The entertainment world is excited because it is Oscars time. Rocket scientists are over the moon because Space-X have just landed another rocket first stage back on earth. Medicine is agog because it’s now possible to make a human with three parents. And in structural engineering… well… we are excited because there’s a new timber standard out!
I can tell you now that it’s a standard you almost certainly never look at. Yet it will affect your work on an everyday basis. This standard is, for the record, “Design criteria for timber framed residential buildings“. It forms the technical basis of many of the span tables and software suites that you use.
It’s a standard that has a long and chequered pedigree. The last version was written in the nineties. In my work library, I have a CSIRO guide to deflection limits that comes from the 1960s. And in my university lecture notes I have a reference to floor loadings derived back in the 1940s that were measured when people were “dancing furiously”. (And just for the record, although I am feeling my age, I wasn’t around then!) Much of the information in those older documents has flowed through to this new standard.
To be frank, not really much has changed from the previous version. It did however, have a different standard’s number – which gives you some sense of how exciting and dynamic the world of a structural engineer is.
There has been some minor messing around with the load combinations for the design of floor members. But since almost all floor all members are governed by deflection rather than strength that’s unlikely to change your span tables or software significantly.
The standard has the long overdue inclusion of wind beams. Personally, it’s a relief to me, because I won’t have to keep explaining what they are to builders and just get either blank or hostile stares back. That is the same look I used to get when formal wall bracing first came into domestic construction. The old-school builders just thought all the engineers were totally out of touch to insist on bracing walls. Now that wind beams are in the standard I am hoping to see more of them on sites and so have less of those difficult conversations with builders.
There is one very subtle difference which won’t turn up in any span table but which is perhaps the most important change of the lot. The original code writers felt it was too risky to write a generic code that could be used for the design of any individual member. It was felt that it was okay to set design criteria for span tables where the members were used in “traditional timber framed construction”. In theory, the code was not meant to be used for anything other than span table development. Back in the 1990s when this previous version was written, “traditional construction” generally still meant fairly short spans.
Fast forward to today and houses are being built on steroids with longer spans, higher loads and more spindly members. This has increased the risk that extrapolation of “traditional construction” to these more extreme buildings could lead to failures or serviceability problems.
So it is perhaps with a measure of courage that the writers of the current code revision have opened it up to be used for the construction of timber farmed houses more generally with the only limit being that they fit within the scope of AS1684.
A good example of this is the design of verandah posts which assumes that the load is “concentrically applied” – that is to the centre of the post. That was always a somewhat bogus assumption which, I believe, was made so that the new (back in the 1990s) designs matched even older span tables. If the design models the reality that verandah beams are generally half-checked on the post you get much smaller allowable post loads and lengths.
This new standard effectively allows that someone might use its basis design a very long deck posts capable of supporting, say, 30 people at the party. With a move away from those “traditional” spans, and if you didn’t know that the load was supposed to be concentric (who would), there is an increased risk of failure. Indeed, if I were to assess the design of the actual construction more rigorously using a detailed engineering approach, there very likely would be an unacceptably high risk of failure.
So, this code potentially will be used for more and more exotic forms of construction as building construction evolves. My message is a simple one in a complex situation: I reckon you should be continually doing a reality check and asking yourself “are my designs that are based on this code consistent with something like traditional construction or should I be really taking a closer look at this?”
Next month, for ongoing excitement in the field of timber engineering, we will look at how to clean your calculator buttons and I will write a poem on my favourite kind of fastener. Eat your heart out, Oscars. You can’t beat that for excitement!