A Laser Scanning Reality Check
Coupled with 3D models, laser scans offer a clear view of reality that is invaluable during building renovation projects.
Laser scanning has enjoyed rapid adoption for building construction based on its ability to capture, document and display digitally the geometry of a 3D space or environment. The decision to use this technology depends, of course, on the type and breadth of the construction project, but scanning proves particularly ideal for extensive building renovation projects.
Laser scanners function by transforming whatever environment they scan into a digital point cloud. This cloud contains millions of data points. The scanner first sends a laser beam over its full field of view. Whenever the laser beam hits a reflective surface, it is reflected back to the scanner. In this process, the scanner records an image plus the X,Y and Z coordinates of each point it measures.
Individual scans are later registered together using common references. The point cloud created comprises all of the scans made for a project and can then be viewed and manipulated.
The resulting collection of millions of data points with X-Y-Z coordinates made through laser scanning is transformed into a 3D model of the building. This 3D model is keenly beneficial for engineers, architects and building contractors. Armed with the 3D representation of the building, these professionals can design the project at hand and make adjustments based on the point cloud collection of measurements.
Laser Scan Uncovers Deflection
The University of Georgia’s basketball coliseum renovation shows how valuable laser scanning can be. The coliseum had been built with a giant arched portico tied into the roof on the entire structure. Campus architects wanted to hang a glass cover on the roof as part of a plan to expand the coliseum. In their plans, architects specified having exterior walls knocked out so that some concourses could be added.
Before these renovations could occur, the university’s architects wanted the coliseum’s roof scanned. According to Donnie Longenecker, president and founder of Athens, Georgia-based Design Technology Consultants, his firm was hired to scan the roof to capture points every 2 feet up along the arched portico. These data points would help structural and civil engineers design the desired cover wall.
Design Technology Consultants and their partners at LandAir Surveying of Georgia scanned the roof, then returned in the summer to perform a verification scan. Longenecker’s team found about 2 inches of deflection on the roof. “The roof lowered by 2 inches because the concrete expanded when it got hot,” Longenecker explained. “Then in the winter it would raise 2 inches. Discovering this deflection was very important for this curtain wall because the whole thing was based on tension, and had we not found that deflection, the whole wall probably would have collapsed and the glass panels would have popped out of it,” Longenecker added.
Scans Crucial for Accurate Design
After learning about the deflection, the coliseum project’s engineers were able to design a system of shock absorbers at the top of the wall where the wall connected with the roof, to allow for the deflection. “We gave engineers the ground elevation at the base of the wall and the cloud data points along the arch in the roof so they could calculate how to fit the wall system to the arch on the building,” Longenecker said. Although the planned scanning of the roof proved challenging, university architects decided it was crucial for capturing the critical elevations so that they could design the curtain wall and make this fit into the existing structure without any modifications.
To scan the coliseum roof, the Design Technology Consultants team used a Leica laser scanner along with Leica TruView, a point cloud viewer that overlays the photogrammetry that the scanner takes when it records each pixel. TruView is a Web-based viewer, which allows the user to move around inside the scanned site and still have photographic realism. As a result, the user can view scanned details, perform measurements, make rough annotations and print PDFs. What is delivered to a client, therefore, is a 3D model. If they have questions about this model, they can pull up the point cloud and very easily see any of the details.
Design Technology Consultants ultimately gave the design team an as-built survey of the coliseum roof — a CAD drawing showing elevation spots along the underside of the roof. By putting this information into their own CAD program, the engineers for the renovation project were able to design the curtain wall accordingly, Longenecker said. He noted that the deflection problem “wasn’t jeopardizing the structural integrity of the building whatsoever.”
CAD Software Yields 3D Model
Ron Mak, a surveyor with Van Harten Surveying Inc., Orangeville, Ontario, Canada, knows, while laser scanning itself is tremendously valuable as a tool for comprehensively capturing an environment as it actually appears, in totality, the CAD drawing is what brings it to life. For example, Mak has used the MicroSurvey Ultimate CAD software with laser scanners. The software integrates point cloud data with CAD drawings. This means Mak can bring existing drawings of surveys, line work and data points into a point cloud. “This allows me to see where the cloud is relative to my linework,” Mak said. Also, if Mak is tracing a cross-section in the point cloud, he can enter this into the CAD drawing as he’s creating it. “As I’m drawing the cross-section in the point cloud, I can see the line appearing on the other screen, in the CAD drawing,” Mak added.
Surveys once were basically 2D drawings with no lines labeled as to what they were. “With the LiDAR solution, we’re able to produce pretty easily a 3D model of the project,” Mak said.
Complex Projects a Good Fit
Another example of just how important the 3D model of the scanned project is comes from Tucson, Arizona-based Darling Geomatics. Darling was tapped by Turner, a North America-based, international construction services company, to produce pipe mapping and as-built drawings of concrete structures that it needed for phased demolition of Terminal 2 of the San Francisco International Airport. The project began in 2008 and was completed in 2009.
Darling Geomatics performed 3D laser scanning of more than 180,000 square feet of Terminal 2. Directly above this terminal was an active FAA control tower, and below floor level was a maze of electrical conduit, communications wiring and highly critical infrastructure. To accomplish the renovation below the main floor of the terminal, Turner had to be sure they weren’t dropping in the wrong conduit, which is why scanning all of the conduits was so critical. “They had to know what they needed to work around; what couldn’t be moved,” according to Ryan Darling, vice president/3D technologies at Darling Geomatics.
For the scanning project, Darling’s Scan Crew 1 used a Leica ScanStation with self-traversing functionality. Scan Crew 2 applied the Leica High-Definition Scanner 6000 in tandem with the survey crew who used a reflectorless robotic total station to carry control. Over 90,000 square feet of crawl space and another 90,000 square feet of the arrival level were scanned in 16 field days.
The final deliverable was a CAD drawing of the structure and piping in Terminal 2’s first and second levels. The Leica TruView was used to produce BIMs (Building Information Models) and a 3D model of the Terminal 2 project. The 3D model had 10 times higher accuracy than other methods and was delivered in one-fourth the time, Darling said. In all, 60 days of design time were cut from the project, he noted. Other benefits included less field verification and fewer RFIs (requests for information).