Concrete-filled steel tube (CFST) arch construction

How bridges get built · Cable-supported and arch bridges · Concrete-filled steel tube (CFST) arch construction

Also called: CFST arch, concrete-filled tubular arch, steel tube arch filled with concrete, stiff-skeleton arch

How it is done

Fabricate rib segments in the shop: chords of about 0.75–1.4 m diameter steel tube with tubular bracing, assembled into truss segments with match-fitted end connections, and trial-assemble adjacent segments.
Erect the hollow steel arch: on falsework for short spans, by vertical or horizontal rotation of half-arches, or by cable crane with tie-back stays from temporary towers for long spans; close it at the crown at the planned temperature.
Release the tie-backs in the analysed sequence (or keep some during filling if the analysis requires) and survey the empty-arch geometry.
Pump self-compacting, low-shrinkage concrete into each chord from both springings toward the crown, one chord at a time in a sequence that keeps the arch balanced; vacuum-assisted pumping and vent pipes reduce voids.
Monitor tube stresses, arch deflection and concrete temperature during filling; the empty tubes carry the wet concrete, so the filling sequence governs their stress.
Check filling by ultrasonic testing and hammer tapping, and repair voids or debonding by drilling and grout injection.
Install bracing, hangers or spandrel columns, cross beams and the deck; for a stiff-skeleton concrete arch, encase the CFST skeleton in reinforced concrete in layers, using forms hung from it.

Key numbers

Span rangeabout 100–575 m (record Pingnan Third Bridge, 575 m, 2020)
Chord tubestypically 0.75–1.4 m diameter (Pingnan: four 1.4 m chords per rib)
Fill concretetypically C50–C70 self-compacting, low-shrinkage mixes
Segment weight per cable-crane lifttypically 50–220 t (Pingnan up to about 215 t)
Rise-to-span ratioabout 1/4 to 1/5 for long-span arches

Where it fits

Deck, half-through and through arches of about 100–575 m over gorges and rivers, especially in mountain regions; sites where segments can be delivered by cableway or by water; the stiff skeleton of very long reinforced concrete arches.

Where it does not

Sites where filling cannot be supervised and tested, ground that cannot take the arch thrust, aggressive environments without a maintained coating on the tubes, owners or codes without a CFST design basis.

Choosing it

Choose a CFST arch over a steel box arch for spans of about 150–550 m where the arch can be erected light and stiffened afterwards: the empty tubes weigh a fraction of the final rib, which shrinks the cableway and tie-backs.
Choose it over a concrete arch built in cantilever when travellers and stays would be very heavy; for the longest concrete arches, use the CFST tubes as a stiff skeleton and encase them.
Prefer a steel arch where filling quality cannot be controlled and inspected, or where the governing code has no CFST design basis.
Prefer a cable-stayed or girder bridge on soft ground: an arch needs foundations that can take the thrust.

Plant, pace and money

PlantTube fabrication shop, cable crane system or rotation equipment, temporary towers and tie-back stays, concrete pumps with vacuum assistance, strain and survey monitoring, ultrasonic testing.

ProductivitySteel arch erection by cable crane at several segments per week; each chord filled in one continuous pour of one to two days; a 400–550 m arch typically takes two to three years from foundations to deck.

CostIndicative: USD 2,500–6,000 per m² of deck for arch, deck and temporary works, varying strongly with the cableway and foundations.

Risks and controls

What goes wrong

Voids and debonding at the crown and below diaphragms, overstress of the empty tubes during filling, shrinkage or heat of hydration loosening the confinement, crown misfit, tie-back or cableway failure, corrosion of the tubes in service.

Quality assurance

Stage analysis of erection and filling, tube strain and geometry monitoring during filling, mix trials for pumpability and shrinkage, ultrasonic and tapping checks of the fill, NDT of tube welds, coating inspection.

Origins

Concrete-filled tubes became an arch-building method in China from 1990, when the 115 m Wangcang Donghe Bridge in Sichuan was completed; spans grew through Wushan (460 m, 2005) and Bosideng at Hejiang (530 m, 2013) to Pingnan Third (575 m, 2020).

Examples

Wushan Yangtze River Bridge (460 m, 2005), Bosideng Bridge at Hejiang (530 m, 2013), Pingnan Third Bridge (575 m, 2020); stiff-skeleton concrete arches such as Wanxian (420 m, 1997) and Tian'e Longtan (600 m, 2024).

Case studies

Wangcang Donghe BridgeChina · 1990115 m span, generally regarded as China's first CFST arch bridge.

Wushan Yangtze River BridgeChina · 2005460 m CFST arch over the Three Gorges reservoir, the longest CFST span when it opened.

Bosideng Bridge (Hejiang Yangtze River Bridge)China · 2013530 m, the longest CFST arch until 2020.

Pingnan Third BridgeChina · 2020575 m; each rib has four 1.4 m chords; sections of up to about 215 t were placed by cable crane with the half-arches anchored back to two truss towers, and vacuum-assisted pumping with a special mix was used to avoid voids and shrinkage.