1LX. J. I. HAYCROFT. 



and will not be astonished at its being criticised, as such is the 

 lot of all novelties, but hopes that critics, before giving their 

 opinions, will take the trouble to thoroughly understand the 

 arguments he uses, as, whilst art is difficult, good criticism is 

 also difficult. 



Discussion. 



During the discussion the following additional information 

 was supplied by the author : — 



The bridge, with which Professor Vierendeel compares his truss, 

 is one of a series erected by the Dutch Government over the 

 Meuse, near Nimegue. It was calculated for a range of stress 

 of 4*11 tons per square inch, on nett section, but the Professor 

 points out that this stress is only the primary stress, deduced in 

 the ordinary way on the hypothesis that the intersections of the 

 lattice bars and booms are articulated, whilst the engineer Van 

 der Kolk has shown* that the actual stress arising from combined 

 primary and secondary stresses may amount to from 6^ to 7f tons 

 per square inch ; yet in the case of the Vierendeel bridge, owing 

 to the secondary stresses being taken account of in the calcula- 

 tion, the stress does not exceed 475 tons per square inch. Both 

 bridges were constructed of iron, of what is known in Belgium 

 as No. 3 iron of ordinary quality, the tests on the metal in the 

 Professor's bridge resulting as follows : — Plates, longitudinally, 

 25-89 tons per square inch ; elongation, 23'5 per cent., and 

 transversely 19-24 tons per square inch, with elongation of 

 3^ per cent., the angle irons being 22-91 tons per square inch, 

 with an elongation of 7*7 per cent. 



Both bridges are single-line deck structures of 103 feet 4 inches 

 span, with a live load of 141 tons per lineal foot. The depth of 

 each truss is 9 feet 10 inches, and distance between centre of 

 verticals is 11 feet 5f inches. The weight of the bridge over the 

 Meuse is 0*64 tons per lineal foot, whilst the Professor's bridge 

 weighs 0*58 tons per lineal foot, or a total weight of 6 "81 tons in 



* Tijdschrifl van het Koninklijk Instituut van Ingenieurs, 1889-90. 



