Instructions on Modern American Bridge Building

Counterbraces. Now, as to the necessity of Counterbracing, there are various opinions. The object of it is to stiffen the truss and check vibrations. If a load be placed over any panel point, it causes that portion of the truss to sink, and produces an elevation of the corresponding panel point at the other end of the truss—thus producing a distortion, which change of form is resisted by proper counterbraces. The strain to which this timber is subjected is caused by the moving load on one panel only—and requires only scantling of the size of the middle braces. These counterbraces should not be pinned or bolted to the braces where the cross—as their action is thereby entirely altered—but it is well to so confine them as to prevent vertical or lateral motion.
 

Shoes. Formerly it was the custom to foot the braces and counters on hard wood blocks on one side of the chord, the vertical rods passing through and screwing against a block on the other side—thus the whole strain tended to crush the chord across its fibres. This is now remedied by the use of cast iron blocks, bearing on one side of the chord, but having tubes extending through to the other side, where the washer plate for the bolts fits firmly on their ends, forming a complete protection, as all the crushing strain is received on the block itself.
 

Width. It now becomes necessary to determine upon the width between the two trusses. For a single track bridge for a railroad, 14 ft. is the usual width adopted, and for a highway bridge, from 12 to 16 ft. When a double track is required, three trusses are usually employed, with a width for each roadway of 14 ft. for railroads.
 

Bolsters. Large timbers 12 x 12, or thereabouts, are laid on the bridge seats of the abutments to support the ends of the trusses, one of these should be directly under each of the extreme panel points. A panel point is the intersection of the centre line of a brace produced, with the centre line of a chord. The rise of a truss is the vertical distance between the centre lines of the upper and lower chords.
 

Camber. Were a bridge to be framed with its chords perfectly horizontal, it would be found to fall below the horizontal line on being placed in its proper position, owing to the closing up of the joints in the upper parts of the structure, and opening of joints in the lower parts, as well as to the compression of the parts. To obviate this defect, it is usual to curve the chords slightly in a vertical direction, by elongating the upper chord, so that the bays or panels are no longer rectangular but of a trapezoidal form—and, as a consequence, the inclined web members are slightly lengthened, and the verticals become radii of the curve. The amount of deviation from a horizontal line is called the Camber.

A table of Cambers for different spans will be found further on, as also a table of multipliers, by which to multiply the camber in order to find the elongation of the upper chord. Part of the Camber table is taken from Trautwine's Engineer's Pocket-Book, (which should be the inseparable companion of every engineer,) and part was calculated for this pamphlet, according to Trautwine's rules. The table of multipliers is Trautwine's.
 

Diagonal Bracing. In order to stiffen a bridge, it should have the two Trusses braced together at the Lower Chords always, at the Upper Chords when practicable—and in case of a deck bridge, where the roadway is supported on the upper chords, it is as well to have rods for vertical diagonal braces, their planes being perpendicular to the axis of the bridge. The more usual form is similar to the web members of the Howe Truss—the rods from ¾" to 1" in diameter, and the braces of 6" x 7" scantling, footed on wooden blocks, usually. It is more usual to have the tie rods of the horizontal diagonal bracing, and the braces themselves, meet in a point about midway of a Truss panel on the centre line, nearly, of the chord. This will generally give a half panel of diagonal bracing near each end of the truss—and it is very usual to have the diagonals foot at their intersection there against a cross timber interposed between the trusses, while the tie rod prevents any spreading.
 

Floor Timbers. The general dimensions of the transverse floor beams, when about 3 feet apart, from centre to centre, are 8" x 14", the largest dimension being the depth. The stringers should be notched to the floor beams about 1" or 2", and should be about 10" or 12" x 14". The cross ties should be 18" to 24" apart, from centre to centre, and be 3½" x 6".

Large, heavy bridges require no fastening to connect them with their seats, but light bridges should be fastened, as the spring on the sudden removal of a load, (as when the last car of a train has passed,) may move it from its proper position.
 

Splices. As the upper and lower chords have to be made in several lengths, securely fastened to each other, and, in order to weaken the built beam as little as possible, it is necessary to adopt some form of splicing whereby the greatest amount of tensional strength may be retained in the chord with the least amount of cutting, and yet have a secure joint. Such a splice is shown in Pl. II, Fig. 4, and below is a table from Vose's Hand-book, giving reliable dimensions.

This manner of splicing requires the back of the splice block to be let into the chord stick, against which it lies, about ¾ of an inch. To show how the various Engineers differ, as to their estimates of the sizes of the several parts of bridges, I subjoin two Tables—one by Prof. G.L. Vose, a well known Engineer, and one by Jno. C. Trautwine, an Engineer of note also—and I would premise that a bridge built according to either would be amply strong.

TABLE FOR DIMENSIONING A HOWE TRUSS BRIDGE.
G.L. VOSE.

 
TABLE FOR DIMENSIONING A HOWE TRUSS BRIDGE.
JNO. C. TRAUTWINE, C.E.

 
Both of these tables were calculated for a single Railroad track, and would answer equally well for a double Highway Bridge. In the bridge according to Trautwine's Table, each lower chord is supposed to have a piece of plank, half as thick as one of the chord pieces, and as long as three panels, firmly bolted on each of its sides, in the middle of its length.

PRATT'S BRIDGE.

This is opposite in arrangement of parts to a Howe Bridge, as the diagonals are rods, and sustain tension, and the verticals are posts, and suffer compression:

The tension on the lower, or compression on the upper chord, will be 300000 X 100 / 96 = 333333 lbs. The dimensions of the chord and splicing would be found in the same manner as for a Howe Truss.
 

Suspension Rods. Fig. 1, Pl. III., represents an elevation of a Pratt Bridge. Now, it is evident that the first sets of rods must support the weight of the whole bridge and its load, which we have found to be 300000 lbs. Each truss will have to sustain 150,000 lbs., and each end set of rods 75,000 lbs. Now, if there are two rods in each set,—each rod will have to bear a strain of 37500 lbs., and this will have an increase due to its inclination, so that the strain on it must be found by the following proportion:

Height : diagonal :: W : W' or
12 : 15.8 :: 37500 : 49375 lbs.

Referring to the Table for bolts, we find that 2⅛ gives a strength a little in excess, and will be the proper size. The next set of rods bear the weight of the whole load, less that due to the two end panels, and so on. Fig. 2, Pl. III, shows the manner of applying the rods. The bevel block should be so fitted to the chord that it will not have a crushing action.
 

Counters. Top and bottom chords are always used in this bridge, and consequently the counter rods have only to sustain the movable load on one panel. The weight of the moving load cannot be more than 2000 lbs. per lineal foot which, for a panel of 10 ft., gives 20000 lbs., or 10,000 lbs. for each set, and if we have two rods in a set, the strain on each rod will be 5000 lbs., increasing this for inclination, we shall have,

12 : 15.8 :: 5000 : 6585 lbs.,

requiring a rod of ¾ of an inch diameter. The posts in this bridge correspond to the braces of the Howe Truss, but being vertical, are not so large.

Subjoined are two Tables, one by Prof. G.L. Vose, and one by Mr. Trautwine, giving principal dimensions for bridges of different spans of the Pratt type of Truss.

TABLE OF DIMENSIONS OF A PRATT TRUSS.
PROF. G. L. VOSE.

 
TABLE OF DIMENSIONS OF A PRATT'S TRUSS.

This table is partly given in Trautwine's Engineer's Pocket Book, and partly made up from directions therein given.

TABLE OF DIMENSIONS FOR SMALL SINGLE TRACK PRATT TRUSSES.

This bridge possesses an advantage over the Howe Truss, for the panel diagonals can be tightened up by screws, so that every part of the truss can be forced to perform its work. In Howe's bridge the adjustments must be made by wedging the braces and counters.

Below are given the dimensions of a Howe bridge on the Vermont Central R.R., at South Royalton, (single track, deck.)

 
The bridge over the White River, on the Passumpsic R.R., is a Howe Truss, strengthened by an arch. The verticals are of wood, and the diagonals foot on steps formed by enlarging the ends of the verticals. The counters are in two lengths, and are adjusted by wedges at the points where they intersect the braces. The bridge is in two spans, and has a double track, and consequently three trusses. There are two timber arches to each truss, and the truss is supported on them by connecting them to the verticals by short cross pieces notched into the posts, and resting on the upper surface of the arches. It is a very stiff bridge, and similar to the one at Bellows Falls, both having their axis oblique to the channel of the stream they cross. The timbers could hardly be procured now, except at great expense.

Diagonals 6 x 8, Rods ⅞. Floor timbers suspended both from arches and truss, 9 x 13; stringers 10 x 14.

In the Cheshire Bridge, the braces are only 20 x 8, and the span is only 175 feet, the number of Panels being 14, as in the W.R. Bridge—the other dimensions are the same. Below are given the dimensions of a Howe Truss of 108 ft. span, weight to be borne on upper chord.

As plank is used for the chords, the pieces must be bolted thoroughly with ⅝ bolts.

A form of bridge that has been used to some extent on the Baltimore and Ohio Railroad, by Mr. Latrobe, is the Arch Brace Truss. In this form of Truss the braces lead directly from the abutments to the head of each vertical; thus the load is transferred at once to the abutments, without passing through a series of web members. The counterbracing is effected by means of a light lattice,—and is applied to both sides of the chords, and the intersections of the diagonals are fastened while the bridge is strained by a load—thus preventing recoil—so that the effect of a moving load is to lighten the strain on the lattice—without otherwise affecting the Truss. There are two models of this style of bridge, to my knowledge; one built by Prof. G.L. Vose, on a scale of ½ an inch to the foot, and representing a span of 150 feet, which supported 2,500 lbs. at the centre, and a movable load of 150 lbs., proving itself to be strong and rigid enough for any thing. The other, on a scale of 1 inch to the foot, and representing a span of 76 feet, was built by the Class of '73, of the Thayer Engineering School, under the writer's direction, and though bearing very heavy weights, has never been thoroughly tested—it has, however, been subjected to the sudden shock of 1040 lbs. falling 20 inches, without injury, several times. Subjoined are the dimensions of the models mentioned.

DIMENSIONS OF A MODEL OF AN ARCH BRACE TRUSS.
G.L. VOSE.

This represented a span of 150 ft, a rise of 20 feet, and a panel of 15 ft. Weight, per running foot of bridge and load, was taken at 3000 lbs.

The method of calculating the dimensions of this truss, from the foregoing data, is as follows. The half number of panels is 5, and the lengths of the corresponding diagonals (neglecting fractions) are

= 25 feet.

= 37   "  

= 49   "  

= 64   "  

= 78   "  

The weight upon each set of braces is that due to one panel, or 3000 x 15 = 45000 lbs., half of this, or 22500 lbs., is the weight for one truss only—and, as there is a brace under each of the 4 chord sticks, we divide by 4, and have 5625 lbs. per stick of the brace;—now, correcting for inclination, we shall have

20 : 25 :: 5625 :   7031 lbs.
20 : 37 :: 5625 : 10406 lbs.
20 : 49 :: 5625 : 13781 lbs.
20 : 64 :: 5625 : 18000 lbs.
20 : 78 :: 5625 : 21937 lbs.

The weights fouud show the compressional strains on the several braces;—and, were the pieces to be proportioned for compression only, their scantling would be quite small—but on account of their elasticity, they require larger dimensions.

These braces should not be fastened to the verticals,—but should be confined both laterally and vertically, where they pass them. The length of beam, for which we have to guard agains flexure, is the length between verticals in any panel.

In panel No. 1, it will be 25 feet,
      "    "     2,    "   "   18   "
      "    "     3,    "   "   17   "
      "    "     4,    "   "   16   "
      "    "     5,    "   "   16   "

Now, using the formula

we shall have, in round numbers, the following dimensions:

For the 1st panel, 25 feet long, 8 x 10
      "     2d     "   37   "     "     8 x 10
      "     3d     "   49   "     "     8 x 10
      "     4th    "   64   "     "     8 x 10
      "     5th    "   78   "     "     8 x 10

For the lattice work, a double course on each side of each truss, in long spans; and a single course, in shorter spans, of 3 x 6, or 2 x 9 plank, bolted at intersections, is sufficient.

GENERAL TABLE OF DIMENSIONS FOR ARCH
BRACE TRUSS. G.L. VOSE.

The arch braces must all foot on an iron thrust block, of which a view is given in Fig. 4, Pl. III; and the centre of pressure of the braces must be directly over a bolster, to prevent crippling.

The several sticks forming a brace must be blocked together at intervals, and when they are spliced,—a butt joint should be used—and it should come in the centre of a panel. Below are given the dimensions of the Thayer Engineering School model.

There are several other forms of Bridge, the most notable among which are the Whipple, McCallum's, Post's, Towne's, Haupt's, and Burr's. But enough has been said to give the student an idea of the general arrangement of the different parts of a Truss, arid to enable him to determine the strains to which the various members are subjected. Nothing will be said in regard to Wooden Arches, as our space is too limited.
 

Pile Bridging. A bridge of this description is useful in crossing marshes, or in shallow water. Fig. 5, Pl. III, gives a good example of this kind of bridge, under 20 feet in height. If on a curve, there must be extra bracing on the convex side.
 

Trestle Work. This is a combination of posts, caps, and braces; and is used for both temporary and permanent works. Plate IV, Figs. 1, 2, 3 and 4, give some of the best varieties in use. Figs. 1 and 2, may be used up to 15 feet in height; Fig. 4, up to 20 feet; and Fig. 3, to 30 ft. The distance apart of the various bents should not exceed 10 or 12 ft., unless bracing is introduced between them, and the bents should always be raised above the ground a few feet on a solid masonry foundation. Want of space forbids any mention of abutments and piers, which really come more properly under the head of masonry.

Iron Bridging is gradually working its way into favor, and will probably eventually supersede wooden trusses;—but in many cases wood is the only material at hand—and therefore some knowledge of Wooden Bridging is desirable. It is intended to follow this pamphlet with a portfolio of sheets containing working drawings of several kinds of Wooden Bridges, taken from actual measurements of some of the best specimens of the different styles of Truss in use.

PRACTICAL NOTES.

When putting a truss together in its proper position, on the abutments, 'false works' must first be erected to support the parts until they are so joined together as to forma complete self-sustaining truss. The bottom chords are first laid as level as possible on the false works, then the top chords are raised on temporary supports, sustained by those of the lower chord, and are placed a few inches higher at first than their proper position, in order that the web members may be slipped into place. When this is done the top chords are gradually lowered into place. The screws are then gradually tightened, (beginning at the centre and working towards both ends,) to bring the surfaces of the joints into proper contact, and by this method, the camber forms itself, and lifts the lower chords clear of the false works, leaving the truss resting only upon its proper supports. The subjoined Table will be found useful in estimating the strains on a truss when proportioning a bridge for any moving load.

Table of weights per running foot of a bridge, (either of wood or iron,) including weights of floor, lateral bracing, &c., complete, for a single track.

The weight of a single track railway bridge may be taken as equal to that of a double track highway bridge,—and the trusses that will be large enough for one will be large enough for the other.

The greatest load that a highway bridge can be subjected to is 120 lbs. to the square foot of surface.

TABLE OF CAMBERS FOR BRIDGE TRUSSES.

 
TRAUTWINE'S TABLE FOR FINDING INCREASE IN LENGTH OF UPPER CHORD BEYOND THE LOWER CHORD ON ACCOUNT OF THE CAMBER.

 
TABLE OF AMERICAN WOODS.

 
The above table is compiled from a much fuller one in Vose's Treatise on R.R. Construction.

 
TABLE OF BOLTS AND NUTS CALCULATED FOR A WORKING STRAIN OF 15,000 LBS. PER SQUARE INCH OF SECTION.

 
TABLE OF SAFE WORKING LOAD IN LBS., FOR HOLLOW CAST-IRON COLUMNS.