How can I find the maximum load capacity at the center point of an 8-foot 2"x12" board?

Question

Also, can the capacity be doubled when sistering another board to it?

Answer 1

Depends on the grade of wood and how you are loading it. Taking a typical 2x12, the allowable bending stress, Fb, is around 500 psi. The actual dimension of a 2x12 is 1.5"x11.25" thus resulting in an section modulus, S, of 1/6*b*h^2 = 31.64 in ^3. Therefore the allowable bending capacity, M, is equal to Fb*S = 15820 lb-in. If a load, P, is applied directly at the center of the board, then the allowable load is P = 4M / L, where L is the length in inches (96 inches). Therefore, the allowable load is 4*15280/96 = 660 pounds. This represents the "safe load" that can be applied. Theoretically, the board will fail in bending around 3 times that amount, or around 2000 lb. The allowable value of 660 lb is what you use when determining how much load to place.

And doubling up will provide double the capacity, given that the concentrated load is shared equally between the two beams. If not, the beams would have to be connected along the length to transfer the load appropriately.

Answer 2

How can I find the maximum load capacity at the center point of four 16 foot 2 X 12 boards SYP screwed together supported on both ends

Answer 3

One question, is this board standing on edge like a typical floor joist, or laying flat like a plank. If the former, then the first answer is correct (and quite detailed). If the latter, the allowable load will be considerably less, you can redo the calculations given in the fist answer, swapping b and h (I got about 88lbs). However, in the case of a plank, if a second board is securely fasted so as to transfer shear loads, the capacity increases four times (note the h^2 section modulus calculation)

Answer 4

Try this Website:


http://courses.cit.cornell.edu/arch264/cuhk/courseNotes/timber/timber.html

and the capacity can be increased if the two boards are attached together with a bonded surface that is strong enough to carry the horizontal shear load due to bending, whether with nails, bolts, epoxy, screws, or whatever.
Whether the load can be doubled or not, you would have to calculate that.

Answer 5

Lumber Loads