What is an Orthotropic Steel Bridge Deck?
An orthotropic steel deck (OSD) is a bridge deck in which a thin steel plate is stiffened by ribs and floorbeams. The deck plate is also the top flange of the floorbeams and of the longitudinal girders. FHWA describes this as an efficient system, because it saves material and adds rigidity.
Name and origin
The word is a short form of orthogonal-anisotropic. The ribs and floorbeams are orthogonal, and the deck has different stiffness in the two directions. German engineers developed the first OSD bridge in the 1930s, and the first deck was built in 1936.
Parts
- Deck plate. The flat steel plate that carries the wheel loads, with a wearing surface on top.
- Ribs. They run along the bridge and are welded to the underside of the plate.
- Floorbeams. They run across the bridge and carry the ribs to the main girders. Other names are crossbeam and diaphragm.
Open ribs are flat bars, bulb shapes, inverted T-sections or angles. Closed ribs are trapezoidal, U-shaped or V-shaped, and the trapezoidal rib is the most common. A closed rib resists twisting better, so it spreads a wheel load to the next ribs. Open ribs give easy access from below for inspection and maintenance.
Use
FHWA names three uses in the United States:
- long spans where low dead load matters
- box girders with slender plates that need stiffening
- the redecking of existing major bridges
Wearing surface
The surfacing on a steel deck is bituminous or polymer. Bituminous systems include mastic asphalt and epoxy asphalt. Polymer systems include epoxy resins, methacrylates and polyurethanes.
Fatigue and inspection
Fatigue is the main design concern. Live loads from single wheels control the design, and the welds are hard to analyse. The rib-to-deck weld and the rib-to-floorbeam connection are fatigue sensitive. FHWA lists these inspection methods in addition to visual inspection:
- dye penetrant
- magnetic particle
- ultrasonic testing, including phased array
- radiographic testing
- pulse eddy current
- acoustic emission testing
An inspector may need access equipment to reach the rib details.
Airfield note
Airfield pavement is not an orthotropic deck. The FAA and EASA airfield sources used for this glossary do not mention the term. It is on this site as a structural vocabulary entry only.
Frequently Asked Questions
- What does orthotropic mean?
- Orthotropic is a short form of orthogonal-anisotropic. The ribs and the floorbeams are at right angles to each other, and the deck has different stiffness in the two directions. German engineers created the word, and a patent was registered in 1948.
- Which rib type is common in modern orthotropic decks?
- Closed ribs, and most often the trapezoidal rib. They resist twisting better than open ribs, so they spread a wheel load to the next ribs. Modern designs usually do not use open ribs under direct traffic loading.
- Why do orthotropic steel decks crack?
- Fatigue causes the cracks. Repeated wheel loads control the design, and the welded details are hard to analyse. The rib-to-deck weld and the rib-to-floorbeam connection are the details that FHWA calls fatigue sensitive.
References
- fhwa-if-12-027
- Manual for Design, Construction, and Maintenance of Orthotropic Steel Deck Bridges (FHWA-IF-12-027) Federal Highway Administration
- fhwa-focus-2012-osd
- FHWA Focus, April 2012: Manual for Design, Construction, and Maintenance of Orthotropic Steel Deck Bridges Federal Highway Administration
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.