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High hopes


A riskfibased approach to new buildings, plus tie force requirements, could prevent progressive collapse, says David Scott.

I am often asked why the US, five years after 9/11, has not followed Europe's lead and developed a code to prevent progressive collapse.

The answer is quite complex.

First, codes in the US are much more prescriptive than those in Europe. They are considered by the legal system as a minimum standard of care, to be followed explicitly. There is some reluctance by engineers to codify, particularly anything as vague as progressive collapse. The implication is that the designer, builder and owner could be held liable. But of course designing against progressive collapse using simple tie rules does not guarantee performance.

A few years ago, I attempted to create a simple model to demonstrate why the old British rules should be used in the US. I took a simple 10 storey building with a 30ft span, (the bread and butter of the US market) and designed the steel connections for the nominal lateral force. We then ran a nonfilinear finite element analysis and removed the midfi column. It failed.

After several attempts to refidetail the connections and change the removal time we concluded that they could not work. Instead we looked at a combination of steel connection and slab tie action. This proved to me that simple tie rules will only work if there is adequate ductility, which is not really addressed in the simple rules.

However, it is not the fear of being sued, nor the questions about how effective the rules can be that prevents the rapid implementation of rules in the US. There is a strong lobby here who question the necessity.

'Show us the risk,' they say.

We can not find any examples of progressive or disproportionate collapses in the US and we have been building high rises since the early 1900s. I do not think it is because US detailing rules are as good as UK progressive collapse rules at preventing this phenomenon fi is Ronan Point the only example here?

Both the World Trade Center and the Alfred P Murrah Federal Building in Oklahoma City could have easily complied with the old UK rules with no different outcome. The performance of these buildings may, however, have been improved by adopting a riskfibased approach, which is essentially the direction the UK is going. Risk is also driving the US approach; all new government buildings now have much more onerous progressive collapse requirements than anything in Europe.

But for normal high rise buildings, US clients and designers are very focused on cost. If the cost of building goes up due to new progressive collapse rules, the concern is that we will end up with less building for the sake of small risk.

All tall buildings should be designed with some nominal tie force requirements. They are cheap, easy to implement and improve robustness. A riskfibased approach may also provide a middle ground for all parties. When I see these buildings supported by tall slender columns that come down on the kerb, I cringe to think that they might have been designed without an alternative load path or damage mitigation.

David Scott is chairman of the Council of Tall Buildings and Urban Habitat, and is principal with Arup in its New York office.

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