Discussion. The New Manchester Road Bridge in the Port of London.

Abstract

I should like to comment on some problems associated with the Scherzer type of bridge with particular reference to experience I have had with a bascule bridge in Bombay which was commissioned in 1964 and has been working satisfactorily ever since. 66.The Author has referred to three difficulties with a Scherzer bridge: the high bearing pressure on the tracks, the heavy foundations needed and the difficulty in making the bridge run straight. The first two aspects are dependent upon the weight of the moving parts. This weight is independent of the type of bridge, but is a function of the span and the lever arm to the counterweight. The greater the lever arm the less force on the bearings or rolling track but the greater is the distance the bridge has to move.67. Nowadays, tracks are much less of a problem than they used to be. Formerly, bascule bridges had built-up webs and a relatively thin track. The heavy rolling pressure used to give rise to working and fretting corrosion between the back of the track and the built-up web. This can be avoided with modern techniques. In the Bombay bridge the rolling surface was 230 mm wide. The curved track was a 520 mm deep casting tapered at the back to the same width as the web plate to which it was welded so that by the time the stresses reached the weld they had been dispersed over a considerable length. The straight track was a casting 460 mm deep, bolted to a girder spanning between the lock wall and a well sunk to rock behind the wall. This track was 450 mm deep, so that the stresses were reduced at the junction with the main girder.68. One problem encountered in construction was in obtaining a uniform bearing between the straight track and the fabricated supporting girder. The track had a machined lower surface but the top flange of the girder could not be fabricated to the same degree of accuracy. Consequently, the track casting was tightened down on to a prepared bed of shims 1 mm thick, holes drilled at intervals through the track and the shims and an epoxy grout injected. This has worked very well and given no trouble.69. It is true that a Scherzer type bridge requires a heavy foundation to take the rolling weight but bascule bridges are normally sited at lock entrances and the lock wall will provide a very good support for a girder to support the rolling weight. In Bombay, the track girder spanned between the lock wall and a well sunk to the rock behind the wall. A rocker bearing was provided over the well and a roller bearing at the back of the wall so horizontal forces were not imposed on the wall. During construction the need to insist on proper testing was illustrated. The roller bearings for the track girders were of high strength steel of small diameter. The specialist fabricator of the bearings said it was not necessary to test them as he had made many of them and there had never been any trouble. Nevertheless, we insisted on a full strength test on all the bearings and some failed. It was found that the specialist manufacturer had inadvertently made the be...