Shallow bearing structural foundation systems function to safely separate habitable areas from the exterior environment and limit damage or distress to interior/exterior brittle building materials. By design, post tension slab on grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting. The structural integrity (strength) of a slab on grade foundation system is determined by its ability to bear allowable design loads, while slab on grade foundation performance (serviceability) measures how closely the foundation system achieves its intended purpose.
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.
When engineering analysis shows the profile of a slab on grade foundation system has a post-construction deflection greater than 1”/360”, it has failed if it can be reasonably linked to more than one “symptom” of deflection present. Typical symptoms would be some type of non-load bearing brittle material fracture such as sheetrock or floor tile cracking. However, to qualify as a performance failure, the foundation may not deflect after construction in a tilting mode in excess of one percent from the original construction elevations. Additionally, no evidence of loss of section or partial collapse can be present, and the foundation system must support its design loads.
Section 5.3 of the ASCE guidelines defines residential foundation performance as “…the capability of the building to serve its intended purpose. Elements of concern are safety, function, durability, and habitability.” When collected data shows no safety or habitability variances and instead indicates the foundation has sustained a deflection greater than 1”/360” together with more than one associated distress symptom, it has a performance (serviceability) failure. These non-structural symptoms (brittle material cracking and separations) technically impair “the capability of the foundation (building) to serve its intended purpose.” As such, Section 7 of the ASCE Guidelines requires repairing this post tension slab on grade foundation system in accord with its section 7.3 “Non-Structural Remedial Measures” of the ASCE Guidelines.