§27-685 Foundation Piers

Foundation piers shall be designed as columns. Reinforced concrete piers shall be designed in accordance with the provisions of reference standard RS 10-3. Reinforced and unreinforced masonry piers shall be designed in accordance with the provisions of reference standards RS 10-2 and RS 10-1. Unreinforced concrete piers shall be designed in accordance with subdivision (b) of this section.

(a) Lateral support. The equivalent unbraced length of a pier supported by lateral soil pressure may be determined by a recognized method of elastic analysis. Alternatively, such a pier may be assumed to be hinged, but laterally braced at intervals equal to the full height of the pier or eight times the least lateral dimension of the pier, whichever is the lesser value. The provisions of subdivision (e) of this section shall apply.

(b) Unreinforced concrete piers. Where unreinforced concrete piers are used, the allowable compressive stress shall not exceed 0.25f'c and the center of cross section of the pier at any level shall not deviate from the line of action of the resultant of all forces (which line of action shall consider the eccentricities due to all loads and moments acting on the pier) by an amount more than one-sixtieth of its height or one-tenth of its least lateral dimension and the ratio of the height to the least lateral dimension shall not exceed eight. For larger values of the ratio of the height to the least lateral dimensions, or for greater eccentricities, piers shall be reinforced.

(c) Metal shells. Where piers are encased by a metal shell, the shell may be considered as contributing to the structural strength of the pier provided that the thickness is one-eighth inch or greater, and subject to the following requirements:

(1) Where boring records or site conditions indicate possible deleterious action on the shell, where any portion of the shell is embedded in ash, cinder fill or garbage fill, where the encased piers are used for support of chemical plants, piles of coal, or under other conditions conducive to chemical seepage or corrosive action, or where the encased piers are used for support of electrical generating plants, the metal shells shall be protected against deterioration by encasement, coating, or other device acceptable to the commissioner.

(2) The area of the metal section of the shell multiplied by the efficiency of the horizontal joints shall be considered as equivalent vertical reinforcement of the pier. The area of the metal section of the shell, multiplied by the efficiency of the vertical joints, shall be considered as equivalent spiral reinforcement of the pier.

(d) Minimum dimensions. The provisions of subdivision (e) of this section shall apply. In addition, the plan dimensions of the pier shall not be less than those of the column above. If the column above is a steel section resting on a base plate, the base plate and pier shall be proportioned for allowable bearing pressures as prescribed in reference standards RS 10-1, RS 10-2, and RS 10-3.

(e) Filling. The provisions of subdivisions (a) and (d) of this section shall apply only where the fill (or backfill) is placed around the pier as controlled fill, and wherein the level of the fill is raised uniformly around the entire perimeter of the pier. Where the fill operation is not controlled the least lateral dimension of a foundation pier shall be twenty-four inches and the pier shall be proportioned for lateral pressure equal to the pressure of a differential height of fill equal to five feet, applied simultaneously with the other loads on the pier specified in article three of this subchapter.