In the current ACI 117 Tolerances commentary, tolerances are defined as “a way to establish permissible variations in dimension and location, giving both the designer and the contractor parameters within which the work is to be performed.” Furthermore, “necessity rather than desirability should be the basis of selecting tolerances.”
The committee recognizes that no structure is exactly level, plumb, straight, or true and that this level of perfection isn't necessary. The question is how to recognize the above and be fair to all parties concerned in construction of a project.
When Eldon Tipping, Structural Services, Richardson, Texas, agreed to become chairman of the ACI 117—Tolerances committee in 2005, he initiated a recruiting effort to enlarge the membership and to bring significant contractor participation to the committee. That effort was successful, and the new group is taking a fresh look at the 117 document with the goal to develop a new standard that would contain more realistic, measurable, and objective tolerance requirements for contractors, designers, specifiers, and owners.
The primary work of the committee, for the next few years, will be to develop and to publish two documents. The first initiative is to develop a standard measuring protocol for evaluation of structures for conformance to tolerances; the second initiative is to address the issue of tolerance compatibility between concrete and materials with which it interfaces.
Building forms to gather information
At present there are few protocols for evaluation of completed structures for conformance to tolerances contained in ACI 117. The development of a standard protocol is an iterative process that involves first development of a procedure the committee thinks will be successful, and then field-testing of that procedure to confirm the desired information results.
The first task undertaken by the committee has been to develop forms for use by the contractor members of the committee to use for collecting and reporting tolerance data on their projects. By field-testing and refining the forms to be used for data collection, it becomes possible to relate the data from one project to another, and to ultimately develop meaningful guidelines.
Tipping says that several contractor members of the 117 committee have volunteered funds and manpower to help with the data collection process. The goal is to collect large amounts of information and measurements of tolerance to validate the measurement protocol and to gain a better picture of what is installed in the field. This effort will provide information about what both works and doesn't work in the real world.
Building a database
Concurrently with the data-collection effort, the committee will be developing a database to organize and to hold the information. Data will be evaluated statistically in order to identify the parameters for acceptable tolerances. If, from a statistical standpoint, the data for certain measurements are distributed normally, then the resulting tolerance might be some multiple of the standard deviation for the data.
The final task for the ACI 117 committee will be to publish guidelines for a large number of tolerance issues. The hope of the committee is that the guidelines that will be developed as a result of this process result in better construction practices and a better understanding of the capabilities of the construction industry. The committee hopes to have a draft of the new document ready by the fall of 2008.
In coming issues of Concrete Construction, this column will report on individual tolerance issues as they are considered by the committee.