Constructing the various elements of a project within the specified tolerances is only one part of a contractor's challenge. The larger, and often more costly, problem is resolving tolerance incompatibilities.
The ACI Committee 117 on Tolerances, under the leadership of committee chair Eldon Tipping, is actively working to address both of these issues. Colin Milberg's work toward establishing standard measuring protocols was described in the July and August installments of this column.
Data collection has begun in earnest and includes not only Milberg's own efforts but also measurements being taken by other committee members on projects around the country. The preliminary results of that effort will be published in a report once the data have been analyzed.
Tolerance incompatibilities are being addressed in a second report with the working title “Guide for Tolerance Compatibility in Concrete Construction.” To coordinate this project, Tipping enlisted the help of David Ballast of Architectural Consulting, Denver, an architect with a long history of working with construction tolerances, and the author of “Handbook of Construction Tolerances.” First published in 1994 by McGraw Hill, the book's second edition was published in March 2007 by John Wiley & Sons.
Ballast says the ACI report is organized into three sections. The first is a list of current industry standard tolerances—where they exist—for the various items involved in concrete construction. It also covers building elements that typically have a concrete interface, such as window frames.
A second section will give suggestions and recommendations on how to accommodate existing tolerances. To expand this section, Ballast actively is seeking anecdotal information from people dealing with tolerance incompatibility issues in the field.
“We welcome input from contractors as to how they solve the problem of concrete and other material tolerances on a daily basis,” Ballast says. He is specifically looking for on-the-job situations—and the solutions that have been applied—in 10 areas: embedded items; elevator cores and hoistways; doors and other openings in slabs and walls; Lenton couplers and NMB splice sleeves; stairs; cladding systems; infill wall systems; accessibility for ramps and stairs; thin flooring; and expansion joints.
The final section of this report will be written by Milberg and provide a commentary on more general approaches to identifying and mitigating tolerance incompatibilities based on academic and statistical approaches.
One other area in which the committee is working includes making adjustments to the ACI 117 tolerance standard, which was updated in 2006, to bring it into line with ACI 301, “Specifications for Structural Concrete.” Tipping says this process is well under way and should have the added benefit of getting the two documents on the same publishing cycle.
If you're planning to be in Puerto Rico for the ACI meeting in mid-October and have an interest in construction tolerances, consider attending the 117 meeting scheduled for 8:30 a.m, Tuesday, Oct. 16.
Concrete Construction readers who would like to share their experiences and how tolerance issues have been resolved should e-mail Ballast directly at firstname.lastname@example.org.