Foundation, Concrete and Earthquake Engineering

### Transfer of Force at Base of Foundation

When the loaded area (area of column pier or base plate) and the supporting area (area at the top of the footing) are equal, bearing stress on the loaded area of the footing shall be equal to or less than 0.85φf’c where f’c is the lower of the strengths of footing or column concrete.

When the supporting area is larger than the loaded area on all sides, the bearing stress on the loaded area shall be equal to or less than 0.85φf’c √( A1/A2), but not greater than 1.7 φf’c where

A1 = area of the lower base of the largest frustum of a pyramid, cone, or tapered wedge contained wholly within the footing and having for its upper base, the area actually loaded, and having side slopes of 1 vertical to 2 horizontal, and

A2 = loaded area at the column base.

Where the stress on the loaded area exceeds the permissible bearing stress reinforcement shall be provided for the excess by extending the longitudinal bars into the footing or by dowels.

Where transfer of force is accomplished by reinforcement, the development length of the reinforcement shall be sufficient to transfer the compression or tension to the supporting member.

Extended longitudinal bars or dowels shall have a total area of at least 0.005 times the loaded area of the column or pedestal, and a minimum of four bars shall be provided. Where dowels are used, their diameter shall not exceed the diameter of the column bars by more than 3 mm.

Column bars of diameters larger than 40 mm φ in compression only may be lap spliced with dowels not larger than 35 mm φ of the necessary area. The dowel shall extend into the column, a distance equal to the development length of the column bar and into the footing a distance equal to the development length of the dowel.