Structural strength considers the capability of a foundation system to support its intended dead and live loads. Structural strength is concerned with stability, component strength, and material soundness. Deflection causing loss of section, loss of shear capacity, and excessive titling indicate potential structural element failure. When the data and engineering analysis clearly indicate this foundation has sustained a non-structural performance (serviceability) failure by deflecting greater than analogous deflection limit of L/360, non-structural remedial measures are appropriate. These measures are governed according to section 7, paragraph 7.3 of the ASCE Guidelines. These guidelines require the engineer to:
- Match the remedy to the type of foundation failure (performance); and
- Assure the proposed repair remediates the underlying cause of foundation failure (non- uniform bearing clay moisture content changes), and assure the repair is economically and technical appropriate for the type foundation failure.
Section 5.3 of the ASCE guidelines defines performance as “…the capability of the building to serve its intended purpose. Elements of concern are safety, function, durability, and habitability.” When formulating foundation repair methods due to inadequate performance, Section 5.8 of the ASCE Guidelines requires an engineer to consider if the repair :
- is cost effective,
- is practical,
- will perform satisfactorily over time,
- matches the client’s needs, and
- is “commensurate with the nature and cause of the [foundation performance] inadequacy, and the seriousness of its consequences”.
For residential foundation performance failures, applying Non-Structural remedial measures and monitoring foundation performance in lieu of structural repairs complies with the requirements of Section 5.8 of the ASCE Guidelines as follows:
- Non-Structural Remedial Measures eliminate the cause & origin of the foundation performance failure (inadequate ground surface water drainage and non-uniform bearing soil moisture content along the foundation perimeter);
- Non-Structural Remedial Measures will help reduce foundation system rotation and vertical displacement resulting from soil shrink/swell cycles by establishing a more uniform foundation bearing soil moisture content distribution around its perimeter;
- Non-Structural Remedial Measures are practical, can be installed while the homeowner occupies the residence, and typically do not require the homeowner to vacate the premises for extended time periods;
- Non-Structural Remedial Measures can achieve their intended purpose over time when installed and maintained according to design specifications.
For example, post-tension foundation performance can be impacted by post-construction activities unrelated to their core design criteria. If rainfall is allowed to pond or collect adjacent to a structure built on expansive soil, the structure may be subjected to unscheduled distress caused by swelling bearing soils due to increased soil moisture content. Lot surfaces must be graded to drain away from the structure in accord with the International Residential Code R401.3. To non-structurally repair a residential foundation system according to section 7.3 of the ASCE Guidelines, the following summarizes a potential set of remedial measures that can be recommended:
- Roof Rain Gutter System (ASCE, section 7.34).
- Drainage Improvements (ASCE, section 7.35). Surface GradingErosion ControlOption A-Surface Water Drainage System
- Option B-Subsurface Water Drainage System
Other types and combinations of repair methods may apply and are contained within the standard.
Note: The currently accepted residential foundation evaluation protocol was published in 2002 by the Texas Section of The American Society of Civil Engineers (ASCE). This protocol, titled “Guidelines for the Evaluation and Repair of Residential Foundations – Version 1” (The Guidelines), establishes a uniform criteria for evaluating the performance and repair requirements of residential foundation systems. The guideline initially seeks to determine a foundation’s structural strength and performance status. Should a foundation system have either a strength or performance failure, this evaluation method provides a set of rational criteria to prescribe an appropriate, yet distinct, repair for each type of failure.