Home About Us News Dammam Bridge: a strategic project in Saudi Arabia

Dammam Bridge: a strategic project in Saudi Arabia

Dammam, the capital of the Eastern Province and the fifth most populated city in Saudi Arabia with 4.1 million inhabitants, is experiencing high levels of industrial and population growth. Standing on the shores of the Persian Gulf, Dammam boasts a strategic vantage point between the Emirate of Bahrain to the south, the industrial city of Jubail to the north and the road leading to Riyadh in the west. This dynamic city is home to the country's oil industry.
In an effort to support its development, the local authorities launched a wide-ranging programme to improve the road infrastructures between Dammam and Khobar, the twin cities adjoining Bahrain.

Such improvements included the construction of the Dammam Bridge, a seafront flyover located at the intersection between the Coastal Road and the Port Road, an area that was constantly gridlocked due to train traffic from the port.

The structure and its design

The structure comprises four parallel viaducts that are designed to ensure that the two central decks shoulder the main traffic coming in both directions from the Coastal Road (North & South Major Bridges) and that the two adjacent ramps provide access to the Port Road.

There were plans to create three lanes for each bridge, except for the South ramp, which only has two. The deck width is consequently 12.610 m and 10.360 m respectively.  The deck features a constant height of 3.0 m along its entire length. 17 segments make up each span, typically measuring 55 m. Span segments are 3.22 m long. Pier segments are 3.32 m long. The structure's vertical profile reaches a longitudinal slope of 4% on the central decks and up to 6% on the access ramps. From a plan perspective, the structure is relatively straight, since it offers a minimum radius of curvature of 1,700 m. The layout of the piers follows the railway line, meaning that the span lengths are highly variable from 16.40 m to 55 m.
The access ramps to the viaduct were created from backfill using Reinforced Earth®, thereby minimising their footprint and offering an elegant architectural motif with their panel facings.

The structure was designed by consulting firm Saudi Consolidated Engineering Company (Khatib & Alami), which focused on a solution with precast segments and prestressed concrete. The spans were fitted with expansion joints at each pier and replaceable external prestressing. Pot bearings created the interface between the superstructure and the piers.  
The general contractor chosen by the Dammam Port Authority to build the bridge was Saudi firm Al Yamama Company for Trading & Contracting (Al Yamama Co.).

An array of bespoke services

Freyssinet was involved in the project as a superstructure specialist, from precasting the segments through to installing and prestressing the spans. The scope and type of services were tailored to the context and expectations of Al Yamama & Co., which was looking for a bespoke service.

Freyssinet dispatched a team of experts to design and organise the segment precasting area. Four precasting units were also designed, manufactured and shipped to Saudi Arabia for assembly and commissioning. The process of adjusting the formwork (using a proprietary geometric design control program) and the concreting operations were supervised by the same employees.

In addition to precasting, Freyssinet was also involved in fitting the spans. A launching gantry was modified and made available for the Dammam project. The gantry had previously been used on the Prai River Bridge in Malaysia and therefore required a complete inspection to ensure that it was appropriately scaled to the Dammam Bridge, while new components had to be designed and manufactured to meet the project's specifications. This construction technique was used for the very first time in Saudi Arabia on this particular project and proved to be indispensable, since traffic along the railway and the Port Road could not be cut off during construction. Freyssinet oversaw the heavy lifting and equipment commissioning operations, as well as installation of the prestressing.

The precasting plant

The segment precasting plant was organised into three areas: the reinforcement cage preparation area, the concreting area and the precast segment storage area.

The first area was specifically dedicated to cutting, bending and assembling the reinforcement bars into moulds matching the exact geometric dimensions of each type of segment, which enabled the teams to create the reinforcement cages. This area featured two tower cranes, which hoisted the batches of reinforcements that been delivered by road. A 20-ton forklift truck was also used to move the fully-assembled reinforcement cages to the concreting area.

The concreting area comprised a space for delivering the concrete and reinforcement cages, a space for the four precasting units and a space for finishing the segments. This area featured a rail-mounted 175-ton gantry crane with a 25 m span above the four precasting units. Given that the weight of the pier segments (151 tons) was considerably higher than the other segments (68 tons for the deviators), it was thought advisable to create a permanent storage area for the pier segments beneath the gantry crane.

The storage area continued on from the concreting area and offered a storage capacity of 102 segments. This area also contained a buffer stock of segments to supply the continuous work zone. An 80-ton gantry crane with a 36 m span was used to move the segments and especially load them onto the trucks.

Precasting units for segments with a three-web structure

The precasting method used for this project was the so-called "match-casting" principle. This technique involves using the previously cast segment as a "casting mould", so that the faces of two adjacent segments always form a precision fit.
This method requires specific formwork – precasting units – which can be used to prefabricate the bridge deck segment-by-segment by following the imposed horizontal and vertical alignment.

714 segments constitute the project's 48 spans as follows:  
- 96 pier segments
- 474 standard segments with a fixed length of 3.22 m
- 96 deviator segments for deviating the external prestressing tendons
- 48 middle segments with a variable length from 1.1 m to 4.0 m

To precast as many segments as possible in the same unit and to avoid the time-consuming process of moving freshly-stripped segments from one unit to another, precasting units were made according to a standardised design:
- One specific unit for precasting the pier segments
- Three so-called "versatile" units for precasting standard, deviator and middle segments

Removable and modular components were designed for the versatile units in order to quickly convert them for each type of segment. This organisation saved considerable time at the height of production, since it concentrated each unit on precasting a span from end-to-end (except for the pier segments, which were cast in a specific unit).

Freyssinet designed and manufactured the formwork to strict quality requirements and applicable standards. Dimensional and geometric inspections were continuously carried out while the metal structures were being fabricated, as well as complete pre-assembly prior to shipment.

A distinguishing feature of this trapezoidal structure is the central web. The web requires a more sophisticated system than usual with the presence of a two-part internal formwork, which must be capable of deploying in a confined area during installation and retracting when stripping away the formwork. Freyssinet overcame all these constraints thanks to specially-cut panels combined with a series of articulated joints, hydraulic (or mechanical) jacks and props.

The match-casting method allowed the teams to precast one standard segment a day. Precasting a pier segment took longer at three days per segment, since prestressing anchors needed to be fitted and the bearing sheaves adjusted. Finally, one deviator segment was cast every two days. Work was organised into day shifts during the winter and night shifts during the summer to allow the concrete to cure.

Mass production started quickly by using hydraulic systems to create automated formwork. The goal of producing 20 segments a week was achieved following a learning curve of only a few weeks. Complete precasting of the structure took 12 months.

A reconditioned launching gantry

The launching gantries used to install precast segments are generally custom-designed, but they can be reused after being technically modified.

In the case of the Dammam Bridge, the length and weight of the spans (1,165 tons with a 55 m span) represented a major constraint, since they were only just compatible with the so-called "span-by-span" construction method.

These baseline data were strikingly similar to the Sungai Prai viaduct in Malaysia (1,500-ton deck with a 50-metre span, which was partly erected by the cantilever construction method), for which Freyssinet had kept the launching gantry. Various modifications were made to adapt the gantry to the Dammam Bridge.

These modifications included:
- An extension to the main beams and local reinforcements to satisfy the new span length of 55 m
- The creation of a pier bracket
- An increase in the winch's lifting capacity from 130 to 160 tons
- A reinforcement to the longitudinal moving system to accommodate the 6% slope

The new design was masterminded by Freyssinet's teams in Thailand. A risk analysis was incorporated into the design phase to identify and correct any hazardous situations. The analysis involved the gantry's designer, manufacturer and user, as well as a safety expert in order to review all the operations performed by the gantry. Their findings were factored into the general design and construction methods for the spans.

Assembly and load testing

The launching gantry was initially assembled on the ground. As many parts as possible were preassembled, so that they could be hoisted into place by conventional means and thereby minimise the amount of work at height, which represents a significant risk factor for operators. The trickiest part of the assembly process involved two 500-ton mobile cranes, which were used to tandem-lift the two 71.52 m long main beams, one-by-one, onto their supporting structures. Additional parts were then cantilever-assembled to restore the launching gantry's total length of 127.35 m. Assembly was completed by installing the wind bracing, winch, front and rear legs, and finally the segment suspension system.

As with any lifting equipment, a proof test was carried out before the launching gantry was used for the first time. This static test was conducted amid special safety conditions and involved loading the launching gantry 20% beyond its maximum service load, i.e. with four segments in addition to the 17 segments forming a 55-metre span. The winch was also tested at 125% of its static capacity and at 110% of its dynamic capacity before work began on installing the spans.

Installing the spans

The spans were installed during a one-week cycle (six days worked) as part of a single shift pattern. The cycle began with the segments, which had been delivered by road, being hoisted and suspended one-by-one from the launching gantry using hangers. This enabled the gantry to take its final deflection before starting to assemble the segments one after the other.

The segments were then assembled with temporary prestressing after epoxy paste had been applied to the surface of the segment joints. The span thus created was still suspended from the launching gantry and had to be "permanently" prestressed in order to become self-supporting.

External prestressing (16 x 27C15 tendons) was also performed according to a highly specific load transfer sequence to accommodate any differences in rigidity between the concrete span and the steel launching gantry and consequently prevent the span from bending during the operation. Once supported by the piers, the prestressed span could be disconnected from the launching gantry.

Finally, the gantry was "launched" to the next span using its main supporting structure, which was fitted with a wheel track and pushing jacks. Before it reaches the next pier, the gantry achieves a maximum cantilever deflection of close to one metre. The front and rear legs offset the deflection and help relocate the main supporting structure during the launching operation. These operations involve moving major loads and call for painstaking preparation.

Special operations

The sequence of events in building the deck also required complex operations with such an oversize launching gantry. The gantry had to be slid sideways once it arrived at the end of the structure, from one span to the next directly adjacent span. Since the decks were not at the same level, the 1,000-ton gantry had to be jacked up 1.5 m before sliding and then lowered down after sliding over to the next deck.

Installation was completed in March 2015 after 14.5 months of construction.

A human adventure

The very type of services performed for this project prompted Freyssinet to form a team with a wide range of complementary skills spanning pre-casting, geometric control, heavy lifting and prestressing.
Fourteen people were mobilised to prepare and set up the project as well as carry out the standard operations. This multicultural team featured people from India, Malaysia, Morocco, Tunisia, Australia, Canada and France. The project's most tremendous achievement was not only meeting the technical challenge, but also creating such a culturally diverse team.

Main stakeholders:
Client:  Dammam Port Authority
General contractor:  Al Yamama Company for Trading & Contracting
Engineering firm:  Saudi Consolidated Engineering (Khatib & Alami)
Consultant:  AMO & Partners Engineering Co.
Specialised subcontractor: Freyssinet Menard Saudi Arabia

Key figures:
48 spans
714 precast segments
4 precasting units
1 launching gantry with a 1,000 ton capacity
1,000 tons of prestressing