News Detail

Tolerance in a Complex World

8/1/2006

Since its creation in 1904, one mission of ACI has been to standardize tolerance criteria throughout the concrete industry. In 1962, responding to its own growth and increasing concern that tolerances were being spread throughout numerous committee documents, ACI created Committee 117, Tolerances, to collect existing tolerances into one document. Nearly half a century later, we are struggling to reach a consensus on the collection and application of tolerance acceptance criteria. Why does this topic create such passion and conflict?

Tolerance disagreements often lead to finger-pointing and disputes among owners, fabricators, designers, and builders alike. Accurately capturing the essence of tolerances in a three-dimensional world is daunting. Historically, ACI 117 has been a collection of specific construction tolerance numbers, not a discussion of how, where, and when these tolerances should be applied, or what to do when an item falls beyond the allowed tolerance.

When presented with a collection of itemized tolerances, evaluation of real three-dimensional conflicts on a project can become confusing. Seemingly innocuous questions such as "Are the individual tolerances cumulative?" and "Does evaluation of a member subject to multiple tolerances result in a rational tolerance envelope?" are likely to result in heated debates.

Changes in the industry with respect to procurement and delivery options, litigation, and the insurance market have further increased the demand for a reliable means of defining, measuring, and ultimately enforcing practical and achievable tolerances.

ACI 117-06 will be released later this year as a first step toward simplifying the definition and application of tolerances. It is, however, only the first step. Over the next 5 years, ACI 117 will undergo a paradigm shift away from collecting specific tolerance numbers to a statistical approach to tolerance acceptance criteria. This manner of thinking starts by addressing the basic questions of how, what, and when:

• How do we establish project control and benchmarks?
• What means are used to collect and record data?
• When are these measurements to be recorded?

This transformation of ACI 117 has commenced and is now well underway. In the past year, the committee has grown from a handful of participants to more than 40 active members. The committee is currently developing a measurement protocol document to clearly define how and when measurements are to be taken. Understanding and accounting for the accuracy of measurements is a key component to evaluation of tolerance acceptance criteria and has historically been overlooked. The committee is also collecting field data to validate the measurement protocol and to allow comparison with tolerances used in the current ACI 117 specification.

To ensure the accuracy and credibility of the protocol and field data, ACI Committee 117 has forged strategic partnerships with the American Society of Concrete Contractors, the American Institute of Steel Construction, and the Precast/Prestressed Concrete Institute, as well as representatives from critical specialty trades such as masonry, aluminum glass and glazing, vertical transportation, and others. I'm encouraged by this living, breathing example of one of my key objectives for ACI-that is, to expand our working relationships with other technical organizations.

Once the measurement protocol and baseline field data have been developed and validated, ACI Committee 117 will move forward with the creation of a tolerance evaluation rationale founded upon statistical analysis rather than hard limits. Statistical rationale for tolerance evaluation is currently used in Europe and Canada and is endorsed by the International Organization for Standardization (ISO) but has not been widely adopted within the U.S.

Within the next 5 years, ACI Committee 117 seeks to create a family of complementary documents that promote awareness of potential tolerance issues for concrete construction and the interface of concrete construction with critical building elements.

To learn more about ACI 117-06 and the future of tolerance evaluation and acceptance criteria within ACI, mark your calendar for the ACI Fall Convention in Denver, CO, November 5-9, 2006. ACI Committee 117 will host a technical session on "The New 117-Where We Are and Where We're Going" to highlight the changes in the recently adopted ACI 117 document and provide insight concerning the goals for future editions of the document.

I am confident that this shift in how we deal with the complex world of tolerances will, over time, have a positive impact on our Institute and our industry.

Thomas D. Verti, President
American Concrete Institute
tverti@pankow.com

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