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Bridge Engineering (5)Substructure ± Abutments and Piers

1. *SubstructureThe portion of the bridge structure below the level of

the bearing and above the foundation is generallyreferred to as substructure.

1.1 Piers: the support, usually of concrete or masonry, for thesuperstructure of a bridge.

1.2 Abutments: the support for the end of a bridge span orlateral support for the soil or rock on which the roadwayrests immediately adjacent to the bridge, functioning as apier and a retaining wall

1.3 Bed blocks over the piers and abutments

2. Piers2.1 * Types of piers

2.1.1 solid: common for railroad bridges;

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2.1.2 cellular;2.1.3 trestle;2.1.4 hammer-headed: avoiding skew spans in passing over

existing highways or railroad tracks.2.2 *Design requirements of piers

2.2.1 The general shape and features of a pier depend to alarger extent on the type, size and dimensions of thesuperstructure and on the environment.

2.2.2 Solid and cellular piers should be provided withsemicircular cut waters to facilitate streamlined flow andto reduce scour;

2.2.2 Solid piers are of masonry or mass concrete;2.2.3 Cellular, hammer-head and trestle piers are of

reinforced concrete;

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2.2.4 It is permissible to use stone masonry for theexposed portions and to fill the interior with leanconcrete, which would save expenses onshuttering and would also enhance appearance;

2.2.5 The cellular type permits saving in the quantityof concrete, but usually requires difficultshuttering and additional labor in placingreinforcements;

2.2.6 The trestle type consists of columns (usu. circular

or octagonal ) with a bent cap at the top.2.2.7 In some recent designs , concrete hinges have

been introduced between the top of columns andthe bent cap in order to avoid moments beingtransferred from the deck to thecolumns;

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2.2.8 For tall trestles as in flyovers and elevated roads,connecting diaphragms between the columns may also beprovided;

2.2.9 Trestle design leads to minimum restriction of waterway.2.2.10 The top width of piers depends on the size of the

bearing plates on which the superstructure rests. It isusually kept at a minimum of 600mm more than the out-to-out dimension of the bearing plates , measured alongthe longitudinal axis of the superstructure;

2.2.10 The length of piers at the top should be not less than

1.2m in excess of the out-to-out dimension of the bearingplates, measured perpendicular to the axis of thesuperstructure.

2.2.11 The bottom width of piers is usually larger than the topwidth so as to restrict the net stresses within the

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2.2.12 It is normally sufficient to provide a batter of 1 in 25on all sides for the portion of the pier between thebottom of the bed block and the top of the well or pilecap or foundation footing;

2.2.13 The main advantage in use of reinforced concreteframed type of piers is due to reduced effective spanlengths for girders on either side of the center line of the pier leading to economy in the cost of superstructure.

2.2.14 Caution in wide adoption of framed type of piers:

__ such framework would be conducive to accumulationof debris and especially floating trees if used in riverssubjected to sudden floods near hills and forests;

__ such designs call for two expansion joints at closeintervals of about 1 to 2m on each pier, resulting in

riding discomfort besides maintenance problems;5 Wednesday, May 18, 2011

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__ the ends of the decking on either side of the pier centerline should be cantilevered beyond the bearings sothat one expansion joint would be adequate .

3. *Abutments3.1 An abutment is the substructure which supports one

terminus of the superstructure of a bridge and, atsome time, laterally supports the embankment whichserves as an approach to the bridge.

3.2 The abutment can of masonry, plain concrete orreinforced concrete .

3.3 An abutment generally consists of the following threedistinct structural elements:

3.3.1 the breast wall which directly supports the dead andlive loads of the superstructure, and retains the filling

of the embankment in its rear;6 Wednesday, May 18, 2011

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3.3.2 the wing walls, which act as extensions of thebreast wall in retaining the fill though not takingany loads from superstructure; and

3.3.3 the back wall, which is a small retaining wall justbehind the bridge seat, preventing the flow of material from the fill on to the bridge seat .

3.4 Design requirements of abutments3.4.1 consisting in assuming preliminary dimensions

depending on the type of the superstructure andfoundation , and checking the stresses at the silllevel;

3.4.2 the front face of the breast wall should have abatter of not less than 1 in 25 , preferably at 1 in

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3.5 *Typical forms of reinforced concrete abutments3.5.1 gravity abutments with wing walls;3.5.2 U-abutment;

3.5.3 spill-through abutment3.5.4 pile-bent abutment with stub wings;

3.5.5 others

3.5.5.1 with a gravity type breast wall;3.5.5.2 the counterfort type;

3.5.5.3 the types to reduce the overturning moment due toearth pressure;

3.5.5.4 the type for overpasses over expressways to enhanceaesthetics;

3.5.5.5 the buried types more adaptable for overpasses with

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3.6 Failure of bridge abutments3.6.1 The breast wall may fail by tensile cracks, crushing

or shear;3.6.2 The wall may tilt forward due to excessive

overturning moment due to earth pressure;3.6.3 The wall may slide forward due to earth pressure if

the vertical forces are inadequate;3.6.4 Failures may occur along a curved surface by

rupture of the soil due to inadequate shear

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