GNR 1870 Kings Cross Roof
NEW ROOF OF PASSENGER STATION, GREAT NORTHERN RAILWAY, KING'S CROSS.
MR. RICHARD JOHNSON, M. INST. C.E., ENGINEER-IN-CHIEF.
The following is an abstract of a paper read by Mr. R. M. Bancroft before the Civil and Mechanical Engineers’ Society, at a meeting held on the 8th of December, 1869.
The Great Northern passenger terminal station at King's Cross was opened to public traffic in 185*2, and its roof constructed during the two previous years, and was at the time a work which created some little sensation, as it was the largest span roof of the laminated type constructed in this country. From designs of French roof* it appears that Colonel Emy, of the French Military Engineers, was the first person to draw public attention to the subject, by applying the system of the laminated timber arches in the construction of the roofs over the Hiding Schools of Marac, near Bayonne, 65 ½ ft. span, and Libourne, 69 ft. span, about the year 1819. They were both semi-circular, and made to form a complete truss to support the covering, by means of principal rafters tangential to the curves, and tied to the semi-circular rib by a number of radiating clipping braces.
The ribs were bent complete without trenails, and were maintained in their form by iron stirrups subsequently placed, and passing round the whole system at intervals, and by bolts going through the laminations in the intermediate spaces. Mr. John Green, in the year 1827, made a design and model for a bridge, with timber arches resting upon stone piers. In 1833 the plan was adopted, and in 1837 it was put into execution at the Ouse Burn Viaduct of the Newcastle and North Shields branch of the North-Eastern Railway.
The late Joseph Locke, Esq., C.E., constructed a bridge upon the laminated arch principle for the Rouen and Havre Railway over the Seine and Epaulet, near Rouen. The arched ribs of this bridge were made 4ft. deep by 1ft. 6in. wide, and formed by bending sixteen planks 3in. thick of Baltic timber, placed concentrically over each other, each course being fastened to those above and below it by oak trenails.
These trenails were made to pass through two courses of planks into the third. The ribs were bent upon a platform on the ground to half the thickness of the ribs, thus dispensing with the necessity of centreing, and were thus placed in position by a travelling crane, and the remaining depth of the rib was completed in place.
In bridges built upon this plan it was found every time the girders or ribs deflected under moving loads the concentric layers of planks detached themselves sllightly from the layers that were respectively above and below them, so that a passage was formed for the admittance of aqueous vapours to the inside of the ribs, where a process of fermentation or "wet rot" rapidly developed itself.
The King's Cross passenger station was erected on the site of the Small-pox Hospital in 1851-2, and, therefore, the roof has stood about eighteen years, a period much longer than the bridges built on the same plan. The principals of this roof were constructed upon a platform on the ground, to which chocks were secured at intervals of 4ft. to 5ft. ; these were set out to a radius corresponding with the innermost layers of the planks, and were secured by means of clamps to the chocks. On tins first layer a second series was laid and fastened with 3in. wood-screws, 8in. apart, and again clamped to the chocks, and so on until the whole thickness of the rib was completed, care being taken that all the planks broke joint.
The ribs were hoisted to place by three derricks, one placed against each wall. and a third in the middle of the span. A band of wrought iron, 4in. by ½ in., was finally placed round the topmost plank, and bolts passed through both iron and wood about 2ft. from each other in the centre of the rib. These laminated ribs in their turn support whole timber purlins, which carry the covering. As to the cause of decay in the roof, in the large quantity of timber used most likely some of it was in a sappy or wet state. The planks being so closely packed together, and painted over as soon as fixed, prevented the exudation of the moisture, and the intense heat of the sun during the summer months, as well as the vapour and sulphur from the locomotives and inefficient ventilation, assisted the fermentation going on inside the ribs, and caused rapid decay.
The scaffold for the construction of the roof, which I shall now briefly describe, contains altogether about 14,000 cubic feet of timber, and the estimated weight of it, when in full work, and including all iron and weights constantly being lifted, is 400 tons.
It takes about seventeen minutes to move it a distance of one bay, or 20ft., out of which time the men take a rest of two or more minutes. The large wrought iron plate girders are constructed of such section that they may be hereafter used in bridges down the line. The stage has been designed so that no hindrance is caused to the traffic constantly passing underneath.