How bridges get built · Cable-supported and arch bridges · Stay cable installation and tensioning
Also called: strand-by-strand installation, parallel strand stays, locked coil stays, stay stressing
How it is done
- Before installation: complete the system acceptance tests (fatigue and leak-tightness to fib or PTI), check anchorage positions and angles against survey, and prepare the stressing sequence from the construction control analysis.
- Lift and hang the HDPE sheath between the deck and pylon anchorages on a messenger or pilot strand, with guide pipes and deviators ready at both ends.
- Install the first (reference) strand and stress it to the computed force with a load cell; pull in each further strand through the sheath by winch or strand pusher and seat its wedges at both ends.
- Stress each new strand with a monostrand jack to the current force in the reference strand (isotension principle); as strands are added the reference force falls, so all strands finish at the same force.
- Check the total stay force on completion (lift-off on sample strands or a multistrand jack) and adjust with the ring-nut anchorage as later stages require.
- After closure and superimposed dead load, make the final adjustment, cut strand tails, fill anchorages with wax or grease, fit caps, and install internal or external dampers and any cross-ties.
- Measure final forces by lift-off and vibration frequency as the baseline for the owner's monitoring.
Key numbers
- Strand15.7 mm seven-wire, 1,770–1,860 MPa, individually galvanised, waxed and HDPE-sheathed
- Stay sizetypically 20–100 strands
- Service stressabout 45 % of GUTS under permanent and live load (fib, PTI); higher temporary limits during construction
- Fatigue acceptance2 million cycles at about 200 MPa stress range, then a tensile test to at least 95 % of actual strength
- Rain–wind vibration controlScruton number of about 10 or more (PTI DC45.1), usually by ribbed or dimpled sheaths plus dampers
- Installationa 60–100 strand stay installed and stressed in one to two days
- CostUSD 15–30 per kg of installed stay including anchorages (indicative)
Where it fits
- All cable-stayed and extradosed bridges.
Where it does not
- Whole-stay lifts of heavy prefabricated strands where a deck crane is not available; skipping force verification.
Choosing it
- Choose parallel strand stays installed strand by strand for most bridges: light handling equipment, individually replaceable strands and force adjustment at any stage.
- Prefer prefabricated parallel-wire or locked-coil stays where a deck crane can lift the whole stay and the client prefers factory-made cables (common in China and Japan).
- Specify ribbed or dimpled sheaths and dampers from the start rather than retrofitting after rain–wind vibration appears.
- On extradosed bridges use saddles with separable strands only where fretting fatigue and slip have been tested.
Plant, pace and money
PlantStrand pusher or winch, monostrand and multistrand jacks, pipe handling frames, force measurement.
ProductivityA stay of 60-100 strands installed and stressed in one to two days by a specialist crew.
CostUSD 15-30 per kg of stay installed including anchorages.
Risks and controls
What goes wrong
- Wedge seating failure, strand damage during pushing, wrong force sequence loading the pylon unevenly, pipe damage, fatigue at anchorages.
Quality assurance
Fatigue and static acceptance tests of the stay system (fib 30 / PTI), force records per strand and per stay, lift-off tests on a sample, anchorage inspection.
Origins
Early cable-stayed bridges used locked-coil ropes or prefabricated parallel-wire cables installed whole; parallel strand stays installed and stressed one strand at a time were introduced by the prestressing firms in the 1980s and have been the default since the 1990s.
Examples
Freyssinet, VSL, DSI and BBR systems on most modern cable-stayed bridges.
Case studies
Fred Hartman BridgeUSA · 1995Large rain–wind vibrations of the stays after opening led to a retrofit with cross-ties and dampers.
Erasmus BridgeNetherlands · 1996Soon after opening, rain–wind excitation set the stays oscillating; dampers were fitted to the stays.
Russky BridgeRussia · 2012Parallel strand stays up to about 580 m long, among the longest installed strand by strand.
Related methods
Cable-stayed cantilever erectionPylon construction: concrete, anchorage zones and geometryExtradosed bridge constructionCable wrapping and dehumidificationStay cable and hanger replacement
Further reading
PTI DC45.1 · fib Bulletin 30 · SETRA Cable stays guide · fib Bulletin 89, Acceptance of cable systems using prestressing steels