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Freedom Tower to be world's safest


ALL KEY NIST recommendations have been included in the design of New York's controversial Freedom Tower, making it the safest megatower ever built, its developer claimed last week.

'We firmly believe that we have correctly anticipated the findings and recommendations in the NIST report-' said Larry Silverstein, who is redeveloping the site of the collapsed World Trade Center towers and neighbouring WTC7.

'The Freedom Tower in particular- will be the safest highrise offive building ever constructed, and serve as the exemplar for the design of safe office buildings, ' said Silverstein.

Structural engineer for the Freedom Tower and the replacement WTC7 is Skidmore Owings & Merrill (SOM).

SOM partner Carl Galioto said that complying with NIST's recommendations on designing for simultaneous evacuation of all floors, while maximising the remoteness of escape stairwells within a conventional building fl oor plan was a major challenge.

'Current New York codes would allow you to have two escape stairs back to back in the core, separated only by a fire barrier, ' he said. 'On WTC7 , we effectively pulled the core apart' (see diagram).

'Elevators open into internal corridors (within the core) protected by smoke stop doors.

'Stairwells are at diagonal corners [of the core] to maxi mise survivability in any extreme event.' Anyone escaping down the stairs will bypass the main lobby and be directed to one of several routes to exit at street level, Galioto added.

Like WTC7, he reedom Tower will have a high-strength C80/95 concrete core, with walls up to 900mm thick. Galioto said the key to the tower's safety is 'lots of system redundancy'.

'For example, there will be two sprinkler standpipe risers within the core, with alternate take-offs at the floors to minimise loss of function if one riser is put out of action, ' Galioto said. 'Water storage is dispersed throughout the core, and the risers are looped at the top so that fl oors above a break in one riser can be fed from the other.' Sprinkler spacing will be much closer than the norm, at one per 15m 2, and sprinklers will have a higher rate of flow. And there will be 60 minutes' supply capacity rather than the normal 30.

Specified fire protection for the structural steel frame is five times higher than current New York code requirements, Galioto said.

Core design is outwardly conventional. Public elevators open out from the core; there are six internal service elevators and two public escape stairwells. Galioto said the stairwell design was highly developed.

'The stairs are 20% wider than the norm to allow users to descend side by side. People entering the stairs do so with the flow rather than against it.

'To help people see, the stairwells are pressurised, and there are battery powered lights to back up the emergency lighting and phosphorescent paint in case the light batteries are fl at.' SOM has also developed the concept of a self sufficient central 'core within the core'.

This includes a dedicated emergency responders' stairwell and protected freight elevators for the supervised evacuation of people with impaired mobility.

All equipment is seismically robust and able to sustain high impact levels.

Galioto said SOM had evaluated the use of protected public elevators, as recommended by NIST, but had concluded that the public was so used to warnings about using elevators in fires that the concept of unsupervised evacuation via the public elevators was not viable.

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