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Cause and effect

Sweeping recommendations from the US team investigating the Twin Towers tragedy could have a lasting influence on codes and building technology worldwide.Dave Parker reports.

In October 2002 the US National Safety Team Act gave the National Institute of Standards & Technology (NIST) a very specific brief. It was charged with investigating the collapse of the three World Trade Center towers and 'identifying, as specifically as possible, areas in current building and fire codes, standards and practices that warrant revision'.

On 30 June NIST published its draft final report into the collapses of WTC1 and WTC2 - the 'Twin Towers' - with the equivalent document on WTC7 due sometime later this year.

Thanks to earlier interim drafts there were no real surprises on NIST's analysis of the collapse mechanism (see box). But NIST concluded with 30 specific recommendations in eight distinct categories, and it is on these that most attention focused.

Given the shock of the events of that day in September 2001 it is hardly surprising that NIST's first group of recommendations should cover increasing structural integrity, especially the prevention of progressive collapse.

NIST points out that most design criteria for progressive or disproportionate collapse, such as those used in the UK, assume the sudden loss of one or two columns at ground level.

It recommends that future codes should consider other possible traumatic events, such as bomb blasts and gas explosions which take place elsewhere in the building, as in the 1968 Ronan Point disaster. NIST does not however believe buildings should be designed for aircraft impact.

Enhanced re resistance is the focus of the second group of recommendations. Least controversial of these perhaps is a call for a major overhaul of 'the century-old standard for fire resistance testing of components, assemblies and systems'. NIST is particularly concerned at the lack of scientific rigour in scaling up small scale furnace test results into ratings for larger elements and uncertainties in the relationships between prescriptive ratings and real fire performance.

Inevitably NIST also sees a pressing need for more sophisticated testing for spray applied fire resistive materials, including tests for abrasion and impact resistance. And in the short term at least it believes the best route for code drafters is the 'nationwide adoption' of the prescriptive 'structural frame' approach, which calls for equivalent prescriptive standards of protection to all steel members that contribute to overall structural stability.

This may seem like a retrograde step, given the advances in performance-based fire engineering seen recently in Europe. But some engineers see this as a safe and practical route for the standard office development, especially coupled with the first recommendation in the next group.

'New methods for fire resistance design of structures' opens with a demand for designers to adopt as a universal objective that uncontrolled building fires should result in burn-out without local or global collapse. Achieving this objective without performance based fire engineering analysis should be an interesting task. But NIST calls only for the development of performance-based methodologies, supported by new test methods and software, all of which will take time even if European research is accepted.

Core design in tall buildings is likely to undergo major changes as a result of several NIST recommendations. A requirement for the escape stairs to be designed for total rather than phased evacuation and to be separated as far as practical will produce bigger, more complex cores, almost universally constructed from high strength concrete. They will contain hardened elevators for firefighter access and the evacuation of 'mobility challenged' occupants, at least two sprinkler supply lines and a fire fighting riser.

Assumptions that concrete will take over as a preferred structural option seem to be premature. NIST has hedged its bets, carefully and publicly declining to recommend any particular material or system over another. It does, however, recommend that more research should be carried out into the actual fire performance of advanced structural steels and high strength reinforced and prestressed concrete.

It is worth remembering that NIST can only recommend, not enforce, and that it will be at least three years before any of its recommendations appear in local US codes. It is also the case that the vast majority of super high rises are being built in the Middle East, Asia and China. What impact NIST's recommendations will have in these areas is still far from clear.

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