Colin Milberg, Ph.D., has a sophisticated, new piece of equipment—a 3-D scanner—and he's starting to gather as-built data for some research regarding tolerances. He's convinced there's a more rational way to come up with better tolerance values that will not only suit designers' needs but also be constructable. By looking at what is being achieved in the field today, he believes “process capabilities” can be developed from which appropriate and achievable tolerances can be set.
Milberg, a member of the ACI Committee 117 on tolerances, says previous tolerances in concrete work have been arrived at by consensus. Basically, designers have said they need a certain degree of accuracy, and contractors have said yes, we can meet that requirement. The problem is that these numbers arrived at by mutual agreement have not necessarily reflected the work that is being done in the field.
That's where the new scanner comes into play. Milberg, who is also an assistant professor in the construction engineering and management program at San Diego State University, along with ACI Committee 117 chair, Eldon Tipping, recently received a grant from the Charles Pankow Foundation to gather field data on concrete construction in progress and compare that to the design. The foundation's grants are given specifically to foster the adoption of innovative engineering and construction technology. This project will demonstrate how effective by a 3-D laser scanner can collect data.
“With this equipment we can gather a breadth of data that can tell us what we're achieving in the field,” says Milberg. For more information on the technology itself, read the 3-D Laser Scanning article.
Milberg will be using a total station to collect a second set of data on some of the same projects. Because the scanner collects far more data than a total station, this dual collection effort will enable him to develop correlation guidelines between the methods. Using that approach, data from other projects collected with a total station can be processed and included in the overall database of as-built information.
The first phase of his research, which he expects to continue through 2007, consists of gathering data on vertical elements—walls, columns, shear walls, and elevator shafts—from several southern California high-rise projects. Subsequent surveys will measure horizontal elements, such as slabs.
“This approach also completely changes the game for flatness measurements,” Milberg says. “We're redefining what we can measure.”
Although his primary focus is on the numbers, Milberg also is collecting data on the methods being used. He hopes that additional information may lead to further insights into which techniques are best suited to concrete work requiring tight tolerances.
This research is one part of Milberg's larger research focus on tolerance management in civil systems, which started with his doctoral work at the University of California Berkeley. His “Tolerance Management Theory” tackles a variety of issues, such as how building systems should be designed and whether tolerances should be tightened. He is using the theory to look into the costs associated with compatibility issues, both in the context of what now is being done in the field and the increased—or possibly decreased—costs that might result from adjusting expectations on both sides of these issues.
“The current opportunity to advance our tolerance practices is great,” says Milberg, “because 3-D scanning technology enables the rapid and accurate collection of three types of data—location, orientation, and form—with a single instrument. And that means we can obtain large amounts of data far more economically than ever before.”
Later this year, both data and emerging results are expected to be made available on a project Web site.