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: Strength of Materials - 2x8 PT Lumber


jerry_nj
01-20-2006, 11:02 PM
I have been using 2x8 inch pressure treated lumber for ramps into my shed. I have been using for an number of years without any problem, but at times I do wonder how much load these planks can take before they brake.

In my application the planks/boards are about 5 feet long and sit with one end on the ground and the other about 4" above the ground on the shed floor. I drive my GT6000 tractor (about 700 pounds?) and myself up these planks. I weight about 250 pounds so I'd estimate the load at about 1000 pounds, or 500 pounds per plank.

Is there a "rule of thumb" for how much load wooden planks can take, carry?

bontai Joe
01-20-2006, 11:54 PM
Depends on how many knots there are and if they are small or large. I use 2 x 8 planks for my truck and have put the same load on them for several years with no problem. Length also makes a big difference. A plank 10 feet long and only supported on the ends will hold a lot less than one 5 feet long

Carl
01-21-2006, 09:23 AM
Yes there is a rule of thumb kinda. But it is a formula. span in feet equal to depth in inches. But that is 2 X 'x' being in the upright position. So it won't work for you.

A quick calculation says that will hold up 200 pounds per plank. Yours is holding up 300+/-. I guess that the safety factor works.

Argee
01-21-2006, 06:57 PM
As a rule...treated lumber is SYP (Southern Yellow Pine) which does not have a great load bearing rating like fir. If you were to cut some tapered stringers on the bottom side, you should have no problem.

Oslo in NC
01-21-2006, 07:05 PM
Jerry, you could put some thin plywood on the plank and screw it down. You could, as Argee said, put a stringer on the bottom. YOu could also put a prop or two mid way. Seems you are going well beyond the load that those planks can handle. Be careful what ever way you choose to go. oslo

Carl
01-21-2006, 07:36 PM
Or just put a block under the center. Hasn't broken yet though. One thing that might be saving it is that as your tractor climbs the ramp it sags. As it sags more of the plank is resting on the ground thus shortning the ramp.

Argee my book gives the strength of southern yellow pine as the same as or a little greater then the fir used in construction. It will vary from species to species, and tree to tree. As Bontai Joe says it also depends on the number of knots too. That is in the grading and the strength tables take that into account.

jerry_nj
01-21-2006, 09:56 PM
All, thanks.

Thinking on what was said, I think I overstated my load. If the tractor plus me is 1000 pounds, the truth is the tractor wheel-base (a Sears GT) is about 4 or 5 feet, so as the rear wheels are starting to come onto the plank, the front wheels are just at the top or just moving onto the shed floor. So, I doubt that the two planks are ever subjected to more than a total of 500 pounds, 250 pounds each. I am not really worried about these breaking they have held up well for many trips. The question really came to mind, given my experience with my tractor/shed planks, would I not be able to get away with using a 2x8 plank between two ladders, say, as a scaffold, to do some house painting from. I now think it is not strong enough, not if the plank is supported at 4 or 5 foot spacing.

Carl
01-21-2006, 10:34 PM
The friend that helped me roof my shed said that his bro-in-law had some scafolding that goes from hangers between two ladders. They use 2X12s. I have a couple left from using on side rails for stairs. 12 footers. I will support one from the ends and see how solid it feels. I think my friend said that there were 2 of them side by side for the working width. My calculations say that 2 of them side by side should support about 240 # in the center of a 10 foot span. But then it could still be kinda bouncy.

bblalocksr
01-27-2006, 07:17 PM
Jerry, there is a book out called the Archithical (sp) Digest it lists all the different lumbers their span strength etc. You may want to get it from the lib. it cost over $100.00 but it will tell you what you need.I have been using 2x8 inch pressure treated lumber for ramps into my shed. I have been using for an number of years without any problem, but at times I do wonder how much load these planks can take before they brake.

In my application the planks/boards are about 5 feet long and sit with one end on the ground and the other about 4" above the ground on the shed floor. I drive my GT6000 tractor (about 700 pounds?) and myself up these planks. I weight about 250 pounds so I'd estimate the load at about 1000 pounds, or 500 pounds per plank.

Is there a "rule of thumb" for how much load wooden planks can take, carry?