Segmental Bridge Erection Gantry

Overview

A segmental bridge erection gantry is a self-launching steel machine that builds long-span concrete bridges from precast box-girder segments. It sits on top of the bridge it is building, spans from one pier to the next, lifts each segment up from a barge or transporter below, and positions it to within a few millimetres so the segment can be glued and stressed to the cantilever already in place. When a span is finished, the gantry walks itself forward to the next pier without any assist crane.

The machine is used for the balanced-cantilever method. Construction starts at a pier and segments are added in pairs, one on each side of the pier, so the growing structure stays roughly balanced about the pier. Each segment is typically 2.5 to 4 m long and weighs 60 to 250 t. The Segment Winch Trolley picks up the segment, runs it along the truss to the free end of the cantilever, and holds it while the joint is made.

Main Truss

The Main Girder Truss is the spine of the gantry. It is a pair of welded box-section trusses, usually 60 to 130 m long, made from Welded Truss Module units of 8 to 12 m that bolt together at site through 30 to 40 mm Bolted Splice Plate connections. The truss must be longer than one span, because while one span is being built the truss has to reach across to the next pier to land on it before launching. Lateral Lateral Bracing Frame members tie the two trusses together and carry wind and torsion loads.

Along the top chord runs a hardened Top Chord Trolley Rail, a crane rail on which the lifting trolleys travel. Plate thickness in the truss modules ranges from 20 to 40 mm in S355 structural steel, and each module weighs 12 to 20 t, which sets the size of the assembly crane needed to erect the gantry at the start of the job.

Support Legs

The Support Leg Set carry the whole machine and the segment being lifted down into the completed deck and the pier. There are normally three: a front leg that reaches ahead to the next pier, a mid leg over the pier currently being built around, and a rear leg standing on the finished deck. Each Leg Column is a box section of 1.2 to 1.8 m square carrying axial loads up to 6000 kN.

The legs are not simply set down. Because the load on a cantilever is heavily one-sided during construction, the legs have to resist uplift and overturning. The Deck Clamping Base clamps to the deck using Through-Deck Clamp Bar tension bars that pass through ducts cast into the slab, holding the leg down against uplift of up to 2500 kN. The Base Spreader Plate under each leg spreads the downward load so the bearing pressure on the young concrete stays below 15 MPa. Each leg sits on Leg Levelling Jack cylinders that level the gantry and transfer load gradually, and a mechanical Mechanical Lock Nut holds the load without relying on hydraulic pressure once the leg is set.

Self-Launching System

When a span is complete, the Self-Launching System moves the gantry forward by one span. The truss rests on Articulated Roller Bogie bogies, each an equalising frame on hardened Launch Roller Wheel rollers running on a stainless-faced Launch Skid Track. Long-stroke Launch Push-Pull Cylinder units pull the truss along: a Friction Gripper clamps the truss web, the cylinder extends to drag the gantry forward 1.5 to 2 m, then the gripper releases and the cylinder retracts for the next bite. A typical launch of one span takes a few hours and is the most carefully watched operation in the cycle, because the front of the gantry is cantilevered far out ahead of the support before it lands on the new pier.

Lifting and Positioning

Each Segment Winch Trolley carries two Hoist Winch units. A winch uses a grooved Grooved Rope Drum storing multi-fall Hoist Wire Rope of 26 to 40 mm diameter, with a 5:1 minimum safety factor on breaking load. A spring-applied Winch Brake holds the segment if power is lost. The segment is picked up through a Segment Lifting Beam spreader whose Pick-Point Shackle shackles match the lifting loops cast into the segment. Fine positioning uses the trolley travel and the hoist together so the new segment lines up with the matching face of the previous one to within a couple of millimetres before the epoxy joint is closed.

Hydraulics, Power and Control

The Hydraulic Power Unit unit runs the legs, the launch cylinders and the grippers. A variable Hydraulic Pump feeds 350 bar through a Proportional Valve Manifold of Proportional Control Valve proportional valves and Load-Holding Check Valve load-holding valves, so the leg cylinders rise together and stay locked if a hose fails. Oil is kept under 60 C by an Air-Blast Oil Cooler.

The Control and Monitoring System is built around a safety PLC in the PLC Control Cabinet, reading Load Cell cells on the legs and hoists, Inclinometer sensors on the truss, and an Anemometer that stops work above the rated wind. The operator works from the Operator Cabin using Ball Bearing supported trolleys and proportional Control Joystick controls, with travel and hoist driven by Servo Motor units through the Electrical Power System system. Erection rates of one segment pair per day are common once a crew is in rhythm, and a long viaduct of several hundred segments can be built by a single gantry over many months.