Discussion. Factors Governing the Choice Between Repairing, Strengthening, and Reconstructing Railway Girder Bridges.

Abstract

(Chief Civil Engineer, Western Region, British Railways) said that maintenance must always loom large in the mind of an engineer in charge of civil engineering assets or liabilities, particularly in a concern which had a long life such as British Railways. 67.In the opening paragraphs of the Paper the Author stated that Waddell had said, 'The life of a metal bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long'. However normal a bridge was, some degree of compromise in its design and maintenance varied widely from good, conscientious work to neglect.68. The analysis of the data concerning the under-bridges and past reconstructions and constructions had given an average estimated life of about 100 years for wrought iron and 85 years for the early steel. The modern steel design had a much longer life, for maintenance was taken very much into account. Fig. 23 showed the general picture of the life of bridges on the Western Region. On the left of the picture was a histogram showing the life of buildings up to 1962. Carrying on from there would be seen a dotted line which showed the time when all the previous bridges would come due for reconstruction. There was a peak in 1980-95 because of the telescoping of the steel and wrought iron bridges which were reaching the end of their life at the same time. It had been decided to flatten this curve out. It would not be possible to deal with such a peak of over 100 bridges a year for 16 years, with a maximum of 110.Probably it would be impossible to cope with that with the present strength of the design staff-although the consulting engineers would no doubt be pleased to hear that. He did not know whether it would even be possible to construct the bridges with the aid of the steel industry.69. Far more important, however, was the factor of occupation of the line. This could not possibly be faced. Consequently, it had been decided to reconstruct some a little prematurely, and this gave never more than 40 bridges a year, which was probably about the right number from the point of view of the programme of line occupation. This meant that many of the bridges being planned for reconstruction later would have to be repaired and examined much more carefully to avoid any risk of premature failure under traffic. Fig. 24 showed a view of the East Largin Viaduct. In this and the adjoining St Pinnock Viaduct, the spans ranged between 40 and 65 ft in length. They had originally been built in 1859 in timber and replaced in 1885 by wrought iron truss girder double-track spans. There were two main girders to each span with cross girders spanning between the vertical web members and carrying intermediate stringers. Transverse timber decking supported on the top chords of the main girders and on the stringers, carried the ballasted tracks. In 1960 examination had 70.