Included in this £6.5 million contract also awarded to Dowsett Engineering Construction Ltd. was the completion of the interchange between the M1 and the M62 at Lofthouse together with the construction of two other interchanges at Newmarket with the junction of the A642 Wakefield - Aberford County Road and at Normanton at the junction A655 Normanton - Castleford Road. The Contract started on the 14th March 1972.
This length of 6.1 miles of motorway also traverses the Yorkshire coalfields and special precautions were taken to minimise any effect shallow workings may have had on the pavement and structures.
The operations included the excavation of colliery waste from adjacent shale tips and the selection of suitable material for incorporation in the works.
Alternative tenders were invited for this contract on the basis of the contractor
- finding his own borrow pits for some 1,000,000 cu. yds. of common fill and
- removing some 1,000,000 cu. yds. of colliery waste from West Riding No 3 tip near Altofts.
The contract was let incorporating the use of colliery waste, was successfully compacted in the embankments, and contributed to the removal of yet another tip with resultant benefits to the environment when final treatment of the tip area is completed.
There are 12 structures including a river crossing which carries the motorway. Altofts River Bridge carries the motorway over the River Calder near to the village of Altofts and also provides for access along both flood banks of the river and for floodwater to pass between the two areas of marshy wash land behind the eastern flood bank.
Each carriageway of the Motorway is supported on two box girders which are continuous across three spans, the main river span being 183ft. and the approach spans 100ft. the main girders are connected at approximately 10ft. centres by cross members with cantilever beams projecting sideways to support the edges of the deck slab. The reinforced concrete deck slab is 9in. thick and forms a composite beam with the steel box girders and cross members.
The bedrock at the site is at a depth of about 30 ft. below the river level and it is overlain by very soft alluvial materials. The foundations are therefore supported on piles and hence the lightweight superstructure resulting from the box girder arrangement gave considerable savings in the cost of the foundation. A further advantage was that during erection the minimum falsework was necessary and no special foundations had to be provided other than a hard standing for mobile cranes.
This bridge was one of the first in the country to incorporate the requirements of the Merrison Report at the design stage. Although the design was complete before the Report became available there was time for the appraisal calculation and the independent check to be carried out before any fabrication was started. Where extra strength was found to be required it was possible to increase the thickness of plates. This avoided the necessity for elaborate and expensive systems of stiffeners which have bedevilled some structures caught during and after fabrication.
Provision was made in the Contract for payment for the preparation of calculations by the Contractors to demonstrate that the method of erection proposed complied with the Criteria for the Assessment of Steel Box Girder Bridges. Also, an item was included for additional steel which may have been added to the structure to ensure compliance during erection, but this was not required.
The main girders are rectangular in section except that the top plate slopes to the cross fall of the carriageway. Stiffening was provided by internal plate diaphragms at about 30 ft. centres in line with cross members between the main girders. Welded plate stiffeners of similar section to the cross members stiffen the top plate at 10 ft. centres between the diaphragm. Thus each cross beam and cantilever is effectively continued through the main boxes by either a diaphragm or a plate stiffener. Further stiffening to the flanges and webs was provided by rolled steel angles and bent plates welded to the main plates. The steel used is weldable structural steel to BS 4360 grades 43A and 50C, the latter being a high tensile notch ductile steel.
The box girders were detailed to be fabricated in 30 ft. long units to be connected on site with friction grip bolted joints. These units were of a size and weight which made transport from the fabricators works to site and the erection very straightforward. The bolted joints were quick and easy to make and eliminated all the problems associated with site welding.
Dowsett Engineering Construction Ltd. as Main Contractors, carried out all the foundation and sub-structure work but the fabrication and erection of the steelwork was sub-contracted to Fairfield-Mabey Ltd. of Chepstow.
Fairfields elected to joint the first two pairs of units at the works so that the approach spans consisted of 60 ft. long units which were erected onto a temporary tower at the joint. The heaviest of these units which included the section over the pier was about 33 tons in weight. These were lifted into position by a mobile crane standing on a levelled area between the piers and the bank seat. (Coles Centurion 112 ton or P. & H. 125 ton). The remainder of the river span was fabricated in 5 of the 30 ft. units and cantilevered from each side to meet at mid span. The centre three units were lifted by a 40 ton crawler crane (ANDES) mounted on a Uniflote pontoon floating on the river.
The design calculations had assumed a rather different method of erection and for this reason the self weight bending moments had to be adjusted after all the units had been bolted together. The assumption made was that the centre 5 units would be bolted together on the ground and then lifted into position as one unit. This method would have automatically produced zero bending moment at the first joint beyond the pier and it was specified that the Contractor's method of erection should produce the same distribution of self weight bending moments. Obviously a cantilever method produced a fundamentally different result. The Contractor's solution to this was to pack up the bearings at the piers during erection and the camber of the bridge was changed to ensure a proper fit in the cantilevered , packed up position. The girders were then jacked down to the correct level on the piers and the effect of this was to modify the bending moments to the specified distribution. The success of this operation was checked by using calibrated jacks so that the reaction at the bearings was known at all times and the reduction in the reaction could be measured.
To enable the jacking down operation to be carried out without damage to the structure the pier diaphragm was reinforced with extra stiffeners at the positions of the jacks. The total reaction at each pier was about 116 tons before lowering the 9¾ in. to the final position.
Fairfield-Mabey fabricated all the steelwork at their works in Chepstow. Inspection and testing of the work was carried out by Messrs. Solus Schall acting as Specialist Steelwork Inspectorate for the Engineer. Each box was assembled from its constituent plates, complete with stiffeners and the corners and diaphragms welded. Adjacent boxes were assembled with joints bolted up to ensure a perfect fit when they were lifted into position on the bridge. After completion each unit was checked for distortion against tolerances calculated as laid down by the Merrison document but no difficulty was experienced in meeting these tolerances. Those panels which include site joints were checked after erection and again no serious problems emerged.
The protective paint system was applied in the works as far as possible, that on the inside being complete except for the joint areas and the outside having two complete coats to be applied after erection. The system adopted was expected to need no maintenance for 5 years and major repainting only after eight years or more. Runway beams for a maintenance gantry were incorporated in the design of the bridge to facilitate re-painting over the river.
The low friction sliding bearings and rocking bearings were supplied by the Glacier Metal Co. Ltd. These have a PTFE pad which slides on a polished stainless steel plate with a coefficient of friction between 2-3%. The deck is fixed at the west piers and thermal movements are relative to that point. Movement at the surface level is accommodated by a Maurer D 60 expansion joint at each end.
The reinforced concrete deck slab was cast in bays arranged in sequence to ensure the proper development of the composite action between steel and concrete. Those bays towards the centre of the bridge were poured by pumping concrete through pipes laid along the tops of the steel beam.
The Contract was completed in August 1974.
As motorists cross the bridge today in a matter of seconds little will they realise of the effort that went into providing this safe secure crossing. | 