Wood Stress Failure
Wood is a stiff, strong material that many builders enjoy working with. In addition, it is a natural resource that, unlike many natural resources, is renewable; you can plant more trees if wood becomes scarce.
Different types of wood can withstand different amounts of pressure; the amount of pressure that causes the wood to break is called its stress failure point.
You need to know the strength of your wood, expressed in terms of how much pressure the wood can take before cracking or breaking, before you begin building.
The amount of pressure under which the wood begins breaking is the wood stress failure point, and you need to consider this point when you design your building so that you don’t put too much pressure on the wood and cause your building to collapse.
Wood stress failure is affected by the fasteners you place in the wood. If you place your fasteners too close together, it puts too much pressure on a small portion of wood. Thus, you’ll see cracking and other signs of wood stress failure around the fasteners.
This problem is solved by using fewer fasteners and putting 2 to 3 inches of space between each fastener so that the pressure is distributed more evenly over the piece of wood.
How to Test
Engineers test various types of wood with a compression machine. The engineer must apply slow, steady pressure to a piece of wood and raise the pressure over time. When the wood begins to crack, the engineer knows she has reached the stress failure point.
Engineers can then report on the stress failure of various types of wood, and you can use that information when planning your building design.
Elasticity is different than stress failure. Wood might sag or bend under the pressure of a large load, but if the load is less than the wood’s stress failure point, the wood will bounce back to its original shape once the excess weight is removed.
For example, if you put too many books on a bookshelf, the wood will sag, but if you then remove some of the books, the wood will stop sagging and become useable again.
You still want to be careful of overloading wooden pieces, because large load pressure can cause stress failure if it is applied for a long period.