DESCRIPTION AND THEORY OF OPERATION
IMPROVED RIBBON BRIDGE (IRB)
RAMP BAY M16
(NSN 5420-01-470-5825) (P/N 12478918);
INTERIOR BAY M17
(NSN 5420-01-470-5824) (P/N 12478919).
Section I. EQUIPMENT DESCRIPTION AND DATA
EQUIPMENT CHARACTERISTICS, CAPABILITIES, AND FEATURES
The Improved Ribbon Bridge (IRB) is a modular bridge having a roadway surface supported by a
floating integral superstructure made of aluminum that functions as a ponton. A complete ribbon bridge
consists of a ramp bay at each bank and the required number of longitudinally connected interior bays
spanning between them. The IRB has a roadway width of 14 ft 9 in. (4.5 m). Adjacent to the roadway is a
4 ft (1.2 m) walkway on the outer pontons. IRB crossing weight capacities are listed in TM 5-5920-278-10.
The IRB can be used for rafting operations by using one or more interior bays joined with a ramp bay at
each end. The IRB-R and IRB-I can be individually assembled longitudinally by the bridge crew, with the
assistance of bridge erection boats (BEB), at a rate of one bay per minute. In addition, IRB-R and IRB-I can
be assembled with Improved Float Bridge (IFB) ramp and interior bays at a rate of one bay every two
minutes or less. An IRB bay is retrievable in five minutes or less. Each bay is transported, launched, and
retrieved in a folded condition on a ribbon bridge transporter.
IRB Interior Bay (IRB-I). The IRB-I is a four-ponton folding module consisting of two inner pontons
and two outer pontons. Each inner ponton is divided into two watertight compartments. The IRB-I unfolds
automatically once released and afloat. IRB-Is are connected to each other by first manually engaging four
upper couplings, followed by two lockpins on the inner pontons. The lockpins act as bearing points between
consecutively joined bays, thus allowing the entire bridge to hinge with the weight of a moving vehicle and
uneven water conditions.
IRB Ramp Bay (IRB-R). The IRB-R is a four-ponton module that functions similar to the IRB-I, but
differs substantially in design. The ramp end of the inner pontons extends lengthwise, beyond the outer
pontons, and slopes down, forming the ramp edge. The sides of the outer pontons are slightly tapered
toward the ramp end, and attaching ramp plates are provided. The IRB-R contains a manually controlled
raising mechanism that is utilized when joined to an IRB-I. The angle or height of the IRB-R can be
adjusted to meet various bank conditions. The IRB-R also contains two lockable, self-draining stowage
boxes recessed in the outer pontons.