The economics of water-proofing solid-floor bridges might properly be considered in the chapter on "Economics of Maintenance and Repairs," or in that of "Economics of Metal Protection"; but the subject is one of such importance that it truly merits special treatment in an independent chapter.

Very few engineers are sufficiently conversant with the water-proofing of bridges to permit of their writing authoritatively upon the subject of its economics, and the author's experience therein certainly has not been very extensive; hence he deems himself exceedingly fortunate in having secured the aid of Mr. J. B. W. Gardiner, President of Gardiner and Lewis, Inc., of New York City, who has made a special study of the subject, extending over several years, and who has very kindly prepared for him the substance of the following dissertation:

The economics of water-proofing means this: Is or is not water-proofing worth while financially? does the effect produced or the protection afforded the structure by water-proofing the floor justify the expenditure involved? It must be assumed at the outset that the same care has been taken with the water-proofing as is taken ordinarily with the steel or other structural material, i.e., that the bridge was designed to be water-proofed, that a careful selection of materials was made, and that those materials were installed by men skilled in that particular trade, or at least were placed under careful supervision. These conditions are no more stringent or unreasonable than those which apply to the design, fabrication, and placing of other materials that go into the structure. The initiatory presumption is, therefore, entirely proper.

Whether water-proofing as applied to bridge floors is a profitable invest- ment depends on several factors. These are:

a. The proportionate original cost of the water-proofed bridge as

compared with the total cost of the bridge without water-

proofing.

b. The probable life of the water-proofing and the probable cost of

its renewal.

c. Its effect on the life of the structure itself and on the cost of

maintenance and repairs.

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