On Thu, 25 Oct 2007 10:23:30 -0500, Ignoramus20839
wrote:

I have steel ramps, but they are very short and suitable for my [low]
trailer only.

I am considering making ramps from wooden boards. Specifically,
picture a ramp that looks like a letter I in cross section. The wide
part of the I is a 2x8. On the top and bottom of the I, there would be
2x4s. The "I", of course, would be laid flat for use.

Here's the ascii graphic:

. .
|==========|
~ ~

The 2x4s would be screwed to the 2x8 with wood screws, and reinforced
with glue.

My question is, what would be the strength of this ramp if it was,
say, 8 ft long. Specifically, how much load could I place in the
middle for some reasonably low deflection (like 1-2 inches).

You forgot an important question.

While the 2 x 4s are probably more than sufficient for most things on
an 8' span, that 2 x 8 is going to be getting a lot of pressure in
it's weakest direction. If you're running something up the ramp that
has a wide tire that can disperse the load across the board, there's
no problem- but if you've got something like a piece of machinery on
narrow casters rolling up the center, there's a reasonable chance of
splitting it. Wood is strong in compression on the endgrain, and in
resisting bowing forces, but pretty weak along the grain.

If you're just making one set of ramps, you might want to consider
using LVL beams instead- they're like a 2" thick piece of plywood that
is glued under immense pressure, and then you'd be really reducing
your chances of splitting the ramp down the middle because the grain
is alternating. They're not the cheapest things in the world, but
neither is 2 x 8 SYP, and you only need one, cut in half.

In article ,
Prometheus wrote:

On Thu, 25 Oct 2007 10:23:30 -0500, Ignoramus20839
wrote:

I have steel ramps, but they are very short and suitable for my [low]
trailer only.

I am considering making ramps from wooden boards. Specifically,
picture a ramp that looks like a letter I in cross section. The wide
part of the I is a 2x8. On the top and bottom of the I, there would be
2x4s. The "I", of course, would be laid flat for use.

Here's the ascii graphic:

. .
|==========|
~ ~

The 2x4s would be screwed to the 2x8 with wood screws, and reinforced
with glue.

My question is, what would be the strength of this ramp if it was,
say, 8 ft long. Specifically, how much load could I place in the
middle for some reasonably low deflection (like 1-2 inches).

You forgot an important question.

While the 2 x 4s are probably more than sufficient for most things on
an 8' span, that 2 x 8 is going to be getting a lot of pressure in
it's weakest direction. If you're running something up the ramp that
has a wide tire that can disperse the load across the board, there's
no problem- but if you've got something like a piece of machinery on
narrow casters rolling up the center, there's a reasonable chance of
splitting it. Wood is strong in compression on the endgrain, and in
resisting bowing forces, but pretty weak along the grain.

If you're just making one set of ramps, you might want to consider
using LVL beams instead- they're like a 2" thick piece of plywood that
is glued under immense pressure, and then you'd be really reducing
your chances of splitting the ramp down the middle because the grain
is alternating. They're not the cheapest things in the world, but
neither is 2 x 8 SYP, and you only need one, cut in half.

I'd also be tempted to bolt the wide side of the ramp 2x8" timbers
together every foot with allthread, fender washers, and nuts. This will
prevent the ramp from splitting into two ramps even if the wood splits.

One could omit the glue, and be able to disassemble the ramp for
transportation and storage.

Joe Gwinn