Wood Strength Question

MFan

Member
Anyone know a quick (and correct) way to calculate strength of dimensional lumber? Such as, what is the maximum weight that a 2x8 can hold at a certain span width, etc.? (Not talking live load/dead load for a roof rafter).


For example, what is maximum safe load on three 2x8"s that have a 9.5 foot span?

Thanks.
 
I am not sure if you are saying it is or is not a rafter. It depends on the species/grade of wood, angle of the rafter and such. I have a big book that includes a small table of that kind of information but I do not find it very helpful since I am using ungraded lumber of species that don't appear in the table. Generally it is best to be quite conservative in my opinion, especially for something that is going to be permanently installed. Another factor is where the load is. A point load in the middle of a span is different from a dispersed load. If you can give some more details of what you are trying to do someone may be able to help you.
Zach
 
Additional information:

I have a heavy object (approx. 1500 lb) to lift off of a trailer. I'm just trying to determine size of lumber needed to support chain fall for lifting object. It's a 10 foot span between uprights.
 
Moment= 1500 x 10/4==3750 ftlb==45,000 in lb
section modulus needed for cheap lumber is S = 45000/800 psi = 56.25 in cubed

2x8 on edge S=bxdxd/6==1.5 x 7.25 x 7.25/6=13.14 in cubed

# needed is 56.25/13.14=4.28 if you use Douglass fir and no knots on bottom four 2x8 would work

2x10 S=1.5 x 9.25 x 9.25/6=21.39 in cube three 2x10 would work
 
My gut feeling is that even three 2X8's is not going to be deep enough, falls into that "Well, I dunno about that" category. I don't think you will have enough depth.
I would go with 2 2X12's separated every two feet by 2X4 spacers.

If I just had 3 2X8's to work with I would offset the middle one by half its depth and fill in below it with 2X4's. That would give a deeper beam. I would orient the half with two boards to the bottom.

Actually if I had any 2X4's to work with along with the 2X8's I would build a triangle with two 2X8's on the bottom and use 1/2 of other 2X8 as an upright and the other 2 quarters as gusset plates to tie in 2-2X4's that makes up the other two sides of the triangle.

This is strictly eyeball type Southern engineering so no guarantees!
 
For a given amount of wood a taller thinner beam tends to have more rigidity as I understand it, so 2 2x12 side by side are much more rigid than 3 2x8. Be careful, it sounds like a somewhat risky business.
Zach
 
I have not done the math. If it where me I would spike the lumber together from both sides with three 16d sinkers every 16 or 20 inches. It is critical the beam is stable and can not twist or tip over. Spike something over the beam where your chain loads. Lift only until your load is not on the trailer, maybe only a few inches. Pull the trailer out from under the load. Never put yourself or anybody in a position to be hurt. You could test your beam on the ground by setting each end on a block and loading it.

I like to say as a joke "Lumber has a tremendous capacity to be overloaded"

carpenter
 
Is the wood white pine, spruce, douglas fir or yellow pine. You would get the most strength with yellow pine followed by white pine followed by douglas fir and spruce last. I'm out of town at present. I have a book with charts with lumber strengths at home. Also if you are using 3 boards together you would get more strength if you would glue them together. If you are really worried about the beam get a piece of 1/4"x6" flat steel and bolt between the boards.
 
Your question is pretty vague. Your three 2x8 would be strongest as an H-beam: two boards flat separated by one board on edge in the center, like an I-beam. The H-beam would be 7 1/2 inches wide and 10 1/2 inches high.

How you attach the load would make a big difference on the wood's capacity. If you just wrap a chain around the H-beam, it would probably bend or crack the top board.
 
the problem with making an I beam out of 3--2x8 is fastening the flange pieces to the web piece--the fasteners have to develop the horizontal shear loads between the web and flanges---also you would need web stiffeners at the supports and the load point
 
It's may understanding that ply wood and OSB is very strong on edge/will not flex. So take 2-2x? and tail them together with a piece of ply wood between them.
 
(quoted from post at 17:19:03 09/11/15) Got any big old shade trees where your at ?

Really. Around here you'd just go cut 3 hardwood trees long enough to get the lift, tie them together at the top and lift away. I've seen people support cars, tractors and heavier stuff that way.
 
Yes I'm sure about the species. With all the home remodeling I do I see Douglas Fir fail more than any other species. I see a lot of homes built about the mid 1950 which the fir framing is pretty much in a state of dry rot. I've even had to replace a few boards in my own home built in the mid 1980's.
 
Ultradog, you are correct. I was in the lumber and related construction business for 30 years. Fiber bend, compression, tension, elasticity or whatever an engineer needs to know is in the WWPA or SPIB design manuals or on-line computer. In the softwood world, yellow pine (followed closely by doug-fir) is at the top of the food chain with western white at the bottom. Eastern woods were somewhat less and agency and were rated by NeLMA (which some of my family belonged to) back then. There are several grade ratings within the species and obviously things like moisture content, knot location and wane also affects strength.
 
>I see a lot of homes built about the mid 1950 which the fir framing is pretty much in a state of dry rot.

No lumber comonly used in construction, including Douglas fir, is subject to dry rot unless water gets to it. Rot is always a symptom of some other problem: Leaking roof, poor drainage, poor ventilation, wood in contact with soil, etc. And fir is not particularly susceptible to rot; it's certainly no worse than white pine.

As for strength, I have two references:
[u:654c4848f0]The Gougeon Brothers on Boat Construction[/u:654c4848f0] says the modulus of elasticity for Douglas Fir is 1.56-1.95, versus a puny .99-1.24 for white pine.
[u:654c4848f0]Ortho's Home Improvement Encyclopedia[/u:654c4848f0] lists the modulus for "Douglas Fir (Larch)" at 1.7-1.8 and "Douglas Fir (South)" at 1.3 (#2), while "Northern Pine" is 1.3 (#2) to 1.4 (#1).
No way is white pine stronger than Douglas Fir.

The only real problem with Douglas Fir is its availability. All the old growth fir is shipped to Japan as whole logs; it's far too valuable to let go through American sawmills. Our local Home Depot will sometimes get in Douglas fir 2x4 studs, when they do I always stock up.
 
The wood I was referring to is normally found in attics where the only water they are exposed to is humidity. In this situation other woods will remain intact if struck with a hammer or pried on with a wrecking bar but douglas fir usually breaks in two very easily. Fir just ages very quickly. Even new wood there is so little difference in the strength the lumber companies often combine white pine, spruce and douglas fir together. If strength is required it's best to look to yellow pine which is considerably stronger than any of others.
 
I lifted a 2300# culvert off of a trailer using 3 12' 4x4s tied together with 1" threaded rod. I used a block and tackle tied to the bumper of a truck for lifting. If I had to do it over again I think I would use a chain fall hoist.

I think you might be better with a tripod arrangement instead of a gantry. The load analysis is considerably easier this way.

Never let anyone get under your load.

Cliff(VA)
 
Stephen, if you don't understand the difference between Douglas Fir and the lesser fir species graded as "Spruce-Pine-Fir", then I have to question how you can correctly identify Douglas Fir.
 
Sorry but I think I can identify wood. I've been working wood for 50 years, 42 professionally. In recent years they have been using Balsam Fir in SPF wood but the old days they used a lot of Douglas Fir.
 

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