Despite the ongoing uncertainty surrounding the contract to build the new 'signature span' for the East Oakland Bay Bridge, construction of the skyway which forms the majority of the length of the new crossing is already considerably advanced.
There is unsurprisingly some concern that unless Caltrans makes a decision about the single-tower, self-anchored suspension span in the very near future, the 2.1km-long skyway could be finished before work on the signature span is even started, leaving a rather embarrassing gap between the end of the viaduct and Yerba Buena Island.
For Bay Area motorists crossing the existing bridge, the only noticeable sign of any construction is the line of tower cranes whose tops can be seen from the carriageway. But the safety barriers prevent most drivers from seeing the hive of activity that is going on just above water level, right next to the existing double-deck crossing.
When the designs for the new Bay Bridge were unveiled, the skyway was regarded as the 'unglamorous' part of the new crossing. The intriguing and unusual choice of signature span tended to attract all the attention from the public, with the skyway seen as a necessary but unremarkable approach viaduct (Bd&e issue no 16).
In fact the skyway construction is a significant construction project in its own right, requiring the erection of precast units weighing up to 800t using four specially-designed 'self-lauching erection devices'. The first couple of segments had been lifted when Bd&e went to press, with work set to speed up once the crew get past the steep early learning curve.
The construction of the new crossing was prompted by the damage sustained to the existing 1930s structure during the 1989 Yerba Buena earthquake. A 15m section of the top deck on the east section of the link collapsed in the earthquake, putting the bridge out of action for some time. The traffic chaos caused by this outage and the subsequent damage to the city's economy gave owner Caltrans serious food for thought, particularly in the light of the fact that seismologists were predicting that a much larger earthquake would hit the region within the next 30 years.
The west side of the crossing, with its back-to-back suspension spans proved suitable for seismic retrofit at an affordable price, however it turned out to be more cost-effective in the long term to replace the east side of the crossing, ending up with a brand new, state of the art structure which could be designed to incorporate the latest thinking in seismic engineering.
The skyway consists of a pair of prestressed concrete bridges; one will carry eastbound traffic, the other westbound, each on five traffic lanes, with an emergency lane and 4.5m-wide pedestrian and bicycle lane on the south side, with viewing platforms.
A total of 28 piers will support the twin bridges, 14 for each bridge, and while the spans nearest the Oakland shore will be built on falsework, the majority will be built using a total of 452 precast segments.
Main contractor for the work is KFM, a consortium of Kiewit Pacific, FCI Constructors and Manson which won the US$1,040 million contract in January 2002.
KFM's scope of works includes access dredging, pile fabrication and driving, steel footing shell placement, pier construction and precast segment erection. The 14-span bridge deck consists of 452 segments, some of them the heaviest ever lifted, and the piles are the longest ever driven in the Bay Area. The skyway contract is due to be completed in February 2006.
When Bd&e visited the site in early June, a whole range of works was under way, giving a good visual explanation of the foundation, pile cap and substructure construction procedure.
The first stage of the work is to construct a steel cofferdam for each pier location - this is done by driving sheet piles into place in situ to a depth of about 9m into the ground. A clamshell bucket is used to excavate inside the cofferdam to the required depth and to level the bed for the prefabricated steel footing that forms the base of the pile cap. A 3m-thick bed of gravel is placed in the cofferdam, with the steel footing on top. Each steel footing measures approximately 19m wide by 7.6m high, and has a 1.5m-thick layer of concrete poured inside it before it is lowered into the cofferdam.
A total of six piles is needed on each of the majority of the piers, with four piers requiring just four piles apiece; the piles extend up to a maximum of about 115m long and are formed of 2.4m-diameter steel tubes which vary in thickness from 75mm at the bottom to 50mm at the top.
Three pile templates are in use on the site; they guide the piles at the correct angle as they are driven into place. One of these 46m-high work platforms is placed around the steel footing on each pier, and ensures that the piles are installed at the right batter. A system of hydraulic gates on the templates allows the crew to adjust the number and angle of piles to be driven, to suit each pier location.
Pile driving is carried out using a Menck hammer - the same machine that was used on the construction of the Jamuna Crossing in Bangladesh - suspended on a General Construction derrick barge, one of the largest of its kind on the west coast of the USA.
Splicing of piles is carried out using specially-designed welding machines; a crew of four men and two welding machines for each splice, with the setting up operation alone taking four hours. Ceramic heating pads are used to preheat the steel around the weld area.
Once a pile is fully driven, it is cleaned out, a concrete plug poured into the base and the water is pumped out. At the top of the pile, eight fin plates are welded on around the circumference; these will form the pile to pile-cap connection, consisting of very heavy welding of about 75mm thick. A reinforcement cage is lowered into the pile, gaps around it are filled with grout, and finally the pile itself is concreted up.
KFM started from the Oakland side of the bay building thee foundations for the westbound bridge, and worked on consecutive piers until it reached the end of the skyway next to the signature span. For the eastbound bridge, it is starting from both ends and working towards the middle of the structure.
The first pile-driving template was lifted into position at the end of January 2003, and it is estimated that construction of foundations for each pier - from foundations to completion of the pier column - takes about nine months in total.
With the piles complete, a further 4m-thick layer of concrete is poured in two lifts on top of the pile footing, on which the piers for the substructure are built. A concrete sleeve built on top of the footing of each pier is designed to be visible above the Bay Area water level, and the pier substructure is built within this sleeve.
One of the main aims in the design of the skyway, says TY Lin International project engineer Sajid Abbas, was to minimise the number of piers in order to minimise the seismic forces that would be transferred to the bridge through these piers. This resulted in an optimum span length of 160m being chosen - to balance the cost of the foundations against the cost of the span.
The philosophy behind the seismic design of the entire new crossing, carried out by TY Lin International, was firstly that the bridge should be considered a lifeline structure, and hence had to be capable of providing full service almost immediately after a 'functional evaluation' level earthquake, in this case an earthquake with a 90 year return period. In the case of a 'safety evaluation' level
