438 BBPOKT— 1887. 



the beam has taken a permanent set of ^", but so far is otherwise intact. 

 Observations are also being made of the behaviour of sixty riveted steel 

 box-girders of 18 feet span, built up of two 12" X 3" channels and two 

 flange plates ; which girders are subject to very many thousand repeti- 

 tions of stress ranging from zero to 29,000 lbs. per square inch. 



The Committee, having carefully considered all the evidence pro- 

 curable up to the present time, have arrived at the following con- 

 clusions : — 



(1) For those cases in which the dead weight is much less than the 

 live load, it is the practice of engineers to adopt a lower workijig stress 

 than five tons per square inch, as permitted by the Board of Trade. 



(2) In those cases where the dead weight is lai'ge compared with the 

 live load, the results of experiments on the fatigue of metals indicate that 

 a higher working stress is permissible with the same degree of safety as 

 with the lower stresses in smaller structures. In small bridges, where the 

 effect of wind pressure is practically insignificant, the maximum stress, 

 being due to the passage of the live load, is of frequent recurrence ; while 

 in large structures, where the wind pressiare is a very important element 

 in arriving at the maximum stress, it is clear from the infrequency of 

 heavy wind pressures that the maximum stress but rarely recurs, and 

 that thus 



(3) If the working stress permissible be arrived at from the con- 

 sideration of the experiments upon the endurance of metals under repeated 

 changes of load, then the proper rolling load to assume is certainly that 

 which may be reasonably expected to come upon the bridge a great 

 number of times. 



(4) With regard to dynamic action, the shocks resulting from bad 

 rail-joints are of importance. Rails in 60-feet lengths are occasionally 

 used over bridges in order to avoid injurious effects from this cause. 



The Committee offer the following recommendations : — 



(a) That in the case of very small girders and cross-girders, when the 

 forces operating upon them ave either all tensile or all compressive, the 

 maximum stress to which wrought iron should be subjected by the quies- 

 cent weight of the moving load, added to the weight of the structure, 

 ought not to exceed 4 tons per square iiich. 



(b) That in the case of bridges or structures of such magnitude that 

 the dead weight is more than twice that of the moving load, the stress 

 upon wrought-iron may be safely increased to nearly 6 tons per square 

 inch. 



(c) That in those members or parts of structures which are exposed 

 to stresses alternating from tension to compression, the maximum tensile 

 stress added to the maximum compressive stress should not exceed 6 tons 

 per square inch, nor the maximum tensile stress or compressive stress 

 considered independently exceed 4 tons per square inch. 



(d) In computing the strength required to resist wind pressure,, 

 considering that very high pressures are of I'are occurrence, the stress 

 upon wrought iron from the effects of wind may safely be taken at 6 tons 

 per square inch ; that 



(e) In steel of suitable quality a stress 30 per cent, greater may be 

 allowed. 



