Modifying Existing Foundations to Carry Additional Load

In many situations designers are faced with the problem of a building addition that requires new columns near, or directly above, existing foundations. Several design alternatives are usually considered, and frequently the desired objective is to minimize the impact on the existing foundations. The impact can be minimized by altering the substructure support system or by altering the design of the new building. Alternatives such as cantilevered grade beam systems supported by caissons, or the utilization of cantilevered beams in the superstructure, allow designers to setback the new column line from the existing foundations. The setback minimizes the effects on the existing foundations. The alternatives are usually explored fairly quickly and are precipitated by the desire to avoid a complicated and, perhaps, expensive analysis/design effort that may involve underpinning of the existing foundation. In some cases, alternatives are limited, and utilization of existing foundations, with appropriate modifications, is the only reasonable approach. Modifications of the existing footing to ensure that the allowable soil bearing capacity is not exceeded and that proper load transfer occurs between the existing foundation and additions thereto are critical to this approach. The analysis effort is heavily dependent on the utilization of current computer technology to quickly ascertain the load effects on existing foundations due to the new building columns. Engineering analysis software can be used to quickly determine the modifications that are required to accommodate the new column loads. During a recent design/analysis effort, current engineering analysis software was employed to ascertain the effects of new column loads on existing foundations. The analysis led to design modifications that allowed the new columns to be placed in close proximity to the existing building columns and placed directly onto existing foundations. The design/analysis process consisted of many important steps that ranged from establishing confidence in the computer model to final design modifications based on stress contour plots and basic strength of materials principles. A description of the analysis process and overall conclusions are presented herein.