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Bridge Abutments

abutment and dimensions

BRIDGE ABUTMENTS.

Form. There are four forms of abutment in use, they are named according to their form as the straight abutment, the wing abutment, the U abutment and the T abutment.

The form to be adopted for any particular case will depend upon the location—whether the banks are low and flat, or steep and rocky, whether the current is swift or slow, and also upon the relative cost of earthwork and masonry. Where a river acts dangerously upon a shore, wing walls will be necessary. These wings may be curved or straight. The slope of the wings may be finished with an inclined coping, or offset at each course. Wing walls subjected to special strains, or to particular currents of water require positions and forms accordingly.

The abutment of a bridge has two offices to perform; (1) to support one end of the bridge, and (2) to keep the earth embankment from sliding into the water.

The abutment may fail (1) by sliding forward, (2) by bulging, or (3) by crushing.

The dimensions of abutments will vary with each case, with the form and size of the bridge and with the pressure to be sustained; the dimensions may be determined by the same formulas as used for retaining walls.

For railroad bridges the top dimensions are usually 5 feet wide by 20 feet long. The usual batter is 1 in 12, for heights under 20 feet the top dimensions and the batter determine the thickness at the bottom. For greater heights, the uniform rule is to make the thick ness four-tenths the height.

Bridge abutments are built of first or second-class masonry or of concrete alone or faced with stone masonry, according to the im portance and location of the structure.