Influence Lines

In preceding articles under this title, brief descriptions have been given of some of the important steel bridge structures either recently completed or at present under construction. The outstanding feature of recent and proposed bridge projects is the very considerable increase in length of span, such increase being rendered possible mainly by the employment of material of greatly enhanced ultimate strength and elastic limit. So long as bridge engineers were limited in their choice of material to ordinary mild carbon steel, both limiting and economic spans for the various types of girders were capable of close definition and the economy of erecting spans appreciably exceeding about 2,000 ft. was at least doubtful. Some credit must be allowed to improved methods of production, better organisation and routine of shop work and more economical methods of erection, but tbese alone are relatively secondary influences when referred to the dominant influence exerted by the availability of the new materials. These new materials comprise various heat treated alloy and carbon steels, and their introduction into practical structures may be said to date from the year 1903. Their progressive employment has been retarded by a variety of causes. Just as the supplanting of wrought iron by mild steel was only consummated after an arduous uphill contest, so the natural prejudice against the employment of the new steels has considerably delayed their adoption. Further, the increased cost and limited demand have also operated adversely. It is, of course, imperative that new materials should not be used, especially for structures the failure of which would almost certainly be attended by most deplorable consequences, until their reliability has been placed beyond the region of doubt, and the marked progress of the last few years would appear to have gone far to establish this desirable condition. An increased demand, as time progresses, should react favourably on the question of cost. The employment of the new materials is associated peculiarly with the development of the largest span structures, since very little, if any, structural economy results from their employment for small span structures of relatively negligible dead weight. Since this latter class constitutes the great majority, the demand for the newer materials, to whatever extent it may increase, must still be relatively limited. Structural economy, apart from monetary economy, may be realised by the employment of high tension steel for the whole, or for portions only, of a structure, and in general, a careful combination of the two considerations will be sought. Again, certain varieties of steel are more suited for particular members and in notable recent structures, what is, in our present state of knowledge, the most useful and economic combination of several different alloys has been aimed at. From the poral aspect, the failure of a small bridge structure is equally to be deplored as that of a la

This new building embraces the following accommoddion :-Basement : Boxmaking and storage, cloak rooms. Ground floor : Wholesale despatch department. 1st floor : Stock room. 2nd floor : Despatch ofices. 3rd floor :Treasury and accounts department. 4th floor: Directors’ offices. 5th floor: Welfare and first aid department, main telephone switch board. Manageria1 dining rooms and cafeteria for office staff.

To the Editor of The Structural Engineer. Sir,-I should be grateful if any member reading this letter would communicate with me should he have had any experience in the use of steel ceilings or steel tiles. I am anxious to ascertain as to whether they are satisfactory in practice.