56

CONSTRUCTION OF THE BUILDING.

Their sufficiency

to support a“ dead weight,’’

and a movingLoad.

Ho,v tried : —

By workmen;

By soldiersmarching;

By rolling round-shot “in situ

Who made by.

the diagonal struts or standards, and ties, average 3‘50 inches. The breaking-weight of the girders is calculated, and has been proved by various experiments,to amount to not less than 30 tons. Every one of the gallery-girders which hasbeen used has been proved upon the ground to a strain of 15 tons; and, mexceptional cases, where it has appeared reasonable to expect that an accumulationof weight would have to be borne, their dimensions of thickness have been in-creased, and the amount of proof has amounted to no less than 22-1 tons.

A few simple figures will clearly exhibit the sufficiency of these girders tosupport the loads that are likely to be brought upon them. A bay of gallery-floor, measuring 24 feet by 24 feet, contains 576 square feet; and it has beenfound by experiment, that it is impossible to load any surface with men to anamount equal to one hundred-weight per foot superficial. Assuming, then,576 cwts., or say 30 tons, to be by any possibility accumulated upon such a bayof gallery-floor, the load will be distributed over four girders, any two of whichhave been found to be fully competent to support the load.

In thus estimating the sufficiency of the girders, the load they might possiblybe called on to support has been considered only as what is called ‘ ‘ deadweight,” or load to which no momentum of any kind had been imparted. Inorder, then, to test them under the action of a moving weight as well, a series ofexperiments was instituted. A perfect bay of gallery, 24 feet square, was con-structed, with connecting pieces, girders, flooring, &c., complete. Its surfacewas first crowded with the contractors’ workmen, as tight as they could be packed.The men were then set to walk over it, run over it, and, finally, to j amp upon itwith all their force.

In order further to observe the effects which would be produced by a load towhich a uniform, instead of an irregular motion, had been conveyed, a number ofsoldiers of the corps of Royal Sappers and Miners were ordered to march over it,to run over it, and, finally, to mark time upon it in the most trying maimer.The result of these experiments developed the correctness of the theory uponwhich the dimensions of the girders had been based, since not the slightestdamage was done to the bay of gallery ; and the fact was fully evidenced, thatthe quality of elasticity or springiness in the floor served to protect the girdersfrom the effect of sudden shocks, and prevented the danger of the communicationto them of the accumulating momentum, generated by the possible isochronousmovements of a crowd.

Emboldened by the satisfactory result of these experiments, a yet more con-clusive series was instituted. An apparatus was contrived by Mr. Field, thelate President of the Institution of Civil Engineers , by means of which it waspossible to draw, at a quick walking pace, over the whole of the galleries onwhich the public would have to tread, a number of 68-pounder shot, collectedtogether so as to produce a uniform load of 100 lbs. per foot superficial. Nodamage whatever was produced by these rude tests, and. they may be consideredto have conclusively set at rest any doubts as to the sufficiency, in point ofstrength, of the gallery-floor, or of the girders which support it.

The whole of these girders are of cast-iron, and, together with the columnsand similar castings, have been made in Staffordshire , at the foundries of the con-tractors, at the London Works, Smethwick, near Birmingham ; at those of Messrs.A. and B. COCHRANE, of the Woodside Iron Works, Dudley ; and at those ofMessrs. Jobson, of Holly Hall, near the same town.