The Conditions of Safety

Our railway bridges owe their safety largely to their being kept in good condition; also, they were costly to build, and are even more costly to replace, so that this portion of our equipment requires the greatest care in inspection and maintenance, as is well known to those engineers, foremen, inspectors, and bridge-men, to whom the safekeeping of bridges is entrusted. On the New Zealand Railways a high standard of bridge maintenance has always been observed, and its economy is now amply demonstrated in the excellent condition of many of our old bridges which still have a long remaining useful life, whereas with less careful maintenance they might easily have been reduced, before this, through corrosion, to an unserviceable condition.

Careful maintenance, however, cannot do more than check the loss of strength with age. While often giving the impression of permanency, bridge structures, like railway rolling stock, or any other industrial plant, are all the time depreciating and becoming obsolete. Their timbers begin to decay, and steel and ironwork becomes corroded, thus losing strength. Obsolescence will often be a powerful though indirect factor in determining the life of a bridge. The utility of a bridge becomes restricted by its load-carrying capacity, and when traffic conditions call for increasing axle loadings, a stage is reached where economy demands an increase of strength in the bridge, and this may necessitate complete reconstruction.