considered in connection with steel bridges, the effect thereof on the concrete floor of such structures being discussed in connection with concrete viaducts. The primary function of water-proofing on steel bridges is to furnish protection to those steel members which, because of the very existence of the solid floor, are not accessible for ordinary maintenance. They cannot be painted or otherwise guarded from moisture—as they are, so must they remain. On railroad structures the flow of water to the drains is seriously interfered with by the ballast, which, to some extent, acts as a dam to hold the water on the surface of the concrete floor. The said concrete floor may thus become saturated, because of the moisture being held for a considerable time against the steel-an ideal condition for destructive corrosion.

Brine-drip from refrigerator cars is a singularly active corrosive which is a cause of trouble and annoyance to bridge engineers everywhere, its effect being very frequently seen on the top flanges of stringers and floor beams, where it is extremely destructive. Not only are these affected, but also in half-through, plate-girder bridges those portions of the webs of the girders which are covered by the concrete are also subject to corrosion, as the joints where the concrete meets the steel invariably open sufficiently to allow water to enter.

It is unnecessary, though, to dilate further on the action of either atmospheric water or brine-drip on steel. The facts are well known and universally recognized, and it is conceded that many bridges have been seriously damaged by rust, even to the extent of having to be replaced. This deterioration can be prevented on the exposed metal by painting; but the only protection that can be given to those members which are not exposed is water-proofing. The actual money damage resulting from failure to water-proof cannot, for lack of dependable data, be definitely stated. Certain it is, however, that, if a steel bridge is left unpainted long enough, the sectional areas of the metal will be so reduced that the structure will become unfit for use and ready for the scrap heap. And if this is true of the exposed members from which the water dries out quickly, how much more true is it of those members which are subjected to a greatly aggravated condition! At just what period inaccessible and unprotected steel will become unsafe no one can say; but the fact that so many and such important members cannot be inspected so as to determine their condition would appear to impose upon an engineer, from the standpoints of both public safety and economy to his client, the duty of protecting the metal by efficient water-proofing. With increasing live loads, it is all the more necessary to maintain the full strength of the steelwork, and not to allow of its weakening by preventable deterioration.

From the foregoing it is evident that water-proofing is a necessity for bridges which contain structural steel embedded in the concrete of the floor, and for those having members encased in concrete or gunite. Failure to follow this practise has caused loss in some instances, the encased