Factors Governing the Design of Large Steel Bridge Projects

This part of the report applies to the design of structural steelwork for a11 types of buildings whether (a) combined as a complete steel frame, or (b) independent units supported on walls and/or walls and pillars.

The beam is probably the most useful constructional unit which the modern engineer has at his disposal. It is, therefore, very necessary that he should be fully acquainted with all its properties and with its behaviour under all kinds of loading. Thanks to early investigators and mathematicians, it is possible to calculate the bending and shear stresses in beams of symmetrical section with a fair amount, of accuracy. Under cerfain conditions, however, it becomes necessary to consider the elastic ability of a beam or of its various parts. Sometimes it is found that the web is too thin and will fail by buckling, or that the flanges have not sufficient resistance against wrinkling and, where there is not sufficient lateral support, a long beam may collapse wholesale by lateral buckling. It thus appears that a study of the elastic or secondary causes of failure is necessary before a complete mastery of the subject can be obtained. This paper deals merely with the lateral stability of beams, but it is not intended to give a complete account of all problems arising out of this subject. An attempt, has been made to put forward some known facts and theories most of which are the result of other peoples' researches: and to apply the results as far as possible to practica1 working conditions. A.N. Procter

IT is first desirable to explain how designers in the past have estimated the forces to be resisted. The few reliable determinations available have forced the designer into two courses, one based upon a horizontal force bearing some arbitrary proportion to g, while the other rests upon a wind pressure acting upon any face of the building under consideration. Following the San Francisco earthquake of 1906, much support was given to the latter method; but it is now held to be untenable on the grounds that the lateral force on a building weighing 1,000 tons would be the same as on one weighing 10,000 tons were the exposed dimensions the same in both cases. This is obviously absurd, as the force of any moving body is proportional to its mass. A.S. Mitchell