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Bridge of dreams

Plans for an ambitious 704km long coastal highway in Nigeria are being worked up with the aid of some complex UK-based bridge design. Mark Hansford reports.

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Bis picture: An artist’s impression of the 2.8km long suspension bridge estuarine crossing of the Bonny River

While plans for a wildly ambitious 704km long eastwest coastal highway in Nigeria remain unfunded, some would say unfundable, detailed design of the route and its 180 bridges has already been carried out.

The route across the Niger Delta, the topography of which is characterised by labyrinths of rivers, creeks and rivulets along the coastal swamps that are heavily waterlogged especially during the raining seasons, demands a lot of bridges - 180 in total with three signature structures.

The largest and most dramatic of these is the 250bn Nigerian Naira (£1bn) estuarine crossing of the Bonny River some 35km south of Port Harcourt. It is set to be a stunning 2.8km long suspension bridge, design of which has called for UK expertise in the shape of consulting engineer Hewson and its structural analysis software firm Midas.

Local firm Pearl Consultants has taken it upon itself to promote the scheme and continues to lobby for funding from multiple sources (see box). In that capacity it has funded feasibility and detailed design work and even gone so far as to spilt the project into contract packages and produce tender drawings and documents for the Nigerian government to use, should it ever get a credible funding plan together.

But what it couldn’t do is design the suspension bridge, and so brought in long span bridge specialist Hewson.

Hewson did initial studies, concept and detailed design of this 1,500m main span suspension
bridge and the approach viaducts, which carry the highway over the mangrove swamps on either side of the river.

The suspension bridge has 240m high concrete towers with two main suspension cables supporting hangers at 20m spacing arranged down each side of the steel box girder deck.

The approach viaducts are prestressed concrete box girder construction with 100m typical span lengths, and an overall length of 4km.

All piers and towers are supported on large diameter bored piled foundations. The deck for the main bridge is independent of the towers and anchorages, with a continuous structure throughout. The main bridge anchorages are large concrete structures supported off a combination of bored piles and diaphragm walling.

The length of the span and the poor quality of the ground combine to make it a challenging design job, says Hewson director Andrew Hodgkinson.

“In view of the large oil and LNG tankers that regularly use this river a navigation channel of 1.1km is required in accordance with international recommendations, necessitating the new road to be carried on a bridge with a main span of 1.5km,” he explains.

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“The anchorages for this bridge were undoubtedly the most complex design issue to overcome given the very poor ground conditions”

Andrew Hodgkinson, Hewson

“We’ve designed it to give a deflected clearance to the deck soffit of 60m at the high water level. It has H-shaped concrete towers rising to 230m above the water level supporting a pair of suspension cables each with a diameter of approximately 950mm. The 35m wide orthotropically stiffened steel box girder deck is 4.8m deep in order to provide aerodynamic stability at critical wind speeds and is supported by vertical hangers at 20m centres.”

Design was carried out using a range of Midas software, starting with global analysis of the bridge structure using a 3D frame within the Midas Civil software package.

Hodgkinson explains a key benefit of Midas Civil is that it includes a suspension bridge analysis wizard that can determine the “initial internal force” conditions under permanent loads that are consistent with the reference geometry upon completion of construction. As the design was developed, this facility was found to be very useful in quickly re-establishing the forces in the structure under permanent loads after changes had been made to the design.

But the anchorage foundations posed the biggest challenge, and where Midas’ software showed its mettle. “The anchorages were undoubtedly the most complex design issue to overcome given the very poor ground conditions and the correspondingly high imposed loading,” says Hodgkinson. “Midas proved to be a reliable and comprehensive suite of analysis software for this design.”

The new highway traverses the delta region of Nigeria where the morphology of the river systems is constantly changing and mangroves grow in abundance.

The concept of the anchorage foundations was developed to cope with the unique geotechnical situation encountered where thick layers of weak soft silty clays and sands overly more competent dense sands and gravels. Given that the foundations will transmit horizontal forces of around 850MN from the suspension cables into the supporting ground, the weak upper strata presented a real challenge to the design team. The solution had to be capable of transmitting the huge horizontal forces into the stronger layers at 20m depth, while limiting long term and transient movement of the anchorage.

The final design involved the installation of four, 120m long diaphragm walls to a depth of 30m connected with an 8m deep capping slab. Below the capping slab, parallel, reinforced concrete diaphragm walls have been used to provide a structural arrangement that is stiff in the longitudinal direction and will carry the horizontal forces through the weak upper layers to a sufficient depth that the movements can be controlled and predicted.

The footprint of each anchorage foundation is 120m by 80m. In addition to the horizontal forces, vertical forces, principally from the weight of the concrete anchor blocks and capping slab, are resisted by the diaphragm walls. They are supplemented by 2.5m diameter bored piles approximately 55m long.

Analysis of this complex soilstructure interaction was undertaken using Midas GTS to create a solid 3D model of the foundation within the soil strata. Soils were modelled using appropriate non-linear models, and the sensitivity of the input parameters was checked in relation to the effect that it had on anchorage movement and thus global stiffness.

This enabled Hewson to predict movement of the anchorage in relation to the various construction stages and service load combinations with a reasonable degree of confidence. The movements have been used to generate foundation stiffness values for input into the Midas Civil global bridge analysis model. The GTS modelling also produced forces and moments for the reinforced concrete design of the capping slab, diaphragm walls and piles.

Tower cable anchorages were also complex. The cables generate a vertical force of 450MN, but are also subjected to large transverse and longitudinal forces due to wind, temperature and other live loads. The saddle is thus a critical component in transferring the massive forces from the cables into the towers and is approximately 5.8m long by 4.4m wide and 3.1m in height. It is to be fabricated using a plate stiffened steel casting in high grade steel.

For this element, Hewson created a plate model of the saddle using Midas Civil that took into account the actual geometry and thickness of the various components. The vertical force was applied as a radial load along the base of the saddle groove.

Additionally, a horizontal pressure of 33% of the ultimate limit state vertical load was applied to both of the saddle trough walls to represent the flattening effect of the saddle. In addition to undertaking yield checks, the saddle stiffeners were checked for buckling, using linear eigenvalue buckling analysis.

Another key aspect of the design was the clamps connecting the vertical hangers to the suspension cables. They work through friction by exerting a large confining pressure on the main cables through tensioned bolts. These hanger clamps have an internal diameter of 950mm and are required to support a maximum load of 15.5MN. The clamps subjected to the greatest vertical loads are those located nearest to the tower - they experience about triple the load of the next nearest clamp - and those nearest to the anchorage. It would not be feasible for all the clamps to be designed for the maximum load, thus it was decided that these would be designed independently and all others would be a standard design.

Given the complex geometry, and in order to produce an efficient design, Hewson chose to use Midas FEA to create 3D models of the clamps. One of the main challenges with the analysis was representing the clamp boundary conditions, as it is only supported by the main cable running through the centre, with bolts clamping the two halves. This was achieved through a combination of fixed supports and compression only elastic links.

 

In the danger zone

Much as many communities on the route of the proposed highway welcome the project, confidence levels that it will actually get built are low due to discontent with government inability to execute projects that will improve their standard of living over the years.

Pearl Consultants has already faced challenges getting this far, not least on security - tribalism is a key feature in the delta. So far 31 personnel have been seized, 21 physically attacked and two killed; site survey work has been halted by community action 43 times.

As always, project challenges shoot up the cost of project execution.

The security issues have triggered cost intensive security plans that included engaging the services of military personnel and gun boats for several months, negotiations and settlement of communities and youth groups, community liaison officers, payment of idle workers and alternative logistics arrangement.

All these challenges, compounded by the accessibility to site locations have already caused severe delays and lengthened project duration.

Contractors will have to contend with these same challenges. In addition, work in some areas will be complicated by scarcity of sand sources for possible use as construction aggregates and embankment material.

But these challenges pale into insignificance compared to funding: according to Pearl’s most recent projections, the road is expected to gulp a total of N1.8tr (£4.1bn) and few volunteers have been found so far.

State governors, oil companies, international donor agencies and other investors are all wary. Paying for the road with deductions from oil revenues - which are predominantly generated from the coastal states - has been mooted.

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