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Acceptable Foundation Deflections

With respect to slopes (foundation deflections) in slab on grade foundations, individuals who ask “How much is too much?”. This will receive a variety of answers depending on who they ask. Within the engineering community, the answer will vary depending on each engineer’s opinions as to the appropriate design assumptions, design procedure, applicable building codes, legal precedents, and market standards. For other types of individuals, such as foundation repair contractors, insurance companies, builders, Realtors, property sellers, property buyers, and lawyers the answer will be affected by their personal experiences and vested interest in a particular situation. 

Slab on grade foundations are built of concrete reinforced with steel. Over time, design criteria have changed to incorporate the experiences of the engineering and building communities. In general, older slabs tend to be thinner and have less steel reinforcing than newer slabs. As a result, expectations for the performance of an existing slab should take into account the age, and hence the then current design criteria, of the slab.

Foundation slabs are not completely rigid or perfectly stationary. As the underlying soils move, slabs will move up and down and may tilt (uniform deflection) or bend (differential deflection). Soils movements are caused by: 1] the shrinking and swelling of expansive soils as they dry and get wet; 2] by the settlement of fill dirt that has not been fully compacted; 3] and the heaving of soils where the land has been cut. As underlying soils move, any slab that they support will shift. Because slabs, and the structures that rest on them, are more rigid than the soils on which they rest, the manner in which slabs and soils interact is complex.

As slabs move, the structures that they support also move. How, and of what materials, a house was built determines to a significant extent how much foundation movement is acceptable. The more rigid and brittle the materials used to build a house are, the more rapidly damage will appear. Hard plaster walls are more brittle than sheet-rock, brick veneer is more brittle than wood siding, ceramic floor tiles are more rigid than vinyl tiles. The age of a home is also important. Obviously, movement is less acceptable in newer homes than in older homes. People often pay a premium for older historic homes fully expecting floors to be out of level and doors and windows to be out of square.

Various bodies have had a hand in determining standards for acceptable foundation movement. Some of the more important ones are:

  1. The American Society of Civil Engineers
  2. The American Concrete Institute (ACI)
  3. The Building Research Advisory Board (BRAB)
  4. The Department of Housing and Urban Development and the Federal Housing Administration (HUD/FHA)
  5. The Post Tensioning Institute (PTI)
  6. The various bodies that prepare and distribute the Uniform building Code (UBC)
  7. The courts
  8. The market place

Most of these standards have a qualitative requirement similar to, or based on, the ACI Building Code Requirements, Section 9.5 Control of Deflections which states “Reinforced concrete members subject to flexure shall be designed to have adequate stiffness to limit deflections or any deformations that may adversely affect strength or serviceability of a structure at service loads.” In plain English, what this means is that a foundation should be strong enough so that it will not break or bend beyond the point where the structure begins to suffer unacceptable damage. What is acceptable is subjective and will vary according to particular circumstances. The role of the market and the courts has been to determine what constitutes acceptable deflection or movement as well as to determine damages. The market makes its determination by adjusting the prices of, and demand for, homes with foundation damage.

The following table summarizes the existing standards for the previously referenced bodies.

Acceptable Foundation Deflections


   Superstructure                    Governing Body
   ______________   ________________________________________________
                                     HUD/
                    ACI     BRAB     FHA     PTI     UBC     Courts
                    (2)     (3)      (4)     (5)     (6)
   Wood             1/240   1/200    1/240   1/240   1/240   Various
   Masonry          1/240   1/300    1/240   1/240   1/240   Various

    Notes:
  1. The ratios shown in the table represent the maximum acceptable vertical difference between two points, divided by the horizontal distance between the two points. The two points must be on the same structure. Given a ration of 1/240, for two points that are 20 feet apart (240 inches) the maximum acceptable vertical difference would be 1 inch.
  2. ACI 318-89 (Revised 1992), table 9.5(b).
  3. Criteria for Selection and Design of Residential Slabs-on-Grade, Report No. 33, to the FHA, page 50. Publication 157 , National Academy of Sciences, 1968
  4. HUD/FHA 4900.1 Minimum Property Standards, 1982 Edition, references ACI-318. Now abandoned. No replacement issued.
  5. PTI Design of Post-Tensioned Slabs-on-Ground, references ACI-318
  6. Uniform building Code, 1988, Section 2307, Table 23-D

In 2009, the American Society of Civil Engineers, Texas Section, issued version 2 of “Guidelines for the Evaluation and Repair of Residential Foundations.” The guidelines include 2 quantitative standards: 1/360 for deflection, and 1% for tilt. The ASCE defines the deflection ratio as the maximum distance from a straight line, drawn between 2 points on a foundation and the surface of a foundation. When applied, the standard, taken in isolation, can mean that slopes in excess of 1 inch over a 15 foot span are acceptable. A 1% tilt works out to a slope of 1 inch every 8 feet, 4 inches. The ASCE standards allow the engineer to exercise discretion when applying the standards. Engineers are also expected to consider a variety of factors, such as the extent of cosmetic damage when applying the standards, making recommendations, and reaching conclusions.

Given the existing published standards, and the ability of most individuals to feel a deflection or slope in a foundation, it is our opinion that, in general, the most appropriate standard for maximum acceptable deflections is 1/360. The age of a home, the price range, and the specific manner of construction also play a role in determining how much deflection is acceptable. In situations where seasonal movement is causing unacceptable cosmetic damage, in a new home, or in a custom designed home, lesser amounts of movement may be unacceptable.

Unfortunately, there is no hard and fast standard for foundation deflections that can be used. Individuals and professionals who deal with foundation movements must use their judgment. Prevailing standards, including what typical buyers and sellers find acceptable, are also relevant issues. For example, purchasers of historic homes tend to be more tolerant of movement than purchasers of new homes.

 

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