existing wood truss (metal plate)
Here is my shot at it:
Download APA's 2008 Panel Design Spec (aka Form D510C I believe).
1. If you look at table 5 for 1/2" sheathing you can get 24/0, 24/16 and 32/16 Rated panels.
2. Look at table 4A -> Panel Axial Tension Ft -> for 32/16 rated the allowable is 2800 lb per foot of panel width
3. Convert that to lb per in of panel width -> 2800/12 = 233.33 lb per in of panel
4. multiply by 2 for each side -> 2*233 = 467 lb per in per pair.
Now how we were suppose to know to use 32/16 rated sheathing I'm not sure as it was not stated but it is the only one that works.
EIT
www.HowToEngineer.com
RE: Analyzing an existing wood truss (metal plate)
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Analyzing an existing wood truss (metal plate)
Thanks again.