Clearance (civil engineering)
In civil engineering, clearance refers to the difference between the loading gauge and the structure gauge in the case of railroad cars or trams, or the difference between the size of any vehicle and the width/height of doors, the width/height of an overpass or the diameter of a tunnel as well as the air draft under a bridge, the width of a lock or diameter of a tunnel in the case of watercraft. In addition, there is the difference between the deep draft and the stream bed or sea bed of a waterway.
For roadways and waterways, the clearance is typically specified as the width/height of a structure that the vehicle needs to pass instead of the difference between the vehicle and the structure.
Railways
In railways, clearance is the difference between the loading gauge and the structure gauge. A clearance standard is established using static rolling stock outline (static gauge) as the starting point. This is a cross-sectional outline of a maximum size rolling stock when it is not running. The standard then defines maximum kinematic rolling stock outline for when rolling stocks are running to account for suspension and lateral motion on the track. This is also known as "kinematic envelope". The standard also defines base operating standard for clearance which is larger than the kinematic envelope. This should be maximum outline of the normal rail operation and can only be infringed in special circumstances. The standard then adds another outline called maintenance intervention standard outline that larger than the base operating standard by defining a safety margin (contingency gap) from the kinematic envelope. When there is an infringement of this outline, a maintenance work is required to bring to clearance standard. This establish the loading gauge. Finally, the standard includes structure outline or structure gauge, leaving a space between the loading gauge and structure gauge as clearance.[1]
Roadways
Vertical clearance
In roadways, vertical clearance is the measurement from the ground or the road pavement to the bottom of overpasses or bridges.[2][3]
American Association of State Highway Officials (AASHO) established Interstate Highway standards which included minimum vertical clearance of 14 feet (4.3 m). The Department of Defense later informed that the clearance was not sufficient for national defense purposes and wanted the vertical clearance to be raised to 17 feet (5.2 m). Eventually, the new standards were approved in 1960 to have the minimum vertical clearance of new structures to be 16 feet (4.9 m). There were up to 2,650 existing overpasses in 1967 that were not in compliance with the new standards. The decision was made to only raise those 350 overpasses that served up to 95% of major military installations. Other overpasses were left to be reconstructed to the new minimum vertical clearance at later times.[2]
United Kingdoms has a standard on minimum clearance of a public highway at 16.5 feet (5.0 m). Any bridges that do not meet the clearance requirement are considered to be "low bridges" and they require to have signage to indicate the clearance.[4]
Bridge and tunnel strikes
Bridge or tunnel strikes are collisions of vehicles with bridge or tunnel structures. These may involve over-height vehicles, or low vertical clearance bridges or tunnels. These accidents occur frequently and are a major issue worldwide.[5]
Horizontal clearance
In United States, the term "horizontal clearance" is used interchangeably with "lateral offset". This is the space from the edge of the roadway that is clear from vertical obstructions such as sign posts, utility poles, and fire hydrants. The horizontal clearance is used in urban environments where these objects are expected to be near roadways. The horizontal clearance are to prevent overhung elements such as side mirrors of large vehicles driven at the extreme edge of the roads to hit such objects. It also allows opening curbside doors of parked vehicles. Minimum horizontal clearance in US standard is 1.5 feet (0.46 m). It is to be noted that horizontal clearance is not the same concept as clear zone which is used in non-urban highways.[6][7]
For roadways that require passing under some structures such as tunnels, there are standards on the entire wide of the roads known as horizontal curb-to-curb and wall-to-wall clearances. American Association of State Highway and Transportation Officials (AASHTO) recommends having minimum curb-to-curb clearance for two-lane highways of 24 feet (7.3 m) and wall-to-wall clearance of 30 feet (9.1 m), while desired curb-to-curb clearance should be 39 feet (12 m), and wall-to-wall clearance should be 44 feet (13 m).[8]
Waterways
In waterways, bridge span clearance is a measurement from water surface to the underside of bridge span. The most conservative clearance uses the water level at the mean highest high water (MHHW), the average value of the highest high tide of a measurement period.[3][9]
See also
References
- ^ "7". Code of Practice: Clearances (PDF). Australian Rail Track Corporation Limited. pp. 4–6. Retrieved 27 August 2023.
- ^ a b "Right of Passage: The Controversy Over Vertical Clearance on the Interstate System". U.S. Department of Transportation Federal Highway Administration. Retrieved 27 August 2023.
- ^ a b Lin, Weiwei; Yoda, Teruhiko (2017). "Introduction of Bridge Engineering". Bridge Engineering: 1–30. doi:10.1016/B978-0-12-804432-2.00001-3.
- ^ Preventing of Strikes on Bridge Over Highways - A Protocol for Highway Managers & Bridge Owners (PDF) (2 ed.). Network Rail on behalf of Bridge Strike Prevention Group. p. 6. Retrieved 29 August 2023.
- ^ Nguyen, Bella; Brilakis, Ioannis (2016). "Understanding the Problem of Bridge and Tunnel Strikes Caused by Over-height Vehicles". Transportation Research Procedia. 14: 3915–3924. doi:10.1016/j.trpro.2016.05.481.
- ^ Clear Zone, Lateral Offset & Horizontal Clearances Discussion (PDF). Delaware Department of Transportation. 25 June 2013. Retrieved 28 August 2023.
- ^ "Clear Zone and Horizontal Clearance". U.S. Department of Transportation Federal Highway Administration. Retrieved 28 August 2023.
- ^ Technical Manual for Design and Construction of Road Tunnels — Civil Elements (PDF) (Technical report). National Highway Institute. December 2009. FHWA-NHI-10-034. Retrieved 28 August 2023.
- ^ Mcinnes, K. L.; Walsh, K. J. E.; Hubbert, G. D.; Beer, T. (2003). "Impact of Sea-level Rise and Storm Surges on a Coastal Community". Natural Hazards. 30 (2): 187–207. doi:10.1023/A:1026118417752.