The full length of the iron and glass vault of the train shed is 689ft (240m). Its span of 245ft 6in (75m) was not an exceptional dimension for a bridge but for an interior it was extraordinary, especially extended in depth to form the widest and largest undivided space ever enclosed.
Image: St Pancras Station trainshed, 1868 © National Railway Museum/Science & Society Picture Library
The roof is made up of a series of twenty five principal arched trusses (which Barlow called ‘ribs), each weighing 55 tons, set at regular intervals, linked by longitudinal purlins and rising to a slight point at the crown. This feature differentiates St Pancras from its predecessors which had unbroken rounded arches, and was adopted to afford the greatest possible protection against the lateral action of the wind, although Barlow added that it improved the architectural effect. It was constructed with an enormous timber scaffold on rails, which could move northwards bay-by-bay, each rib taking over a week to assemble.
Image: Steam powered winch being used to move scaffolding, C1867 © National Railway Museum/Science & Society Picture Library
The elevation of the platform above street level and the intention to use the resultant space below for commercial purposes influenced the design and construction of the track and platform deck. This was constructed of wrought iron girders in a grid of primary and secondary beams; the spaces in between comprising riveted wrought iron plates each slightly convex in section. The deck was supported by 688 cast iron columns. A further advantage to this form of construction was that it formed a ready-made tie sufficient for an arched roof crossing the station in one span, obviating the need for intermediate columns.
Image: Engraving of the construction of St Pancras Staton, The Illustrated Times 1867 © National Railway Museum/Science & Society Picture Library
The brick side walls of the undercroft incorporated substantial piers opposite each line of columns and main girders, with arches between the piers. Drawings show the piers were connected at foundation level by inverted arches. Alternate piers enclose the bases (boots) of the wrought iron roof trusses which terminate at the level of the main girders. At this level each base is secured to the brick foundations by four vertical holding down bolts, 3 in diameter and 24ft long.
Image: St Pancras under construction, 1860s © National Railway Museum/Science & Society Picture Library
The iron work was manufactured by the Butterley Iron Company which was formed in 1790 as Benjamin Outram & Co to exploit the mineral resources of the Erewash Valley. Coal and ironstone reserves were found in close proximity and the third ingredient for iron making, limestone, was available from Crich. In its heyday the company had a world-wide role.
Image: Arch base, showing the stamp of the Butterley Iron Company © HS1 Ltd
The roof was originally covered with Welsh slate and timber-framed glazing laid out in a ‘ridge and furrow’ formation, allowing rain to funnel away through the furrows.
A large proportion of the 60 million red facing bricks used in the construction of St Pancras were supplied by Edward Gripper’s Nottingham Patent Brick Company, producers of machine made bricks of consistent quality. When demand outstripped supply, additional bricks were supplied by Tucker and Sons of Loughborough. The soft red rubbing bricks used in the arches and quoins were supplied by a number of brickyards. Even the mortar came from the Midlands.
Image: Side wall arch articulated with rubber bricks © Paul Childs
In the undercroft the brickwork was of a more functional utilitarian nature, being continuous arched structures on the east and west walls. These were built of dark multicoloured stock bricks laid in lime mortar with half inch joints. It is likely these bricks were made and fired on site.
Image: Brickmaking for St Pancras Station, 1867 © National Railway Museum/Science & Society Picture Library
The stone used in the original construction came from the Midlands: Bramley Fell, or Derbyshire gritstone specified for the foundations; Ancaster and Ketton, both limestones, and Mansfield Red, a sandstone, were used for interior and exterior elevations and Mansfield White for stairs and steps.
(www.midlandrailway-butterley.co.uk The Wyvern Winter 2011)