Existing Wood Trusses?
Existing Wood Trusses?
(OP)
I have a job where a client wants to add some solar panels to the roof of his house. The existing house is 26'-6" wide and the roof is framed with trusses at 2'-0" o.c with 2x4 chords and web members. The home was built in the 80's in the Northeast where the ground snow load is currently 30psf and has a 5 pitch.
After inspection/measuring the truss was placed into a computer program and run with the new loading conditions (running both balanced and unbalanced conditions together with attic storage loading). I am getting what I would think are some peculiar results. From what I can tell, the truss system is way over designed. My max compressive load is around 2,500# giving me a fc=425psi. The existing members are Hem-Fir #1 with an Fc = 1350psi*1.15*1.15 = 1785psi. Is it common for a truss like this to be overdesigned?
I am used to seeing trusses with members stressed at 99%-101%.... but I can't really think of anything the mfr could have done to increase the efficiency of the system (not like they could use smaller chords or space the trusses further apart).
After inspection/measuring the truss was placed into a computer program and run with the new loading conditions (running both balanced and unbalanced conditions together with attic storage loading). I am getting what I would think are some peculiar results. From what I can tell, the truss system is way over designed. My max compressive load is around 2,500# giving me a fc=425psi. The existing members are Hem-Fir #1 with an Fc = 1350psi*1.15*1.15 = 1785psi. Is it common for a truss like this to be overdesigned?
I am used to seeing trusses with members stressed at 99%-101%.... but I can't really think of anything the mfr could have done to increase the efficiency of the system (not like they could use smaller chords or space the trusses further apart).





RE: Existing Wood Trusses?
Be careful of the connector plates, as most will also be oversize (due to what they had in stock) but some may have been at the maximum allowed stress already.
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: Existing Wood Trusses?
I ran some rough numbers and I am getting about 5800# max compressive force in the top chord.
RE: Existing Wood Trusses?
When designing wood trusses, does the manufacturer design the chords for combined axial load and local bending?
RE: Existing Wood 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.
RE: Existing Wood Trusses?
Double check your numbers - but if memory serves - a HF#1 truss should be good to about 30' to 32'!! Depends on design and loads!!
RE: Existing Wood Trusses?
This may sound like a dumb idea, but I am seeing that the capacity of the truss members is really susceptible to local bending between the chords. If I could eliminate 50% of the local bending these trusses would be able to support a more load then they were originally intended for. Now I am not trying to increase the load on the truss by 50% but actually trying to increase the load by about 7% (the weight of the solar panels).
Does this seem logical?
RE: Existing Wood 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.
RE: Existing Wood Trusses?
Any time I ask two engineers to analyze a structure and they get with in 5-10% of each other - I am pretty happy!!
RE: Existing Wood Trusses?