Expert: Dan Griffin - 12/17/2009

Question
QUESTION: My goal is to add a hay loft to the small (24'x24') barn I just built. It is supported by nine telephone poles 12' oc, (avg 8" diameter).  The center post is being used as an anchor for temporary bracing to each center pole to prevent wall bowing, but it and the outer posts will eventually carry beams (doubled 2'x12's, estimated capacity 3691lbs over 12' according to the wood beams load tables from awg.org) which will create four 12'x12' loft floor sections. Based on the height of the roof, I estimate being able to stack about 100 bales per 12'x'12' section. The bales I buy typically weigh about 50lbs. 5000lb over 144 square feet gives a minimum required capacity of about 35psf, so I am rounding up to 40psf for my estimations. The span calculator at awc.org does not accommodate this dead load, so I have been trying to extrapolate from what I do know to determine the appropriate sizing and spacing of my floor joists. According to the calculator, a 1'oc floor of 2x6's would only carry half the required load. I own a lot of 12' 2x6's, and I am committed to finding a way to build the floor without buying more lumber. I have thought about three possibilities.

1. Going to 6" oc: 6" spacing is not available in the span calculator, and simply halving the joist spacing does not seem to directly correlate to doubling the allowable load or span, so I don't know how to make this estimate.
2. What about doubling the 2x6's on 12' centers? The wood beams load tables show that a 4x6 is more than twice as strong as a 2x6, so this might satisfy my needs unless there are variables I am not taking into account.
3. What about doubling the 2x6's vertically by edge-gluing them and incorporating blocking into the floor? The wood beams load tables show that a 2x12 is nearly 4x stronger than a 2x6 - would this would allow for greater than 12" joist spacing?

I'm aware of the difference between actual and nominal dimensions - I'm using the estimates above with some confidence since this structure is not intended for living quarters and is somewhat more forgiving of deflections and so on.

Would you recommend one of these solutions over the others? Would you recommend something else entirely? Thanks very much for your time and assistance!

ANSWER: Doug, a few thoughts. . . good alfalfa bales that I had to stack ran 80 to over a 100 pounds.  I like to use these span tables because they let you adjust more details:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Southern+Pi

You do need to verify the grade of lumber that you have and the species.  You do need to verify the total weight you are considering and if this will be a uniform load.  If you are not planning to check with a structural engineer, at least check with a local lumber yard and use some way to monitor the deflection when you load this structure.  Bearing and fastening will be crucial to hoping this will work.

I cannot tell you that, sight unseen, this will work.




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

QUESTION: Thank you for getting back to me. I haven't seen an 80/100lb bale since I was a kid. They just don't bale them that tight around here. While I feel pretty confident about my weight estimate I will increase it by 50% for safety. I will also double check the grade and species and of 2x6's, but the calculator you refer to is the one I used which doesn't go up to 40psf dead load or allow for smaller than 1'oc spacing. If you are aware of a tool or set of tables that would allow for these values I'd appreciate a link or reference. I will definitely do as you suggest and measure deflection during loading. In general, can you tell me if it is valid to consider properly doubled members to be equivalent to a solid member of the same resulting dimension? Thanks!

Answer
Doug,your hay is a live load, as any live load is the weight applied to the structure =  it usually implies something movable that allows the weight to come and go, just like your hay.  Dead load has to do with the actual weight of the material that makes the rated assembly - the joists, the decking, etc.  The table allows you to  take the live load to 100 lb/SF.  The strength of structural members does not gain substantially by being wider, it gains tremendously verical depth, hence a 2x12 on edge is much stronger than 2x4 on edge.  A 4x4 laying down is not much stronger than a 2x4 on edge.