When trestle-bents become unduly wide, a vertical column is to be placed midway between the legs so as to divide up the transverse and horizontal sway-bracing.

Care must be taken to provide properly for expansion and contraction at column feet both transversely and longitudinally.

In elevated railroads, the towers can be placed at about every fourth span or, say, every one hundred and fifty feet, or can be dispensed with altogether, when the conditions so require, by strengthening the columns properly to resist traction, thrust of braked trains, and the longitudinal component of diagonal wind-pressure.

ADJUSTABLE MEMBERS.

It is preferable to avoid altogether the use of adjustable members in trusses, as well as in sway-bracing. If the structure must be made as cheap as possible, adjustable counters may be employed; but it is advisable to confine their use to diagonals in towers of swing-spans and in lower lateral systems of deck-bridges.

CAMBER.

All trusses must be provided with such a camber that, with the heaviest live load on the span, the total camber shall never be quite taken out by deflection. With parallel chords, sufficient camber will be obtained by making the top-chord sections longer than the corresponding bottom-chord sections by one eighth (1/8) of an inch for each ten (10) feet of length. One half of the camber after a span is swung is to be taken out of the track by dapping the ties, unless this would cut too deeply into the timber.

Plate girders and shallow, open-webbed, riveted girders should not be given any camber.

EXPANSION.

Every span must be provided with some means of longitudinal expansion  and  contraction  due to  changes  of  temperature