Analyzing an existing wood truss (metal plate) 1

[AdamJSR]

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

 

[RFreund]

TX - couple of questions in regards to your response:
1. Could you provide some further info on the field press?
2. Why couldn't you use it for the attic trusses?
Thanks!

EIT

http://www.HowToEngineer.com

 

[woodman88]

charliealphabravo - That is a good source of basic information on repairing trusses.

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.

 

[AdamJSR]

Charlie, Thanks for the reference.

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

 

[RFreund]

It me sometime to figure this one out when I came across it as well so don't feel bad.
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

http://www.HowToEngineer.com

 

[woodman88]

If you look at figure 5: Finalized repair, in the article, it states the grade of the plywood.

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.

 

[AdamJSR]

Thanks guys. I focused so much on trying to do teh math that I didn't even really read the specs on figure 5!

Thanks again.