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.

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 establishes the loading gauge. Finally, the standard includes structure outline or structure gauge, leaving a space between the loading gauge and structure gauge as clearance.

In roadways, vertical clearance is the measurement from the ground or the road pavement to the bottom of overpasses or bridges.

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.

Australia defines minimum vertical clearance based on types of roads. The minimum vertical clearance is 5.4 metres (17 ft 9 in) for main roads and highways, and 4.6 metres (15 ft 1 in) for other local roads with road authority approval. For high and very high clearance roads, the values are between 5.9 metres (19 ft 4 in) and 6.5 metres (21 ft 4 in).

Eurocode 1: Actions on structures has a definition of "physical clearance" between roadway surface and the underside of bridge element. The code also defines the clearance that is shorter than the physical clearance to account for sag curves, bridge deflection and expected settlements) with a recommendation of minimum clearance of 5 metres (16 ft 5 in).

In the Philippines, the Department of Public Works and Highways sets minimum vertical clearance at 4.88 metres (16 ft 0 in) above national highways for structures like flyovers and bridges, with an additional 0.15 metres (5.9 in) allowance for future road surfacing. For pedestrian footbridges, the minimum is 5.33 metres (17 ft 6 in). Structures over navigable waters have a different formula based on the highest water level, vessel height, and an allowance of 1 metre (3 ft 3 in).