Cylinder piers are frequently used when the sectional area of a single solid pier is not necessary to stability. These consist of a pair of cylinders arranged so that each may carry the ends of the trusses upon one side of the bridge, and are connected by bracing. near the top to give rigidity transversely to the length of the bridge. They are either thin steel shells filled with concrete, or monolithic con crete shafts, reinforced near the outer surfaces.
210. Stability of masonry pier is a vertical column carry ing both vertical and transverse loads. The vertical loads carried by any horizontal section of the pier consist of the weight of the super structure with its live load and the weight of the pier above the section considered. The effect of impact is not usually considered, although a small allowance for impact is sometimes added for the upper part of railroad bridge piers.
The wind and current pressures are horizontal forces which tend to produce bending moments in any horizontal section of the pier, and in the foundation, in a direction normal to the bridge. The wind load upon the superstructure and upon a railway train upon the bridge may he taken the same as in designing the superstructure. Wind upon the end of the pier is commonly taken at about 20 pounds per square foot of vertical section for semicircular ends, but may he reduced to 15 pounds for pointed ends, and should be increased to 30 pounds for rectangular piers.
The pressure of a current of water upon the end of a pier cannot he accurately determined; in pounds per square foot of vertical sec tion, it is frequently taken at about (where r is the surface velo city of the stream in feet per second) for curved or pointed starlings and about twice this amount for rectangular piers. The center of pressure is assumed to be at one-third the depth from the surface to the bottom of the stream.
The pressure exerted by ice depends upon the thickness of the ice and the shape of the up-stream end of the pier, and is greatest when the ice is breaking up and a large body of floating ice is being cut by the pier. Where ice 10 or 12 inches thick may form, a pressure of 45,000 to 50,000 pounds per foot of width of pier is often assumed, considered as concentrated at the level of high water. For other thicknesses, the pressure is somewhat proportional to the thickness.
Where heavy ice is likely to form, the use of ice breakers, or starlings with edges inclined to the vertical (as shown in Fig. 125) may materially decrease the pressure. The tractive force in the piers of a railway bridge is a horizontal force acting parallel with the length of the bridge at the level of the rail, and therefore duces moments in the horizontal sections of the pier and tion at right angles to those due to wind and current. The tive force is commonly taken at 2/10 of the moving load on one track.
It is essential to stability that the maximum compressive stress upon any horizontal section clue to the vertical loads combined with that clue to the moments of the horizontal forces shall not exceed the safe compressive strength of the masonry. The maximum unit pres sure upon the foundation must not exceed a safe value. No tension should exist in the masonry at any section under any possible loading, unless it be reinforced concrete designed for tension, and compression must always exist over the whole area of the foundation.
The horizontal forces must not be sufficient to produce sliding upon any joint in the masonry or foundation, or to shear any section of concrete.
Ordinary solid piers dimensioned to give sufficient bearing area at the top and slightly bat tered will usually be amply strong. The distribution of loads over the foundation should, however, be care fully looked after.
Large masonry piers are frequently built, hollow. The masonry under the base plates of the superstructure is considered to act as columns which transmit the vertical loads to the foundation, and the central part of the pier is regarded as bracing to stiffen the and carry the lateral loads. A part of the masonry at the center of the pier may he left out without appreciably reducing its strength, thus reducing the weight upon the foundation and saving a considerable volume of masonry or concrete. Such an arrangement is shown in Fig. 126.
piers of reinforced concrete have been occasionally used. These have been designed in a number of ways, columns being used under the base plates of the superstructure, connected in some way by reinforced bracing. The exterior shape of these piers below high water is made the same as solid piers in order to produce minimum disturbance of stream flow.