Shear failure with post and beam home plans is much more difficult to envision than bending failure. In fact, with light frame construction, shear failure seldom comes into play, whereas it is an important consideration for heavy timber framing, particularly with a very heavy load such as an earth roof or a steam train.
One good way of explaining shear is to think of it as a combination of compression and tension stresses. Remember that the top surface of a beam is in compression, the bottom surface is in tension, and the centroid (middle part of the beam) is neutral (so also called the neutral axis.)
Some authors, without explaining the relationship between shear and tension and
compression stresses, describe shear as the tension for all of the wood fibers of the beam to "shear off," particularly at the edge of the post or wall support. While not complete, the analogy is true enough for our purposes, and may be easier to
understand than lots of stress analysis.
However – and this is the strange and interesting part – the structure at the top is actually stronger on shear. The effect of the sliding feature of the wood fibers over the neutral axis is increased, because the compression stresses on the top surfaces of the two spans are causing a tremendous tensile stress at the top of the rafter directly above the girder.
Think of it: If the two spans in the post and beam home plans are each trying to pull away from each other, because of the load on each span, those wood fibers at the top of the rafter (over the center) are working really hard not to break on tension.
All of this translates to lower shear strength at this location. In the top picture, shear stresses over the supports are clearly the same at all four shear locations, expressed by the fractions l / 2 in each case. But, in the bottom picture, the shear stresses are expressed as% at the walls at the right and left but increase to Ys where the long rafter is supported by the girder in the middle.
The upshot is that shear strength is gained by using two ten-footers instead of a single zo-footer supported in the middle. It is also true, as we have said, that bending strength is slightly diminished in the former example, and deflection is increased – but if the weak point in the engineering happens to be in shear, the former example may be better.
This situation may work in our favor, when you consider that two ten-footers are much easier
to handle – and certainly less expensive – than a single twenty-footer.
Deflection is similar to bending … But different. Bending concerns us most when it translates into bending failure, which is a bad thing. With deflection, we can tolerate certain amounts of it in certain circumstances. Springiness – or stiffness – in a floor is a characteristic of deflection. When dealing with post and beam home plans know that cracking plaster on a ceiling, or separation of covered sheetrock joints, is an indication of excessive deflection.