I’ve noticed that builders often gang 2x dimensioned lumber to create a beam or girder. I’m wondering if the stiffness of such a beam could be increased by ganging ¾” plywood, albeit more expensive?
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Sure it would, but it's a bit
Sure it would, but it's a bit uncommon so you may have a hard time finding a table that indicates load versus deflection.
Ply is often used as a gusset to stiffen a joist after a plumber has his way with it.
Thanks Scott,
I was just looking at LVL's and trying to figure if it would be cheaper for me to do my own laminations of ¾" plywood ripped at 15 and 15/16's untill I could achieve a length of 36' - 8" .
The problem I ran into was the cost of the 24 sheets of plywood it would take to build a 12 x 16 beam. The other thing I was unsure of is if LVL's are laminated like plywood ... every other sheet with opposing grain, or if every lamination is long grain. So, I can find Fb & E for sawn lumber and engineered lumber, but so far I'm not finding anything on ganging plywood.
Have you looked at LVL's.
Have you looked at LVL's. The laminations are not layered 3/4" plywood. Each layer is long and staggered, glued under pressure. Nothing like building up plywood pcs.
Actually there are specifications for plywood box beams, although not what you are thinking about.
other than that it makes the most sense to simply use lvls or versalams or glue lams.
Without doing the calculations I can't answer the question. If I remember correctly, when designing box beams, you only use the aggregate thickness of the parallel ply's (the perpendicular ply are not considered) when calculating the bending moment. It's been a long time since I design any box beams or stressed panels.
Oops! You've done it again.
Oops! You've done it again. After reading the comments, it seems that the smartest thing I can do is to talk to the design professionals, explain what I'm trying to do and see what they have to offer. Thank-you muchly.
Well, you could do that... or you could just buy the engineered beam that is MADE for long distances.
What you can build is never going to be as strong or durable as something made in a factory using industrial glues and presses.
All those sistered built up beams you see from old construction... normally they are replaced with new engineered beams like gluelams or LVLs because they just don't hold up over time.
You are absolutely right. Thanks for your insight. Also, IdahoDon gave me pause to think about Span Tables for box beams, which I found at http://www.apawood.org/pdfs/managed/Z416.pdf
I had never given much thought to box beams, they were just always there. This little explaination opened my eyes and solved my problems for 12' and 16' headers. That only leaves a beam to carry the ceiling joists and the roof, and a girder to carry the floor joists and everything else. APA-The Engineered Wood Association had the rest of the information needed.
Wow! Thanks!
Plywood only has 1/2 +/- of the grain running along the "length" of the plywood - it would likely provide less increased stiffness than using 1" by (3/4" nominal)
Your right,rjw.
I've only
Your right,rjw.
I've only learned that since the second day of this thread. I'm convinced that ganging plywood was just fools gold, and would be more expensive than buying an LVL.