It may have been just a happy coincidence, but my guess is that somebody set it up on purpose. Either way, having the regularly scheduled meeting of the ACI Tolerances committee in a 19th floor room facing an adjacent construction site was the perfect setup.
Meeting in April at the ACI Convention in Los Angeles, the committee heard a report from Ward Malisch on the American Society of Concrete Contractors' tolerance manual now being assembled. This research and documentation effort, the attendees learned, is likely to dovetail nicely with the three ongoing ACI 117 projects.
David Ballast reported that the committee's Tolerance Compatibility Document, begun 1½ years ago, soon will be going to ballot. Colin Milberg summarized the measurement protocol development project as being well under way. Data collection continues, and an online data collection sheet now is available for committee members' use. Analysis of the data collected so far also has begun, which should provide some interesting information at the next committee meeting, in St. Louis.
Following those reports, the committee dug into some real work on the third project, revision of the 117 tolerances standard. Although the group discussed many of the more than 300 comments resulting from the review by ACI's Technical Activities Committee (TAC), committee members left with plenty to do in the coming weeks as the document is balloted once more.
After looking at so many words on paper, it was refreshing to close the meeting with a demonstration of the 3-D laser scanner that Milberg and his students from San Diego State University are using to collect as-built data. In a matter of a few minutes, scanning experts Quinn Booker and Daniel Shirkey set up the scanner on the balcony outside the meeting room facing the conveniently located high-rise project next door. The speed of setup was only the first of several amazements.
After positioning on the tripod, the device leveled itself far more rapidly than I was ever able to level a transit. Booker and Shirkey then made software preparations on the attached laptop, which included defining the portion of the image to be scanned. A click of the mouse and the scan began.
Referring to the building image that was quickly appearing on the screen inside, Milberg first explained the mechanics of the data collection process. One of the things that interested those in attendance was how easily the data integration is achieved from multiple instrument setups, thanks to the use of small white balls that are placed as targets before the scanning begins. Milberg also demonstrated several of the ways the software can render the data. (For a further explanation, search for “Extracurricular Learning” at www.concreteconstruction.net.)
Another intriguing demonstration followed later in the afternoon. After a panel discussion on tolerances and compatibility in one of the Hyatt ballrooms, the scanning team set up two scanners. Booker and Skirkey set the Trimble scanner, which uses a time-of-flight approach, to capture a portion of the ceiling and two walls, and the green laser began zipping back and forth across the back wall. About three minutes later, the data had been collected and the color image on the screen was complete.
Next the team started the Faro scanner, which is phase based, and within about a minute we were viewing a gray-scale image of the full ballroom on the screen. Phase-based scanning collects an equivalent amount of data much more quickly than time-of-flight scanning. The trade-off comes in range capabilities—the Faro is effective to about 80 meters, while the Trimble has a working range of 350 meters. The demonstration of the two machines was both dramatic and instructive. Among other things, it helped me to better understand what's going on when a desktop flatbed scanner turns a print photo into a digital image.
What does all this have to do with tolerances? Simply this: Our ability to build more accurately depends upon our ability to measure the finished product. Using 3-D laser scanning technology makes it both possible and practical to measure our work more quickly than ever before. Moving forward, that is what will enable the 117 committee to develop reality-based tolerances.