Independent Piers

INDEPENDENT PIERS. The art of constructing foundations on a compressible soil was brought to a high degree of development by the architects of Chicago between 1870 and 1890, when the principal buildings were founded upon a bed of soft clay. The special feature of the practice in that city is what is called "the method of independent piers" ; that is, each tier of columns, each pier, each wall, etc., has its own independent foundation, the area of which is proportioned to the load on that part. The interior walls are fastened to the exterior ones by anchors which slide in slots.

The opposite extreme is to rest the structure upon a plat form of concrete, timber, or steel beams so strong as to resist local settlement. This method is not usually successful; and when it is successful, it is exceedingly expensive and usually needlessly ex travagant. The post-office building erected in Chicago in 1875 rested upon a bed of concrete 3 feet thick over the entire site; but the concrete was insufficient to resist the unequal loading, and the building settled so badly and so unevenly that it was necessary to demolish it after it had stood only seventeen years. It is said that some noted buildings in Europe rest upon beds of concrete 8 or 10 feet thick.

In a number of the cities of the United States are monumental buildings whose exterior walls rest upon continuous platforms built of heavy longitudinal and transverse steel beams, the object being to prevent even small cracks in the masonry by unequal settlement. In most cases, it is not expected that the platform will prevent all settlement, or even any unequal settlement; but it is intended that the platform shall be strong enough to bridge over any weak spot in the foundation and make the slope in the foundation from the point of greatest settlement to the point of least settlement so gradual as not to cause cracks in the stone facing, particularly in lintels and sills.

The preceding discussion refers to the total weight that is to come upon the foundation. The pressure of the wind against towers, tall chimneys, etc., transfers the point of

application of the load to one side of the foundation, and may affect the stability of the structure.

Amount of Wind Pressure.

The maximum horizontal pres sure of the wind is usually taken as 50 lb. per sq. ft. on a flat surface perpendicular to the wind, and on a cylinder at about 30 lb. per sq. ft. of the projection of the surface. The pressure upon an inclined surfacg, as a roof, is about 1 lb. per sq. ft. per degree of inclination to the horizontal.

For example, if the roof has an inclination of 30° with the horizontal, the pressure of the wind will be about 30 lb. per sq. ft.

Overturning Effect.

The method of com puting the position of the center of the pressure on the foundation under the action of the wind is illustrated in Fig. 81, in which ABED represents a vertical section of the tower; a is a point horizontally opposite the center of the surface exposed to the pressure of the wind and vertically above the center of gravity of the tower; C is the position of the center of pressure when there is no wind; N is the center when the wind is acting.

For convenience, let P = the maximum pressure on the foundation, per unit of area; p = the pressure of the wind per unit of area; H = the total pressure of the wind against the exposed surface; W = the weight of that part of the structure above the section considered,—in this case, AB; S = the area of the horizontal cross section; I = the moment of inertia of this section; 1 = the distance A B; h = the distance a C; d = the distance N C; M = the moment of the wind.

When there is no horizontal force acting, the load on AB is uniform; but when there is a horizontal force acting—as, for ex ample, the wind blowing from the right,---the pressure is greatest near A and decreases towards B. To find the law of the variation of this pressure, consider the tower as a cantilever beam. The maximum pressure at A will be that due to the weight of the tower plus the compression due to flexure; and the pressure at B will be the compression due to the weight minus the tension due to flexure.