| Velocity in feet per second. | Deflection in inches. |
|---|---|
| 0 | 0.19 |
| 22 | 0.23 |
| 40 | 0.22 |
| 73 | 0.25 |
Cast-iron girder, thirty feet span, load thirty-three tons.
Mr. W. H. Barlow, (Eng.) observed, “that in case of a timber viaduct, a freight train, at a low speed, produced a certain deflection; but an extra train, with a much lighter engine, seemed to push the bridge like a wave before it.”
The Britannia tubular bridge was depressed three fourths of an inch by two locomotives and a train of two hundred and eighty tons standing still; but at seventy miles per hour, the deflection was sensibly less.
CHAPTER IX.
IRON BRIDGES.
“A little knowledge is a dangerous thing.”
213. Within the past ten years iron has been brought extensively into use for railroad bridging; and when employed by those who understand its chemical and mechanical nature is unequalled for strength, durability, and elegance of appearance; but when, as is too often the case in America, it is intrusted to men who neither know nor care for any thing but the price they get for it, nothing can be more unsafe. No material requires so complete a knowledge of its properties, to be safely used, as cast-iron.
NATURE AND STRENGTH OF IRON.
214. The table below shows the properties of the several descriptions of iron used in engineering.
| Wrought Iron. | Cast-Iron. | Iron Wire. | Boiler Plate. | Designation of the quality. |
|---|---|---|---|---|
| 480 | 450 | 480 | Weight per cubic foot in lbs. | |
| 15000 | 4500 | 25000 | 12740 | Resistance to extension in lbs. per sq. inch. |
| 11000 | 25000 | 7500 | Resistance to compression in lbs. per sq. in. | |
| .0000066 | .00000608 | .00000685 | .0000066 | Expansion per degree Fahrenheit in lengths. |
| .0000000424 | .000000106 | .0000000446 | .0000000524 | Extension per lb. per square inch. |
| .000000149 | .000000083 | .000000189 | Compression per lb. per square inch. | |
| 90 to 66 | 20 to 111 | 127 to 75 | Ratio of extensive to compressive strength. | |
| 12500 | 17500 | Resistance to detrusion, or shearing. | ||
| 55 | 31 | Relative transverse strength. | ||