Full staging method (ground falsework)

How bridges get built · Cast-in-place concrete decks · Full staging method (ground falsework)

Also called: FSM, falsework, conventional cast-in-place, scaffold construction

Full staging method (ground falsework) — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Prepare the temporary works design and its independent check, with ground information for the falsework foundations (plate load tests or a geotechnical assessment).
Prepare the foundations: a drained, compacted granular platform, concrete strip footings or sleepers, or short piles on soft ground; protect them from washout.
Erect the towers or props, bracing, primary and secondary beams, and set the soffit to the casting levels: design profile plus deck deflection, falsework shortening and expected settlement.
On doubtful ground, preload the falsework (water tanks or sandbags) to about the wet concrete load and survey the settlement before fixing the soffit levels.
Fix reinforcement, ducts and anchorages, inspect them against the drawings and issue the permit to load (hold point).
Pour in the planned sequence (for boxes, bottom slab and webs first and top slab second; for continuous decks, span regions before support regions), watching settlement tell-tales against agreed stop criteria.
Cure and stress the tendons at transfer strength, which lifts the deck's weight off the falsework; then release the jacks or wedges from midspan towards the supports.
Dismantle under a permit to strike and move the falsework to the next span if it is being reused.

Key numbers

Span10–50 m; deck usually less than about 10–15 m above ground
Soffit loadabout 15–40 kN/m2 for solid slabs and box decks (concrete 25–26 kN/m3)
Minimum lateral design load2.5 % of the vertical load (BS 5975), plus wind
Settlement allowance in the casting levelstypically 5–25 mm on prepared ground
Cycle30–45 days per span; faster with two falsework sets

Where it fits

Spans 10-50 m, deck height under about 15 m, dry ground, overpasses closed to traffic below, culvert-type crossings, short valley viaducts.

Where it does not

Water (the game's crane_ground and falsework checks), live roads and railways below, soft clay under towers, tall decks.

Choosing it

Choose it over an MSS when spans are few (under about 8–10), vary in length or width, or are curved, and the ground below is dry, firm and free to occupy.
Prefer piled falsework or a trestle in shallow, slow water; prefer an MSS, launching or precast girders when the ground is unavailable or the viaduct is long and repetitive.
Prefer precast girders when traffic below must keep running or the deck is higher than about 15–20 m.
Choose falsework for complex geometry (ramps, variable width, skew, flares) where standard precast elements do not fit.

Plant, pace and money

PlantFalsework towers or props, steel beams, formwork, concrete pump, mobile crane.

Productivity30-45 days per span for the full cycle; faster with two sets of falsework.

CostFormwork and falsework USD 150-300 per m2 of deck; total deck USD 900-1,600 per m2.

Risks and controls

What goes wrong

Falsework collapse during the pour is the leading cause of fatalities in concrete bridge construction; settlement of tower foundations; premature striking.

Quality assurance

Independent temporary works design and check, foundation bearing verification, pre-pour inspection with a hold point, settlement monitoring during pouring.

Origins

Falsework is the oldest way to build arches and beams, inherited from timber centering; proprietary steel scaffold and shoring towers replaced site-built timber from the mid-twentieth century, and the UK Bragg Committee (1975) and BS 5975 set the modern procedures for it.

Examples

Most short concrete overpasses; in the game it is the default for short dry spans and is barred in water and over rail.

Case studies

Yerba Buena Island transition structures, San Francisco-Oakland Bay BridgeUSA · 2013Cast-in-place post-tensioned box girders built on falsework on the island's steep slopes, linking the new self-anchored suspension span to the tunnel.

Loddon Bridge, ReadingUK · 1972Falsework collapsed while the deck was being concreted, with fatalities; with other collapses it led to the Bragg Committee report and BS 5975.

In the bridge game

What Bridge Master applies when you pick it

Deck types: RC slab, Voided slab, RC T-beam, RC box, PSC box (cast in place), Rigid frame, Concrete arch, PSC box, balanced cantilever, Extradosed, Network tied arch. Spans 10-40 m, up to 55 m.

+25: Dry firm ground, low deck: falsework is the cheap answer.
-50: No falsework in water.
-50: No falsework over track.
-40: No falsework over live lanes unless the road can close.
-30: Tall towers get expensive and unstable.
-20: Falsework foundations settle on soft ground.
-15: Long viaduct: MSS or precast is faster.
-20: Spans past 50 m need heavy towers and long cycle times; MSS, launching or precast is cheaper.
-15: Falsework fills the street: noise, footprint and traffic diversions for months.
not allowed: Ground falsework in a river.
not allowed: Ground falsework over a live railway.
not allowed: Ground falsework cannot stand in water; use piled falsework in shallow water or a method that needs no ground support.

Try it in the game →