BRIDGE ANCHORAGE (Contd)
TOWER LOCATION (Contd)
Distance from the bridge centerline to the tower. When using an overhead
cable system as an upstream anchorage system, the tower is placed some distance
upstream from the bridge centerline. Conversely, if the overhead cable system is
used as a downstream anchorage system, the tower is located some distance
downstream from the bridge centerline. This distance, the Bridge to Tower
Offset (O1), can be calculated as follows:
If 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 bank
height in feet.
This distance provides a suitable slope for the cable running from the bridge to
The use of a deadman on each shore is the preferred method of securing the
overhead cable(s). (Refer to figures 14 and 15.) These deadmen provide the holding
power for the entire overhead cable system. Because of this, accurate deadman
design is critical. Construct deadmen using logs, rectangular timber, steel beams, or
similar objects, buried in the ground with a guy line or sling attached to the
deadmans center. The holding power of a deadman is affected by the frontal
bearing area, mean (average) depth, angle of pull, deadman material, and soil
a. Available materials. The first step in designing a deadman is to identify the
dimensions of all available materials. Generally, select the timber with the largest
timber face or a log with the greatest diameter. Use the largest dimension of the
proposed deadman as the deadman face (Df). The smaller dimension is defined as
the deadman thickness (Dt).
b. Depth of deadman. To determine the depth to which a deadman should be
buried, three rules must be considered.
(1) There must be at least 1 ft (0.3 m) of undisturbed soil between the ground
water level and the bottom of the deadman. Therefore, the maximum mean
deadman depth (DDmax) is defined as:
DDmax = GWL 1 ft (D f)
where GWL is the depth of the ground water level in feet. Df is the
size of the deadman face in feet, and DDmax is the maximum mean
depth of the deadman (the maximum depth that the center of the
deadman can be buried) in feet.