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Stronger regrowth

Technical feature WTC7

Buildings have already started to rise from the ashes of Ground Zero.

From Manhattan, Diarmaid Fleming reports on progress.

Whatever changes September 11 might finally bring to New York City design codes or the future for tall buildings, reconstruction of the World Trade Center 7 (WTC7) building is certainly setting it apart from the original.

It is still unclear exactly what triggered the collapse of the 47 storey WTC7 after it had burned unchecked for seven hours (see box). Rebuilding it was a top priority for the city. WTC7 straddled an electricity substation servicing much of lower Manhattan and the need to reconstruct the electricity distribution network has overridden debate on the future of Ground Zero.

Neither client Silverstein nor structural engineer Cantor Seinuk would discuss the new superstructure design, but it will be significantly more robust than its predecessor.

Instead of drywall (plasterboard) protection to the elevator shafts and escape stairs, the designers have opted for a rectangular reinforced concrete core with walls 700mm thick.

Enhanced redundancy has been designed into the new steel frame, allowing up to two columns to be lost without collapse.

All column/beam joints will act as moment connections providing full fixity to columns and girders. The sole function of these will be to allow alternative load paths to develop if columns fail. Lateral loads on the building will be carried by the reinforced concrete core.

Concrete was also preferred for the complex five storey structure which will support the main office tower and house electrical substations and transformers. It is believed this was as much for its blast resistance as any dissatisfaction with the performance of the massive steel trusses on the fallen building.

Office floors, as before, will be of composite construction, consisting of 75mm metal decking with 62.5mm concrete topping.

The decking will be supported on 525mm steel 'filler' beams at 3m centres, spanning 13.75m from the core to the perimeter beams. Total floor area is just over 150,000m 2, including about 7,000m 2for the electricity substation.

Rebuilding began in July, which is remarkable considering the clear up at Ground Zero had just finished. Completion is expected by the beginning of 2005.

On site, work is still in the ground. Geotechnical consultant Mueser Rutledge is supervising the installation of 100 'caisson' piles by local contractor Urban Foundations.

Leading the efforts are Mueser Rutledge principal George Tamaro - fresh from his vital role in stabilising the WTC 'bathtub' - and his protégé and number two Pablo Lopez. Construction manager for the site is Tishman Construction.

It is a return journey for Tamaro. He worked on foundations for an office tower around the substation in the late 1960s and came back again in 1983 after the developers of WTC7 decided to build a much larger and higher tower on extended foundations.

'In fact we're able to reuse some of these original bored piles, ' Tamaro says. 'We exhumed some and did metallurgical tests on the steel H section cores to determine if the fire had had any effect and discovered they were okay.

'Some of the concrete strengths were a little low, but this was only encasement concrete, not in the core.'

This time bored piles 900mm in diameter - known in the US as drilled-in caissons - are driven up to 21m to rockhead then socketed up to 5m into the mica schist bedrock. A 530mm by 430mm, 1t/m steel H-section is dropped into the casing with a tremie pipe strapped to its web. Self-compacting concrete is pumped down the tremie. No extra reinforcement is used, despite compressive loads of up to 2,500t.

Finding locations to install the piles is no easy matter. While demolition and recovery work at Ground Zero has moved on to reconstruction of the PATH train infrastructure, the area is still a hive of activity. Relics of previous work mean a myriad obstacles to be avoided.

In addition, the back of the 'bathtub' wall runs along the edge of the site and anchors installed last winter to stabilise it are in the rock running under the WTC7 site.

'It's like trying to put additional spines in the back of a porcupine - trying to thread needles through needles, ' Tamaro says.

'We will be very careful in our initial alignments, and we're prepared to reinstall anchors in the event we hit any, ' he adds.

Utility reinstallation near the site also competes for space.

Lopez says: 'Once WTC7 collapsed, the utility companies just severed the ends and re-routed them, so work to install new services is proceeding alongside us.'

About 100 new piles are to be installed, and it is hope to reuse at least 15 of the originals. 'By the time we're finished, if you took a compass to site, there's probably going to be enough steel here to send it crazy, ' quips Tamaro.

Collapse conundrum

Perhaps the most puzzling event of September 11 was the spectacular collapse of WTC7. No modern steel framed high rise building is known to have collapsed completely and suddenly as a result of fire alone. Even the Federal Emergency Management Agency report into the WTC disaster failed to come up with a convincing scenario to explain the apparent failure of one of the massive steel trusses in the first seven floors, which then caused progressive collapse (NCE 9 May).

These trusses transferred loads from the 40 conventional office floors above to an irregular grid of pile foundations in and around an existing electricity substation.

Within this transfer structure were also located standby diesel generators and transformers, with substantial fuel stores at ground level.

FEMA's theory is that at some point after a blaze had been started by debris from the Twin Towers, the fuel line supplying one of these generators partially ruptured while the emergency fuel pump kept running.

This would have sprayed diesel fuel into the inferno, creating high enough temperatures for long enough to soften the heavy steel of the transfer truss.

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