66 More structures Alex Turner Dec 2016
At our December meeting we were pleased to welcome, for the third time, Alex Turner, a retired senior civil engineer with Network Rail. This time he concentrated on the structures which are an essential part of any rail network. These include the bridges and viaducts which carry the tracks over roads, rivers and valleys as well as those which allow the tracks to pass under roads and other railway lines. There are over 40,000 such under and over bridges on the rail network in the UK, many of which were built when the first railways were constructed in the 19th century. The earliest bridges and viaducts were in the form of single or multiple arches constructed using bricks or local stone. On the larger structures the bricks were often made on site using locally sourced clay. Most of the construction work was carried out by hand without any assistance from machines and yet the workers managed to achieve a very high standard of workmanship. As the industrial revolution developed new materials such as cast iron, wrought iron and later steel became available for use in bridge construction; nowadays new structures are usually built using either reinforced concrete or steel, or sometimes a combination of both materials. Timber was also used in the early days and was used for example on the river crossings on the Godmanchester to St. Ives line. More recently reinforced plastics have been used to construct a number of footbridges. Not all the bridges in service can carry the heaviest trains and a classification system has been developed which indicates the weight carrying capacity of a particular bridge in terms of the types of locomotives which can safely cross it.
As on any transport system the safety of the travelling public is of paramount importance and bridges and other structures need to be inspected regularly and any defects rectified. It is usual for a visual examination of a structure to be carried out every 1-2 years with a more detailed hands-on examination say every 6 years. The results of these examinations are used to determine the priority and extent of any repairs that might be needed. Defects can arise from a number of sources such as mistakes in design, poor workmanship, deterioration of materials and damage from accidents such as might occur in a collision or a derailment. One particular source of damage results from so called “bridge bashing” where an underbridge is struck by an overheight vehicle or the load it is carrying. Many of the earlier bridges had to be built over existing roads and to save the cost of building a high embankment to take the railway over a road a reduced headroom with a lower embankment was often adopted. The clearances of such bridges are usually lower than current standards and it is not unusual to read of cases when a double decker bus has taken a wrong route and had it’s upper deck removed. Such strikes can be dangerous if the bridge superstructure, and the rail track it is carrying, is displaced sideways with the possibility of a train being derailed. Network Rail has developed a protocol to speed up the decision making process as to when trains can be allowed to use a “struck” bridge, although a detailed inspection, using specialist access equipment may be needed before any decision can be made. A number of years ago there was an attempt to reduce the number of “bridge bashing” incidents by improving the warning signage for low headroom bridges, painting the superstructures with bold yellow and black paint and installing sacrificial beams in front of the bridges. Electronic devices which activate an illuminated warning sign when an overheat vehicle breaks a light beam are also used as in the case of the A14 viaduct at Huntingdon station.
Alex illustrated his talk with a number of slides as well as an extract from a DVD about railway structures which is shown to new entrants to Network Rail. As usual he was informing as well as entertaining and the audience very much enjoyed hearing from someone with first hand knowledge of his subject.