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Astructure gauge, also called theminimum structure outline, is a diagram or physical structure that sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. It specifies the height and width of station platforms,tunnels andbridges, and the width of the doors that allow access to awarehouse from arail siding. Specifications may include the minimum distance from rail vehicles torailway platforms, buildings, lineside electrical equipment cabinets,signalling equipment,third rails or supports foroverhead lines.[1]
A related but separate gauge is theloading gauge: a diagram or physical structure that defines the maximum height and width dimensions inrailway vehicles and their loads. The difference between these two gauges is called theclearance. The specified amount of clearance makes allowance forwobbling of rail vehicles at speed or the shifting of vehicles on curves; consequently, in some circumstances a train may be permitted to go past a restricted clearance at very slow speed.
The term can also be applied to the minimum size of roadtunnels, the space beneathoverpasses and the space within thesuperstructure ofbridges, as well asdoors intoautomobile repair shops,bus garages,filling stations,residential garages,multi-storey car parks,overhangs atdrive-throughs andwarehouses.[citation needed]
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 forsag curves, bridgedeflection and expected settlements with a recommendation of minimum clearance of 5 metres (16 ft 5 in).[2] In UK, the "standard minimum clearance" for structures over public highways is 16 feet 6 inches (5.03 m).[3] In United States, the "minimum vertical clearance" of overpasses onInterstate Highway System is 16 feet (4.9 m).[4]
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