Existing Wood Trusses?

[SteelPE]

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).

 

[woodman88]

Wood trusses are designed per the lumber available to the manufacturer. They may have had only the #1 HF for chords in stock at the time they were built. If you check the next truss configuration with less panels, you should get an overstressed design.
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.

 

[AELLC]

You need to compare actual fc with Fc', the reduced allowable based on slenderness of the truss members. There is also a section in the Code which calculates the slenderness effects of a 2 x 4 truss top chord which has sheathing nailed to it but I don't know it off-hand because I haven't used it in years.

I ran some rough numbers and I am getting about 5800# max compressive force in the top chord.

 

[SteelPE]

OK, I believe I was being lead down the wrong road by some people and now I am getting a design that makes more sense.

When designing wood trusses, does the manufacturer design the chords for combined axial load and local bending?

 

[woodman88]

Yes.

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.

 

[MiketheEngineer]

Woodman is right -- Since 2x4s are generally the smallest available - these type of trusses are often well over designed. And many people have found that buying #1 or even DSS (SYP) is cheaper because there is much less culling of crap wood!! I have one client that only buys SYP DSS - He claims he saves a fortune in labor, call backs, etc. He also uses Microlam or equal in all his kitchens and bath studs. Saves a fortune on installs and no call backs!!

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!!

 

[SteelPE]

Woodman/Mike,

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?

 

[woodman88]

Repairing the top chord for bending is easy. It is the effect of the extra loading on the connector plates (heel and bottom chord splice, mainly) that is the problem. Assuming that the bottom chord lumber still works.

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.

 

[MiketheEngineer]

7% - probably OK. Plates might be getting pushed a bit - but they do have a Safety Factor of about 2:1 or better.

Any time I ask two engineers to analyze a structure and they get with in 5-10% of each other - I am pretty happy!!

 

[SteelPE]

We are trying to keep the increase to less than 5%. We are going to take a look at the truss using the current required code loadings as they stand today..... then we are going to check the truss for the revised loads of adding the solar panels. If the increase in the member forces is less than 5% then according to the IEBC we do not need to strengthen the existing truss.