The Transverse or Lateral Strength of any Girder, Beam Brest-summer, Lintel, etc., is in proportion to the product of its breadth and the square of its depth, and also to the area of its cross-section.

The best form of section for cast-iron girders or beams, etc., is deduced from the experiments of Mr. E. Hodgkinson, and such as have this form of section I are known as Hodgkinson's.

The rule deduced from his experiments directs that the area of the bottom flange should be 6 times that of the top flange - flanges connected by a thin vertical web, sufficiently rigid, however. to give the requisite lateral stiffness, and tapering both upward and downward from the neutral axis; and in order to set asidethe risk of an imperfect casting, by any great disproportion between the web end the flanges, it should he tapered so as to connect with them, with a thickness corresponding to that of the flange.

As both cast and wrought iron resist crushing or compression with a greater force than extension, it follows that the flange of a girder or beam of either of these metals, which is subjected to a crushing strain, according as the girder or beam is supported at both ends, or fixed at one end, should be of less area than the other flange, which is subjected to extension or a tensile strain.

When girders are subjected to impulse*, and are used to sustain vibrating loads, as in bridges, etc., the best proportion between the top and bottom flange is as 1 to 4: as a general rule, they should be as narrow and deep as practicable, and should never be deflected to more than one five-hundredth of their length.

In Public flails, Churches and Buildings where the weight of people alone is to be provided for, an estimate of 17S lbs. per square foot of floor surface is sufficient to provide for the weight of flooring and the load upon it.

In churches, buildings, etc., the weight to be provided for should be estimated at that which may at any time be placed thereon, or which at any time may bear upon any portion of their floors; the usual allowance, however, is for a weight of 280 lbs. per square foot of floor surface for stores and factories, and 175 lbs. per square foot when the weight of people-alone is to be provided for.

In all uses, such as in buildings and bridges, where the structure is exposed to sudden impulses, the load or stress to be sustained should not exceed from 1-5 to 1-6 of the breaking weight of the material employed; but when the load is uniform or the stress quiescent, it may be increased to ½ and ¼ of the breaking weight.

An open-web girder or beam, etc., is to be estimated in its resistance on the same principle as if it had a solid web. In cast metals, allowance is to be made for the loss of strength due to the unequal contraction in cooling of the web and flanges.

In cast-iron, the mean resistance to crushing or extension is as 4.3 to 1, and in wrought iron as 1.35 to 1; hence the mass of metal below the neutral axis will be greatest in these proportions when the stress is intermediate between the ends or supports of the girders, etc.

Wooden girders or beams, when sawed in two or more pieces, and have slips set between them, and the whole bolted together, are made stiffer by the operation, and are rendered less liable to decay.

Girders cast with a face up are stronger than when cast on a side, in the proportion to l to .96, and they are strongest also when cast with the bottom flange up.

The following results of the resistances of metals will show how the material should be distributed in order to obtain the maximum of strength with the minimum of material:

The best Iron has the greatest tensile strength, and the least com pressive or crushing.

The most economical construction of a girder or beam, with refer-ence to attaining the greatest strength with the least material, is as follows: The outline of the top, bottom and sides should be a curve of various forms, according as the breadth or depth throughout is equal, and as the girder or beam is loaded only at one end, or in the middle, or uniformly throughout.