Discussion: Railway Underline Bridges: Developments Within Constraints of Limited Possession.

Abstract

The Paper mentions that we live in a period of change and there are already two areas where the Paper requires amendment. Firstly, the requirements to cater for accidental loading have been updated to give a more realistic representation of the forces involved. Secondly, the requirements to limit track deflexions and twist as trains pass over bridges have been updated.65. High early strength concretes giving 20 N/mmz in 24 h have been developed by British Railways Research Department and this, coupled with the vastly increased craneage capacity available, has led to a number of new techniques being developed.66. One of the most common forms of construction has been the box girder bridge which gives minimum construction depth and ease of erection but care must be taken in detailing especially when considering the waterproofing. On spans up to 20 m, prestressed beams with precast concrete infill between them to give effective match-casting have been used successfully. The major benefit is that no concreting is required during the possession.67. The problems of using air entrained concrete need to be highlighted. It is important to realize that air entrainment can cause a loss of strength of concrete (up to 15 N/mm2 for a 50 N / m Z nominal mix at 10% air entrainment) and in principle with levels higher than 7%, air entrainment should be looked on as a limit above which concrete should not be put into the works.68. The contractor-client relationship is of great importance, during possession work, when problems have to be solved at very short notice. This requires a high level of co-operation by staff on both sides and this has been a feature on many projects carried out for British Rail especially on the London Midland Region.69. There have been a number of guidance notes issued by the Health and Safety Executive. These cover areas where British Standards and other documents do not cover a safe method of working. They are very relevant in many areas of bridge construction and, although much of what is covered should be normal practice in a well organized contractor's oflice, they provide a useful check particularly where late changes are suggested.70. With the combination of the form of contract and the fixed possession times for Railway Bridgeworks, the opportunity for contractors to propose amendments to the permanent works is often excluded. Perhaps contract periods should be more flexible, to enable contractors to apply their ingenuity to these structures; with such a heavy bias to method related design.