Medium Girder Bridge

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A 16-Bay with LRS Medium Girder Bridge across the Kazer River, Mosul, Iraq, 2003.

The Medium Girder Bridge (MGB) is a lightweight, hand-built, deck type, two-girder bridging system designed by UK firm WFEL[2] for the British Army in 1971. The two longitudinal girders, with deck units between, provide a 4.0 m (13 ft 2 in) wide roadway. It can be built in various configurations to provide a full range of bridging capability for use both in the forward battle area and in the communications zone. Speed of erection by the fewest number of soldiers is its major characteristic. The MGB also requires very little maintenance once erected, is air transportable in either stand palletized loads or in partially assembled bridge configurations, and all US components will fit MGBs in use by allies (except for the launching nose cross girder posts)

The bridge can be supported on unprepared and uneven ground without grillages. It is constructed on one roller beam for single-story construction; two roller beams, 4.6 m apart, for double-story construction; and on three roller beams when constructing a double-story bridge over 12 bays long. The ends of the roller beams are supported on base plates and each can be adjusted in height. No leveling or other preparation of the ground is required. Single-span bridges are launched using a centrally mounted launching nose.

1 Parts
2 Configurations
3 Construction
4 References
5 See also

[edit] Parts

The MGB parts are fabricated from a specially developed zinc, magnesium, and aluminum alloy (DGFVE 232A). This enables a lightweight, high strength bridge to be built. All except three parts weigh under 200 kg. Most parts can be handled easily by four soldiers. The three heavier parts, used in limited quantities, are six-man loads.

Top Panel — Used to build the bridge girders. There are 7 panel points on each top panel; it is 6 ft 4 in long, 2' 1³⁄₈" wide, 1' 9⁵⁄₈" high (1930 mm × 645 mm × 549 mm), weight 385 lb (175 kg).
Bay (same as above)
Bottom Panel — Used as a brace for bridge girders. it is 6 ft 5 in long, 2' 3" wide, 3' 7³⁄₈" high, weight 435 lb.
End Taper Panel — Used as a bottom brace between the junction panel and bankseat beam. is 13' 2⁵⁄₈" long, 2'4" wide, 1'6" high, and weight 600 lb. It is one of three components that requires 6 Marines to carry.
Bankseat Beam — Used to keep the bridge girders properly spaced and provide connection for the ramp units. It is one of three components that requires 6 soldiers to carry; it is 13' 3¹⁄₂" long, 1' 9³⁄₈" wide, and 1’ 6" high and weighs 570 lb.
Ramps — Two types US and UK ramps. UK. or short ramp is and US. or long ramps are one of three components that requires 6 soldiers to carry. UK ramps are 264 lb. and the US ramp is 400 lb. Each type provides an approach to the bridge, 7 are required at each end. UK ramps are used for single story only.
Deck Unit — 9'1" long, 1' 5¹⁄₄" wide, 6⁷⁄₈" high, weight 163 lb it requires 2 soldiers to carry. This component fills in the gap between girders. Four deck units are required per bay of bridge. In single story construction we only use the ramp unit, bankseat beam, top panel and deck unit.
Junction Panel — Used as a brace between the sloped and level part of a double story bridge. It is 5'³⁄₄" high, 3' 5¹⁄₂" long on top, 2' 2³⁄₄" long on the bottom, 2' 1¹⁄₂" wide, weight 478 lb.
Sway Brace
Curb

[edit] Configurations

[edit] Single Story

Up to 9.8 m span with a MLC[3] of 70

The single story MGB bridge is constructed using top panels which are pinned together to form two girders and joined at each end by a bankseat beam creating a rigid framework. This type of bridge is used for short spans that will carry light loads. Single-story bridges can be constructed by 9 to 17 soldiers.

[edit] Double Story

Up to 31.1 m span with a MLC of 70

In the double story MGB bridge, the girders consist of top and bottom panels, with junction panels and end taper panels forming the sloping end of the bridge. In both cases, ramp, deck and kerb units complete the construction. The heavier duty double-story configuration is used for heavy loads or longer spans. The normal building party for double-story bridges is 25 soldiers.

[edit] Double Story with Link Reinforcement

Up to 49.4 m span with a MLC of 60

The MGB Link Reinforcement Set (LRS) consists of reinforcing links which are 3.66 metres (12 ft) long, plus short links of 1.82 metres (6 ft), which are pinned together to form chains under each girder.

The chains are suspended 2 metres (6 ft 6 in) beneath each bottom chord of the bridge and tensioned after building, by pulling the reinforcing posts into the vertical position. This creates a fully reinforced structure.

The Link Reinforcement Set (LRS) is constructed when a long, high class type of bridge is required. The LRS deepens the girder and transfers the load throughout the length of the bridge. This type of construction requires a building party of 34 soldiers, and is built on three roller beams.

Although using an LRS provides a longer bridge span, it lowers the Military Load Class from 70 to 60. This prohibits the heavier vehicles in use by the military from crossing.

[edit] Multi-span Bridges

2 span double story up to 51.5 m with a MLC of 70

3 span double story up to 76 m with a MLC of 70

The MGB Span Junction Set consists of span junction posts, which are pinned together at the top and connected at the bottom by hydraulic articulators.

The Span Junction Set gives the MGB Double Storey bridge a multi-span capability and allows bridges to be constructed over supports which are either fixed or floating. These may include any combination of existing supports, pontoons, existing or improvised piers and the MGB Portable Pier.

MGB Double Storey multi-span bridges usually take the form of two or three span structures rated at MLC 70. The two span bridge can have an overall length of up to 51.5 metres (169 ft), while the three span can be 76 metres (250 ft). This requires a total crew of 40 personnel—24 for the main bridge, 8 for the MGB Portable Pier and 8 to install anchorages.

[edit] Floating MGB

Up to MLC of 60

Floating MGBs can be constructed in single or double storey configurations using the same components as the dry bridge configurations:

Double story construction allows landing bay spans up to 26.5 metres (86 ft 11 in) and is suitable for conditions where there is considerable rise and fall in water levels.

Single story construction provides either floating bridges or ferries for load classes up to MLC 60.

Single and double storey Floating MGBs can be built using standard MGB superstructures, carried on MGB Pontoons with single story hinge bays or double story Span Junction Sets to provide articulation. The length of these bridges is limited only by the amount of equipment available.

MGB pontoons

The MGB Pontoon is fabricated from marine grade aluminium alloy.

Two pontoons are coupled back to back to create each pontoon pier. Three such piers make up one landing bay raft. Powered pontoons are driven by a 75 hp diesel engine with a water jet propulsion unit. Fully laden pontoons can operate in currents up to 2.5 m/s (4.9 knots).

[edit] MACH MGB

MACH MGB (Mechanically Aided Construction by Hand) is a semi-mechanized bridge building system, which reduces the size of construction crews from 25 to 9, for similar build times.

This is achieved by prefabricating MGB components into modules in a separate assembly area. The bridge is then constructed using a suitable crane or CALM vehicle.

MACH MGB uses standard MGB components supplemented by special components designed to assist mechanical handling.

The advantages of MACH MGB over other purpose-built mechanized systems are:

Any suitably sized crane can be used.
Reduced manpower compared to standard MGB.
The bridge can still be built by hand if the crane or hydraulics are incapacitated.
MACH MGB is field proven and used worldwide.

MGB handrail

The MGB handrail is designed to provide a significant increase in awareness of roadway width for both military and civilian drivers. It consists of vertical connecting posts with longitudinal hand rails, creating a continuous barrier along the edge of the bridge. Handrail components can be carried on an MGB palletor unit transport.

[edit] Construction

Bank seat beam of the Medium Girder Bridge. Used to roll the bridge out over the gap. Kazer River, Mosul, Iraq, 2003.	Soliders (299 MRBC USAR[1]) attach the link reinforcement set to the underside of the bottom panel	Aerial view of the erection site of a Medium Girder Bridge over the Kazer River, Mosul, Iraq, 2003.	The MGB launching nose suspended over the gap. Kazer River, Mosul, Iraq, 2003.
Soldiers (299 MRBC) steady the launch nose of a Medium Girder Bridge during erection over the Kazer River, Mosul, Iraq, 2003.	Aerial view of the nearly complete erection of a Medium Girder Bridge over the Kazer River, Mosul, Iraq, 2003.	A view of the sway brace system of a Medium Girder Bridge before the decking is placed. Kazer River, Mosul, Iraq, 2003.	A fully erected Medium Girder Bridge crosses its first traffic over the Kazer River, Mosul, Iraq, 2003.