Railway station roofs
These early examples of long span structures were generally arched, but from 1830 the truss became popular.
The structures considered to date generally derive their ability to span considerable distances by being arched, but from 1830 onwards the truss gained favour as a structural form for iron and steel bridges and in the long span roofs for railway stations and exhibition buildings. But the techniques for analysing trusses were not refined until about 1850 and this explains why some of the early forms of truss are over complicated and poorly conceived. They were often highly redundant and in many cases were a combination of arch and truss, the arched upper chord serving to provide enhanced stiffness. The development of the railway roof structures in the UK can be illustrated by five particularly important structures. The earliest of these examples is the roof for Euston Station in 1837, and the structural form evolved to that used at St Pancras in 1868.
The roofs at Euston station are believed to be the first all iron roof trusses.
The roofs at Euston station, built between 1835 and 1839 are believed to be the first all iron roof trusses. The roof consisted of two 40ft. spans supported on slender cast-iron columns. Designed by Charles Fox, working under Robert Stephenson, they were built using rolled iron T sections for the rafters and compression members and rolled bar for the tension members. Forging was a commonly used method for shaping iron and the connections for this roof were made by forging and drilling ends to the bars to take bolts. Once put together the truss could be adjusted by wedged connectors (turnbuckles to provide tension in the rods were a later development). Although no patent has been found for the rolling of Ts, which are naturally more difficult to roll than simple angles we know that they must have been available by 1830s when this roof was built. This form of construction remained in use for some time and roofs of the period survive today so that an example can still be seen in some of the roofs of Crewe Station (1867).
When the roofs at Euston were designed locomotives were relatively small and burned coke, but the developing technology brought about two changes. Locomotives became larger and began to burn coal. Consequently the roofs became dirty from the smoke and looked uncomfortably low against the scale of the new locomotives. In 1862 the roofs were jacked up and pedestals inserted between the top of the columns and the trusses.
Turner devised a new model long span structure which formed the basis for several notable station roofs, including Liverpool Lime Street and Birmingham New Street.
An accident at Euston when a derailed train brought down a column and part of the roof suggested a need for clear spans in stations and to achieve this Turner produced a kind of combined truss and arch. This comprised an arched top chord formed of rolled deck beams tied with an arched rod. To keep this crescent shape, diagonal 'struts' (Turner's term for them) were set between the two chords. Turners arch was formed from deck beams, in fact a bulb T. Regarded as a chimera by the railway company, they demanded load tests on a full scale arch - a test that was more than successful and once built this structure rapidly became a model for others of much greater span such as Liverpool Lime Street (1849) and Birmingham New Street (1854).
Turner's roof at Liverpool Lime Street was particularly daring but he would have been in a confident mood, having completed Kew Palm House a year earlier. He proposed that the roof should cover the whole station in a single span of 153 ft. 6 in., more than twice the span of any railway roof built up to that time. Sutherland describes Lime Street as the, "first really long span iron roof anywhere in the world". With that borne in mind it is perhaps not too surprising that the railway company required full-scale tests. Once these tests were completed the construction of the roofs proper got underway and work was completed in 10 months.
The roof structure designed by Turner for Liverpool Lime Street should probably be best described as an arched truss. Sliding joints at the supports were provided to prevent lateral thrusts being transferred to the supports and thus arching action was avoided.
The St. Pancras Station roof was a trussed arch spanning 73m and tied below floor level to accommodate the thrusts.
In contrast to Liverpool Lime Street, the St. Pancras Station Roof, London, of 1868, designed by Barlow and Ordish was a trussed arch spanning 73m. In order to contain the outward thrusts generated by this enormous arch the feet extend down below floor level where they are tied. Despite arguments (including reduction in foundation costs) made by Barlow and the Midland Rail Company to support the need for the size of structure chosen it does seem that a desire to build the world's longest spanning structure was behind it all.
It is interesting to note that although load-testing was allowed for in the contract price for St. Pancras the tests were never actually carried out. This is a good measure of the general level of confidence which iron had gained, by that time, as a reliable and strong structural material.
Many interesting variants of long span iron structures can still be seen in British railway stations.
There are many variations on these forms seen in the roof structures of railway stations throughout the UK. For instance the roof at Carlisle Station has deep, narrow trusses, fabricated with flat bar. The potential lateral instability problem is addressed by a system at right angles to the main span which is braced by rigid circular elements.
