Self-anchored suspension bridge erection

How bridges get built · Cable-supported and arch bridges · Self-anchored suspension bridge erection

Also called: self-anchored suspension, SAS erection, deck-first suspension construction, load transfer by hanger jacking

How it is done

Build the towers and the end piers that will receive the cable anchorages in the deck; check the end piers for uplift and the deck ends for the concentrated anchorage forces.
Erect the stiffening girder on temporary supports (falsework towers or trestles) at its cambered erection profile, or by launching or cantilevering with temporary stays where falsework is impossible; complete the anchorage zones at the deck ends.
Install catwalks and form the main cables by PPWS or aerial spinning, anchored in the deck ends; set the strand sag to the free-cable geometry computed for this unloaded state.
Compact the cables, fit cable bands and install the hangers, connected loosely to the deck.
Transfer the load: shorten the hangers by jacking in a computed multi-cycle sequence so the cables pick up the deck progressively and the girder lifts off the falsework without overstress, the deck compression rising as the cables tension.
Survey cable and deck geometry, hanger forces and falsework reactions after each cycle, and move the tower saddles as the cable force grows if the design uses movable saddles.
Remove the temporary supports, complete the deck, surfacing and cable protection, and record final hanger forces.

Key numbers

Main spanabout 100–400 m (San Francisco–Oakland Bay Bridge self-anchored span 385 m)
Cable sagoften deeper than the span/9 to span/11 usual in earth-anchored bridges, to limit deck compression
Deck compressionequal to the cable's horizontal force, so it grows with the square of the span for a given sag ratio
Load transfera few weeks of hanger-jacking cycles, after which the falsework is removed
Costabout USD 6,000–15,000 per m² of main-span deck (indicative)

Where it fits

Spans of about 100–400 m in soft ground, deep water or urban sites where earth anchorages are impractical; signature crossings where a suspension profile is wanted; sites where falsework under the deck is acceptable (land, shallow water, moderate spans).

Where it does not

Spans beyond about 400 m, where deck compression becomes excessive; deep navigable water where falsework under the whole deck is impossible; programmes that cannot accept the deck-first sequence.

Choosing it

Choose it over an earth-anchored suspension bridge when the ground cannot take a gravity or tunnel anchorage (soft soil, deep water, dense urban sites) and the span is under about 400 m.
Prefer a cable-stayed bridge for most spans of 150–400 m: it can be cantilevered without falsework under the deck and is usually cheaper; the self-anchored form is chosen for appearance or site reasons.
Prefer an earth-anchored suspension bridge for long spans, where deck compression and the deck-first sequence make the self-anchored form uneconomic.
Budget the falsework honestly: in deep or navigable water, temporary supports for the whole deck can cost as much as the towers.

Plant, pace and money

PlantFalsework towers or trestles under the whole deck, land or floating cranes for deck erection, catwalks with PPWS or spinning plant, hanger jacking systems with load cells, survey and monitoring.

ProductivityDeck erection on falsework at steel-girder pace; cable formation 2–6 months; load transfer a few weeks of jacking cycles.

CostIndicative: about USD 6,000–15,000 per m² of main-span deck including towers and cables, usually above a cable-stayed alternative of the same span; the falsework is a large share.

Risks and controls

What goes wrong

Falsework failure or settlement before load transfer, overload of individual hangers or falsework bents during the jacking sequence, deck buckling under cable compression in temporary states, cracking of the deck-end anchorage zones, cable geometry errors because the free cable has no load to correct it.

Quality assurance

Construction control analysis of every load-transfer cycle with hanger force and falsework reaction monitoring, survey of cable and deck after each cycle, NDT and inspection of the deck-end anchorages, hardness and embrittlement testing of high-strength rods and fasteners.

Origins

The concept dates from the mid-19th century (Josef Langer in Austria); the Cologne-Deutz Bridge (1915) and Pittsburgh's Three Sisters (1924–28) were early large examples, and the type returned with Konohana in Osaka (1990), Yeongjong in Korea (2000) and the San Francisco–Oakland Bay Bridge east span (2013).

Examples

Konohana (Osaka, 1990, 300 m), Yeongjong Grand Bridge (Incheon, 2000, 300 m, road and rail), the San Francisco–Oakland Bay Bridge self-anchored span (2013, 385 m), and Pittsburgh's Three Sisters bridges of the 1920s.

Case studies

Konohana BridgeJapan · 1990300 m main span with a single main cable in the bridge axis; one of the first modern self-anchored suspension bridges.

Yeongjong Grand BridgeSouth Korea · 2000300 m main span carrying road and rail on a double-deck steel truss, with spatially inclined (three-dimensional) cables.

San Francisco–Oakland Bay Bridge self-anchored spanUSA · 2013385 m main span from a single tower about 160 m tall; the steel box deck was erected on temporary falsework in the bay, the single main cable looped around the west end, and the deck load was transferred to the cable by jacking the hangers. Cracked high-strength anchor rods delayed the opening.