If you've ever stood under a partially erected viaduct and looked up at the steel beast rolling overhead — you already understand why launching gantries are both impressive and unforgiving.
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.
The basic idea: position over the pier, lift segments, apply epoxy joints, stress with post-tensioning, then launch forward to the next span.
Types of Launching Gantries
Overhead (OLG): The gantry sits above the deck. Segments are lifted from below. Common on viaducts with limited ground access.
Underslung (ULG): The gantry hangs below the deck level. Better for curved alignments.
For most Indian HSR and metro projects, overhead gantries dominate.
The Erection Sequence
A typical span erection sequence:
- Gantry positioning — front support sits on the forward pier
- Segment feeding — brought by ground transporter or deck-mounted feeder
- Pier segment lift — the heaviest piece; sets geometry for everything that follows
- Progressive assembly — segments hung and assembled symmetrically toward mid-span
- Post-tensioning — tendons stressed in two or three stages
- Bearing transfer and launch — span lowered onto permanent bearings, gantry moves forward
What Goes Wrong
Geometry drift — if pier head levels aren't precisely set, you'll have alignment problems span to span. Match-cast geometry is unforgiving.
Gantry launch incidents — the launch is the highest-risk operation. Every support change must be a verified step, not an assumption.
Epoxy joint failures — epoxy has an open time of 45–90 minutes. In Indian summers, that window shrinks dramatically.
Stressing errors — wrong duct threading, wrong elongation measurement. Have your stressing records independently checked.
Productivity: What's Realistic
On a standard metro or HSR viaduct in India:
- 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 project start
The Things Nobody Tells You
The gantry crew is everything. A well-trained crew with an experienced gantry master will outperform any schedule.
Segment yard to site logistics is a hidden bottleneck. A gantry waiting for segments is money burning.
Gantry maintenance is non-negotiable. Weekly inspections are not optional.