It is to be noted that, in bridges built in the late 80's and early 90's, hard grades of steel were frequently used for the pins.
In timber-trestle bridges, the stringers in bending usually show low classification. On account of there being three or more sticks acting together, it is permissible to use a higher unit stress for trestle stringers than for a single stick, as the average strength for the several pieces exceeds that of the poorest one. On account of the exposure to the weather and the deterioration which gradually takes place, the allowed unit stress in timber stringers should be reduced as the age of the bridge increases. Where timber bridges are thoroughly inspected and defective material is promptly replaced, and where they are subject to the same general consideration as given above for metal bridges, the following unit stresses might be taken as a safe practice for maximum fiber stress in stringer bridges without allowance for impact:
For stringer bridges six years old, 2,000 lbs. per sq. in., and reduced about 100 lbs. per sq. in. for each year following.
The above figures are based on Douglas Fir or dense yellow pine and for climatic
conditions prevailing in the North Central States. In more arid regions where longer
life of timber may be expected, the reduction in stress for age need not be so rapid.
On account of the comparatively short life of timber bridges and the ease with which
they can be renewed, there is not generally the same urgency in establishing maximum-safe-stress limits as in the case of the more permanent metal bridges. In timber truss-bridges the lowest classification usually occurs in the floor beams, truss rods, and diagonal braces.
It has been found that metal bridges suffer frequently from corrosion in the top
flanges of stringers and floor beams, on account of the action of brine drippings from refrigerator cars.
In bridges where the ties are supported on shelf-angles riveted to the webs of the
girders, the shelf-angles frequently show considerable corrosion and tend to break in
the root of the angle.
In pin-connected trusses, excessive wear sometimes takes place in the pin bearings, particularly in draw bridges.
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