Well it stood up for the past 20 years
The Insight
Structural engineers walk a fine line when designing structures to ensure they are safe, robust and durable while still being economical. Financial constraints can put pressure on designers to make structures leaner and this also has environmental advantages by reducing materials. Unfortunately, downward pressure on construction costs, has proved expensive for the industry, both financially and reputationally. Despite this, it is not uncommon to hear complaints of overdesign, particularly when designing works for existing structures. “Why do we need to that? It has stood up for the past 20 years without a problem?!”
The Impact
Structural engineers are required to design structures to prescribed standards.
Typically, structures are designed using the ‘Limit State’ method, with design for structural strength being to ‘Ultimate Limit State’ (ULS). Requirements under typical ULS design methodology involve putting together combinations of loads and using these to calculate design actions (bending, shear, axial loads) on the element under review. The load combinations typically either include applying load factors which increase the design action or using loads with higher return periods. The capacity of the element is also calculated and a reduction factor is applied to give the design capacity. Fundamentally, the expected design actions are increased to allow for uncertainty and for extreme weather events, and the expected capacity has been reduced to allow for material and manufacturing inconsistencies.
It is an intentional, calculated, prescribed over-design. Our robust standards mean that the ‘holes in the cheese’ rarely line up.
For example, in cyclonic areas such as Darwin, ULS design of most buildings uses a statistical 1 in 500-year wind event which is equivalent to a mid-range category 4 cyclone. Recent cyclones in Darwin have not been anywhere near this level. Cyclone Marcus, which passed through Darwin in March 2018, was a category 2 cyclone. With wind speeds only about half of the required design wind speeds, the actual wind loads were only about a quarter of the design wind loads.
The fact that buildings stood up and roofs stayed on (which they typically did except for damage by falling trees) says nothing about the compliance of the structures. However, if there had been significant damage, this would have been incredibly concerning given this was nowhere near a design event.
The fact that a structure has stood for the past 20 years without a problem is not an indication that the engineer was being overly cautious, nor is it an indication of a compliant design. It just means that in the past 20 years, the holes in the cheese didn’t line up.