CENTRES.
209. Centres are temporary wooden frames, used in the construction of stone arches. Their duty is to hold the masonry, while it is unable to support itself.
For arches from five to fifteen feet span, a centre made of boards or planks, fig. 87, is all that is necessary. For longer spans, when the ground beneath the arch can be used, the form, fig. 88, answers well. When there is no support but the abutments or piers, something similar to fig. 89 must be adopted. This is the plan adopted by George Rennie, chief engineer at the Waterloo bridge over the Thames at London.
Fig. 87.
Fig. 88.
Centres are strained in a different manner as the arch progresses; first at the haunches, and last at the crown. Excess of weight at any point causes a settling at such, and a rise takes place at some other place. By loading the arch temporarily, such motions are checked.
Fig. 89.
These frames are placed vertically upon the pieces F F, which being connected with the braces D D by the folding wedges c c, admit of adjustment of the height of the centre. The distance between the ribbed frames depends upon the form of the arch, and the span, or upon the weight to be supported; varying from one to four feet. The centres are covered with a course of narrow plank, placed parallel with the axis of the arch, upon which the voussoirs rest.
210. The method of putting a bridge upon the masonry is shown in figs. 90, and 91; the former when the road-way is upon the upper, and the latter when upon the lower chord.
Fig. 90.
Fig. 91.
211. In figs. 92 to 100, are given several plans for spans from five to seventy-five feet. Fig. 92, shows the simple beam braced beneath with diagonal plank; the bolts passing through the ties, stringers, and braces. The stringers are bolted to the wall plates, and when the bridge is upon a curve notched also, by cutting the bolster. Fig. 92 A shows the plan. This form answers for openings from five to twenty feet. From fifteen to thirty feet, we may use figs. 94, 95, 96, and 97. From twenty-five, to fifty and sixty feet, figs. 93, 97, and 98. And from fifty to seventy-five feet, figs. 99 and 100.
The following tables give reliable dimensions for bridges upon the above plans.
Fig. 92. 5 to 20 feet.
Fig. 92 A.
| Span. | Bolsters. | Stringers. | Ties. | Braces. | Bolts. |
|---|---|---|---|---|---|
| 5 | 12 × 12 | 12 × 12 | 6 × 10 | 2 × 10 | 1 inch |
| 10 | 12 × 12 | 12 × 13 | 6 × 10 | 2 × 10 | 1 inch |
| 15 | 14 × 14 | 12 × 18 | 6 × 10 | 2 × 10 | 1 inch |
| 20 | 14 × 14 | 12 × 24 | 6 × 10 | 2 × 10 | 1 inch |
The ties being notched three inches on to the stringers, without cutting the latter.
Fig. 95. 15 to 30 feet.
| Span. | Rise. | Bolster. | Stringer. | Braces. | Rod. |
|---|---|---|---|---|---|
| 15 | 6 | 12 × 12 | 12 × 12 | 2—5 × 6 | 1 inch |
| 20 | 7 | 14 × 14 | 12 × 12 | 2—5 × 8 | 1¼ inch |
| 25 | 8 | 14 × 14 | 12 × 15 | 2—5 × 9 | 1⅜ inch |
| 30 | 10 | 15 × 15 | 12 × 18 | 2—5 × 10 | 1½ inch |
Fig. 96. 15 to 30 feet.
| Span. | Rise. | Stringer. | Post. | Rod. |
|---|---|---|---|---|
| 15 | 5 | 12 × 12 | 8 × 8 | 1⅛ |
| 20 | 6 | 12 × 13 | 9 × 9 | 1¼ |
| 25 | 7 | 12 × 15 | 10 × 10 | 1½ |
| 30 | 8 | 12 × 18 | 10 × 12 | 1⅝ |
Fig. 94. 15 to 30 feet.
| Span. | Rise. | Stringer. | Braces. | Rods. | Lattice. |
|---|---|---|---|---|---|
| 15 | 5 | 2—8 × 8 | 2—5 × 5 | 1 | 2 × 6 |
| 20 | 6 | 2—8 × 9 | 2—5 × 6 | 1¼ | 2 × 8 |
| 25 | 7 | 2—8 × 10 | 2—5 × 8 | 1⅜ | 2 × 9 |
| 30 | 9 | 2—8 × 12 | 2—5 × 9 | 1½ | 2 × 10 |
Fig. 97. 15 to 30 feet.
| Span. | Rise. | Stringer. | Post. | Rods. | Braces. |
|---|---|---|---|---|---|
| 15 | 5 | 2—8 × 8 | 8 × 8 | 1 | 4 × 5 |
| 20 | 6 | 2—8 × 9 | 9 × 9 | 1⅛ | 4 × 6 |
| 25 | 7 | 2—8 × 10 | 10 × 10 | 1¼ | 5 × 6 |
| 30 | 9 | 2—8 × 12 | 10 × 12 | 1½ | 6 × 6 |
Fig. 93. 25 to 50 feet.
| Span. | Rise. | Chords. | Braces. | Posts. | Rods. |
|---|---|---|---|---|---|
| 25 | 8 | 2—6 × 8 | 6 × 6 | 6 × 8 | 1 |
| 40 | 10 | 2—7 × 9 | 6 × 7 | 8 × 8 | 1⅛ |
| 50 | 10 | 2—8 × 10 | 6 × 8 | 8 × 10 | 1¼ |
Fig. 97. 25 to 50 feet.
| Span. | Rise. | Chords. | Posts. | Braces. | Rods. |
|---|---|---|---|---|---|
| 25 | 8 | 2—6 × 8 | 8 × 8 | 5 × 5 | 1⅛ or 2—⅞ |
| 40 | 10 | 2—7 × 9 | 9 × 9 | 5 × 8 | 1⅜ or 2—1⅛ |
| 50 | 10 | 2—8 × 10 | 10 × 10 | 6 × 8 | 1¾ or 2—1¼ |
Fig. 98. 25 to 50 feet.
| Span. | Rise. | Chords. | Posts. | Braces. | Rods. |
|---|---|---|---|---|---|
| 25 | 8 | 2—6 × 8 | 6 × 8 | 5 × 5 | 2—1 or 1—1⅜ |
| 40 | 10 | 2—7 × 9 | 6 × 9 | 5 × 8 | 2—1⅛ or 1—1⅝ |
| 50 | 10 | 2—8 × 10 | 6 × 12 | 5 × 10 | 2—1¼ or 1—1¾ |
Fig. 99. 50 to 75 feet.
| Span. | Rise. | Chord. | Posts. | Braces. | Lattice. |
|---|---|---|---|---|---|
| 50 | 8 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 2 × 6 |
| 60 | 9 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 2 × 6 |
| 75 | 10 | 3—8 × 10 | 2—8 × 10 | 3—6 × 8 | 2 × 6 |
Fig. 100. 50 to 75 feet.
| Span. | Rise. | Chords. | Posts. | Braces. | Centre braces. | Rods. | |
|---|---|---|---|---|---|---|---|
| No. 1. | No. 2. | ||||||
| 50 | 8 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 5 × 5 | 1½ | 1 |
| 60 | 9 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 5 × 5 | 1⅝ | 1⅛ |
| 75 | 10 | 3—8 × 10 | 2—8 × 10 | 2—6 × 8 | 5 × 6 | 1¾ | 1¼ |
212. In dimensioning small bridges, like the above, in estimating the maximum load, more regard must be given to the weight of momentary loads than (as in large bridges) the weight per lineal foot, as the weight of the bridge itself, when under fifty or sixty feet span, is inconsiderable. The greatest load that can come upon a single post or rod, is that from the driving wheels of a locomotive. If the whole engine weighs forty tons, there will be ten tons on each pair of drivers, or five tons or 11,200 lbs. on each wheel; which, being applied over a length of ten feet only, may be considered as at a single point, and all parts must be able to bear such load. In large spans, where the weight is great, if the truss is strong enough to support the bridge and load, it will safely resist the effects of a sudden application of passing trains.
Note.—On the Static and Dynamic deflection of Bridges. Considerable variance of opinion exists as to the relative deflection of bridges, produced by stationary and by moving loads. Neither experiment nor theory has exactly settled the point.
Experiments upon the Elwell bridge, (Epsom and Croydon Railway, England).
| Velocity in feet per second. | Deflection in inches. |
|---|---|
| 0 | 0.215 |
| 25 | 0.215 |
| 31 | 0.230 |
| 32 | 0.225 |
| 54 | 0.245 |
| 75 | 0.235 |
The bridge being a cast-iron girder of forty-eight feet span, load thirty-nine tons.
Experiments on the Godstone bridge, (S. E. R. R. England).
| 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.