How a Launching Gantry Actually Works: A Field Perspective

What Is a Launching Gantry?

A launching gantry (LG) is a steel structure — typically 1.5 to 2.5 times the span length — that straddles two or more piers to erect precast segments span by span.

Types

Overhead (OLG): Sits above deck level. Common on viaducts with limited ground access.

Underslung (ULG): Hangs below deck level. Better for curved alignments.

The Erection Sequence

1. Gantry positioning over the forward pier
2. Segment feeding via ground transporter
3. Pier segment lift — sets the geometry for everything that follows
4. Progressive assembly from both ends toward mid-span closure
5. Post-tensioning in two or three stages
6. Bearing transfer and gantry launch to next span

What Goes Wrong

Geometry drift — if pier head levels are not precisely set, you get alignment problems span to span.

Gantry launch incidents — the launch is the highest-risk operation. Every support change must be a verified step, not an assumption.

Epoxy joint failures — open time shrinks dramatically in Indian summers. Have an abort procedure ready.

Productivity: What's Realistic

• 1 span per week — realistic with an experienced crew
• 1 span per 5 days — achievable on straight alignments
• 1 span per 10 days — typical learning curve at start of project