Mar 17 2004
In today's timber buildings, the trend is toward larger and more efficient constructions, often with aesthetic features. As part of this development it's also important to find effective ways to join different building components into a supporting shell--in other words, to find a good jointing technique.
There are several ways of joining construction elements of wood, using glue, bolts, screws, and nails. Nailed joints are simple to produce and have the advantage of spreading the load across large areas of the wood, thus reducing the risk of local splintering. The drawback of nailed joints is that a great many nails are needed if major forces are to be conveyed.
When a nailed joint gives, the break is usually gentle and controlled, a so-called ductile failure. If the nails are too close to each other or if there are too many nails in a small area, a phenomenon called block failure can occur. This break is sudden and uncontrolled (that is, brittle) but it occurs only when the joint is overloaded with the grain of the wood, which is relatively uncommon.
In a supporting structure brittle breaks should be avoided, quite simply because they occur entirely without warning. If the building collapses, the people inside will have no signs that the shell is giving in.
In the dissertation “Plug shear failure in nailed timber connections - Avoiding brittle and promoting ductile failures”, Helena Johnsson at Luleå University of Technology, Sweden, treats the subject of brittle block failures, describes their characteristics, and develops a model for calculating when block failure will occur.
Block failures are not described in the Swedish construction norm today. In the European building norm there is an informative annex in which block failure is described, but the findings of the dissertation, to be publicly defended, show that this description is insufficient. The dissertation defines the limits for when block failure can occur, and recommendations are made about how to avoid this form of joint failure to the greatest possible extent.
Source: Alphagalileo and The Swedish Research Council