Post And Beam Home Construction: Compression

Compression for a post and beam home construction — in wood, not my father's car engine — can be thought of as the tendency to crush or compress under a load.

The actual crushing or compressing does not have to be measurable to be real. If I stand on a stout — say, 12-inch diameter by 12-inch high — oak chopping block, my weight puts that chopping block in compression, even though I am having no measurable impact upon it.

My entire family could balance atop the block to no effect, yet the block is definitely in compression.

It might seem that such a stout block would never fail under compression, and yet it can under extreme circumstances.

In October of 2003, my friend and I rotated a 2O-foot long 2o-ton stone on a i2-inch-wide pivot made of a dense hardwood, an incredible concentrated load.

Yes, the pivot eventually failed — it was crushed and ruptured apart, finally — but we did manage to swing the stone through almost 90 degrees of arc before it did.

The stresses on posts or columns for post and beam home construction are due mostly to compression, particularly if the line of thrust from above is straight down through the center of the post.

(Our chopping block example, incidentally, is simply a short stout post.) However, if the line of thrust wanders out of the middle third of a post — or a wall — then the side of the post or wall where the load is concentrated is in compression, while the side away from it is in tension.

Compression and Tension in Beams For Post And Beam Construction

Beam is a good catch-all word to identify a (usually) horizontal timber whose job it is to carry a load across a span.

Girders and floor joists are common specific examples, as are lintels over doors and windows. Even though many roof rafters are pitched to some degree, they perform as beams, too, although other thrust considerations come into play.

Let's load a simple but imaginary beam to see how it works.

We'll make it a rather flimsy beam so that its exaggerated performance will show what's happening. Imagine a 12-foot long two-by-eight plank spanning — flatwise — from one support to another. If the ends of the plank are each bearing on a foot-wide concrete block, the clear span between supports is ten feet.

Now I'll step on to the center of this "beam," rather carefully, with my i/o-pound weight. Obviously, the plank sags in the middle, and quite a bit. But it probably doesn't break, even though it has me a little worried. What is happening is that the underside of the plank is stretched under my weight; that is, it is in tension.

At the same time, the molecules on the top surface of the plank are trying to crush together; it is in compression. Allowing that this is true — and it is — it follows that an imaginary line along the center of the plank's thickness is neither in compression nor tension. This line is known as the centroid or the neutral axis.

An imaginary beam as here described would be very springy, somewhat like a trampoline.

Move one of the supports inward four feet, and we are on the way to inventing both the cantilever and the diving board. Interestingly, when the beam is cantilevered by placing my weight at its free end, the top surface is now in tension and the bottom surface is in compression.

Instinctively, we know that to lay a "beam" flat like this is — well — stupid. Obviously, if the plank were rotated 90 degrees along its transvese axis — so that it looks like a proper floor joist — it would be very much stronger against bending pressures. It would feel quite stiff to walk along, providing I could maintain my balance for 10 feet. We may think that we know this instinctively, but I submit that it is a matter of our post and beam construction experience more than instinct.

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