TUBULAR BEAMS. In addition to what has been said on the subject of the resistance of hollow girders, [under Otitnrsis assn IirsisraNcE], it may be desirable to state that the investigations by lir. Tait of the experiments made by Mr. Fairbairu have led to the recognition of the following laws upon the subject:—I. The strength varies nearly as the area of the top, or of the bottom flange, multiplied by the depth, divided by the distance between the points of support, and affected by a coefficient deteriniheel by experiment. 2. When the depth and distance between the points of support are the mine, the breaking weights are as the areas of the top, or of the bottom parts. Mr. Tait gives, as the practical formula derived from these laws ; calling w the breaking weight of a rectangular tubular beam ; a, the area of the bottom web; d, the depth of the beam; 1, the clear bearing, and c, a coefficient found to be equal to 13 tons; all the dimensions being in inches; then c . 3. In hollow cylindrical beams formed of thin plates, tho breaking weight in tons is equal to the continued product of the sectional area, by the depth, and by a constant (14 tons), divided by the distance between the supports; and Mr. Tait asserts that square tubular beams possess litimes the strength of cylindrical ones : if this be correct, rectangular cells at the top of is large tubular girder are preferable to circular ones. 4. In hollow elliptical beams the breaking weight in tons is equal to the continued product of the sectional area, the depth and a constant (15 tons) divided by the distance between the supports : all these dimensions being, as before, in inches. The formula becomes of course, with the change in the value of the coefficients, the same a as above,w= c andas it is so much easier in rectangular beams to modify the area of the bottom flange, so as to ensure in it the requisite ratio between it and the top flange, there must evidently be an advantage in their use.
The tubular girders for beams of wrought iron appear to have been used on a large scale, for the first time, in tho bridge turn pike road at Blackburn, by Mr. William Fairbairn ; since that period, the system has been extended by other engineers in a most extra ordinary manner, as In the instances of the Britannia and of the Salt ash bridges ; and at the present day the tubular wrought iron beams are very often substituted for the cast iron girders which were, previously so much used in general construction. In the latter case, the superior lightness of the wrought iron tubular girders, and their greater powers of elasticity, render them preferable to the weightier, and more brittle and uncertaiu, cast iron ; but the wrought Iron tubes do not admit of artistio treating in the same manner that cast iron ones do, and therefore are less fitted for picturesque structures. The works of Messrs. Iludgkinson, Tate, Clark, Fairbairn, Morin, Love, &c. must be consulted by the student of this branch of the arts of con struction ; and in the Annales des Ponta et Chauseees,' and in the ' Journal des Travaux Publique do la Belgique,' may also bo found some interesting, practical, and theoretical articles on the subject. The Swiss and Americium engineers have for many years used species of tubular girders in their timber bridges of large span ; but these were designed upon the ordinary principles of carpentry. as applied to ordinary framing, and did not depend for their strength in any important manner upon the cross bracing and root which converted them virtually Into tubular beams.