Guide for Performance Evaluation of Slab-on-Ground Foundations
|Publication Date:||1 January 2018|
• Provides guidelines to aid in the evaluation of the performance of slab-on-ground foundations;
• Provides guidelines to aid in the evaluation of the performance of residential and other similarly constructed low-rise buildings with slab-on-ground foundations;
• Applies to post-tensioned and non-post-tensioned slab-on-ground foundations; and
• Applies to all soil conditions.
This document is not intended to fully describe the causes or mitigation of foundation movement; however, a brief overview of the causes and mitigation of foundation movement is provided in Chapter 3.0.
The concepts found in this document may be applicable for evaluating other construction techniques as deemed appropriate by the licensed design professional (LDP). The concepts found in this document may be supplemented by additional evaluation techniques and procedures.
Slab-on-ground foundations serve two primary functions:
1. To provide a floor surface for the building; and
2. To transmit the loads from the structure to the soil.
The design of slab-on-ground foundations is governed by IBC1 and is designed by PTI DC10.5-12.2 Slab-onground foundations are not designed to control soil movement. Rather, they respond to soil movement. They are not infinitely stiff or immovable; therefore, they will experience out-of-plane curvature (also known as deflection or bending) and planar tilt. The opportunity for soil movement and subsequent foundation movement begins the day the foundation is placed, and continues throughout the life of the building.
The 2015 International Building Code1 gives guidance on foundation performance in Sec.1808.6.1, which states that a foundation "shall be designed to resist differential volume changes and to prevent structural damage to the supported structure. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure." Based on the aforementioned Code excerpt, cosmetic distress is allowed, while functional distress should be limited and structural distress should be prevented.
Visual observation of distress is one method for evaluating foundation performance as outlined in Chapter 4.0. It is based upon the fact that foundation movement can cause visible distress in the foundation, structure, and/ or finishes of the building. A floor elevation survey is another method for evaluating foundation performance, as outlined in Chapter 5.0. These floor elevations and surveys are defined as the gathering of data points completed under the responsible charge of an LDP, and are not required to be stamped by a licensed surveyor. This data can be used to estimate the foundation movement as outlined in Chapter 6.0.
The determination of whether a foundation performance is "acceptable", "unacceptable", or "marginal" should depend on the associated levels of cosmetic distress, functional distress, structural distress and the estimated foundation movement described by curvature and tilt as outlined in Chapter 7.0.