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Sixties' revival in New York

PILING & DEEP FOUNDATIONS

Foundations for the new building on the site of the collapsed WTC 7 in Manhattan include some of the original piles. Diarmaid Fleming reports from New York.

One of the most puzzling events of 11 September last year was the spectacular collapse of World Trade Center 7 after it had burned unchecked for seven hours following the terrorist attacks.

No modern steel-framed high rise building is known to have collapsed completely and suddenly as a result of fire alone. A 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.

Rebuilding WTC7 was a top priority for New York. The 47-storey building straddled an electricity substation serving much of lower Manhattan. Although the future of the rest of the area is still a matter of intense debate, the need to rebuild the electricity distribution network has overridden any hesitation.

The replacement structure will be taller and have a smaller footprint than the original.

Construction began in July and is expected to finish in 2005. Based on the layout of the new building and the loads it will impose, it was decided that about half of WTC7's piled foundations could be reused if they were undamaged.

Determining the extent of any damage was the first task for New York consultant Mueser Rutledge Consulting Engineers, led by geotechnical partner George Tamaro.

Tamaro's work on the site - like his vital role in stabilising the WTC 'bathtub' foundation to avert a flooding catastrophe (Ground Engineering March 2002) - is a return to old haunts. He worked on the foundations for an office tower around the substation in the late 1960s, and returned in 1983 after the developers of WTC7 decided to build a much larger and higher tower.

WTC7 was supported by about 125, 30-42inch (762-1067mm) diameter vertical and raked New York City-type caissons socketed into the hard mica schist bedrock about 23m below street level.

The piles were designed with capacities ranging from 454t to 4264t and mostly penetrated a superficial fill layer and natural silty sand, with rock sockets between 1.2m and 8.5m long. Piles typically consisted of a driven steel outer shell filled with concrete and reinforced with a wide flange steel H-section core.

Visual inspection immediately after the building debris had been removed did not reveal any obvious damage to the piles.

Foundations remained buried and protected beneath the concrete pile caps and foundation mats, which had only been damaged near their top surfaces.

New York based testing firm Lucius Pitkin then investigated the extent of damage to the foundation steel and concrete, taking samples and carrying out insitu metallurgical tests.

'We exhumed some of the piles and did tests on the steel H-section cores to determine if the fire had any affect and discovered they were okay, ' says Tamaro.

'Some of the concrete strengths were a little low, but this was only encasement concrete, not in the core.' Mueser Rutledge associate Alice Arana adds that because the piles were in such good condition, they could be treated as 'new' and so calculations for vertical loads were straightforward.

Lateral load resistance calculations were more difficult, she explains. WTC7 had raked piles to resist lateral loads, with the vertical piles only supporting vertical loads. Because of the difficulty of installing new raked piles in the underground maze of piles, ground anchors and services, the new structure has to rely fully on vertical piles fixed to a thick concrete foundation mat to resist both lateral and vertical loads.

The vertical piles derive their lateral stiffness and capacity from composite action of the concrete, the steel core and the steel shell in bending. The cross-sectional properties (and hence the bending stiffness) of the original piles are very similar to that of the new ones and they will share 'as equal partners' with the new caissons in resisting the lateral loads, Arana says.

While the new piles could be designed to ensure there is sufficient capacity in combined axial load and bending to resist the applied vertical and lateral loads, the original piles were not designed for this role.

Another factor in determining the lateral load capacity of the original piles was the presence of splice welds in the piles' steel cores and shells. The former were designed only to resist axial loads, the latter to resist loads caused by pile driving.

Most of the piles being reused are those installed in the 1960s and design information is sketchy. Without the full details and locations of the splice welds, engineers had to take a conservative approach.

The first decision was to reduce the loads these piles needed to resist. Instead of fixing them to the foundation mat like the new piles, they are pinned to the mat.

Remarkably, this small change in the connection detail reduced the lateral loads on the original piles by more than 60%.

It was then assumed that a splice weld existed in both the casing and the shell for each pile, but probably not at the same elevation. Each pile section was checked for two conditions. The first check assumed the core beam splice had no bending capacity and that the pile consisted of a composite section of the concrete and the steel shell; the second that the shell splice had no bending capacity and that the pile consisted of a composite section made up of the concrete and the steel core.

The lesser of the two calculated capacities was determined to be the design capacity of the piles.

Fortunately none of the existing piles were found to be overstressed using this method.

MRCE is now supervising the installation of the 100 new piles by local contractor Urban Foundations for client Silverstein. The structural engineer is Cantor Seinuk and the construction manager is Tishman Construction.

Urban Foundations is installing a similar system to the original, with 900mm diameter piles installed down to bedrock, with sockets up to 5m long. A 530mm by 430mm, 1t/m steel H-section is dropped down inside the casing with a tremie pipe strapped to its web. Self-compacting concrete is pumped down the tremie to form the pile.

No extra reinforcement is used, despite compressive loads of up to 2,500t. But finding places to install the piles is not easy. 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 industry. Relics of previous work mean a myriad of obstacles to be avoided.

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

The Ground Zero bathtub wall runs along the edge of the site and the anchors installed last winter to stabilise it are fixed in the rock beneath WTC7. They had to be fitted in around temporary anchors installed during construction of the bathtub in the late 1960s, Tamaro explains.

Detecting where the anchors are is nigh impossible. 'There really is no practical way to find them - we will be very careful in our initial alignments, and we are prepared to reinstall anchors in the event we hit any, ' he adds.

Utility reinstallation near the site also competes for space.

Tamaro says: '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.'

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