How bridges get built · Steel erection · Orthotropic steel decks
Also called: orthotropic deck, steel deck plate, closed rib deck
How it is done
- Detail for fatigue first: a deck plate thick enough for wheel loads (16 mm or more in truck lanes), closed trapezoidal ribs typically 6–8 mm thick, and rib-to-floor-beam cut-outs to current guidance.
- Fabricate panels on automated lines: weld U-ribs to the deck plate with about 70–80 % partial-penetration welds (or double-sided or full-penetration welds where specified), controlling distortion with pre-camber and fixtures.
- Inspect rib welds: macro-etch sections from run-off plates to prove penetration, phased-array UT on the specified proportion, and deck plate flatness checks.
- Assemble panels with floor beams and diaphragms into box or deck sections in the shop or a site yard; trial-fit adjacent sections where site joints are welded.
- Erect the sections (cantilever, floating crane, strand jacks or launching) and make site joints: deck plate butt-welded on ceramic or steel backing, ribs spliced with bolted plates or welded through access holes.
- Inspect site welds (UT or radiography of deck butt welds), grind where specified and repair the coating.
- Blast and prime the deck plate, apply waterproofing and surfacing (epoxy asphalt, mastic asphalt or a polymer overlay) within temperature and humidity limits, and test the bond.
Key numbers
- Deck plateolder 10–12 mm plates cracked under heavy trucks; 16–18 mm is usual in truck lanes today
- Closed ribs6–8 mm thick, about 300 mm wide at the top, at about 600 mm centres; floor beams at about 2.5–5 m
- Rib-to-deck weldat least about 70–80 % penetration of the rib wall, without melt-through
- Weightdeck plate with ribs about 180–250 kg/m², against 500–600 kg/m² for a 200–250 mm concrete slab
- Surfacingthin, typically 40–80 mm
- Costfabricated USD 6,000–10,000 per tonne; deck USD 2,500–4,000 per m² (indicative)
Where it fits
- Suspension and cable-stayed main spans, movable bridges, deck replacements where weight must fall.
Where it does not
- Heavy-truck routes with thin deck plates (older 10–12 mm plates cracked; 16–18 mm is current practice in truck lanes, and around 14 mm is acceptable only under thick asphalt), poor surfacing bond.
Choosing it
- Choose it over a concrete deck slab where dead weight governs: long-span suspension and cable-stayed main spans, movable spans, and deck replacements on old bridges.
- Prefer a composite steel-concrete deck for cable-stayed spans up to about 500 m, where the fatigue and surfacing risks of an orthotropic deck are not repaid by its lower weight.
- Prefer thick deck plates (16–18 mm) or a bonded high-performance concrete overlay where truck volumes are high; cracking risk falls steeply with plate thickness.
- Prefer bolted rib splices over welded ones where site weld quality cannot be assured.
Plant, pace and money
PlantAutomated welding lines, large-section transport, floating or deck cranes, UT inspection.
ProductivityPanel production is shop-driven; site welding a few splices per day.
CostUSD 6,000-10,000 per tonne fabricated; deck USD 2,500-4,000 per m2.
Risks and controls
What goes wrong
- Fatigue cracks at rib-deck and rib-floorbeam welds, surfacing failure, distortion from site welding.
Quality assurance
Weld penetration verified by macro tests, phased-array UT on 100 % of rib welds where required, surfacing bond tests.
Origins
Developed by German engineers after the Second World War to save steel in rebuilding the Rhine bridges; closed trapezoidal ribs became standard in the 1960s, and the Severn Bridge (1966) brought the streamlined orthotropic box to suspension bridges.
Examples
Akashi Kaikyo, Great Belt, Sutong, Russky; the game's steel_box_orthotropic type.
Case studies
Severn BridgeUK · 1966988 m suspension span, the first with a streamlined orthotropic steel box deck, which cut deck weight sharply; fatigue cracking of welds and hangers later required major strengthening and hanger replacement.
Golden Gate Bridge deck replacementUSA · 1982–1986The original concrete roadway was replaced by prefabricated orthotropic steel deck panels installed at night while the bridge carried traffic by day, reducing the dead load on the cables.
Great Belt East BridgeDenmark · 19981,624 m main span with a streamlined welded orthotropic box deck erected in large sections lifted from barges.
Related methods
Cantilever erection of steel (derrick and lifting frames)Cable-stayed cantilever erectionFloating crane erectionSurfacing: asphalt, SMA, concrete and thin overlaysRepainting steel bridges with containment
Further reading
FHWA Manual for Design, Construction and Maintenance of Orthotropic Steel Deck Bridges · Eurocode 3-2 · AASHTO LRFD Bridge Design Specifications, Article 9.8.3 (orthotropic decks) · EN 1993-2 Annex C (recommended details for orthotropic decks) · Wolchuk, Design Manual for Orthotropic Steel Plate Deck Bridges (AISC, 1963)