Launching girder (beam launcher) for precast girders

How bridges get built · Precast concrete decks · Launching girder (beam launcher) for precast girders

Also called: beam launcher, girder launcher, overhead girder erector

How it is done

Assemble the launcher on the abutment or first span and commission it; prove trolleys, winches and the self-launch sequence with a load test.
Support it on the pier ahead (front leg), the pier just completed (middle) and the deck behind (rear), and check the pier caps or brackets for the leg reactions.
Deliver girders along the completed deck on bogies, checking the erected girders and deck for the bogie loads, or lift them from the ground at the rear.
Pick each girder with the two trolleys, carry it forward, shift it transversely (by the trolleys or by side-shifting the launcher) and lower it onto its bearings; brace it at once.
Complete the span with diaphragms and bracing, then return the trolleys for the next girder.
Self-launch to the next span with the trolleys placed as counterweight, within the wind limit and after a launch checklist (hold point), and land the front leg on the next pier cap.
Cast the deck slab behind the launcher as the spans are completed.

Key numbers

Span25–50 m; girder weight 50–200 t
Launcher lengthabout twice the span; self-weight 150–400 t
Cycleone span of 4–10 girders in 1–3 days
Break-evenroughly 8–10 spans or more
Costlauncher USD 1.5–4 million; erection USD 60–150 per m2 above crane erection

Where it fits

Viaducts with 25-45 m spans over water, wetlands, railways, urban roads; more than about 8-10 spans; wetland viaducts (with top-down delivery).

Where it does not

Very short viaducts, spans beyond the launcher's rating, girders wider or heavier than the trolleys allow.

Choosing it

Choose a launching girder over cranes when ground access is blocked (water, wetland, rail, traffic) and there are roughly ten or more spans.
Prefer a span-by-span segmental gantry when spans exceed about 45–50 m or a box deck is wanted.
Prefer full-span precast erection when there are hundreds of identical spans in a straight corridor.

Plant, pace and money

PlantLaunching girder 60-100 m, 200-400 t, pier brackets, deck transport bogies.

ProductivityOne span per one to three days.

CostLauncher USD 1.5-4 million; erection USD 60-150 per m2 above crane erection.

Risks and controls

What goes wrong

Launcher instability during self-launch, bracket failure, girder swinging in wind.

Quality assurance

Launch procedure, bracket load checks, wind limits.

Origins

Beam launchers evolved with precast girder viaducts in the second half of the twentieth century and are now standard on expressway, metro and rail viaducts across Asia and the Middle East, where long viaducts cross ground that cranes cannot use.

Examples

Sea viaducts in the Gulf, Bangkok elevated expressways, Chinese high-speed rail approaches.

Case studies

Samruddhi Mahamarg (Mumbai-Nagpur Expressway), ShahapurIndia · 2023A girder launching machine collapsed during erection, killing 20 workers: a reminder that launcher supports, self-launch and load transfer need formal procedures and independent checks.

In the bridge game

What Bridge Master applies when you pick it

Deck types: PSC I-girder, PSC U-girder, Steel plate girder, Spliced PSC girder, Precast segmental box, Steel box girder, Voided slab, Full-span precast box. Spans 25-50 m, up to 55 m.

+30: No ground access, many spans: beam launcher.
+20: Top-down over the wetland.
-30: Too few spans.
-15: Cranes would be cheaper.
-20: Launcher struggles on tight curves.

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