Bridge roadway expansion joints are provided to accommodate the thermal changes in the superstructure, and, in the case of prestressed-concrete bridges, to accommodate creep shortening of the superstructure as well. They are required at abutments that are restrained against longitudinal movement and at the end of supported superstructures free to translate due to provision of expansion bearings. In some long-span steel bridges, expansion joints and expansion bearings must also accommodate change of length of span due to live load deflections. Expansion joints are not required in short bridges where movement is small— for example, in steel bridges with span less than 50 ft (15 m)—or in longer bridges where the superstructure is fixed to the abutment (jointless bridges or integral construction). For these longer bridges, designs that eliminate or minimize bridge expansion joints, without introducing problems in the approach roadway or causing distress in the superstructure or substructure, are favored. (See Arts. 4.15 and 4.16.7.)
Expansion joints, or, more accurately, rotation joints, are also provided where the deck is made discontinuous, or a hinge is provided, in anticipation of settlement of the end of a span.
Where expansion joints are required, they should be sized to accommodate the anticipated movement with a liberal allowance. A joint-sealing device such as a strip seal can be destroyed by one occurrence of a record cold period. For deep simple-span girders, the joint movement due to live load rotation of the end of the span should be included. Specifications for installation of joints should take setting temperature into account. A table giving required joint opening dimensions for different ambient temperatures is preferred over an equation for adjustment of a fixed dimension that is applicable to a given temperature. In areas where roadway deicing salts are not applied, it may not be necessary to seal expansion joints. Even in this case, though, sealed expansion joints will prevent intrusion of foreign objects, which can damage the bridge by causing excessive local pressure, and will prevent accumulation of debris on bridge seats.
Large-capacity open expansion joints can be fabricated using steel plates with meshed fingers, the so-called finger joint. Finger joints have served well for many years on many bridges. The plates used in these joints must be thick to withstand the direct cantilever wheel loading to which the fingers are subjected. The two halves of finger joints are massive steel fabrications, but they can be gas-cut with accurate dimensional control. In snowplow areas, where the ends of the fingers may be snagged by the plow blade, the use of a finger joint should be avoided, or the ends of the fingers should be rounded downward. In areas where joints should be sealed, the finger joint surface may be left open, but an elastomeric trough should be installed beneath the joint.
Bicycles should not be permitted on bridges with finger joints, because the wheels can drop into the space between fingers, causing injury to the rider. Conversely, open finger joints should not be used on bridges on which bicycles are permitted.
In areas of salt application, expansion joints must be sealed. Northern states have incurred tremendous cost to repair damage to superstructures and substructures in the form of steel corrosion, prestressed-concrete beam deterioration, and concrete spalling due to salt drainage through open or inadequately sealed expansion joints.
