CHAPTER III.

TRUE ECONOMY IN DESIGN.

Treatise after treatise has been written upon the subject of economy in superstructure design, but unfortunately the result is simply a waste of good mental energy; for the writers thereof invariably attack the problem by means of complicated mathematical investigations, not recognizing the fact that the questions they endeavor to solve are altogether too intricate to be undertaken by mathematics. The object of each investigation appears to have been to establish an equation for the economic depth of truss, or that depth which corresponds to the minimum amount of metal required for said truss; and, to start the investigation, it seems to have been customary to make certain assumptions which are not even approximately correct, For instance, the principal assumption of several treatises in French and English is that the sectional area and the weight of each member of a truss ate directly proportional to its greatest stress; or, in other words, that in proportioning all members of trusses a constant intensity of working stress is to be used, while in reality for modern steel bridges the intensities vary from, say, 6000 pounds up to 15,000 pounds, or, when impact is provided for, up to 18,000 pounds, and when both impact and wind stresses are included, up to nearly 24,000 pounds. Again, no distinction is made between tension and compression members, and no account is taken of the greatly varying amounts of their percentages of weights of details.

There is, however, one mathematical investigation concerning economic truss depths which, in the author's opinion, is approximately  correct,  and  which  is  based  on assumptions

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