TIMBERWORK.
284. The timber-work supporting the rails consists either of cross ties of wood, hewn flat on top and bottom, of dimensions from 6 × 7 to 7 × 9, and 2½ or 3 feet longer than gauge; or of longitudinal sawed timbers rectangular in section, placed directly beneath the rail, and giving it a bearing throughout the whole length.
Longitudinal bearings seem to possess no advantage over cross ties, but are subject to some decided disadvantages. In case of removal, two rails at least must be taken up to admit of the replacing a timber; while with cross ties any one may be taken out and replaced without even affecting the immediate passing of a train. A continued bearing is no better than a broken one, as the strength of the timber itself offers very little resistance to the weight of a locomotive. Strength is not to be expected in the timber-work; it is only the elastic medium between the rail and the ground serving to maintain the rail in a proper position. The strength is in the rail. The distance at which to place cross ties depends upon the weight of engines traversing the road, the nature of the ballast, and the strength of the rail; somewhere between two and four feet from centre to centre.
The amount of superficial bearing which the timber-work ought to give per lineal foot of rail, is differently estimated by different engineers.
Upon the 4′ 8½″ gauged roads of America, 1¾ square feet per lineal foot of rail has been allowed.
Several of the English roads give the following:—
| Name of road. | Gauge. | Square feet per lineal foot of rail. |
|---|---|---|
| London and N. W. Railway, | 4′ 8½″ | 3 |
| Great Western, | 7′ 0″ | 2½ |
| S. and W. of Ireland, | 5′ 3″ | 3⅔ |
| Midland G. W. of Ireland, | 5′ 3″ | 27 12 |
If ties are made eight inches wide, and eight feet long; we have the following amounts of bearing surface per lineal foot of the rail with different distances of the ties.
| Distance C to C of tie. | Superficial feet per lin. foot. |
|---|---|
| 2 | 2.66 |
| 2½ | 2.13 |
| 3 | 1.78 |
| 3½ | 1.53 |
Of course the longer the tie is made the greater may be the distance between, provided the rail will bear it. Mr. Peter Barlow, in his report of August, 1835, to the directors of the Liverpool and Manchester Railway, fixes the following dimensions for superstructure.