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The poles in a pole barn provide both vertical load bearing and resistance to wind blowing perpendicular to the wall. 3 2x6 boards will come out to 4.5 inches by 5.5 inches. A 6x6 is 5.5 by 5.5 inches. So the "laminated post" has only 85 % of the cross sectional area. However, the design value for compression parallel to the grain for a Number 2 6x6 Hem-fir post is 575 PSI. The size adjusted value for a 2x6 Hem-Fir is 1430 PSI. So the laminated post has almost twice the gross bearing strength. Gross load bearing is usually not the issue in a column of any significant height. The ability to carry weight depends upon the height of the column and the stiffness of the post in both the in-plane and out-of-plane directions. In-plane stiffness can be improved with horizontal bracing, such as the horizontal girt boards typically applied. A set of three nailed 2x6's has a dubious shear strength, not to mention a smaller dimension in one axist than the 6x6 post. So it becomes important to orient the 2x6 laminated post such that the 5.5 inch dimension is perpendicular to the plane of the wall. Our laminated column is very weak in the 4.5 inch direction, and needs to be laterally restrained. Horizontal girts typically used to carry the sheat metal walls can provide adequate in-plane restraint, assuming a spacing of 4 feet on center (or thereabouts) and reasonable nailing and/or metal plate connectors are used. Gluing the plies of the 2x6's together could provide additional shear strength in the weak axis, but is not necessary if the girts are present. The embedded poles are usually also asked to provide resistance to wind blowing perpendicular to the plane of the wall. Here again, the laminated post is a clear winner over a 6x6, but ONLY if the 5.5 inch dimension is oriented perpendicular to the wall. Consideration should be given to use of two posts, oriented in opposite directions, at each building corner. I don't understand how to "alternate the grain" of the three laminations. All three boards should have the grain should oriented parallel to the length of the post (i.e the board is vertically loaded in the same way as the tree it once was). One could alternate the curvature of the growth rings, but this provides no particular benefit other than possibly reducing cupping or twisting of the composite beam as it dries. But such an issue seems unlikely for pressure treated wood, given the moisture control used in its manufacture. It is important to stagger any joints in the individual plies relative to the joints in the other two plies.
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