By Kenya Engineer
Hot News!
The construction of Masalani suspension bridge on road E873, across the Tana River, 30 km South of Hola town is now complete. The contract was awarded to Associated Construction Co (K) Ltd and supervision was carried out by the Chief Engineer (Roads) for the Ministry of Roads and Public Works.
The bridge is a self anchored suspension bridge on which it was hoped to use bridge components from the old Sabaki Bridge. However, the use of these old bridge sections was discarded after it was established that most of the steel components were unserviceable.
The bridge spans 134m with outer spans of 17m and 25m and a central span of 92m. The carriage way width is 3.7m. The substructure comprises two abutments and two piers in reinforced concrete construction, founded on 600mm diameter concrete bored piles averaging 14m below the pile cap.
The bridge deck is made up of compound girder sections with 2 No 1420mm x 250mm welded plate girders connected with a 533 x 210 x 92 universal beam.
Two such girders were interconnected with a cross beam and bracing arrangement, over which a decking of 9mm thick steel plate troughing 200mm deep was provided. 300mm of concrete was cast over the steel troughing. Two towers were provided in compound sections with the main legs being in 686 x 254 x 125 kg/m and 2 No 406 x 178 x 74 kg/m universal beams.
The main suspension cables system is an assembly of 7 No 45mm diameter fully locked coil strands, clamped at hanger bar locations within the central span. The hanger bars are 40mm diameter bars with turnbuckles at the lower end.
The cables are anchored to the bridge deck beams at the abutments in a system comprising anchor blocks, U_bars, bearing pins and 40mm link bar arrangements. The bearing for one abutment is a fixed pin bearing while for the piers and the other abutment are link plate bearings. Lateral restraint supports are provided at the beam ends on the abutments and piers.
Messrs Associated Construction Co (K) Ltd. carried out detailed designs to establish missing bridge details that were originally not provided as most parts were to be reused from the old bridge parts. The design formed the basis of developing fabrication drawings for the steel components.
Samples of cast parts were obtained from the existing components and physical and chemical tests analyzed in laboratory to compare their materials specification with the design analysis carried out.
Fabrication of the steel components was carried out by Yerevan Steel Ltd. of Dubai who fabricated the girders, deck troughing, cross beams and bracings, towers and bearings. They also organized for the manufacture of the steel cast saddles and bearing pins.
The construction design was undertaken by Dorman Long International Ltd of the UK who carried out the necessary design for the permanent erection of the bridge.
The cable system was supplied by Bridon International Ltd. of the UK who manufactured the main cables strands and anchors blocks. The U-bars and cable clamps were manufactured by Forge Lifting Gear Ltd and William Cook Cast Products of UK, respectively.
The erection of the steel girders, the deck towers and cable was carried out by Associated Construction Co (K) Ltd. using their local resources. As this is a self anchoring suspension bridge, temporary support trestles were first provided by driving 300mm and steel pipe piles at designated location.
Trestle extensions were then provided to the soffit level of the lifting jacks assembly. The bridge deck beams were conveyed into position by use of a 40 ton capacity pontoon barge with trestle and placed on a jacking frame assembly on the temporary supports. The towers were lifted in the assembled form by a crane and positioned on the piers and held in place by temporary stay beams.
In order to achieve the correct deck profile, the following procedure was used in the cable erection. The towers were set to the designed initial pull back inclination.
The first cable was set to the free cable geometry, ensuring the pre-set saddle marks on the cable corresponded with the saddle centre mark and the design level of the cable at the center is achieved. As a confirmation of the cor
|
