ON THE DESIGN OF GIRDEE BRIDGES. 477 



Capt. Henry James, of the Portsmouth Dockyard ; and three civil engi- 

 neers, Messrs. George Rennie, William Cubitfc, and Eaton Hodgkinson. 



After examining the leading engineers and ironfounders of the day 

 as to their experience and practice, and making numerous and elaborate 

 experiments, the Commissioners reported that ' legislative enactments 

 which would fetter scientific men in the development of a subject as yet 

 so novel and rapidly progressive would be highly inexpedient.' 



They, however, made certain recommendations with respect to cast- 

 iron bridges, which are substantially the same as the present rule for 

 cast-iron structures — tbat the breaking weight should be six times the 

 moving load added to three times the dead load. 



They also made a further recommendation, applicable to all elastic 

 horizontal bridges, that provision should be made for the increase of strain 

 in bridges under 40 feet long when subject to a rapidly moving load, as 

 indicated by the increased deflection in sucb cases — a recommendation 

 whicb until recent years was lost sight of. 



These recommendations were, immediately upon the publication of 

 their report, embodied by the Railway Commissioners in a circular letter 

 of instructions to their inspecting officers. 



On the day following the date of this letter Capt. Simmons inspected 

 the Torksey Bridge, a wrought iron box-girder bridge of two continuous 

 spans of 130 feet, designed by Mr. (now Sir) John Fowler, and objected 

 to it as of insufiicient strength. 



Its rejection was discussed by the Institution of Civil Engineers, and 

 the Railway Commissioners were accused of applying to wrought iron 

 the recently published recommendation of the Commission on IroUj 

 requiring a factor of safety of six for cast iron bridges. 



At the same time a lengthy correspondence took place between 

 Mr. Fowler and the Commissioners, and Captain Simmons decided — after 

 the examination of such examples as were available — that the bridge 

 should be strengthened so that the strain should not exceed five tons per 

 square inch. 



In fixing this he admitted that there was no decided authority upon 

 the subject, and that the variation in the circumstances of the construction 

 of bridges prevented the application of an invariable law. He recognised 

 as a principle the variation of the admissible strain in a bridge according to 

 the proportion of live to dead load — a point which has been recently revived. 



In the end — after a special examination to test the continuity of the 

 spans — the bridge was accepted with a strain under the most favourable 

 estimate of slightly over six tons per square inch. 



Between 1850 and 1858 the rule for cast iron bridges appears to have 

 found general acceptance, while there is conclusive evidence of the ab- 

 sence of a defined limit for the strain in wrought iron structures. 



In 1858 a wrought iron tubular girder bridge over the Spey was 

 brought before Captain (now Sir) Henry Tyler some time before its com- 

 pletion, and a lengthy correspondence ensued between Captain Tyler and 

 Mr. Fairbairn with reference to its strength. This correspondence Captain 

 Tyler laid before the Board of Trade, who, on March 30, 1859, issued a 

 circular letter of instructions fixing five tons per square inch as the 

 proper limit of strain for wrought iron. Upon this Captain Tyler based 

 his rejection of the bridge on April 30. 



It is very important to note that this rule was explained at the time 

 by Captain Tyler to represent a factor of safety of four for combined 



