Expert: Daniel Humphrey - 1/27/2008

Question
I went to this calculator.
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

My question is: For a 2x6 results, what span would I gain if 3/4 inch plywood were glued between two 2x6 joices?
Would it be 1.5 times, 2 times etc?
Thanks Bill Whatley  

P.S. your answer will not get to my computer,(addresses only) however, I will get notice and will download it from my servicer.


Answer
Bill,

As you probably know, when you add one additional thickness, you add just one more unit of strength. To be conservative, you would consider that the plywood adds another .25 unit of strength to two 2 x 6 pieces.

Wood has different kinds of strength that we're interested in for structures: the ability to carry loads, stiffness, and flexibility. Some wood that is stiff and resists flexing has a relatively low breaking strength, etc.

Plywood is sometimes used as a stiffener to take bounce out, but it doesn't have more breaking strength than regular wood, and sometimes less. In the sandwich you're talking about, the 2 x 6 joists on each side keep the plywood vertical so it doesn't deflect. In that upright position, plywood is very stiff and may add more than .25 units in resisting deflection, but may not increase breaking strength by much.

Somebody probably has put together a sandwich like that and measured its different strength properties.

Daniel