146 SECTIONAL ADDRESSES. 



Structures. 



The indebtedness of structural engineering to mathematics and to 

 experimental science has already been suggested, but these powerful aids 

 to progress had to be supplemented by the development of new materials 

 and by a direct experimental attack before the remarkable achievements 

 of recent times were possible. 



In this respect it is well to go back occasionally to the work of William 

 Fairbairn, who was president of this section in 1862. He carried out his 

 classical experiments during the forties of last century to determine stable 

 forms of compression elements for structures, and upon his experiments 

 practice followed rather slavishly for more than forty years. The failure 

 during erection of the cantilever of the Quebec Bridge dramatically forced 

 attention upon the problem of the stability of compression elements. 

 Again a series of experiments filled the gap in knowledge and led to 

 success. Further work on a large scale, the cost of which can probably 

 only be undertaken by public authorities, is desirable. During the last 

 twelve years attempts have been made, with considerable success, to 

 design all-metal aeroplanes and rigid airships, the compression elements 

 and beams of which must of necessity be as light as possible. With a 

 fair degree of precision the external forces can be determined by experi- 

 ments upon models. The structure of an aeroplane has a certain degree 

 of ' statical indeterminateness,' but the forces and the bending moments 

 in the elements can be approximately obtained, and what might be 

 called the primary stresses can be found with the same degree of approxima- 

 tion. Three serious difficulties immediately, however, faced the designers : 

 (1) materials of a suitable character and in a suitable form for constructing 

 the very light but strong elements, (2) the lack of technical skill and 

 suitable machinery for building up the elements, (3) the lack of knowledge 

 and the almost insuperable difficulty of determining by analytical methods 

 the secondary, and what might be called stresses of even higher orders 

 of indeterminateness. The only way was a courageous combination of 

 experiment and mathematical investigation. The one without the other 

 was useless, but by a combination of the two rapid progress was made, 

 and to-day all-metal planes are being constructed of steel or of duralumin 

 which are as light, more durable, and stronger than the planes made of 

 the best spruce. 



No new fundamental knowledge, such as the splendid contribution 

 of Navier or of Euler to the theory of elasticity, has come out of the work, 

 but taking these fundamental formulae as faithful guides the experiments 

 have led to the developments of forms which, allied to the ingenuity of 

 skilful designers, have made it possible to build machines fully equal to 

 the very onerous conditions of lightness, strength, and durability imposed 

 upon aircraft. Fairbairn and Hopkinson, when experimenting on the 

 forms to be adopted for the compression elements of the Britannia Tubular 

 Bridge, were faced with almost the same difficulties, but their problem was 

 simplified in one respect. They were able to use wrought-iron plates and 

 angles sufficiently thick and rivets of sufficient diameter to make it 

 unnecessary to consider the instability that may occur in the plates 

 between adjoining rivets or in the web of the angles. This is not the 



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