Expert: Bruce E. Johnson - 3/21/2010

Question
QUESTION: Hello,

I am building a pole building and have been trying to find information to compare the structural abilities of a #2 fir/larch 2x12 versus a 1 3/4 LVL beam.  Specifically, the plan that we are using was from a building that was originally built as a 36x48 building that we will be building as a 30x36.

For the rafter beams, the original plan called for a 1 3/4 x 11 7/8 LVL to span between 12 foot spaced apart posts (4:12 pitch) on each endwall and two LVL's nailed together for the interior posts.  Since we have made the building smaller we will be spanning 10 feet, not 12.  

What I would like to do, if it is structually sound, is use two 2x12s nailed/bolted together instead of the LVL for the end walls and three 2x12s nailed/bolted together for the interior dble LVL.  If this is possible, I need to find some information/proof to be able to submit to the building inspector.

Any information you can give me would be appreciated.

Thank you.

Todd

ANSWER: Hi Todd, go to the following website.
www.cwc.ca/Resources/E-Tools/SpanCalc.
The span calculator will provide you with your answer based on your roof loads.  Just plug in the information and it will calculate your maximum spans.  But most ridge beam requirements are based on the size of the rafter and the length of the end cut of the rafter that attaches to the ridge beam.  In other words if you make your 4/12 cut on the end of a 2x8 rafter, your ridge beam should be at least the size of that rafter end which will be larger than the 2x8 because it is cut at an angle.  If your rafters sit on top of the ridge beam then the span table calculation should work fine.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Bruce,

Thanks for your response,  Unfortunately, I don't think I made the building's design very clear.  The roof isn't designed as a ridge beam/rafter system.  It's a pole building that is designed with 4 "rafter beams" (for a building 36 ft deep) that attached to 6x6 posts spaced 12 feet apart front to back, and 10 feet apart side to side (the building is 30 feet wide). The rafter beams run from the eave to the ridge.  Then 2x8 purlins hang on these rafter beams at a spacing of 23" O.C. The purlins actually run parallel to the ridge/eave.  A steel roof is then attached the purlins.

We are using an engineered plan that a friend of ours used to build their building, though we are cutting the foot print down since we don't need the size of the building he had and need to save on costs.  The plan says it wants 1 3/4 x 11 7/8 LVLs used as these 4 rafter beams, but LVLs are expensive.  I wanted to use doubled up 2x12s, to save on costs.  And as I said, this rafter beam is now only spanning 10' between posts and not 12' as it was on the original plan.  The building inspector said that would be okay, but we would need to provide some official documentation that a dble 2x12 would bear the same load as the 1 3/4 X 11 7/8 LVL.

I was hoping I could find some reference online that would give me this information, some table or chart that says over a 10 foot span a 2x12 will bear X amount of load, and an 1 3/4 x 11 7/8 LVL will bear X amount of load.  But I haven't found anything that has given me information that straight forward.  

Thanks.

Todd

Answer
Hi again Todd, I am not an engineer.  However I don't see why, with a 10' span, you can't use a double 2x12 for this application.  A 4x12 beam can effectively span 12' and has been doing so for a long time.  The rule of thumb is one inch of height per foot of span for 4x material.  However if you are getting a building permit you will need a structural engineer to seal your plans anyhow so it wouldn't hurt for you to call one up and have him sign off on it, seal it, so you can submit it.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com