ROADS AND BRIDGES Bridges for Crossing Open Drainage Ditches For railway and primary road crossings permanent structures of steel, concrete, and treated timber are required. The substructure should be so placed as to allow unobstructed waterway at times of greatest flow. Abutments should be located so as to prevent water flowing behind them as a result of erosion of the banks. Foundations should be at least 3 feet below the bed of the ditch in material other than rock, hardpan, or hard clay and should be deep enough to be below the erosive action of the current. Piers or bents should not be placed in the watercourse. The types best suited for permanent crossings are reinforced concrete box and arch culverts, slab and T-beam spans, steel girders, low trusses, and treated timber trestles. For secondary and farm-road crossings the most usual types are simple beam spans or timber trestles. Simple beam spans are suitable for narrow ditches whose banks offer firm support for the mud sills or blocks and are subject to little erosion. Short, longitudinal mud blocks give greater and more uniform bearing area than a sill placed directly on the ground. They will last longer if placed on gravel or broken stone. A clearance of 12 inches should be provided between stringers and the ground surface to prevent decay. Timber trestles should consist of a channel span and approach spans on each side of sufficient length to place the ends of the trestle out of range of erosion. Two spans with a bent at or near the center of the ditch should not be used because of the obstruction to flow. Single-span trestles with timber back walls should not be used, because the back walls decay rapidly, causing excessive maintenance cost. The floor system of a timber trestle may be supported on either pile or frame bents, but pile bents are seldom economical on small jobs. Frame bents consist of three or more posts, a cap, and a sill which may be placed directly on the ground on mud blocks or on concrete piers or pedestals. They should be provided with double bracing bolted at the ends to the cap and sill and at intermediate points to each post. The caps and sills should be dapped to receive the ends of the posts, to which they should be securely drift-bolted. The sills should also be anchored to concrete piers or pedestals by means of bolts placed in masonry and extending into the sill. Concrete foundations should be high enough to keep the timber sills above the ordinary water level and have sufficient bearing area at the bottom to prevent settlement. Stringers should be spaced not greater than 27 inches center to center and preferably between 18 and 24 inches. The inside stringers should have full bearing on the caps and be separated at the laps in order to avoid the retention of moisture between them. The outside stringers may be placed end to end over the center line of the cap. Double 2 by 4 inch bridging should be used when the width of stringer is less than one-third the depth. Spans under 15 feet should have one line of bridging; 15 to 20 foot spans, two lines; above 20 feet, three lines. Floor planks should be spiked to each stringer and have a thickness of not less than 3 inches for highway crossings or 2 inches for private crossings, but preferably 3 inches for a 5-ton loading. U. S. D. A.-10-1-26 12623°-27-17 253 2x12 Flooring X10 Cap 210 2x12" Mud Blocks Tamped Gravel 0.80 88"Post 0.05 8% 12" Mud Silla HALF SECTION HALF END VIEW 21 8 boowboi bas enigno ART Pour corro bind so tud (B) Tinber bridge suitable for crossing drainage ditch U. S. D. A.-10-1-26 The usual type of handrail has one rail spiked to the top of posts and two side rails, one at the top of posts and one halfway between the floor and top of posts. They should be bolted to the outside stringers with at least two bolts through the lower end of the post and stringer. A wheel guard should be placed on each side of the roadway and be of sufficient width to prevent contact between vehicles and handrails. Stringers for timber trestles; stress 1,200 pounds per square inch [Truck load, 80 per cent on rear axle; axles, 12 feet center to center; wheels, 6 feet center to center; impact, 30 per cent] 16-foot roadway 14-foot roadway 10-ton truck: 5-ton truck: 3-ton truck: Stringers A fiber stress of 1,200 pounds per square inch may be allowed for the following kinds of timber: Western larch, Pacific post oak, bur oak, bald cypress, mountain-region Douglas fir, and redwood. 4 by 16 53. 3 10 4 by 14 46.7 6 by 14 63.0 8 4 by 16 42.7 Number Span Stringers for timber trestles; stress 1,600 pounds per square inch [Truck load, 80 per cent on rear axle; axles, 12 feet center to center; wheels, 6 feet center to center; impact, 30 per cent] A fiber stress of 1,600 pounds per square inch may be allowed for the following kinds of timber: Tanbark oak, white oak, Cuban pine, long-leaf pine, coast-region Douglas fir, and short-leaf pine (treated). U. S. D. A.-10-1-26 |