316 DR. FAIRBAIRN ON THE EFFECT OF IMPACT, VIBRATORY ACTION, 
fair margin of strength ; but subsequent considerations, such as generally attend a new 
principle of construction with an untried material, induced an increase of strength, and, 
instead of the ultimate strength being six times, it was increased to eight times the 
weight of the greatest load. 
The stability and great success of these bridges gave increased confidence to the 
engineer and the public, and for several years the resistance of six times the heaviest 
load was considered an amply sufficient margin of strength. 
Owing to the success of these undertakings, there was a general demand for wrought- 
iron bridges in every direction, and numbers were made without any regard to first 
principles, or to the law of proportion necessary to be observed in the sectional areas 
of the top and bottom flanges, so clearly and satisfactorily shown in the early experi- 
ments. The result of this was a number of weak bridges, many of them so dispro- 
portioned in the distribution of the material as to be almost at the point of rupture 
with little more than double the permanent load. These discrepancies, and the erroneous 
system of contractors tendering by weight, led not only to defects in the principle of 
construction, but the introduction of bad iron and, in many cases, equally bad work- 
manship. Now there is no construction which requires greater care and more minute 
attention to sound principles than wrought-iron girders , whether employed for bridges of 
large or small span or for buildings. The lives of the public entirely depend upon the 
knowledge and skill of the engineer, and the selection of the material which he 
employs. 
The defects and break-downs which followed the first successful application of 
wrought iron to bridge-building led to doubts and fears on the part of engineers ; and 
many of them contended for eight, and even ten times the heaviest load as the safe 
margin of strength. Others, and amongst them the late Mr. Brunel, fixed a lower 
standard ; and I believe that gentleman was prepared in practice to work up to one-third 
or two-fifths of the ultimate strength of the weight that would break the bridge. 
Ultimately it was decided by the authorities of the Board of Trade, but from what data 
I am not informed, that no wrought-iron bridge should with the heaviest load exceed a 
strain of 5 tons per square inch. Now on what principle this standard was established 
does not appear ; and on application to the Board of Trade the answer is, that “ The 
Lords Commissioners of Trade require that all future bridges for railway traffic shall 
not exceed a strain of 5 tons per square inch.” 
The requirement of 5 tons per square inch on the part of the Board of Trade is not 
sufficiently definite to secure in all cases the best form of construction. It is well 
known that the powers of resistance to strain are widely different with wrought iron, 
according as we apply a force of tension or compression ; it is even possible so to dis- 
proportion the top and bottom areas of a wrought-iron girder calculated to support six 
times the rolling load, as to cause it to yield with little more than half the ultimate 
strain or 10 tons on the square inch. For example, in wrought-iron girders with solid 
tops it requires the sectional area in the top to be nearly double that of the bottom, to 
