THE DESPERATE search for those buried under the rubble that was once the World Trade Center (WTC) has hinged on the judgement calls of engineers.
Structural engineers working up to 16 hours at a time are questioned relentlessly by the emergency services and contractors on the site: can we move this piece of steel? Will this stretch of rubble support a crane? Can we work underneath the facade of this building when it has huge pieces of steel debris protruding from the cladding?
Decision making has had to be faster and based on much scanter information than engineers are used to, said Guy Nordenson of Guy Nordenson Associates, who volunteered to help on the site.
But in the first frantic week there were no reported serious injuries during the search and rescue operation.
At Ground Zero, the site of the collapse, standard 23t elements of WTC facade are precariously stacked as high as six or seven storeys. Four holes at the end of each element reveal that the bolted connections just came away during the collapses on 11 September.
Equipment monitors the dangerous rubble piles day and night and engineers beneath have worked feverishly to advise the four main contractors on the site - Amec, Bovis Lend Lease, Turner and Tishman - where to site cranes to lift the steel away.
At what was the main WTC plaza, the ground level floor slab has collapsed into the huge shopping mall underneath making it treacherous to get 1,000t plus cranes near enough to speed debris clearance. In the first week the only major crane in action was an 800t unit between the North and South Plaza Buildings west of the site.
A lack of drawings of the underground levels has also made it difficult to place cranes (see p12). Engineers have worked with the Metropolitan Transit Authority which has marked tunnel routes to be avoided through the middle of the plaza and along neighbouring Church Street.
Crane loading has been provided in some areas by backfilling the debris with earth and laying huge pieces of steel on top. One such operation was needed on the basement slurry wall around the WTC complex before a crane could be loaded across it.
After a week and a half of preparation, designing and fabricating steel crane supports, massive cranes were ready to go in.
As NCE went to press more than 20 cranes were installed on site.
An 800t crane was placed last Thursday; a 1,000t crane last weekend and a 1,500t crane earlier this week.
The object is to edge ever closer to the apex of the site from Church Street to the east and West Street opposite.
Huge piles of rubble on Vesey Street north of the collapsed 47 storey Number 7 World Trade Centre building hamper access to the site. Neighbouring streets are meanwhile clogged with vehicles and equipment.
Organisation of the engineering teams had to be changed quickly in the face of all these logistical challenges, said Arup New York senior associate Nancy Hamilton who left her role on the £1bn terminal at JFK airport to join the site.
'The main problem has been the handover of shifts from one team to another, ' she says.
'Sometimes our team's shift would finish while we were right in the middle of solving a complex problem and there would be a long overlap time while the problem was explained to the incoming team.
'To begin with some problems would get cycled through a few shifts before being solved and some engineers would be on site for up to 16 hours while problems were handed over.'
Co-ordinating consultant Thornton Thomasetti has responded by extending shifts from eight to 12 hours to give more chance of a problem being dealt with in one shift.
Engineering teams have also been matched up with a particular contractor to give continuity.
'Different consultants were saying different things to the contractors which was very confusing, ' said Arup New York principal David Scott whose team is now working with Amec.
The move reflects how Ground Zero has become 'more and more like a huge construction site, ' says Arup New York principal Leo Argiris. 'People are realising that there are not days but months of work ahead of them and the adrenalin is wearing off.'
'When you first arrive you feel overwhelmed. It takes time to get your bearings and the only way to deal with it is to focus your thoughts on one specific problem rationally and clearly.'