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Old 11-27-2015, 02:06 PM   #21
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Originally Posted by mkguitar View Post
a marine grade or baltic birch is hardwood.
True

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many ply woods are made from strands of soft woods and is not meant to resist compression
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.
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Old 11-27-2015, 02:51 PM   #22
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The hardwood/softwood thing is really a descriptive definition as to whether the tree is a confer or has leaves that drop every year. Sometime in the far distant past, someone noticed that the conifers seemed to be softer than the deciduous trees and we got the naming. It certainly isn't true that hardwoods are all harder than all softwoods. Is oak harder than pine, yep. Is aspen or poplar harder than fir or yellow pine, absolutely not.

In reality, you won't break a solid wood ramp made out of any of them, unless you are bridging a gap with them, which you shouldn't be doing anyway. In wood ramps, I would bet 99% of the accidents are driving off the end of them or rolling back off them because they usually don't have stops at the end, or tire cradles, like the manufactured ones do.

The manufactured ramps are a totally another story, and I have heard of several of them that have collapsed, both in the plastic and in the formed steel. The analysis I saw on some of them indicated some common traits (other than overloading, which isn't really a trait, just dumb). They did not have a bottom connecting strut from the front to rear, on the sides, so they just kind of caved in, allowing the ends to move out (usually the ramp end). The second common trait was that many of them had been damaged do to a drive off. It is typical in a drive off to put a huge hit on the front end, center, of the ramp with the bottom seam of the rocker panel. This seam is almost always a multilayered, spotwelded, piece that can be very strong. On many vehicles it is even the dedicated jack point. When that knife edge comes down on the top of the ramp with weight and momentum of the vehicle moving it, it hits really hard. Plastic ramps are very susceptible to this kind of damage. I have a set of quite heavy, tied bottoms, 3 ton, formed steel ramps with a deep crease in each of them. It is dead center at the fronts and bent them almost 1/2". I only use them to stabilize a wheel while on jack stands, if I need to, so they never see much load. The damage was done 20+ years ago by on of the kid's boyfriends that came over to change his oil, and was sure he knew how to drive a manual transmission car up onto ramps. The car was a tiny Ford Fiesta(?) that barely weighed 2000#.
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Old 11-27-2015, 03:37 PM   #23
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Almost all dimensioned lumber that the ramps shown so far in this thread are classified as softwoods. As for plywood, the advantage of marine grade is that it will last longer in the presence of moisture. As much as one would use them it probably makes no difference.

Wincrashers ramps are most forgiving in that you can drive up and if you overshoot the top the descent is just as gentle.
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Old 11-27-2015, 03:56 PM   #24
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It certainly isn't true that hardwoods are all harder than all softwoods.

In wood ramps, I would bet 99% of the accidents are driving off the end of them or rolling back off them because they usually don't have stops at the end, or tire cradles, like the manufactured ones do.
Just think, Balsa is a hardwood!

Booster is right, in that the biggest threat to safety is how one uses (or mis-uses) the ramps.

Maybe make the bearing surface of the ramp long enough to allow the use of chocks?
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Old 11-27-2015, 05:26 PM   #25
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Maybe make the bearing surface of the ramp long enough to allow the use of chocks?
The best ones I have seen have done just that, with a relatively tall block permanently attached to the front and a 1" board across the rear for a cradle.
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Old 11-27-2015, 09:46 PM   #26
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I've got some plastic, (reinforced with strands of something in it), 10" tall ramps, that have braces across and diagonally on the underside all tied together that are rated at 12k each. Very light weight. They have vertical stops molded into them at the end to keep from rolling off unless the driver can't handle the vehicle somehow.
I've run our Sprinter van and numerous vehicles up on them over the years with zero issues. Believe I paid about $25.00 for the pair on some kind of special at Autozone 10+ years ago
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