BRIDGE ANCHORAGE (Contd)
d. Anchorage of Floating Bridges
All military bridges must be held in position by some system of anchorage.
Anchorage systems can be classified as short-term or long-term. Short-term
anchorage generally refers to a method of holding a bridge in position for a limited
period of time. Assault bridges, such as the ribbon bridge, are normally anchored
using only short-term means. This work package describes the method of anchoring
such bridges using BEBs. Lines of communications bridges, such as M4T6 and
Class 60 bridges, remain in position for longer periods of time. For these bridges,
long-term anchorage systems are necessary. Anchorage of Floating Bridges
describes the design and construction of these long-term anchorage systems.
DESIGN OF LONG-TERM ANCHORAGE SYSTEMS: BASIC CONSIDERATIONS
The design of any system of anchorage is influenced by several factors, including:
Width of the river
Velocity of the rivers current
River bottom conditions
Height and slope of riverbanks
Depth of the groundwater table
Each of these factors must be considered when deciding upon the type of
anchorage system to be installed. Generally, the velocity of the river and the river
bottom conditions will have the greatest impact upon the type of long-term
anchorage system that will be selected for a given site.
The three basic components of all long-term anchorage systems include approach
guys, an upstream anchorage system, and a downstream anchorage system.
Approach guys are cables which prevent the bridge from being pushed away from
the shore as a result of the impact of vehicles driving onto the ramps of the bridge.
One end of each approach guy is attached to each side (upstream and downstream)
of the first bay of bridge on both ends of the bridge. The other end of each approach
guy is secured on the shore, normally using chain picket holdfasts. Place approach
guys at approximately a 45-degree angle with the bridge centerline. (Refer to figure
6.) A minimum of 1/2-in. (12.7-mm) IPS cable should be used for each.