CHAPTER XIV.COUNTRY BRIDGES AND CULVERTS.
Bridge building is a profession of itself, and some of the great bridges of the world are justly regarded as among the highest achievements of mechanical science and skill. But it is proposed to speak in this work only of the cheap and simple structures for spanning small streams. The measure of the strength of a bridge is that of its weakest part. Hence, the strength of a plain wooden bridge resting upon timber stringers or chords, is equivalent tothe sustaining power of the timbers in the middle of the span. The longer the span, other things being equal, the less its strength. The following table shows the sustaining power of sound spruce timber, of the dimensions given, at a point midway between the supports:
Lengthof Span.Width and Thickness of Timber.6 by 8 inches.6 by 9 inches.6 by 10 inches.6 by 12 inches.Feet.Pounds.Pounds.Pounds.Pounds.102,8002,6924,5006,480122,4003,0423,7505,400142,0582,6043,2164,632161,8002,2802,8084,050
A stick of timber twenty feet between supports, will bear a load in its center only one half as great as a timber of the same dimensions, ten feet between supports. Thus four timbers six by twelve inches, in a span of sixteen feet, would bear a load of eight tons; in a twelve foot span, the same timbers would support a weight of nearly twelve tons.