III.-DESIGN OF PERMANENT BRIDGES.
(1) Basic considerations. The problem of bridge design properly regarded, is first of all a hydraulic and only secondly a structural one. Neglect of this truth fosters uneconomic design such as the adoption of spans longer than would be needed if attention were given to increasing the rate of discharge of the stream channel by making it smoother, steeper, straighter and better guided-improvements often attainable
by a judicious use of wing walls, aprons and spurs. It is too often taken for granted that the object of a bridge is to carry a track over water, whereas the proper point of view is that the object of a bridge is to pass water under a track. Bridges in different countries reveal this difference in the habit of thought of their respective designers. Thus, in the plains of North China, all bridges have their inlet and outlet channels guided on each side by substantial rubble-pitched spurseveral chains long; in Sumatra the principle is carried still further and the Dutch engineers have 0ccasionally made works amounting almost to a canalisation of the river for a short distance above and below a bridge.
John Edwin Holmstrom