Bending Induced Shear Stress in Multiple Layer Timbers
Bending Induced Shear Stress in Multiple Layer Timbers
(OP)
We have a relatively large column load that will be applied temporarily and we are thinking of using 12"x12" x 5' timbers to distribute the load. Two layers of timbers will be used and the two layers will be laid perpendicular to each other (bottom layer running north-south and top layer running east-west). Some discussion has come up about whether to divide the horizontal shear stress between both layers since they are laid perpendicular. We are in agreement that if they are laid parallel the shear stress can be halved.
Thoughts?
Thoughts?






RE: Bending Induced Shear Stress in Multiple Layer Timbers
RE: Bending Induced Shear Stress in Multiple Layer Timbers
RE: Bending Induced Shear Stress in Multiple Layer Timbers
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Bending Induced Shear Stress in Multiple Layer Timbers
RE: Bending Induced Shear Stress in Multiple Layer Timbers
I assume the bottom layer (N-S) is 5' sq., that's 5' long 12x12's x 5 pcs. wide. The second layer need not be wider than the base plate on the column, in multiples of 12" wide pcs., of course. The soil bearing pressure will not be uniform, thus the need for the beam on elastic found. analysis. The pressure will, of course, be highest immediately under the column. If you have uncertain soil conditions, so that the soil is not well compacted below the mat, you may want to consider a 3'x3' mat for the lowest layer, made up of 6x6's. These would be driven down, causing consolidation of the soil in the most highly loaded area. Then the 5'x5' mat would come into play, for the total load, at a lower stress level; and the top layer, maybe 3'x5' distributes the column load to the 5'x5' mat. Each layer must be analyzed individually for its bending and shear stresses as a function of some sort of beam on elastic found. analysis. This can more than likely be done long-hand with some good engineering judgement and the advice of the Geotech. Engr. on the job. I wouldn't spend more than $60k on FEA on this one.
RE: Bending Induced Shear Stress in Multiple Layer Timbers
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Bending Induced Shear Stress in Multiple Layer Timbers
Provide a sketch.
BA
RE: Bending Induced Shear Stress in Multiple Layer Timbers
A base plate 5 feet square would not be reasonable or practical unless the column was, well, 3.5 to 4 feet square. How big is it at the bottom?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask