Estimating wooden ramp strength

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Regarding use of 2x4s vs. 2x6s on sides, I decided to use 2x4s and center support (support the ramp in the middle), which I can do with cinderblocks or jackstands, etc. Makes ramps a lot easier to deal with.
i
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On Thu, 25 Oct 2007 11:48:26 -0500, Ignoramus20839
<snip>

Just as a reference, my father made some ramps of 3"x12"x~10' rough sawn live oak he used for over 30 years to load levee rollers on the bobtail IH. The levee rollers were concrete spools with heavy angle frames that weighed about 4800 lbs. We'd put three on the truck and deliver them around to implement dealers, which is why I had a commercial license at 16. Dad had mounted a hydraulic winch in the bed, and we'd tilt the dump bed about 5 deg. up to load and unload. Those old loading ramps never broke, although they did develop a raised ridge over time down the middle. I guess something to do with the way they were sawn.
Every time I took a load through the old Baytown tunnel, I'd have to stop for an inspection. The chain boom handles had to be safety wired, 2"x4" cleats nailed behind the rollers, etc. I guess they didn't want to see three rollers unload down the tunnel as I exited.
The ramps were also a bear to handle, being pretty damn heavy. The attachment to the bobtail (single rear axle truck) was just a small angle bolted to the ramp set in a groove formed by a piece of bar welded to the back of the bed.
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On Oct 25, 5:39 pm, Ignoramus20839 <ignoramus20...@NOSPAM. 20839.invalid> wrote:

While you are playing with the design, think about a couple of blocks you could put between about the middle of the ramp and the ground. If you had a block in the middle of the span, it would be like two 4 foot ramps in series. And I think the capacity would be much higher. So you could use you original design which would be lighter to set in place.
Dan
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Good point. It is not trivial, though, to add that middle support that would be usable with different heights. But it is a very good thought, and a solvable problem.
i
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Ignoramus20839 wrote:

Actually, it is pretty trivial, just add an attachment point for basic RV stabilizer jack stand. Something like this: http://www.campersworld.com/product-view.php?product_id332:4204
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On Oct 25, 7:22 pm, Ignoramus20839 >

You don't have to have it height adjustable. Just a reasonable size so it will fit under the ramp. If the ramp is being used for a low thing, the support may be close to the high end, and the unsupported length a two foot span and a six foot span. If it is being used on a very high thing, the support may be close to the ground end, and again you have a two foot and six foot span. But when you play with the program you will see a six foot span is much better than an eight foot span.
Of course cutting in to two four foot spans is even better, and those aluminum support jacks look like a nice solution.
Dan
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Using deflection as a criterion for beam design is not to be condoned.
The beam MUST be designed with an allowable BENDING stress in the top and bottom flange of the beam.
Once this stress is at an acceptable level can the design be checked for deflection and, if too large, the beam be made stronger TO REDUCE THE DEFLECTION.
Starting with deflection one has no idea what the bending stress is and whether it has exceeded the allowable limit.
Since I generally do not design in wood..... if someone will supply the info on type and grade of lumber, and allowable bending stress (from a building code, perhaps) I would let you know the load capacity.
Wolfgang
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snipped-for-privacy@hotmail.com wrote:

The building codes for residential wooden structures do not in general specify allowable bending stress, they specify allowable deflection.
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snipped-for-privacy@hotmail.com wrote:

I think that Machinery's Handbook has strength and modulus data on wood.
One thing to consider is tipping due to an offcenter load, if the road surface is too soft and the ramp too narrow.
Joe Gwinn
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Joseph Gwinn wrote:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
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On Oct 25, 2:22 pm, Ignoramus20839 <ignoramus20...@NOSPAM. 20839.invalid> wrote:...

Car jack stands work well for this as long as you tie the ramps to the vehicle and use a crosswise plank between the stands and the ramps. The ramps may twist if supported only by the narrow stand tops, and when the vehicle sinks from the load, the ramp ends lift off the ground and slide. Put some tiedown eyes on the sides of the ramp.
Jim W
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Why the "I" on the ramp? Especially at the ground end, it serves no purpose that I can see and only makes it harder to get things started up the ramp.
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Why not nix the idea of the 2X4's on the sides and do this instead. Make your own metal ramp ends that you fit to the ends of the ramps and then rout slots on the underside of the boards where you run a couple of 1/4 or 3/8 rods that are fastened to the metal ends. This way the lumber will be in compression, and the steel will restrain the wood from flexing too much.
I did something similar to some saggy rafters and was able to jack the whole roof straight.
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Roger Shoaf

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I like this idea, though the earlier idea of bolting on angle is probably easier.
Alternatively, consider changing from box store softwood to a locally milled rough hardwood like oak or ash. The wood itself is stiffer and you'll gain a little more stiffness from it being thicker (close to a full 2").
--Glenn Lyford
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As I sse the problem with the angle idea is that when the ramp is loaded, the bolts you use to secure the angle to the wood will want to twist in the wood, ant that would as I see it be the start of a failure.
By using the steel on the underside as I suggested the sag would be limited to the ability of the steel to streach against the unwillingness of the wood to compress against the end grain like a wall stud.
You could induce a little preload when welding the rods to the end caps if you flipped the ramp upside down and between two fulcrum points. If you welded one end, and then placed a weight on the center the wood would flex down, (Actually "up" in its usual orientation.) and then you could weld the other end. When the weight was released, the rods would already be in tension.
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Roger Shoaf

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