Fortunately for the stability of these high buildings, the effect of wind pressures had been studied in this country in the designs of the Kinzua, Pecos, and other high viaducts.
All this had been thoroughly worked out and known to our engineers before the fall of the Tay Bridge in Scotland. That disastrous event led to very careful experiments on wind pressures by Sir Benjamin Baker, the very eminent engineer of the Forth Bridge. His experiments showed that a wind gauge of 300 square feet area showed a maximum pressure of thirty-five pounds per square foot, while a small one of one foot and a half square area registered gusts of forty-one pounds per square foot.
The modern elevated railway of cities is simply a very long railway viaduct. Some idea may be gained of the life of a modern riveted-iron structure from the experience of the Manhattan Elevated Railway of New York. These roads were built in 1878–79 to carry uniform loads of 1600 pounds per lineal foot, except Second Avenue, which was made to carry 2000. The stresses were below 10,000 pounds per square inch.
These viaducts have carried in twenty-two years over 25,000,000 trains, weighing over 3,000,000,000 tons, at a maximum speed of twenty-five miles an hour, and are still in good order.
Bridge engineers of the present day are free from the difficulties which confronted the early designers of iron bridges. The mathematics of bridge design was understood in 1870, but the proportioning of details had to be worked out individually. Every new span was a new problem. Now the engineer tells his draughtsman to design a span of a given length, height, and width, and to carry such a load. By the light of experience he does this at once.
Connections have become standardized so that the duplication of parts can be carried to its fullest extent.
Machine tools are used to make every part of a bridge, and power riveters to fasten them together. Great accuracy can now be had, and the sizes of parts have increased in a remarkable degree.
We have now great bridge companies, which are so completely equipped with appliances for both shop drawings and construction that the old joke becomes almost true that they can make bridges and sell them by the mile.
All improvements of design are now public property. All that the bridge companies do is done in the fierce light of competition. Mistakes mean ruin, and the fittest only survives.
Having such powerful aids, the American bridge engineer of to-day has advantages over his predecessors and over his European brethren, where the American system has not yet been adopted.