Making sure that tall towers are safe

America's building owners urged to make sure that tall towers are safe.

Through the agency of the Commerce Department's National Institute of Standards and Technology, the United States Government is calling on all American organisations that develop building and fire safety codes, and the State and local authorities that implement them, to make changes in standards and practice that fully reflect the findings of NIST's three-year inquiry into the World Trade Center disaster of 200l-09-11.

If this call is taken as seriously as the U.S.

Government is demanding, it will lead to the most widespread and in-depth investigation into the safety of America's tall buildings that the country has so far known.

NIST is urging that immediate consideration be given by building and fire safety communities to 30 recommendations to improve structural stability, reduce fire risk and facilitate evacuation from buildings under stress.

Building owners and public officials have been asked to evaluate the safety implications of these recommendations for their existing inventory of buildings and to take the steps necessary to mitigate any unwarranted risks without waiting for changes to occur in codes, standards and practices.

The agency's final recommendations will not be published until mid-September at the 'Putting Recommendations into Practice' conference intended to bring home the importance of its conclusions.

Prior to that a six week period has been allowed for submission of public comment.

NIST makes no criticism of the structural design of the doomed towers, indeed it points out that they withstood the aircraft impacts and would have remained standing were it not for dislodgement of thermal insulation (fireproofing) and the subsequent multi-floor fires.

"The robustness of the perimeter frame-tube system and the large size of the buildings helped the towers withstand the impact".

"The structural system redistributed loads without collapsing in places of aircraft impact, avoiding larger scale damage upon impact".

But NIST is convinced that had its recommendations been in place at the World Trade Center the casualties would have been a great deal lower than were suffered at the time.

As things were, it is a fact that approximately 87 per cent of the estimated 17,400 occupants of the towers at the time the aircraft struck got out successfully.

For those located below the impact floors, the proportion of people saved was 99 per cent.

On the issue of structural integrity, first in eight groups of recommendations, the draft report says that standards for estimating the load effects of potential hazards such as progressive collapse and the design of structural systems to mitigate them need to be improved.

The aim is to prevent progressive collapse (the mode in which the World Trade Center towers finally failed) through the development and nationwide adoption of consensus standards and code provisions.

'Rigorous enforcement of codes is critical'.

Dr S Shyam Sunder, deputy director of NIST and lead investigator in the agency's building and fire research laboratory, insists that rigorous enforcement of building codes and standards is critical in order to ensure the expected level of safety.

"Unless they are complied with, the best codes and standards cannot protect occupants, emergency responders or buildings".

Also under the heading of structural integrity, NIST is asking for nationally accepted performance standards for wind tunnel testing of prototype structures and estimating wind loads and their effects on tall buildings.

An appropriate criterion should be developed, it says, to enhance the performance of tall buildings by limiting how much they sway under lateral load design conditions such as winds and earthquakes.

There are four recommendations for enhancing the fire resistance of buildings: the first of these says that when designing tall buildings, the codes should provide for timely access by the emergency services and full evacuation of the occupants, the extent to which redundancy in active fire protection systems should be credited for occupant life safety and the need for redundancy in fire protection systems critical to structural integrity.

In this connection, NIST recommends evaluating and improving the technical basis for determining appropriate construction classification and fire rating requirements, especially for buildings greater than 20 storeys in height, and making code changes now as far as possible.

The other recommendations in this group ask for a better method of fire resistance testing than the century-old standard for components, assemblies and systems, the development of test methods for performance of spray-applied fire resistance materials, and nationwide adoption of the 'structural frame' approach to fire resistance ratings, so that structural members connected to the columns would carry fire resistance rating as high as the columns.

Procedures and practices used in the fire resistance design of structures should, says the NIST report, be enhanced by requiring that uncontrolled fires result in burnout without local or global collapse.

This effort should include the development of new fire resistive coating materials and technologies and fire performance evaluation for conventional and high performance structural materials.

An important feature of this search for better fire resistance would be evaluation under conditions expected in building fires of the performance of advanced structural steel, reinforced and prestressed concrete and other high performance material systems.

The culmination of this distillation of research experience is a call to action: building and fire safety communities are to give immediate and serious consideration to these recommendations in order to achieve improvements in the way buildings are designed, constructed and maintained, and the ways in which they are used and vacated in emergency response procedures.

NIST to provide technical assistance.

Dr Shyam Sunder said that NIST is assigning top priority to working with the building and fire safety communities to ensure that there is complete understanding of the recommendations and to provide technical assistance.

After the final report is issued in September, the agency will have powers under the National Construction Safety Team Act to conduct, enable or encourage the conduct of appropriate research, and to promote adoption of these recommendations by the Federal Government and its agencies.

This will be a massive task, applying as it does to tall buildings all over the United States.

If implemented with the vigour intended, it should produce a major enhancement of safety in high places, be they in domestic or workplace use.

It will certainly give a much needed boost to public confidence in tall buildings.

As part of its overall World Trade Center outreach plan, NIST has given detailed briefings on tall building safety to the promoters and designers of the WTC redevelopment, including Silverstein Properties, the Port Authority of New York and New Jersey and the New York City Directorate of Building.

Plainly the replacement for the towers which went down in such tragic circumstances in September 2001, if not by then the tallest in the world, should at least be the safest.

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