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
- Gantry positioning over the forward pier
- Segment feeding via ground transporter
- Pier segment lift — sets the geometry for everything that follows
- Progressive assembly from both ends toward mid-span closure
- Post-tensioning in two or three stages
- 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
Written by
Arjun Mehra
Civil Engineer · Construction & Erection