Form follows function on a stunning road bridge being designed for the Jordanian capital, Amman. Andrew Mylius reports.
Srinivasan, technical guru and chairman of consultant Dar Al-Handasah, rebuffs suggestions that the bridge he has designed to link wealthy north Amman with deprived districts in the south is pure aesthetic indulgence. Swooping sculpturally across the Wadi Abdoun valley, which bisects the Jordanian capital, the dramatically curved deck is supported by parallel cable stays fanning out from three slim, elegant Yshaped towers. But the structure is an essay in economy and functionality, Srinavasan contends.
Housing and commercial development flanking Wadi Abdoun is dense. To minimise land take and visual intrusion Dar put approaches to the bridge - which will carry twin dual lane carriageways - into tunnel. However, connections with existing junctions to the north and south dictated a shallow geometry - the tunnels will have to be delivered using cut and cover construction rather than full face excavation.
Demolition of existing structures to allow for excavation of the tunnels was out of the question. It would be politically unpopular, time consuming and costly - construction is to be shoehorned into a tight 21 month programme and total costs for the bridge are expected to be no more than £6.25M. The only practicable locations for the tunnels were offset, ruling out a straight, linear connection. Site conditions dictated the bridge's curvaceous alignment, says Srinivasan.
Dar set out to minimise the number of supporting members to keep visual impact low and avoid large scale ground works on the steeply sloping valley sides. Amman is also in a Grade 2 seismic zone and the bridge has been geared to resist a one in 120 years earthquake. The consultant has produced a cable stayed design in reinforced concrete, with the three towers breaking the bridge's 412m length into two main spans of 132m and two shorter side spans measuring 74m each. Deck level will be 45m above ground level at its central point and 30m above ground at the side towers. The towers will rise 26m above deck level.
Dar has inclined the cable plane to maximise equilibrium.
'Lateral movement of the deck is controlled by the cradle effect of the cables, ' Srinivasan explains.
Towers themselves are in plane with the cables to eradicate avoidable bending moments.
The deck runs through and is integral to the towers, which stem from a single pier like the arms of a capital Y. Loads placed on the bridge's central tower are symmetrical. However, the deck's curves place huge rotational forces on the first and third towers. 'The deck wants to rotate because of its curvature, pulling the towers over to one side, ' says Srinivasan.
To resist torque applied by the deck, the towers will be prestressed, with tendons tied back to 3m deep pile caps. Ground is marl and the central pier will be founded on 1.3m diameter bored piles up to 20m long. Piles for the first and third piers will need to be 30m long. The first and third piers will be rooted still more firmly to the spot by 54 raked anchors apiece.
For ease and speed of construction, towers are symmetrical about the bridge's centre line and laterally, allowing repeat use of formwork. Each tower arm consists of two slender, parallel elements connected by a reinforced concrete web. And each of these elements has a constantly changing profile, diminishing in section over its height.
At deck level it is a straightsided ellipse with radiuses of 900mm and 500mm. At the top, the section is circular, with a 500mm diameter. 'Mass is where it is needed most, ' Srinivasan notes.
Bespoke steel formwork, cut and shaped using computer aided design and fabrication software found in state of the art shipbuilding, will be used to construct the towers in 3m lifts. To accommodate the towers' constantly changing geometry, different formwork will be needed for each lift, but each section will be used four times per pair of towers. Formwork elements will be used 12 times in total.
One of Srinivasan's main concerns during construction is that tolerances are carefully controlled. In such slender towers it is essential the prestressing ducts are correctly aligned. And 'the contractors will have to watch closely for creep and shrinkage, ' he cautions. Dar has specified C60 grade concrete incorporating microsilica for durability and strength, and PFA to reduce shrinkage and improve pumpability.
'If the contractor is able to successfully do the towers then the rest is relatively straight forward, ' Srinivasan summarises.
Match casting of the bridge's deck will be undertaken off site.
An entire 3m long deck segment will weigh 135t and, for ease of transportation and handling, will be broken up into three parts; two 50t side elements complete with parapet, and a 35t central 'beam' section. The three elements will be lifted and stitched together insitu to ensure perfect alignment, before glueing and post-tensioning of the deck takes place. Again, fine tolerances will have to be achieved.
Half a degree of error in the casting yard would throw out alignments on site.
There are four deck segments between stays and Dar has designed a lifting and assembly gantry to enable erection. The gantry structure will be bolted to a completed 12m section of deck while new segments are raised into place, stitched, glued and stressed to form a suspended cantilever. Once the next cable stays are in place the gantry will be moved forward and the process repeated.
If all goes to plan, deck erection will start from the first towers while the last towers are still being cast. Erection of the 120m of deck suspended from each tower (60m each side) will take about four months, predicts Srinivasan.
The towers will not be prestressed before deck erection gets under way. The delicate operation of tuning the entire structure will take place by degrees, as the deck is advanced, increasing curvature and torque.