Quote:
Originally Posted by mkguitar
a marine grade or baltic birch is hardwood.
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True
Quote:
Originally Posted by mkguitar
many ply woods are made from strands of soft woods and is not meant to resist compression
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NOTE: It has been a
very long time since my structural engineering classes.
That statement is not true.
The issue here is that most tables for lumber and plywood give values for compression
parallel to the grain. However, I am having trouble finding values for compression
perpendicular to the grain for plywood (our situation with an RV on top of a stack of ply pieces).
Soft woods
can and do resist compression. And the classification of hardwood vs softwood has nothing to do with the "hardness" of the wood. As to softwood compression perpendicular to the grain, think of all those 2x sill plates in walls made of Southern Pine. Do you see them failing, being "crushed"? NO, they do just fine. Still, wood is always stronger in compression parallel to the grain than perpendicular to the grain. Think of a bundle of straws - more resistance standing on end than lying on their side.
I found a table that lists these values for solid wood, perpendicular to the grain, but in metric. Converting, white pine is 435 psi. That's still pretty good for this use. Now ply is usually made up of a variety of woods, but I could not find appropriate values for ply. For reference(compression parallel), Virginia pine is 913 psi, Doug fir is around 750 psi. And we are talking psi - pounds per
square inch. So if the contact area of a tire is 10" wide x 2", 20 square inches, that = 15,000#. Even with a value of 500psi that still equals 10,000#.
So what does this mean? Well, you could hire a structural engineer to figure this all out; I don't remember how the stacking up of all those ply layers will affect the overall strength (should increase it). However, often practical experience works. A number of people on this forum are stacking up (and connecting all those plays together with glue and/or mechanically is important, too.) up plywood and it's working fine for them. I would be more worried about making sure the stacked ramps are wide and long enough and the base for them is level and solid (avoid coarse gravel driveways on a slope) Still suggest you use exterior grade and don't leave them out in the weather.
If you want hardwood, you could use something like solid White Oak with a compression value of 1073 psi.