History and Development of Bridges in Kenya

Last Updated 14 years ago by Kenya Engineer

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.

The history of Bridges in Kenya stretches before the colonial times when the indigenous community would use tree logs and rock boulders or rock fill across ‘unfriendly’ sections of the footpath. The ‘unfriendly’ sections included swamps, rivers and gorges. The crossing points along the footpaths largely followed the narrow crossings and where the banks were stable like where the river/ gorge bank has rock outcrops would be preferable.

During the colonial times the same crossings were mostly improved to modern bridges. Others were found stable with capacities to handle vehicles. Some of these timber bridges are to date found within the forest zones like around Mt. Kenya especially in Nanyuki region.

In coastal areas due to the wide crossing waterways a system of floating bridges was used. An example is the Nyali bridge which was a floating pontoon bridge linking Mombasa Island to the Kenyan mainland.

The bridge linked the Mzizima district of Mombasa to Nyali, and was built in 1931.In 1980, the bridge was superseded by the New Nyali Bridge (located approximately 0.55 miles (0.89 km) to the north), leaving the steel bridge to be dismantled for scrap. The western (Mombasa) approach to the bridge is the only remaining part of the bridge but one of the pontoon mooring anchors is on nearby display at the Tamarind Restaurant

Another bridge in the coastal region is The Makupa Causeway which links Mombasa Island to the Kenyan mainland. The road runs for approximately 2.4km between the Magongo roundabout and Makupa Roundabout. The causeway dissects Tudor Creek to the east and Port Reitz Creek to the west. The road on embankment replaced the ocean bed crossing that was only used during the day when the tides are low or by use of boats when the tide is high.

The Classified Road network in Kenya is approximately 63000Km. Out of thisKenya National highways authority is responsible for the National highways approximately 14000 km. The process of Bridge inventory and condition survey to establish the number of bridges, the type of bridges and their condition within the Authority’s jurisdiction is underway.

Further, under the ongoing reconstruction programme, the old and deteriorated bridges are undergoing reconstruction. Of importance are the environmental concerns in a number of blocked waterways like Mbita and Makupa causeways in Kisumu and Mombasa respectively. These two causeways are earmarked for improvement to bridges which will allow proper circulation of water.

Types of Bridges

Generally there are four main types of bridges based on their construction method:-beam bridges, cantilever bridges, arch bridges, suspension bridges

Beam bridges

They are the simplest in structural forms. They are supported by an abutment at each end of the deck. Since no moments are transferred through the support their structural type is known as simply supported. Bridges designed for modern infrastructure will usually be constructed of steel or reinforced concrete, or a combination of both. The concrete used can either be reinforced, prestressed or post-tensioned.

Constructions could have various beams side by side (with a deck across the top of them), to a main beam either side supporting a deck between them. The main beams could be I-beams, trusses, or box girders. They could be half-through, or braced across the top to create a through bridge. Beam bridges are not limited to a single span. Some viaducts such as the interchanges under construction in Nairobi – Thika highway and Eastern by pass at North Airport road and City Cabanas have multiple simply supported spans supported by piers.

Beam bridges are often only used for really short distances because, unlike truss bridges, they have no built in supports. The only supports are provided by piers.

Some of the major bridges in the country under this category include: Nyali and Mtwapa bridges both in Mombasa city. The construction of the two bridges started in 1978 and completed in 1980 and was done by Sumitomo Construction Company under the supervision of H.P. Gauff.

New Nyali bridge:-It is 391.65 metres long and 26.3 metres wide with a total of six lanes. The main bridge is 330metres prestressed continuous box girder over three spans with the centre span of 150 metres. The outer spans are 90 metres each. The bridge is founded on piled foundation. Nyali Bridge includes an approach bridge with three spans totaling 61.65 metres whose superstructure is simply reinforced concrete beams and the deck. Since its completion in 1980, no periodic maintenance was done to the bridge until 2005 when major repairs to the bridge were carried out. The repairs included post tensioning the box girder and shear strengthening using crack sealing and use of steel plates on the internal face of the box girder.   

Mtwapa bridge:-The bridge consists of three spans totaling 192 metres long and approximately 15 metres wide carrying two lanes. The superstructure is a prestressed box girder with the centre span of 111.9 metres. The outer spans are 40 metres each. The substructure consists of normal reinforced concrete Abutments and piers founded on piles. The construction of Mtwapa bridge was completed in 1980.

Inspection on the bridge in early 2000 found that the centre span had deflected. Like the Nyali Bridge renovations were not done until 2005. A highly specialized periodic maintenance was carried out when the bridge was observed to have developed a deflection of about 192 mm. The maintenance works involved an installation of additional post tension cable to avert further deflection of the bridge and the deflection was held in position.

Kilifi Bridge is the longest bridge in Kenya with a total length of 420 metres. The superstructure is a prestressed continuous box girder carrying two lanes. The bridge has three spans. The construction of Kilifi Bridge was completed in 1991.

At Sabaki, the mouth of River Athi (Sabaki at the coast) the current concrete bridge replaced a steel suspension bridge in 1996. The steel suspension bridge was eventually transferred and used at Masalani Bridge. The bridge length is approximately 184 metres with three spans. The superstructure is prestressed box girder carrying double lane. The substructure is reinforced concrete founded on piles.

Cantilever bridges

They are built using cantilevers—horizontal beams supported on only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses. Cantilever bridges are constructed using much the same materials & techniques as beam bridges. The difference comes in the action of the forces through the bridge. The largest cantilever bridge is the 549-metre Quebec Bridge in Quebec, Canada. In the country, the nearest cantilever bridge from Nairobi is across River Athi along Athi River – Namanga road.

Arch bridges

Arch bridges are those that use arches as the main structural component. Basic arch bridges are differentiated by the number of hinges used to allow the bridge to accommodate different loads and stresses. Some arch bridges don’t include any hinges at all. Arch bridges include those where the arch is underneath the bridge, not above it, provided that the trusses are arranged vertically and not diagonally

In Kenya most of the arch bridges are found along the Kenya- Uganda Railway line. A typical example of arch bridge is Railway over bridge across Uhuru Highway. Like beam bridges, arch bridges can be made of both steel and concrete.  

Suspension bridges

In their simplest form were originally made from rope and wood. Modern suspension bridges use a box section roadway supported by high tensile strength cables. The suspension design is the other design for very long bridges. Here pylons or towers are erected and wires hold the deck of the bridge in place on the deck’s underside

In the early nineteenth century, suspension bridges used iron chains for cables. The high tensile cables used in most modern suspension bridges were introduced in the late nineteenth century. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.

In Kenya two common bridges of the type are found across Tana River in the North Eastern Kenya. Bura Bridge is a cable stay bridge constructed in 1985. It has three spans totaling 118 metres. The cable anchorage is external. Masalani Bridge is a self anchored suspension bridge across Tana River at Hola. It has a span of 135 metres with the centre span of 92 metres. The construction of Masalani Bridge was completed in 2007. Another suspension bridge in Kenya is Thua Bridge in Kitui.

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