Guide for Performance Evaluation of Slab-on-Ground
Foundations1
1.0—Introduction
This Document:
- 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.
The concepts
found in this document may be applicable for evaluating other construction
techniques as deemed appropriate by licensed design professional.
Slab-on-ground
foundations serve two primary functions:
- To provide a floor surface for the building; and
- To transmit the loads from the structure to the soil.
The design of
slab-on-ground foundations is governed by IBC and is designed by PTI
DC10.5-12. Slab-on-ground 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 Code 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.
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.
2.0--Definitions
Distress is a general term used to describe cracks,
separations, misalignments, ghosting doors, sticking doors, sticking windows,
open joints, wavy or bulging materials, and so on. Distress should fit within one or more of the
following three categories:
- Cosmetic Distress is defined as distress that is often noticeable by the building’s occupants but does not affect the function of the building or affect the ability of the foundation or structure to transmit loads.Examples of cosmetic distress that may be caused by foundation movement include but are not limited to minor separations and cracks in the walls, floors, ceilings, and brick veneer; open joints in interior casing and baseboards; or uneven reveals around doors or windows.It must be noted that cosmetic distress can be caused by other factors such as material shrinkage and/or differential thermal movement, which are unrelated to foundation movement.
- Functional Distress is defined as distress that affects the use of the building.Examples of functional distress that may be caused by foundation movement include but are not limited to doors or windows that stick, or will not close or open; doors that close or open on their own (that is ghosting doors); open exterior caulk joints; noticeable floor slopes; vertical offsets on walking surfaces sufficient to cause tripping; slope of intended level surfaces such as countertops, tubs, showers; reduced, or reversed slope of roof gutters, plumbing drains, or as-built sloped areas such as garage, patio, and porch floors.It must be noted that functional distress can be caused by other factors unrelated to foundation movement.
- Structural Distress is defined at distress that affects the ability of the foundation or structure to transmit loads.Examples of structural distress that may be caused by foundation movement include but are not limited to separations, cracks, or distortions to structural support members such as studs, columns, beams, joists, or foundation elements such that the members can no longer safely carry their design load based on strength and/or stiffness.Tilted masonry veneer could be considered as structural distress, even though it is only carrying its self-weight, provided that it can be justified by structural calculations and associated physical distress is present.
Elevation of a data point is the measured vertical height. Elevations are typically measured and
recorded to the nearest 0.1 or 1/8 in. (3mm).
Foundation
Movement is a general term used to describe the cumulative movement
of a slab-on-ground foundation. Foundation movement cannot easily be determined from any one singular
floor elevation survey. Foundation
movement can only be estimated from a Time Change Elevation Plan.
Licensed
Design Professional (LDP) is an engineer or architect who is
licensed to practice as defined by the applicable statutory requirements of the
professional licensing laws of a state or jurisdiction.
3.0--Brief Overview of Foundation Movement
The following
is only a brief overview of causes and mitigation of foundation movement, as
this document is not intended to fully describe the causes and mitigation of
foundation movement.
3.1 Causes of Foundation Movement
Nearly all
foundation movements occur due to movement of the underlying soils. There are many types of soil movement. Several of the more common types of soil
movements include settlement, soil shrinkage, subsidence, and soil swelling.
3.2 Mitigation of Foundation Movement
The first step
to mitigate foundation movement is to design and construct the foundation based
on adequate geotechnical information using recommended site preparation
procedures. The geotechnical design
should incorporate appropriate soil and climatic data. The structural design
should use the provided geotechnical parameters, satisfy Building Code
requirements, and incorporate local industry standards of construction.
Mitigating
foundation movement after construction is completed can often be accomplished
by one or more methods. Examples include
but are not limited to maintaining and/or improving proper grading and drainage
around the foundation, adding roof gutters with downspout extensions, adding
underground yard drains, implementing a foundation watering program during
extended periods of drought and maintaining, and/or removing landscape
plantings near the foundation. For
significant foundation movement, mitigation techniques may include structural
repairs (pier and piles), moisture barriers, and root barriers.
3.3 Evaluation of Foundation Movement
Aspects of
foundation movement that should be considered in the method presented herein
include, but are not limited to:
- All foundations on expansive soils are expected to move as a result of soil volume change (soil swelling and shrinkage).This expected movement may be sufficient to cause some cosmetic or functional distress.The design procedures for slab-on-ground foundations assume some movement will occur.The design procedures will result in foundations with sufficient strength and stiffness to allow some cosmetic distress, limit functional distress, and prevent structural distress.
- Foundations are seldom constructed perfectly flat and level.
- When there is a difference in floor elevations from a single survey and there is no associated cosmetic distress, then the difference in elevations is most likely due to original construction tolerances and/or concrete shrinkage edge curl rather than foundation movement.
- When foundation movement criteria contained herein for curvature and tile are exceeded, there should also be noteworthy associated distress that can be correlated to the out-of-tolerance measurements
- It is not unusual to have minor sheetrock cracking, brick cracking, and/or other material separations in a building supported by a slab-on-ground foundation.These cosmetic distresses area often caused by factors other than foundation movement.
- Foundation movement can be caused by soil volume changes resulting from shrinkage and swelling of expansive soils, settlement, improper subgrade preparation, improper soil fill retention, abnormal soil conditions, and abnormal weather occurrences.Foundation movement can also occur due to non-soil-related causes.These include temperature and shrinkage stresses and inadequate quality of construction or design.
- There is an initial movement or “settling in” period of the foundation that occurs after occupancy.Most slab-on-ground foundations go through a 12-to 24-month adjustment period as yards, landscaping, and watering programs are established.
- There are seasonal soil movements in certain climates, which are due to seasonal weather changes that affect soil moisture content.
- Foundation movement can be caused by unanticipated issues that are NOT included in the initial design.Examples include but are not limited to improper introduction of landscaping and trees, removal of established trees and other vegetation, plumbing leaks, over or under irrigation, inadequate site drainage, improper roof gutter systems, faulty pool or pool deck installation near foundation excavations, and improper long-term homeowner maintenance.These issues need to be addressed by the contractor and by the building owner through construction and maintenance procedures.
- Localized cosmetic and functional distress can occur even though the curvature and tilt criteria listed herein are satisfied.This type of distress can be caused by localized areas of improper subgrade preparation, improper soil fill retention, poor quality of construction, poor quality of construction materials, improper introduction of landscaping and trees, removal of established landscaping and trees, inadequate site drainage, improper roof drainage systems, plumbing line leaks near foundation excavations, improper long-term homeowner maintenance and/or other obvious deficient factors.
- Localized slope is sometimes incorrectly used as a measurement to quantify localized anomalies.Localized slope is NOT included as a foundation movement criteria in this document because it can be a combination of curvature and tilt.Foundation movement is estimated in this document using the two separate and distinct criteria of curvature and tilt.
4.0—Data Acquisition
4.3 Visual Observations
…It should be noted that not all distress
is the result of foundation movement; other causes of distress include but are
not limited to differential thermal movements, normal and expected concrete
shrinkage, displacement of the wood framing due to external loading or moisture
reduction, inadequate frame design, improper attachment of interior partitions
to roof trusses, improper fastening of gypsum board ceilings, poor quality of
construction, building material shrinkage, and poor-quality building materials.
It is
recommended that visual observations be documented for future reference and
comparison. Distress fits within one or
more of the following three categories: Cosmetic Distress, Functional Distress, and Structural Distress.
- Guide for Performance Evaluation of Slab-on-Ground Foundations— 2018 --PTI DC10.8-18—Post-Tensioning Institute