ws 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 3/4 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.

End Centre End Centre Counter
Span. Rise. Chords. Post. Post. Rod. Rod. Rod.

50 10 2--8x10 5 x 5 4 x 4 2--1-3/8 2--1 1--1-1/2
75 12 2--8x10 6 x 6 5 x 5 2--1-5/8 2--1 1--1-1/2
100 15 3--8x10 7 x 7 6 x 6 2--1-3/4 2--1 2--1-1/8
125 18 3--8x10 8 x 8 6 x 6 3--1-7/8 3--1 2--1-1/3
150 21 4--8x12 9 x 9 6 x 6 3--2-1/8 3--1 8--1-1/8
200 24 4--8x16 10 x 10 6 x 6 5--1-7/8 5--1 3--1-1/8

TABLE OF DIMENSIONS OF A PRATT'S TRUSS.

| | | Upper | Lower | Main Brace Rods. | Counter | | | |
Clear| | No.| Chord. | Chord. | | Rods. | | | Posts. |
Span |Rise| of |---------|---------|--------------------|-----------| | |----------|
in | in |Pan-|No.| |No.| |No.| Size |No.|Size.|Num| |No.| Size|No.|Size. |
feet.|feet|els.|Pcs|Size.|Pcs|Size.|Ctr|Centre|End| End.|ber| Size. |End| End.|Ctr|Centre|
-----|----|----|---|-----|---|-----|---|------|---|-----|---|-------|---|-----|---|------|
25 | 6 | 8 | 3 | 4x5 | 3 | 4x10| 2 |1 | 2 |1-3/8| 1 |1-7/16 | 3 | 4x5 | 3 | 4x4 |
50 | 9 | 9 | 3 | 6x7 | 3 | 6x10| 2 |1-3/16| 2 |1-1/8| 1 |1-5/8 | 3 | 6x6 | 3 | 6x5 |
75 | 12 | 10 | 3 | 6x9 | 3 | 6x11| 2 |1-5/16| 2 |2-1/2| 1 |1-7/8 | 3 | 6x7 | 3 | 6x5 |
100 | 15 | 11 | 3 | 6x10| 3 | 6x12| 2 |1-7/16| 2 |2-7/8| 1 |2