BRIDGES

A bridge consists of a superstructure and substructure. The weight of the superstructure and the loads imposed on it are taken by the supports of the bridge and transmitted to the foundation. To design the foundation, it is necessary to know both the magnitude of the forces and also the manner in which they are transmitted to the foundation. From this part of view, there are three categories of bridges :

1. The vertical loads acting on the superstructure are transmitted vertically to the foundations.

2. Besides the vertical forces transmitted to the foundations by the supports, the horizontal thrust pushes the supports outwards.

3. The vertical forces are transmitted to the foundations vertically, but for stability the superstructure has to be anchored to rock on a large concrete mass, and there are forces that tend to pull out the anchorage.

The superstructure may be steel, reinforced concrete, or timber. It rests on or is fixed to the abutments. Abutment refers to a terminal support of the bridge. Obviously, a bridge has two abutments, and the traffic has to pass over both abutments in order to enter the bridge and leave it. The abutment may be an embankment of variable height on merely the ground surface with perhaps a little grading. Abutments generally are made of concrete, plain or reinforced, although in some bridges, other materials are used, e.g. steel or, in old bridges, rough ruble masonry. The concrete abutments sometimes are faced with 'dimension stones'.

If the bridge consists of several spans of equal or variable length, the intermediate supports (between abutments) are piers. A multispan bridge may consist of a number of mutually independent girders supported at both ends on the piers and abutments, or a long girder may cover several spans ("continuous" girder). In all cases of girders, the girder must be allowed to move a little for temperature expansion. For this purpose, the girder should be fixed firmly on an abutment or pier and placed on rockers or rollers on other supports.

Figure represents a cantilever bridge with two piers and two abutments. The piers essentially carry the weight of the superstructure, and the structure may be so balanced that the load on the abutments is negligible. The girders have protruding arms (cantilevers) and carry a relatively small, simple beam at the center of the bridge.

Arch bridges may be steel, concrete on timber. An arch bridge may be provided with a tie that takes up the horizontal thrust, H, caused by the arch instead of the abutments doing so. A rigid frame bridge may be steel or of reinforced concrete and is commonly of one or two spans.

A suppression bridge consists of two cables, generally sprung of strong wire, which rests on saddles firmly fixed at the top of the steel towers. The loaded cables have the tendency to pull the towers inward, and to oppose this tendency; the cables are anchored either in natural rock or in massive block of concrete that holds down the end of the cables.

Abutments of a Bridge

If the bridge access is an embankment, the abutment has to hold it back to prevent the earth from moving into and obstructing the waterway between the bridge supports. The abutment has to offer a seat for the superstructure and at the same time be a retaining wall for the embankment. Designing swing abutments does this.

The plan of footing for a beveled–wing abutment is shown in figure. The straight-wing abutments are somewhat weaker than the beveled-wing; the wings of the latter reinforce the straight retaining wall.

When the land is inexpensive, U-shaped abutments can be used. In this type only the central part of the embankment is contained between the wings, and the slopes are permitted to fall outside.

Besides these simple abutment types, there are a number of other arrangements serving the same purpose.

Piers of a bridge

These intermediate bridge supports are built mostly of concrete with granite facing. Occasionally steel is used or even timber in bridges formed by piles protruding over the high water level. As a rule, the longer the stream, the higher the piers and the deeper the foundations. However, in wide shallow rivers, the piers are generally low and foundations rather shallow. Highway piers are long perpendicular to the general direction of the bridge. Railroad bridges generally are much narrower, their width depending on the number of tracks they have to carry. Small bridges generally have no piers but in rare cases may have one or two. Figures represent a solid concrete or masonry shaft for a medium –Size Bridge. It has triangular (or rounded) ends directed against the current, though in many cases, the piers have symmetrical ends. The pier is battered on all sides, though in modern bridge-piers, the downstream end is often vertical. The pier may be provided with a starling, most of which should be located below the high water level. The function of the starling is to regulate the passage of water, and, particularly, to serve as an icebreaker in the spring.

A few types of hollow piers for medium sized bridges are shown in figures. In a long structure with a considerable number of spans, piers similar to those shown in figures are generally called bents. Piers for longer bridges generally are hollow and somewhat similar to in shape to the dimumitive bents shown in figure. They consist of combinations of high vertical shafts, straight, stepped or circular (cylindrical) with portals, and other architectural features.

In the design of bridge foundations, the settlement of the bridge and its stability should be considered. The total vertical load on the foundation of a bridge consists of the dead load plus the vertical live load. If the bridge supports stands on a spread foundation, the total load (in pounds) divided by the area of contact of the foundation (in square feet) gives the soil pressure in pounds per sq. foot on the base of the support. The soil preserve should not exceed the value of the unconfined compression strength of the underlying materials.

Stability of the bridge is affected by lateral forces and scour (a type of water erosion). The lateral forces, of which wind is the most important, essentially cause an overloading of the foundation on the lee side. This overloading is of short direction only and, practically, does not affect the settlement of the bridge, but in exceptional, vary rare cases; it may cause the support to tip. The maximum shearing stress acting on the supporting soil materials should not exceed their shearing strength, using small safety factor.

Besides wind, other lateral forces acting on the bridge are presence of the running water, wave action, ice and drift presence, and shocks from passing vessels if the piers are unprotected by special fenders of piles driven around the pier. In railroad bridges, the longitudinal forces, mostly due to braking of the trains, may be of importance. For modern bridges supports on deep foundations the lateral forces are of little consequences, except they may be critical for weak timber piers.

Scour

When the bridge supports and often a portion of the access embankments are placed in a waterway, it becomes narrower. Thus scour is caused because the water velocity increases and the bed deepens until some state of equilibrium is reached. Scour may also be induced, by rectifying a meandering stream in soft alluvial deposits. Such a rectification is combined with an increase of the flow gradient (because of the shortening of the distance) and hence in velocity. Scour may be observed also in stream under natural conditions without any bridges. In all cases, scour is intensified during the high water periods.

There is no efficient method to prevent scour. One of the palliative methods is placing riprap around a pier; another one is to drive piles under a pier to a depth greater than required by stability of the pier. Piles thus driven, which will be exposed between the periods of high water, should be protected against dry rot, for instance by impregnation. Though the terms "scour" and "erosion" are practically synonymous in geo-techniques, there is little refinement in their usage, e.g. scour at the bottom of the channel and erosion of the banks of the channel.

Abutment foundations: In the geo-technical investigations for a bridge abutment, the influence of water in scouring in softening and swamping the soil should be duly considered. Abutments are built mostly on spread foundations, though pile foundations also have been used.

Pier Foundations: The piers of a bridge are more subject to the action of lateral forces and scour than the abutments and often are founded on deeper foundations. There are water piers, permanently or periodically standing in water, and land piers, or via duet-type piers. The piers of the via duet type should be founded below the frost line in the same way as building facings, and if the highest ground water level is above the base of the facing, the resultant, decrease of the bearing power of the foundation should be taken into account by the designer when proportioning the pier. Water piers which stand in water only periodically should be founded below the frost line or at a level which ensures a good seat of the pier and safety from erosion during the high-water period, whichever of these two levels is lower. In any case, the bottom of the pier footing even for a small bridge should be at least 3 ft or so below the finished grade or the bottom of the stream.

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