Concrete arch on centering and the Melan (stiff skeleton) method

How bridges get built · Cast-in-place concrete decks · Concrete arch on centering and the Melan (stiff skeleton) method

Also called: arch centering, centring, arch falsework, Melan arch, stiff skeleton arch, embedded steel arch

How it is done

Design the centering (timber or steel trusses, towers or a lattice arch) and its foundations, and compute its deflection and settlement for each casting stage; set the formwork to the arch profile plus elastic shortening, creep and centering deflection.
Erect the centering from the ground or from temporary piers in shallow water, or assemble it on shore and float it into place.
Cast the rib in segments placed symmetrically from both springings towards the crown, leaving short gaps that are concreted last to limit shrinkage cracking and follow the centering's deflection; place precast voussoirs in the same symmetric order.
Survey the centering after each stage and keep the loading symmetric, adjusting the sequence if deflections depart from the prediction.
Close the crown; where specified, use jacks at the crown to introduce thrust and lift the rib off the centering, compensating its shortening.
Decenter when the rib reaches the specified strength by lowering sand boxes, wedges or jacks in stages while monitoring the crown and springings; shift the centering to the next rib or span if it is reused.
Build the spandrel columns and deck symmetrically. In the Melan method, first erect and close the steel skeleton (often concrete-filled tubes placed by cable crane), then encase it in concrete in several rings or stages so the skeleton is never overloaded.

Key numbers

Span on centeringcommonly 50–300 m (Gladesville 305 m, 1964)
Stiff-skeleton (Melan) spans420 m at Wanxian (1997) and longer since
Rise to spantypically 1/4–1/8
Centering reuseone centering shifted sideways or floated between ribs or spans (Plougastel, Gladesville)
Decenteringstaged, after the rib reaches the specified strength, with the crown drop predicted and measured

Where it fits

Concrete arches of about 50–300 m where the centering can be founded: dry valleys, shallow water, or a centering floated into place
Several parallel ribs or repeated spans, so one centering can be shifted and reused
Melan (stiff skeleton): long concrete arches of 300–450 m and more over deep gorges, with the skeleton erected by cable crane or cantilever

Where it does not

Deep gorges and fast or navigable water where tall centering cannot be founded or would block the channel
Single one-off spans where cantilevering with tie-backs avoids the centering cost
Programmes that cannot accept the casting sequence and the decentering hold points

Choosing it

Choose centering over cantilevering with tie-backs when the ground or shallow water under the arch can carry it, and especially when it can be reused across parallel ribs or repeated spans.
Prefer cantilevering with temporary stays for deep gorges, navigable channels and single long spans where tall centering is costly or impossible.
Prefer the Melan (stiff-skeleton) method for very long concrete arches where a light steel skeleton erected by cable crane is cheaper than either centering or heavy stays; check the skeleton for every concreting stage.
Choose precast voussoirs on the centering (Gladesville) over cast-in-place rings when lifting is available and speed and quality control matter.

Plant, pace and money

PlantTimber or steel centering (trusses, towers or lattice arches), sand boxes or jacks for lowering, concrete pumps or a cable crane, crown jacks where used, survey and strain monitoring; for the Melan method, the steel tube skeleton, cable crane and pumping to height.

ProductivityBuilding the centering often takes as long as casting the rib; a rib is cast over weeks to months, and later ribs on a reused centering go much faster.

CostIndicative: arch with its temporary works about USD 2,500–5,000 per m2 of deck for 100–300 m spans; the centering is a major item, so reuse across ribs or spans decides the economics.

Risks and controls

What goes wrong

Centering collapse or excessive deflection under unsymmetrical loading (Sandö, 1939)
Shrinkage and early thermal cracking in long rib pours, hence segmental casting with closing gaps
Crown drop on decentering larger than predicted
Melan method: buckling or overstress of the slender steel skeleton if the encasement sequence is not followed

Quality assurance

Independent check of the centering and its foundations, symmetric casting sequence with centering survey after each segment, rib strength confirmed before decentering, staged lowering with crown and springing monitoring, and strain monitoring of the skeleton in Melan construction.

Origins

Centering is as old as masonry vaulting; in concrete, Freyssinet floated one timber centering between the three 186 m arches of Plougastel (1930), and Gladesville (305 m, 1964) assembled precast voussoirs on a steel centering shifted sideways for each of its four ribs. Josef Melan's embedded steel arch system of the 1890s is the basis of the stiff-skeleton method used for long Chinese arches such as Wanxian (420 m, 1997).

Examples

Plougastel (France, 1930), Sandö (Sweden, 1943) and Gladesville (Sydney, 1964) on centering; Wanxian (China, 1997) by the stiff-skeleton method. Small and medium concrete arches are still built on scaffold centering wherever the valley floor is accessible.

Case studies

Plougastel Bridge (Pont Albert-Louppe), Elorn estuaryFrance · 1930Three 186 m reinforced concrete arches, a record when built, cast on one timber centering floated on barges from span to span.

Sandö Bridge, Ångerman RiverSweden · 1943264 m arch, the longest concrete span until 1964; its first timber centering collapsed in 1939, killing 18 workers.

Gladesville Bridge, SydneyAustralia · 1964305 m, the longest concrete arch when built: four ribs of hollow precast voussoirs placed on a steel centering that was moved sideways for each rib.

Wanxian Yangtze River Bridge, ChongqingChina · 1997420 m concrete arch built around a steel-tube truss skeleton erected by cable crane and encased in concrete in stages; long the record for concrete arches.