Engineers are forever ridiculed and vilified for the size of the structural elements they design, yet why are they rarely persuaded to make them smaller?

To answer this we first need to understand the materials being used. Lets take timber as an example; it’s used everywhere because it’s light, economic and easy to form and assemble on site.

A few years ago the timber classifications changed from the recognisable SC3 and SC4 to the new classifications of C16 and C24. Previously the numbers were just identifiers, now the numbers represent the timber strength. However, in the design we take the strength of a C16 timber as only 5.3N/mm2. Some say that this is an arbitrary factor of safety of 3 but we need to look deeper than just the numbers, actually as deep as the cell and grain structure of the timber.

The benefits of timber, i.e. its lightness, ease of cutting and simple fixings are the result of grain structure, or more specifically its variable grain structure, directly influence its strength properties. Timber is not homogeneous; discontinuities in a trees growth rate, the presence of branches, knots and variable grain size all affect strength. Even the processing of the trunk into recognisable section sizes affects the uses of the timber.

To simplify these strength effects, hundreds of timber sections have been tested in the laboratory and the results show that, for a C16 piece of timber, 95% of the tested samples will pass with a strength of 5.3N/mm2.

Next time you hear someone decrying an Engineer because of the size of some timber, spare a thought for those materials factors that nature controls.