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lumber strength


 
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John Niolon
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PostPosted: Tue Aug 26, 2008 6:15 am    Post subject: lumber strength Reply to specific post Reply with quote

you guys know everything so here"s a pop quiz

what is the difference in breaking strength of a plain old pine 2x10 vs a 2x12 ?? In a ramp situation using this lumber I know the 2x12 will probably be stronger but how much...???

I"ve looked for specs on tensile strength and such but it"s all engineering/math double speak and I don"t know what a modulus is anyway...

plain english... how many pounds to break it across the wide face ??? I know greenness and grade and moisture content weigh in...but if I buy a 2x10 and a 2x12 at Home Depot, what weight will break them ???

john
 
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IH2444
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PostPosted: Tue Aug 26, 2008 6:23 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

Depends a lot on the individual board. How many knots, etc...
 
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JML755
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PostPosted: Tue Aug 26, 2008 6:42 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

john niolon,
If I understand correctly, you want to lay the boards flat (not on edge) to make a ramp, correct? Like a ramp to load a tractor or mower onto a trailer or truck?

If so, I don't know of any testing done on the boards this way. Lots of data on span/load specs for various size lumber since this is used in building structures like floors/celings/roofs. I think any good carpentry book includes this stuff.

As for ramps, I use 2-2x8x10' to load stuff into my pickup. Heaviest is a large brush mower and the ramps flex pretty good with my weight plus the mower weight. You can beef up the ramps by using angle iron along the edge or putting support strips lengthwise on the bottom.

What kind of ramps are you talking about? How much weight? How long?
 
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D13
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PostPosted: Tue Aug 26, 2008 7:05 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

I use 2 ramps made of 2x10" oak , 3' long, to load 7000# tractors on my trailer. When i use pine, each ramp is 2 wide 2x6 with a 2x4 joiner down the crack, and it's all that it can do to handle 4000#.

My Electra 225 went thru the 3 year old treated 2x6 pine bed of my trailer. I rebedded in rough oak and 15 years later I have had no problem.
 
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Coloken
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PostPosted: Tue Aug 26, 2008 7:13 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

The 2 by 12 will hold about 20 percnt more than the 2 by 10. Laying flat. thats very rough cause a 2 by 10 isn't 10 and a 2 by 12 isn't 12. Assuming same lumber. Very, very approximate. Actually, its the vertical thicknees where the strength increases. Double the width doubles the strength. Double the vertical thickness gives 4 times the strength (I think).
 
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Stan in Oly, WA
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PostPosted: Tue Aug 26, 2008 7:18 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

Hi John,

David-OR is the person who can answer this for you. He's not the only engineer who contributes to this forum, but he's the person who most consistently deciphers these kinds of problems into language that the rest of us can make sense of.

Since many of the forces acting on wood are expressed in PSI, it's simple enough to do the math and see that a 2X12 should be about 21% stronger than a 2X10 per unit of length (using the actual rather than the nominal widths.) The force that it would take to break either a 2X10 or a 2X12 being used as a ramp would depend of the length of the board, of course. A 4' 2X10 supported at the ends is going to support a lot more weight than a 16' one, isn't it?

All the best, Stan
 
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john in la
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PostPosted: Tue Aug 26, 2008 9:07 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

While I can not answer the question; I can tell you these guys will need to know the length of span and most likely the degree of incline to give you a weight.
 
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JDemaris
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PostPosted: Tue Aug 26, 2008 9:18 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

There is no such thing as "plain old pine." There's a big difference in strength between southern yellow pine (very strong) and white pine (very weak). Also, many inbetweens e.g. Scrub Viginia pine red pine, jack pine, etc.
 
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VaTom
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PostPosted: Tue Aug 26, 2008 5:11 pm    Post subject: Re: lumber strength Reply to specific post Reply with quote

My Forest Product Laboratory (USDA) handbook lists several pines with "Work to Maximum Load". 5.2 in-lbs/cu in to 13.2 in-lbs/cu in, for clear wood.

Your boards have about 14.25 (1 x 9) and 17.25 (1 x 11) cu in. If you take an average of pines to be 9.2, your boards will support 131.1 and 158.7 in-lbs.

This is for slow loading. Hickories average about 3x the capacity.
 
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RobMD
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PostPosted: Tue Aug 26, 2008 7:05 pm    Post subject: Re: lumber strength Reply to specific post Reply with quote

It also depends if you get no. 1 or no. 2 pine. no. 3 will probably break horribly.
 
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36 coupe
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PostPosted: Wed Aug 27, 2008 2:24 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

Treated lumber seems to be lower grade.Lot of twists and knots.steel ramps are the best way to go.A broken plank will dump equipment on its side.
 
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Billy NY
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PostPosted: Wed Aug 27, 2008 4:02 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

Maybe consider an engineered wood product as well, like a microlam or and O.S.H.A approved scaffold plank, when using lumber I tend to double up, especially for a ramp, extra bracing and shores to reduce spans whenever possible.

Prior to the OSHA approved planks, scaffold plank was 2x10 actual, rough cut clear spruce, I would double those up, at 16'-0" long, from about 4 feet high to the ground, was a single axle flatbed truck, I could raise the body to help a little, a 3000 lb. car would make em deflect a bit but never any trouble. WOuld have been safer to also put an intermediate shore under the middle, they would not have deflected at all by splitting the span in 2.

No matter what, NEVER trust an arrangement like this, when lumber fails, most times it snaps quickly, no time to react, so always play it safe. It is surprising how strong some species are, especially orientated on the side, but one thing to remember, avoid any with imperfections and know that a species like SYP, can be and is brittle, especially after being Kiln dried, they use different grades for roof trusses, I worked in a lumber yard delivering trusses years back and this species can and will fail on its side very quickly, PT wood is often times SYP, so it's kiln dried, then pressure treated, dried then saturated, sometimes making it unpredictable, I don't like it on the flat unless it has a short span.
 
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David - OR
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PostPosted: Wed Aug 27, 2008 6:46 am    Post subject: Re: lumber strength Reply to specific post Reply with quote

This problem is harder than it sounds.

As an engineering material, wood is highly variable from one piece to the next. The laboratory breaking strength of a "perfect" small piece of wood is 5 to 6 times the allowed usable design strength of even a very good grade of lumber. There is a fairly involved series of adjustments applied to come up with a "safe" load for any particular size, species, lumber grade, and even duration of load.

As the others mentioned, the load needed to break a ramp depends on the length of the ramp. In fact, it depends on the CUBE of the length of the ramp, so the difference between a 6 foot ramp and a 12 foot ramp is a factor of 8; the 12 foot ramp will carry just 1/8 the load. So a rather critical detail is missing.

A 2x12 is nominally 11.25 inches wide, a 2x10 nominally 9.25 inches wide, so the difference in strength is naively 21 percent. It isn't that simple, though, because the probability of a defect (knot, check, or split) gets higher as the plank gets wider.

To get a first cut guess on something like this, I usually calculate the amount of bending, rather than trying to figure out the theoretical breaking strength. If you use something like Span/128 as a safe amount of bending, the stresses are at least in the ballpark for being OK. The amount of bending depends on the stiffness of the wood (that dreaded modulus of elasticity), and not so much on lumber grade, etc.

Beam bending for a point load is given by the formula:

Displacement = W * L * L * L / (48 * E * I)

Where E is the modulus of elasticity and
I is the moment of inertia.

For a plank, I is given as:
I = b * h * h * h / 12

A 2x10 has I = 9.25 * 1.5 * 1.5 * 1.5 / 12 = 2.6
A 2x12 has I = 11.25 * 1.5 * 1.5 * 1.5 / 12 = 3.16

E is around 1200000 for ordinary "pine"
or around 1600000 for Southern Yellow Pine

Putting the formula to work, assume a 6 foot long ramp. Allow span/128 deflection, or 72/128 = 0.5625 inches.

0.5625 = W * 72 * 72 * 72 / (48 * 1200000 * 2.6)

Solving for W gives

W= .5625 * (48 * 1200000 * 2.6) / (72 * 72 * 72)

W = 225 pounds. A 2x12 would be good for 273 pounds.

Stepping up to Southern Yellow pine or Douglas fir with E = 1600000 gives 300 and 364 pounds

At this point you are probably objecting: "I know from my own experience that boards are stronger than that. That's ridiculous. Just 225 pounds?? On a 6 foot long Board??"

And in reality they "probably" would take more load. But the "extra" strength depends on exactly what that particular board looks like, and can't easily be predicted in an engineering calculation.

In the end, one thing is clear. Don't try to load a tractor onto a trailer using "ramps" made of 2x10 boards. Maybe a quad, but not a tractor.
 
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