The following is a description of a system of construction in roofs, invented by M. Laver, architect to the king of Hanover. It is also applicable to bridges, and similar works.
This new principle consists in a combination of the two principal forces of materials—that of resistance to compression and resistance to tension. The first of these forces has been used from the remotest periods in the construction of bridges, and arches in general ; the second has been more lately employed—at least in Europe—for the construction of sus pension bridges by the application of chains. The first requires great masses of materials and strong abutments ; the second requires less materials than the first, but secure fasten ings, for the chains are frequently obtained with difficulty. Very sensible vibrations and undulations are experienced where this last mode of construction is employed.
The disposition of the principal parts of this truss is, that of two segments of a circle, placed with the concave sides opposite each other, and tied together at the extremities, being further connected together, or rather kept apart, from each other, by vertical and diagonal struts. Imagine two bows so disposed ; it will be perceived, the chain a k g, fastened at the extremities of the upper bow, acts with the positive force of tension, which the strongest materials possess, varying from 10 to 20,000 lbs. for every square inch of the transverse section of the several kinds of wood employed in construction, and from 20 to 100,000 lbs. for the. several metals.
The bow, a d g, by its resistance to compression, serves to prevent the chain, a k g, from contracting or drawing together the extremities a and g. The lower bow a k g, acting as a chain, prevents the upper bow, a d g, from pushing cr press ing out at the points of support. The vertical and diagonal struts unite in a firm manner the two bows, and the two forces thus neutralized form a complete whole, that sustains itself, and can neither thrust out nor draw in.
It must be observed, first, that the strength of the chains, which act in a similar manner to that of suspension bridges, depends upon the depth of the versed sine, and that the more they deflect below the horizontal line, or chord of the are, the stronger they would be ; secondly, that the upper bow, owing to the elasticity of the material, must absolutely have the convex form ; so that, when any great sudden weight is thrown on it, causing the lengthening of the chains by tension, and the shortening ot' the upper bow by compression, the upper bow may not be reduced to a horizontal line, a n g, beneath which it could no longer serve by resistance to the statical equilibrium of the construction ; thirdly, that the method of combining the extremities, a and g, of the bows, must depend upon the materials employed : for instance, in a wooden bridge, the notching and scarfing at the joints of the ditRtrent pieces of wood ought to be calculated and exe cuted to the force which they have the power of resisting.
The rules and forms most applicable to these joints have also been proved by the experiments of the inventor.
This system of construction is applicable to roofs and bridges of every denomination, such as draw and swing bridges, but especially for suspension bridges, where the locality on either side, or on both, does not admit of secure fastenings for the chains. For covering large rooms, riding-schools, and other openings of large space, it is par ticularly usefill; also the erection of scaffolding, and ladders of large dimensions, and to the stiffening of beams, masts, and supports in general.
The application of this system to roofs and floors of large span, is extremely economical and useful, and by simple modifications serves for the covering of large spaces, with out any intermediate points of support, and also presents this farther advantage, that from its vertical pressure, it requires no other support than walls of moderate thickness. When applied to floors, bridging-joists will remedy the inequality of the surface in the beam itself In roofs of larger span, the posts may be continued upwards, so as to receive the purlins, and when continued downwards, serve to hold up the ceiling, whether flat, vaulted, or mixed.