High-speed precision scanning technology brings cost-effective precision to structural steelwork at Nashville’s Music City Center
By Don Talend
December 21, 2011 - The 1.2 million-square-foot Music City Center in Nashville will have plenty of design features and spaces for visitors to talk about when it is scheduled to open in February 2013. A multifunction exhibit hall covering 350,000 square feet, or about eight acres. A 57,500-square-foot grand ballroom and 18,000-square-foot junior ballroom. Sixty meeting rooms occupying 90,000 total square feet. Several sustainable features that put the new convention center on track to achieve Leadership in Energy and Environmental Design (LEED) Silver certification from the U.S. Green Building Council, such as a 175,000-square-foot green roof designed to mimic the rolling hills of Tennessee and a 360,000-gallon collection tank that will store rainwater from the roof that will be used to irrigate outdoor landscaping and flush the hundreds of toilets in the building.
None of these features will be located where they should be if the steel contractors on the project—including Schuff Steel Atlantic—do not install 11,000 tons of structural steel where the official building survey dictates. Schuff is primarily erecting structural steel that will form the interior backbone of what is arguably the facility’s most distinctive architectural feature: a 162-foot wall at the north end of the main structure that rises out of the main roof and resembles the body of a guitar from a bird’s-eye view. The metal panel wall will enclose the grand ballroom on the ninth floor.
Laying out the structural steel for the radial shape of the guitar wall presents Schuff with a significant challenge. The contractor is using high-speed precision scanning technology to meet it.
Preventing a snowball effect
Viewing three computer screens showing two-and three-dimensional models of the structural steel in his office a few blocks from the Music City Center, Schuff Project Superintendent John Fugera noted that the design tolerance is one-quarter of an inch and three-eighths of an inch around glass. A failure to adhere to the tolerances would cause an undesirable snowball effect, Fugera pointed out. Work from contractors installing glass, cut stone and metal panels would also be off. “It affects all of the follow-on trades, so it’s crucial that everything is where it needs to be,” he said. “We drive the bus—if we’re wrong, then everybody else is going to have issues down the road.”
In March 2011, Schuff began laying out and erecting the steel framework for the guitar wall on the north side of the building. The contractor worked in a clockwise direction from the north side, ensuring that every quarter-inch of the steel beams was positioned precisely in the as-built surveys it had contracted to provide. Schuff used a total station for this task on the straight wall section it started on. “It wasn’t until we got into the curvature section that we figured that the scanner would give us a lot more accuracy,” Fugera said of Topcon Positioning Systems GLS-1000 and GLS-1500 laser scanners rented out by Earl Dudley, Inc., a surveying and geospatial equipment dealer with five locations in the Southern United States. Given the design of the radius walls, determining the X, Y and Z dimensional locations of every quarter-inch of the steel beams using conventional surveying equipment would have been too costly, according to Fugera.
This process would have been especially cumbersome on the east side of the Music City Center. Fugera noted that the guitar wall on this side featured eight radius variations and five pitch variations at the roof. He added that he did not plan on using scanner when Schuff began its work on the project. “Once I started getting drawings on the eight radiuses and I saw the complexity of it, I insisted on getting the scanner.” Schuff rented both the GLS-1000 and the newer GLS-1500 from Earl Dudley in fall 2011.
It was Fugera’s first actual use of laser scanning, although he was familiar with the technology. “I’ve never used it before, but I’ve seen it,” he said. “I’ve always wanted to get involved in it; I’m a technological guy and anything that can speed up the progress and get us more accuracy than a guy with a tape and a level—I’m all for it.”
Fugera estimated that it would take at least two weeks to shoot the estimated 2 million points required to as-built survey Schuff’s steelwork on the long east side of the guitar wall, which has an estimated 4,000–5,000 connections to the main steel structure. “That’s just two weeks gathering all of the information,” he said. Fugera estimated that it would likely take three or four more weeks to determine where Schuff’s steelwork was located relative to the main steel structure. The laser scanner also allowed surveying from ground level, whereas the total station would have been operated on an aerial lift at various levels.
Chris Clay, RLS, Schuff’s licensed surveyor on the project, had operated the total station on the project and, like Fugera, this was his first project using scanners. Two weeks to shoot points on the east side with a total station sounded a bit generous, he said. “Shooting 2 million points—I don’t know that you could get it in two weeks,” he said. “Even if you stayed out here 24/7, you’d run out of battery on your total station,” he added with a laugh. Clay agreed with Fugera that laser scanning was the only realistic way to develop an as-built survey of the steelwork. “This is the largest structural job I’ve worked on,” Clay said. “I’ve worked more in the civil field, subdivisions, roadwork. I’ve surveyed some commercial work—strip malls, stuff like that—but not to this scale. This is a lot more intricate. That’s where that scanner comes in. Trying to collect that information with just the total station and a data collector—no chance.”
More sophisticated BIM
High-speed precision laser scanning is allowing contractors like Schuff to save a great deal of time in developing as-built surveys and correct construction errors, preventing other trades from compounding the errors with their work. As a result, the use of the technology saves the entire building team—and, ultimately, the building owner—time and money. It serves as a key element in Building Information Management (BIM), a discipline that continues to expand its technological sophistication—and feasibility for surveyors and contractors.
In March 2010, Topcon unveiled the GLS-1500, which collects points at a rate of 30,000 points per second—10 times that of the GLS-1000—and a range of 150 meters. Topcon Precise Scan Technology is designed to allow high-accuracy measurements over a wide range of distances. Lens array optics technology maintains distance accuracy from 1–150 meters and additional ranging past 330 meters is possible. Having experienced an exponential increase in as-built surveying productivity with the GLS-1000 vs. using a total station, Schuff had begun to double its productivity with the newer instrument in the past several days.
In late fall 2011, Schuff was erecting steel near the southeast corner of the facility and continuing to work its way around in a clockwise direction. Clay set up the GLS-1500 outside of the guitar wall at ground level. He identified several magnetic targets that had been placed on several steel beams forming a large horizontal rectangle on the eighth level. The target locations were surveyed with a total station and tied to the grid formed by control points based on the official building survey and a three-dimensional BIM model of the entire building. He measured the center of the targets from building control points using a total station and scanned the targets. The GLS-1500 resectioned in order to establish the position of the instrument relative to each target. By doing this, Clay tied the target locations to both the building model and the official survey grid.
“This is where [the GLS-1500] comes in handy—the capability of really tight tolerance checks and the speed of being able to get it done quickly,” Clay said. He added that a specially designed tilt bracket boosted productivity and safety by allowing scanning from one level. He tilted the scanner at a 45-degree upward angle and entered the project number into the GLS-1500 keypad, including his initials in the event that Fugera had questions about the locations of any points in the cloud later. This scanning sequence covered the eighth level on the south side of the building so Clay named the first scan 8S-1 and the targets T1, T2, etc. Clay generally scans the targets from left to right and then bottom to top. He maintained that sequence at this location.
First he dialed the top of the GLS-1500 turret head to the left using jog wheels located on either side of the turret head. He used the instrument’s sighting columnator to aim the GLS-1500 toward the upper right corner of a steel frame for a parapet located just above a block wall to his left. After waiting several seconds for the instrument to calibrate, he scanned the first target. Next he swung the turret head to his right, toward a target located on a vertical steel beam on a flat wall just below the guitar wall. “We’ll scan all of the targets first, then we’ll set up a scanning area and all of the points will be inside of that area,” he said.