The new bridge crosses Shatt El Arab river in Southern Iraq, 420km south of Bagdad and 50km from the river delta into the Indian Ocean, in the city of Basrah where the country’s main port is located.
Southern Iraq is the land of Sumerians, the people that developed an outstanding civilisation five thousand years ago. As precursors of engineers they invented the wheel, created a large network of irrigation channels, and were builders of ziggurats, huge constructions of clay bricks built up to 90m in height.
In the modern context of southern Iraq the aim of the new connection is to create a link between the two sides of the river to carry a four-lane highway and two footways. The bridge has a total length of 1,188m and consists of a central cable-stayed bridge and two approach viaducts. It was designed by DMA De Miranda Associati, and built for the Province of Basra by contractor Maeg Costruzioni.
The cable-stayed bridge has three spans – a central span of 150m and side spans of 69m, making a total length of 288m; the viaduct spans vary in length from 39m to 69m, and combine to a total length of 900m. The two main towers of the cable-stayed bridge rise 40m above the deck, and each tower supports 14 stay cables; seven on the main span and seven on the side span. The central axis of the whole bridge and viaduct has a constant horizontal bend in plane, with a radius of 5,500m, due to the alignment of the new approach roads.
The crossing, founded on piles of 1.8m - 2m diameter, posed an interesting challenge for engineers from the beginning, due to the fact that the bridge is located in a seismic area with peak acceleration of 0.5g, where minimum and maximum air shade temperatures are respectively +6ºC and +50ºC and on a project where many factors changed during the design of the bridge.
When the design was already at an advanced level, in September 2013, a set of results from the geotechnical investigation were made available, showing it was necessary to increase the pile depth with reference to the initial assumptions: in fact the depth of the piles had to be increased from the initial 30m to between 40m and 50m.
The foundation blocks are based on bored reinforced concrete piles that extend up to 50m deep in a soil that is composed of three layers: an upper, 25m-thick layer of clay and silt with a soft consistency; a middle layer of medium-dense sand and a lower layer of very dense sand.
The flood level reading referred to average sea level, since 1990 was +2.4m with a maximum tide +1.75m. All 24 piers are formed of reinforced concrete which is partially post-tensioned and which vary in height from 7.37m to 26.6m and with cross-sections differing between the piers of the viaduct and those of the main river bridge; the former have a circular cross-section with plane surfaces perpendicular to the bridge axis and vertical continuous slots in the external surfaces, designed also to locate the drainage pipes.
The piers in the river are distinct from the other piers, having a different cross-section with vertical squared pilasters that accentuate their slenderness and distinguish the main bridge from the approach viaduct.
The piers are based on foundation blocks in reinforced concrete designed using the same concept as the piers: on land, where the blocks will be buried, the geometry is a simple parallelepiped with tapered top, which was easy, cheap and fast to build. For the blocks of the river bridge, an effort was made to make them light and give them a rounded geometry with tapered both top and bottom, leading to a light, visual appearance which almost suggests a floating structure.
The deck structure is a steel-concrete composite construction which is 2.2m deep and 18m wide on the viaduct, expanding to 21.5m wide for the cable-stayed spans to accommodate the stay cable anchorages.
The longitudinal steel structure of the deck consists of four beams: the outer two have inclined webs to improve aerodynamics of the deck, while the internal webs are vertical. The structure that gives horizontal stiffness to the deck is reticular, and will be hidden by a perforated steel sheet. The top deck slab is built with C40/50 concrete.
There are two towers on the central axis of the bridge, that are designed as vertical, linear steel structures rising 35m from the top of the deck, each supporting 14 cable stays. The structure of the tower is deliberately simple to permit fast construction, but once they have been built, they will be covered with a perforated steel shell and an illumination system will be placed between the structure and the shell, giving the masts lightness and brightness and improving their aesthetics. There will be also lights at top of masts, to meet navigation requirements.
Stay cables are formed by parallel strands, galvanised and individually sheathed, covered by a HDPE tube with helical ribs.
All steel structures and towers will be painted using the same procedure; first they will be sand-blasted to Sa 2.5 (ISO 8501) after which a 75 micron layer of primer of inorganic zinc with ethyl silicate will be applied. The intermediate layer is 125 microns of epoxy polyamino-amide, and the finishing layer is two coats - 25 micron applied in the shop and 30 micron in the field – making up 55 micron total thickness of fluoropolymers.
The construction method of the new Basrah Bridge was conceived to address two main challenges: firstly to concentrate as much of the assembly/construction activities of the deck girder as possible in a local, protected, well-equipped and accessible area; the second objective was to avoid any interruption or interference with vessels in the navigation channel both during and after the bridge construction. Both objectives were achieved by creating two construction yards at the abutment locations, and designing an erection procedure based on double incremental launching of the fully-assembled steel structure, including towers and stay cables.
The longitudinal alignment of the deck consists of two ramps with curved profiles and high radius of curvature, and by a central section, over the river, with a smaller radius of curvature.
The presence of this section with a sharper bend created another challenge to be resolved. In fact longitudinal launching usually requires a constant longitudinal curvature in order to match the alignment of supports with the girder alignment which has to slide above the joint alignment.
A special solution was adopted to resolve this problem; the girder was articulated by using a set of hinges so that its profile became closer to the support profile; once the front section had been launched, it was lowered, and the final profile was achieved.
The bored piles were first constructed, after which the piers were built – and at the same time, the deck elements were being delivered to the jobsite where they were assembled on ground, with the help of gantry cranes on rails – one on each side of the river behind the bridge abutments.
Then the steel deck was assembled in 10m to 12m-long segments on provisional blocks with rollers on top; the segments were moved from the assembly area with the help of the same gantry cranes.
Once assembled, the deck was launched longitudinally over the top of the completed concrete piers, equipped with rollers on top, by means of a strand-jacking system,.
The cable-stayed bridge had an additional mounting system for the towers and the stays: each mast was placed horizontally on the deck, then a provisional steel truss structure was mounted on the tower, and by means of a strand-jacking system the tower was rotated upwards by 90° to reach the vertical final position.
At this point another temporary steel structure was mounted in front and one at the back of the tower to create a trussed structure that integrates the deck and the tower, making it possible to launch the tower longitudinally into its final position together with the rest of the deck.
As previously noted, the front part of the deck was hinged to the rear part during launching, in order to partially realign the profile of bottom flanges, which in service has a different curvature in the vertical plane.
The stay cables were installed at the beginning of the launching operations, to permit working at low level, in protected conditions. After the launching of the two 600m-long decks, the front part of the deck was lowered, by rotating around the temporary hinges; the stay cables were tensioned, the temporary truss was released, and the deck slab poured in a progressive sequence, in parallel with the second tensioning phase of the stay cables.
Mario de Miranda is partner engineer at De Miranda Associati