A small loop road off the highway north from California's Golden Gate allows tourists to stop and look back over the steep hillsides to the glorious bridge and the San Francisco skyline beyond. They can also see that underneath the road they have just driven over, the structure is being torn apart.
The work is deliberate, of course.
And it is taking place within engineered boundaries, possibly even stricter than were envisaged when work started.
Contractor Balfour Beatty Construction Inc asked for, and was given, more conservative design rules for the temporary works. The traffic is safe.
But it may not always be so, especially should there be an earthquake.
Since the Bay area lies within a Zone Four seismic classification, the highest in America, this is a virtual certainty.
The 1987 Loma Prieta shake that pancaked highway viaducts and severed the Bay Crossing underlined the vulnerability of the city's external links. Work is under way on retrofits, beefing-up or replacing structures to cope with future shocks.
'We were lucky, ' says Ewa Bauer, deputy district engineer for The Golden Gate Highway & Transportation District, an independent authority which controls and funds the bridge. 'Lomo Prieta was far enough away to cause just a shaking.'
But the bridge may not resist a larger or closer event and since it is the only northern route into the city for emergency crews, that is potentially disastrous. It must not only remain standing but be operable within hours.
A three phase upgrade has been the subject of a study by consultant TY Lin (CEI August 1996) and is now under way for the main bridge, and the two approaches, north and south. Oddly it is these last which are weakest, rather than the suspension span and towers which can sway in an earthquake.
The weaker north end is being strengthened first. At $56M, the cost is cheaper than the $100m plus estimate for the southern approach, a factor in the timetabling since federal money will only become available later. Current work will be paid for by tolls.
The upgrade involves the deck, towers and foundations of the viaduct linking the Marin County abutment to the big anchorage housing. Five 54m spans carried by a 7.7m deep steel lattice truss supported on steel towers up to 43m high make up the viaduct.
'There is a strict sequence of work, ' says Bill Ogle, project manager for Balfour Beatty. 'The towers have to be replaced and that means a certain amount of strengthening to the truss was required first.'
Altogether 900t of steel has to go in to replace or add to existing lattice members in a 'surgical rebuilding' which will add strength to the truss and also tie the spans laterally into one continuous element. This will then 'float' on bearings on the new towers which will have seismic isolators at the base. A bearing on the abutment will allow for expansion.
But when work began in July 1997 there was a significant environmental problem. The bridge's famous 'gold' colour, actually a rusty red, comes from use of a lead-based paint, drops of which have splashed on to the ground for 50 years during routine painting operations. A subcontract to remove the top 300mm to 400mm of the ground took seven months.
Once completed, an initial 135t of new steel will have been introduced into the truss. 'This is complicated because each added member requires a separate calculation for its effects on the structure, ' says Ogle. 'And since you often can't bolt on to the same points as the old, the load transfer can be quite different.'
There is also the constant live load of six lanes of traffic running above to consider. Local firm Tucker Associates has worked with Balfour Beatty's own inhouse engineers on the calculations.
'We also had to devise a scheme to stabilise the whole laterally when members were out, ' says Ogle. Bracing is added around the voids as elements are released and replaced.
Both the initial work, and the main strengthening now underway, involves intricate manual work, with the new stronger steel elements man-handled around and through the criss-crossing lattices using block and tackle. The elements can weigh up to 2.7t each. Some 90 or so ironworkers have been on the job at peak periods.
'There is also a lot of plate work and busting out of rivets, ' adds Ogle. 'I think we have had about 70 000 rivets.' A large element of the work is drilling holes for the new bolts.
Meanwhile below, the towers must be replaced and the foundation strengthened. First task was the construction of foundations for the huge tubular section temporary towers which support the truss while the old towers are taken out one by one. 'We are using 1.8m diameter augered piles about 25m deep and socketing 4m into fairly competent rock, ' explains Ogle.
'That was difficult not just because of restricted headroom but because we have to work around the existing towers. ' Temporary towers are located 8m either side of the permanent towers along the length of the approach.
Strengthening of the trusses at these points is completed prior to the jacking operation - again under live traffic - to transfer the load from the old permanent tower.
Flame cutters and steel cutting hydraulic shears take out the old tower.
So far one has been removed. 'The biggest problem was the 200mm bearing pins, ' says Ogle. 'They were deteriorated and where we were going to re-use them we will now have to get new ones.'
As the old towers are removed, work can begin on the foundations.
Initially the plan was to replace and rebuild them but a new scheme means they can simply be beefed up. Bauer explains: 'We can simply create a ring of 600mm diameter piles around them.
Then a new reinforcement mat goes over the lot and a new bigger pilecap is cast.'
On to the new foundation go the seismic isolators, each nearly 2m in diameter, and then the new steel towers are erected. Steel is fabricated by the Portland Oregon Ironworks and shipped to a central Californian paintworks before coming to site.
Changes to the foundations and superstructure have meant a considerable increase in the contract price from $31M to $57M. An allowed extension because of a longer than expected clean up of the paint-contaminated ground has pushed completion date back to October 2001.
By that time phase two work on the southern approaches and the difficult Fort Point Channel arch will be under way. This should attract federal funding to supplement the funds raised by the current $3 crossing tolls and bond money raised against that cashflow.
According to Bauer, work on the northern approach will count in the dollar for dollar calculation which requires the Golden Gate Highway & Transportation District to match the federal funds. The 15 strong Golden Gate team will carry out construction management and supervision on this second phase.