TRANSACTIONS OF SECTION G. 1185 
are still required before any universally accepted rules can be laiddown. Probably 
at the present time the engineers who have given the most attention to the subject 
are fairly in accord in holding that the admissible stress per square inch in a 
wrought-iron girder subject to a steady dead load would be one and a half times 
as great as that in a girder subject to a wholly live load, and three times that 
allowable in members subject to alternate tensile and compressive stresses of equal 
intensity, such as the piston-rod of a steam-engine or the central web bracing of a 
lattice girder. If the alternations of stress to be guarded against are not assumably 
infinite in number, but only occasional—as in wind bracing for hurricane pres- 
sures, or in a vessel amongst exceptionally high waves—then the aforesaid ratio 
of 3, 2, and 1 would not apply, but would more nearly approach the ratios 6, 5, 
and 4. 
Hundreds of existing railway bridges, which carry twenty trains a day with 
perfect safety, would break down quickly under twenty trains per hour. This 
fact was forced on my attention nearly twenty years ago by the fracture of a 
number of iron girders of ordinary strength under a five-minute train service. 
Similarly, when in New York last year I noticed, in the case of some hundreds of 
girders on the ‘ Elevated Railway,’ that the alternate thrust and pull on the central 
diagonals from trains passing every two or three minutes had developed weaknesses 
which necessitated the bars being replaced by stronger ones after a very short 
service. Somewhat the same thing had to be done recently in this country with a 
bridge over the Trent, but the train service being small the life of the bars was 
measured by years instead of months. If ships were always amongst great waves 
the number going to the bottom would be largely increased, for, according to 
Mr. John, late of Lloyd’s, ‘many large merchant steamers afloat are so deficient in 
longitudinal strength that they are liable under certain conditions of sea to be 
strained in the upper works to a tension of from 8 to 9 tons per square inch, and 
to a compression of from 6 to 7 tons’—stresses which the experiments already 
referred to prove would cause failure after a definite number of repetitions. 
Similarly, on taking ground or being dry-docked with a heavy cargo on board, it 
has been shown that vessels are liable to stresses of over 1] tons per square inch 
on the reyerse frames, but no permanent injury results from such high stresses, 
because the number of repetitions is necessarily very limited. 
It appears natural enough to everyone that a piece even of the toughest wire 
should a quickly broken if bent backward and forward to a sharp angle; but, 
perhaps, only to locomotive and marine engineers does it appear equally natural 
that the same result would follow in time if the bending were so small as to be 
quite imperceptible to the eye. A locomotive crank axle bends but 3, in., and a 
straight driving axle the still smaller amount of + in., under the heaviest bending 
stresses to which they are subject, and yet their life is limited. During the year 
1883 one iron axle in fifty broke in running, and one in fifteen was renewed in 
consequence of defects. Taking iron and steel axles together, the number then in 
use on the railways of the United Kingdom was 14,848, and of these, 911 required 
renewal during the year. Similarly, during the past three years no less than 228 
ocean steamers were disabled by broken shafts, the average safe life of which is 
said to be about three or four years. In other words, experience has proved that 
a very moderate stress alternating from tension to compression, if repeated about 
one hundred million times, will cause fracture as surely as a sharp bending to an 
angle repeated perhaps only ten times. 
I have myself made many experiments with a view to elucidate the laws 
affecting the strength of iron- and steel-work subject to frequent alternations of 
stress. Perhaps the most suggestive series was one in which I subjected flat steel 
bars about three feet long, in pairs, to repeated bendings until one bar broke, and 
then testing the surviving bar under direct tensile and compression stresses to 
ascertain to what extent the metal had deteriorated. It had come under my 
notice, as a practical engineer, that if the compression members of a structure were 
unduly weak the fact became quickly evident, perhaps under the test load ; but if, 
on the other hand, the tension members were weak, no evidence might appear of 
the fact until frequent repetition of stresses during several years had caused them 
1885. 4G 
