Strength of lumber question

Ultradog MN

Well-known Member
Location
Twin Cities
See image below.
Let's say I have a 4X8 rough sawn red (Norway) pine beam.
It is supported on two steel saw horses 6'6" apart.
The beam is 14' long.
The weight is on the very ends of the beam - represented by the downward arrows.
How much weight will it carry?
Links, charts, tables or even "swags" are welcome.
Thanks

<Img src="https://photos.yesterdaystractors.com/gallery/pieces/c51322_lrg.jpg">
 
It's essentially a pair of cantilever beams at each end; one from the end to the horse, the other from the horse to the middle of the beam.

I don't know if it will break in the middle from bending or at the horse from shear. I'll guess 3 tons at the end will shear it off at the horse because it has a little more torque (3 ft 9 in to the end) than from the horse to the middle (3 ft 3 in)
 
Depends on a few things. Is the [pressure on the 4 inc or the 8 inch sides of it. If on the 8 inch side it is more then twice that of the 4 inch. It is also said that in a fire a wood beam like that is stronger then a metal beam that can hold the same weight since the metal one will start to bend before the wood one will give due to burning.
 
ask Tom Silva in This Old House.

I would imagine that some where on google would be your answer. Just don't ask an engineer because it would be cheaper to use steel.
 
I'm no help other than to say that you will need physical properties for the pine beam. I don't have those, but if properties are known it is a pretty straight forward beam calculation. Haven't had to solve one of these since freshman or sophomore year of college (~30 years.) If nobody comes up with an answer, I'll give it a shot later.

Not sure if my message a while ago got sent to your spam folder, but would like to converse on my shower question at your convenience.

Kirk
 
Yeah,
Pushing 40 years ago as a millwright apprentice We had some simple bending moment problems to solve. We were given a booklet on strengths of channel iron, I, H and other shapes of steel beams to refer to when solving problems.
There was even a section on common species of wood beams - fir, pine, oak, etc.
Of course the booklet, along with my memory of most of that stuff is long gone.
I sent you an email.
 

Back in the day when I studied architecture , we also learned about building materails . I forget the tables , but do know that red pine is not a very stong material , and 2 rough sawn 2x8 laninated on edge is much stonger than that 4x8 beam .

The strengh is always in the web that's why I BEAM floor joist are so strong even though they only use 3/8 aspinite and 2x3 studs .

Larry --Ont.
 

Back in the day when I studied architecture , we also learned about building materails . I forget the tables , but do know that red pine is not a very stong material , and 2 rough sawn 2x8 laninated on edge is much stonger than that 4x8 beam . My first house had 2x6 white pine floor joist and they all squeked and sagged , of course that was way before the building code was introduced.

The strengh is always in the web that's why I BEAM floor joist are so strong even though they only use 3/8 aspinite and 2x3 studs .

Larry --Ont.
 
Here's the equation you'd use for that problem. From what I could tell the modulus of elasticity of red pine is around 1.6x10^6 psi and the tensile strength is around 450 psi. Using these values and a "Z value" of 4 inches (half the beam depth) should let you figure what load gives you a fiber stress of 450 psi on the top surface of the beam.
Symmetric overhung loads on a beam equation
 
OK Ultradog you really stirred my motivation. First a disclaimer; it has been 60 years since engineering school and doing this stuff everyday but I'll hazard an educated guess. Assuming an allowable stress for Norway pine of 800 pounds per square inch and symetrical loading my calcs say 765 pounds. One caveat if I remember correctly is green and dry wood do have different properties. The formula that I used is Bending Moment is equal to Allowable Stress times Section Modulus. And Bending Moment is the Unknown Load (P) times distance (45") from the horse or support to the end of beam. Section Modulus is Moment of Inertia (width of beam 4" times depth of beam 8" cubed divided by 12) and this all divided by 4. Hope this helps. At this stage I'm long retired and would welcome comments.
Bonnan
 
Brendon's calcs and mine are the same we just differ in our assumption regarding the natural strength of the wood. My reference says 800 and he is using 450 psi. Wood probably varies that much so do your own testing.
 
You want guesses? I bet it would hold a small garage....

Real answers: Do you have a Pocket Reference book? I googled and it looks like they have some info in there about wood beam strength. No idea if it has what you need and my copy is at work but google showed hits under "pocket reference book wood beam strength" and said the book has info on 76 species of wood. I would get a book if you dont have one, About $10 and very handy even if for this question you come up blank, you will still use it elsewhere.

You can also google "red pine beam strength" and get a book scanned from an university where they did tests on wood beams. Its the second hit on google by Henry Taylor Bovey, 1898.

Last, any calculations you get you need to figure in the bounce of running down the road, thats really gonna cut into the weight limit because its an unknown. Drive slow the numbers are not too bad, somebody getting paid by the job will go as fast as he thinks he can get away with it so he makes more per hour of his time, then things get dicey.

Good luck.
 
is the 4x8 laying flat or on edge? what is the nature of the loading--meaning short term or long term?
are you looking for the safe carrying weight or weight approaching failure?
 
Like you say, the little bit of looking I did resulted in a fairly wide range of values for the strength of a particular species of wood. The 450 psi I picked out probably has a significant safety factor cooked into it to account for the variability.
 
Ultradog, I searched for my latest WWPA product use manual and absolutely can't find it. Here is a pdf you can download for free or your local lumberyard will have species specific ratings - normally in the back of their truss supplier catalog. How much weight are the saw horses rated for?


"www.bearcreeklumber.com/.../wwpaWesternLumberSpanTables.pdfABOUT WWPA. "
(WWPA charges about $12.00 for this info but using the Bearcreek address is free.)
 
[i:654c4848f0]"You want guesses? I bet it would hold a small garage...."[/i:654c4848f0]

Ha!
I've been found out.
The 6'6" is the distance across the bed of my trailer.
Garage is 14' wide.
I have it sitting on three of those beams.
Beams I've had for 30 years but they've never been outside - zero rot and no major knots or defects.
Deflection when I lowered the garage onto them was negligible.
I think they will be fine even on rough roads.
The other part of it is I don't know how much the garage weighs.
I could compute the weight of all the pieces but my hunch is it's about 2+ tons.
It sank the trailer springs less than loading my Ford 4000 = 4600 lbs.
If folks here thought the beams would support a ton apiece I would be glad.
I think they will.
The mover said they would haul it no problem.
But what does he know - he's only been doing it for 30 years.
I thought I would check with the experts too - seeing how you you guys are so close at hand.
Thanks to all.
 
14 feet (168 inches) less the 6.5 feet (78 inches) is 90 inches. square root of 90 is 9.487, so they will hold as much wood as 9.487 woodchucks can chuck if a woodchuck could chuck wood...
 
Cannot be answered. You have to specify which end rests on the horses, the 4" side or the 8" side. You have to answer how long duration is. I am not sure what red Norway pine is, but I would have to further research this. Cannot be answered.
 
You have to further specify the dimensions of the steel where it rests on the sawhorse, to detemine possibility of crushing failure. Cannot be answered.
 
At a WAG, I'd say it will work for you. 1 ton would have been my best guess without looking at tables. That said....... the dynamic forces that could be involved in hauling over rough roads multiply things a great deal. If you had another beam it would be safer.. I do think a ton is getting to the upper limit per beam... then you'd want a margin of 2:1 or so at least I think.

Rod
 
Jerry I think you would be fine if you put a 2 x 6 stiff leg from the center of your boards to the center of the roof. Like an upside down T.
Ron
 
(quoted from post at 14:26:39 07/27/15) Jerry I think you would be fine if you put a 2 x 6 stiff leg from the center of your boards to the center of the roof. Like an upside down T.
Ron

Why how put the stiff leg to the floor of the trailer?

I don't think he needs it.
 
The loads being out on the ends it would be good to put a 1/3 of the load in the center. I think he does.
Ron
 
That's an outstanding idea.
Easy to do and would carry a lot of weight.
All the lumber I'm using to support it on the trailer
I will remove when I set it back down in the new location and reuse on the new addition - so no waste or extra expense.
Thanks.
 

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