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 form a 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.

Clear Weight of
Span.  Bridge. 
Tons. lbs.

25       .266    596
30       .281    629
40       .313    701
50       .343    768
60       .374    838
70       .404    905
80       .434    972
90       .464   1039
100       .494   1106
120       .554   1241
140       .614   1375
150       .643   1440
160       .673   1507
170       .703   1575
180       .733   1642
200       .792   1774
225       .867   1942
250       .940   2105
275      1.013   2269
300      1.087   2435

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.

  Span.  Camber.  Span.  Camber.  Span.  Camber.  Span.  Camber.
  feet.  Inches.  Feet.  Inches.  Feet.  Inches.  Feet.  Inches.

25 0.8 75 2.5 175 5.8 275 9.2 30 1.0 100 3.3 200 6.7 300 10.0 50 1.7 120 4.0 225 7.5 325 10.8 60 2.0 150 5.0 250 8.3 350 11.7

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