Bending Induced Shear Stress in Multiple Layer Timbers

[dannoMD11]

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?

 

[Lion06]

Is this bearing on ground or are the timbers intended to span an opening? If they are bearing on ground, you have to do a beam on elastic foundation to even get a shear stress to compare to the allowable.

 

[dannoMD11]

I agree a beam on elastic foundation is the way to go on an undisturbed site with adequate geotechnical information, however, we are on a construction site and extensive cut and fill has been performed in the area. The timbers will be placed on the ground and the mat length outside the column was used to calculate the bending and shear stresses in the mat based on the assumption of uniform pressure distribution. When I do this the bending stress is well below the allowable value. The horizontal shear stress is too high.

 

[msquared48]

Even considering double the allowable shear for no knots/checks, etc?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

How big is the foot that sits in the timber? I'm assuming there are a total o 10 of these timbers? 5 in each direction?

 

[dhengr]

Danno:
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.

 

[paddingtongreen]

I don't think you can halve the shear when they are perpendicular, the shear is carried by the top members, to the lower members. You could bracket the design with a couple of scenarios, even soil bearing for a worst case for the timber and say a forty five degree line down from the edge of the baseplate for soil bearing.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

So, what is the footprint of the column? Is the columns sitting totally on one 12x12 or do you have a baseplate which spreads the load out? It makes a considerable difference in the answer to your question.

Provide a sketch.

BA

 

[msquared48]

Usually, in order to not over stress the wood in bearing, and when using wood grillage in this fashion, you would use a series of W shapes to spread out the load to a tapered wood grillage.

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