Expert: Daniel Humphrey - 5/27/2010

Question
Hello........

i have a problem i am trying to figure out, and wondered if you might be able to help.......

I am building a small barn for our horses and i am putting in storage for hay above.....it is only 20' wide making it 19'2" span.  i am looking at around 60-70 lbs per square foot of, i call it live load, the hay, and was figureing 10 psf dead load......  I can't seem to find any information regarding size of joist or spacing for this load........  Our old barn has 3, true 7*7's oak spaced evenly but they sag and are expensive to get them in the length i need.  

I was looking at 2x12 fir(or y. pine) spaced either 16" or 19.2" apart, 3/4 sub floor with 7/16 topping it...........i really want to put up a sound floor(and cost effective), but don't want to be disapointed when i start putting up the hay........If i need to plan on a carrier beam under the joists i can, but need to know ahead of time.......

The fondation is 8" concrete walls for the first 2 feet, and average of 10" of concrete floor on soild compacted stone.  

if need be i was think about doubling up every other joist if need be.

I truely understand if you cant help me, but if you have a direction i can turn to look for any information i need it is greatly appreciated.

Brian Alt

Rural Ohio,45822



p.s.........no building codes here where
i live, except septic....

Answer
Dear Brian,

If you are anticipating only 60 psf of live load, then a normal commercial-type floor would be fine.  Here is a span table:

http://www.raisedfloorlivingpro.com/spantables-2.shtml

Your idea of using 2x12s on 16" centers, maximum should work. I would probably move them to 12" oc just to get the higher weight capacity.

Since you're in Ohio, where lots of barns were built by rule-of-thumb, I think you might enjoy this: To span 20 feet, the old builder's rule-of-thumb would be to build a hay mow floor with the equivalent of a 2x20 joist every two feet. They just used a standard 2" joist for most floors and made the depth 1/12th of the span.

Since 2x20 is not a workable dimension, the carpenters would take the 2" width and multiply by 20" cubed, which yields a stiffness figure of 16,000. So the carpenter would look for workable dimensions that would yield the same stiffness figure. A 2x12 has a stiffness figure of only 3,456. So the old-time carpenter would throw in four or five of those every two feet, just to make the equivalent of a 2x20 on 24" centers. Instead of making all those 2x12s, it's easier to put in a 9x12 every two feet. The performance advantage of a timber over a bunch of 2x12s is that the timber can sag when the hay is wet without trying to flip over and break; so the barn builder would use timbers there whenever possible, and select for flexible wood. Your 7x7 oaks are small and bent, but they didn't break!

As timber framer, I've used this old rule-of-thumb for many floors and have found it to exceed engineered specs. I think it is because the old builders anticipated wet hay being laid up there.

Daniel