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As a rule single beam hangers and large single counters are to be avoiaed on 
account of the great bending moments wliich they produce upon the pins. 
The size of the pin for the liip joint depends greatly upon the arrangement of 
the bars which it couples. In a double intersection bridge where tliero are two liip 
verticals, two long main diagonals and two short ones meeting at the hip, the best 
arrangement is to put one pair of diagonals on the outside of the chord and the 
other pair inside, close to the bearing, the verticals coming next and being kept apart 
by a filler. If the moment on a liip pin be very great, the use of a steel pin 
will prove advantageous in reducing the size of tlio eye-bar heads. 
Except when the cliord pins are small it is not necessary to consider the effect 
upon them of the stresses in the lateral rods, but whenever possible the latter should 
be so arranged that the effect of tlie stress in the outer ono will be to diminisli the 
horizontal component of tlie moment on the pin caused by the stresses in tlie truss 
members, i. e. if tlie tondency of tlie cliord and web stresses is to bend the pin convex 
to the middle of the span, tlio outer rod, wlien bent eyes are employed, should point 
towards tlie middle ; but, if it bo to bencl the pin concave to the micldlo of tlie 
span, the outer lateral rod should point towards the nearer pier or abutment. 
The ends of pins have to be reduced in diameter, so that the nuts and pin pilots 
may be screwed thereon. Care must therefore be taken in proportioning small pins 
to see that sufficient area be left under the root of the thread to resist the tension on 
that section caused by tlio greatest transverse components of the stresses in tlie 
lateral rods. The principal objection to tlie use of large pins is not always tlie undue 
weight of the pins theruselves, but tlie increased size of tlio chord and tie-bar Leads, 
and the room that they take up. 
On the other Land, it is not always desirable to use the smallest possible piu, 
as the width of the bearing is an inverse function of the diameter of the pin : so if, 
owing to the necessity of a large number of rivets, the re* enforcing plates be long, 
it might be economical to increase the diameter so as to reduce the width. Thicken- 
ing the heads of eye bars lias an mjurious effect on the pins, although a beneficial 
one upon the heads, for the lever arms of tlie stresses are thereby increased. 
Bridges with weak pins will not necessarily fail by the rupture of the pins. 
Tlie reason for this is thus stated by Professor Bim •:“ Tlie distortion of tlie pin 
beyond tlie clastic limit will relieve the outside eye bars of a large portion (in some 
cases, perhaps all) of the stress in them. This result will produce a redistribution 
of stress in the eye bars, by which some will be under strained, and the others cor- 
respondingly overstrained. Thus, although the pin may not wholly fail, tlio safety 
of the joint will be sacrificed by the overstrained metal in the eye-bars. n 
The preceding portion of this chapter may be termed tlie theory of pin propor- 
tion and the subsequent part the practice. 
The ordinary method of pin proportioning is to figure the diameters of a few 
principal pins, and to make tlie others of tlie same sizes. Tims, by inspection, can 
be found which pin near the middle of the bottom chord is subjected to the greatest 
bending- moment. If there be an even number of panels in the span, it will be the 
middle pin; but, if there be an odd number, it may be the first, or second pin from 
