Shag Pierce gave me my first real-world lesson in construction tolerances. In the summer of 1983, I had just graduated from engineering school. I was working as a surveyor's assistant for a large contractor on a concrete bridge restoration project near Pittsburgh, and Pierce was the field engineer.
We used engineer's folding rules and steel tapes marked in hundredths of a foot, but at the same time would use the terms eighth (as in, eighths of an inch) and hundredth interchangeably. Falling in line with protocol, I only occasionally wondered where the extra 4/8, or missing 4/100, ended up.
Pierce took the responsibility of mentoring his young assistants seriously. About a month after I joined the crew, we were setting elevations for the stay-in-place pans for the deck paving and he gave me a turn at being the instrument man. Throughout the afternoon, I took the readings as he moved the rod from one station to the next. At each, I called out “plus an eighth,” or “plus a quarter,” or “zero,” as Pierce marked it so the pan man would know how much to shim. Back then it was all done optically, using a standard surveyor's transit. I was surprised by how thick the markings on the rod appeared in the transit's sight. It hadn't been that ambiguous in surveying class. Which one should I call it, I wondered, the one above the crosshairs or the one below?
Finally we finished for the day, packed our gear, and began the long walk off the bridge. After a couple of minutes, Pierce said to me, “Now, when you're sitting in an office some day, just remember how hard it is to get within an eighth of an inch.”
This inherent imprecision in the construction industry can lead to big problems where concrete and other building elements meet. Such is the reason the American Society of Concrete Contractors (ASCC) and eight other cosponsoring groups in February 2005 convened the Inter-Industry Working Group on Reducing the Cost of Tolerance Compatibility Problems. In that two-day gathering, presentations and roundtable discussions made it clear that “even when all components are within their industry-specified tolerances, incompatibility of the tolerances for interfacing materials can result in components that don't fit.”
That statement of the underlying problem comes from the Summary Report for the meeting of an Inter-Industry Working group on reducing the Cost of Tolerance Compatibility Problems. The full report makes for some interesting reading and is available on the ASCC Web site, www.ascconline.org, by clicking “Resources” then “Hot Topics.”
The assembled group–of nearly three dozen—included experts on manufacturing and installation of steel framework, curtain wall systems, floor coverings, and, of course, concrete construction. Numerous specific tolerance-related problems were discussed, including placement of reinforcement, tendons, anchor bolts, and weld plates; floor flatness; and elevators.
As an example, the National Elevator Industry's tolerance for vertical alignment of elevator shaft walls is a maximum of 2 inches. However, that is at odds with ACI 117-90, “Standard Specifications for Tolerances for Concrete Construction and Materials,” which permits a 6-inch maximum tolerance.
The group's conclusions were general in nature. Tolerances need to be discussed and coordinated. They also should be realistic and measurable and the methods for doing that should be defined upfront. And several different interface details may be required for use between a given component and the various structural systems.
The report also emphasizes that design details are critical, noting that mistakes in the field generally can be avoided by careful field engineering and inspection before placement of concrete and protection of installed fittings after placement.
In summary, the report acknowledges that even when tolerances are met “some rework will still be required before the project is satisfactorily completed. To avoid excessive rework, tolerances in each specification division need to be harmonized.”
Tolerances are critical and must be addressed. But like many other things, this issue boils down to communication. After all, Pierce said I should remember how hard it was to get within an eighth of an inch; he never said it was impossible.