TM 5-5420-278-100035 000035 00-20BRIDGE ANCHORAGE (Contd)TOWER LOCATION (Contd)Distance from the bridge centerline to the tower. When using an overheadcable system as an upstream anchorage system, the tower is placed some distanceupstream from the bridge centerline. Conversely, if the overhead cable system isused as a downstream anchorage system, the tower is located some distancedownstream from the bridge centerline. This distance, the Bridge to Tower Offset (O1), can be calculated as follows:NOTEIf the near- or far-shore bank or tower height differ,this step must be performed separately for each shore.If the bank height (BH) is less than or equal to 15 ft (4.6 m), then:O1 = H + 50 ft (15.3 m)where H is the actual tower height in feet.If the bank height (BH) is greater than 15 ft (4.6 m), then:O1 = H + BH + 35 ft (10.7 m)where H is the actual tower height in feet and BH is the actual bankheight in feet.This distance provides a suitable slope for the cable running from the bridge tothe tower.DEADMAN DESIGNThe use of a deadman on each shore is the preferred method of securing theoverhead cable(s). (Refer to figures 14 and 15.) These deadmen provide the holdingpower for the entire overhead cable system. Because of this, accurate deadmandesign is critical. Construct deadmen using logs, rectangular timber, steel beams, orsimilar objects, buried in the ground with a guy line or sling attached to thedeadman’s center. The holding power of a deadman is affected by the frontalbearing area, mean (average) depth, angle of pull, deadman material, and soilconditions.a. Available materials. The first step in designing a deadman is to identify thedimensions of all available materials. Generally, select the timber with the largesttimber face or a log with the greatest diameter. Use the largest dimension of theproposed deadman as the deadman face (Df). The smaller dimension is defined asthe deadman thickness (Dt).b. Depth of deadman. To determine the depth to which a deadman should beburied, three rules must be considered.(1) There must be at least 1 ft (0.3 m) of undisturbed soil between the groundwater level and the bottom of the deadman. Therefore, the maximum meandeadman depth (DDmax) is defined as:DDmax = GWL – 1 ft – (D f)2where GWL is the depth of the ground water level in feet. Df is thesize of the deadman face in feet, and DDmax is the maximum meandepth of the deadman (the maximum depth that the center of thedeadman can be buried) in feet.
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