. Railroad structures and estimates . mp; X. Rv. CONCRETE OVERHEAD BRIDGES. 121 Concrete Overhead Bridges on the L. & N., Fig. 30. — Thestructure is built of reinforced concrete providing 28 ft. roadwayand two 6 ft. sidewalks, carried on four bents of two columnseach, the three spans being 33 ft. each and the clearance underthe bridge to rail 22 ft. The bridge is designed for a live load of 100 lb. per squarefoot of roadway and sidewalk, or a 35,000 lb. road roller on theroadway and 100 lb. per square foot on the sidewalks. Thematerial being clay the footings are spread, those supportingthe en
. Railroad structures and estimates . mp; X. Rv. CONCRETE OVERHEAD BRIDGES. 121 Concrete Overhead Bridges on the L. & N., Fig. 30. — Thestructure is built of reinforced concrete providing 28 ft. roadwayand two 6 ft. sidewalks, carried on four bents of two columnseach, the three spans being 33 ft. each and the clearance underthe bridge to rail 22 ft. The bridge is designed for a live load of 100 lb. per squarefoot of roadway and sidewalk, or a 35,000 lb. road roller on theroadway and 100 lb. per square foot on the sidewalks. Thematerial being clay the footings are spread, those supportingthe end bents being carried down 4 ft. below the ground line andthose under the intermediate bents 6 ft. This structure required 28 tons of steel and 250 cubic yards ofconcrete, 1:2:4 mixture. The approximate average cost forestimating for the bridge only is $6000 or about $ per squarefoot taking 40 ft. by 100 ft. as the area covered. If the abovebridge had to carry street cars the cost in reinforced concretewould be about $ HALF ELEVATION CROSS SECTION ON C. L. Fig. 30. Concrete Overhead Bridge, L. & N. Ry. WOODEN BRIDGES. Howe Trusses. — While timber bridges are not used to thesame extent to-day as in former years, there are still some placeswhere good timber is abundant and cheap, where the cost ofdelivering steel would be high and the probable traffic light. If properly detailed with moderate spans, any strength re-quired in such structures can be developed and when suitablyprotected they will last for many years and may, under certainconditions, be favorably considered both for railway and high-way traffic. 122 WOODEN BRIDGES. The structure is usually built with a large excess of strengthof the Howe or Towne lattice type. The chords and braces are made of timber and the verticalrods of steel usually upset, with cast-iron blocks at the angles ofbraces, which are bolted or doweled into the main best class of timber is used with as few splices as possible.
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