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Bridge Piers and Abutments

pier, top, structure, sometimes, foot and stream

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BRIDGE PIERS AND ABUTMENTS Locations and Dimensions for Piers.—In fixing the locations for piers of a bridge, there are a number of factors which it may be necessary- to take into consideration. In a navigable stream, they trust be arranged so as to obstruct the channel as little as possible and meet the regulations imposed by the Government. This may sirnetimes determine positions, length of span, and height of structure. The waterway requirements and possibility of the piers restricting the waterway to a serious extent must always be considered. The character of the foundation along the line of the bridge, and probable difficulty of placing foundations at various locations may sometimes influence the choice of positions for piers.

Financial considerations are always important. The total cost of the structure including piers and superstructure should be the mini mum consistent with properly meeting the other requirements. The cost of superstructure increases approximately- as the square of the length of span, while the cost of piers may be nearly proportional to their number. An arrangement may therefore be worked out in each instance which will give a minimum of cost for the entire structure.

Aesthetic considerations may also have an influence on pier loca tion; the appearance of the structure is always an important matter and may sometimes control the design. The arrangement of spans to secure symmetry in the whole structure, with proper placing of dominating features ought to be carefully considered.

The shape to be given to a pier is determined by the requirements of each particular case. It trust be designed safely to transmit to the foundation, the loads brought upon it, and to resist any lateral pressure due to wind or current, and the form to be given a horizontal section should offer as little resistance as possible to the flow of the stream in which it may he placed.

The most common forth for piers in streams is that of a rectangle of length a little more than the width of the bridge, with triangular or curved ends. The pointed ends below high water are known as

starlings, and are intended to reduce the disturbance to the stream flow and sometimes to act as ice breakers. Sometimes starlings are used only on the up-stream end of the pier, but more commonly the horizontal section is made symmetrical. The down-stream starling ser.-es to prevent eddies below the pier, and to equalize the load over the foundation area. Starlings are necessary ohly below high water, and the upper part of the pier is sometimes made rectangular, but more commonly the ends are semicircular (see Fig. 123) or the shape of the starling is continued to the top.

Fig. 124 shows a simple form of concrete pier in which a triangular starling is used upon the up stream encl only and is continued to the top of the pier. In Fig. 123 the horizontal section of the starling is composed of two inter secting circular arcs of radius equal to the width of the pier, while the upper part of the pier has semicircular ends.

The dimensions required for the top of a pier are usually fixed mainly by the area of the ings needed for the ture. A coping not leas than 1 foot in thickness is placed on the top of the pier, projecting 3 to 6 inches beyond the top of the masonry beneath. The dimensions of the top of the pier should be such that the base plate of the super structure shall not come within 4 to G inches of the edges of the masonry under the coping. The width of the top of the pier under the coping is required to be at least 4 feet, and at least 1 foot more than is needed for the base plate.

A batter of at least .', inch to 1 foot, or sometimes 1 inch to 1 foot, is given to the surfaces of the pier. Footing courses may be employed at the base of the pier to distribute the loads over a larger area of the foundation, being commonly stepped off, projecting about a foot horizontally and with a depth about twice the width. of reinforced concrete the projecting steps may be designed as cantilever slabs.

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