Span. Rise. Chords. Ties. Braces. Lattice.
50 10 2--8 x 10 1--8 x 10 2--6 x 6
75 12 2--8 x 10 1--8 x 10 2--6 x 6 2 x 9
100 15 3--8 x 10 2--8 x 10 3--6 x 6 or
150 20 4--8 x l2 3--8 x 10 4--6 x 8 3 x 6
200 25 4--8 x 16 3--8 x 10 4--6 x 9
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.
Height Ins. 12
No. Panels 8
Chords Ins. 2--1 x 1/2
Posts Ins. 1--2/3 x 5/6
Braces Ins. 2--1/2 x 1/2
Lattice Ins. 1/4 x 1/2
Width Ins. 13
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, and 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 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.