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Home   LiDARmag     

How to Laser Scan a Bridge in Five Days Print E-mail
Written by Vicki Speed   
Thursday, 30 April 2015

A 1.399Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

When the magnitude 6.9 Loma Prieta earthquake struck Northern California on Oct. 17, 1989, the San FranciscoOakland Bay Bridge (SFOBB) system, known locally as the Bay Bridge, was one of the major man-made structures that sustained damage. A section of the east span's upper deck collapsed onto the lower deck, killing one person and forcing a month-long closure of one of the most important transportation arteries in the region.

California Department of Transportation (Caltrans) officials and project partners needed to know how much movement had occurred across the original east span due to the collapsed and damaged deck sections. Unfortunately, as-built measurement records of the Bay Bridge and the other major bridges in the Bay Area did not exist, so obtaining a quantitative assessment on the entire original east span truss section deformation was not possible.

It was a dilemma that Caltrans District 4 Right of Way Field Surveys and District Office Chief Nelson Aguilar, PLS, was determined to prevent in the future.

As construction began in 2002 on a new 10,176 foot east span of the Bay Bridge, a groundbreaking technology was beginning to draw interest in the surveying and engineering community. 3D laser scanning, pioneered by Ben Kacyra as High Definition Surveying (HDS) under the brand Cyrax (acquired by Leica Geosystems in 2001), promised to revolutionize as-built documentation by using lasers for 3D reality capture.

Research conducted by the Advanced Highway Maintenance & Construction Technology (AHMCT) Research Center at the University of California at Davis in conjunction with Caltrans in 2006-2007 found that using 3D laser scanners could "dramatically improve safety and efficiency over current survey methods."¹ The results of the research, published in a June 2007 report, provided the standards and guidelines Aguilar needed to deploy laser scanning on a major Caltrans project.

In April 2008, during the preliminary work on the Bay Bridge South-South detour project, the Caltrans team used an early version of the Leica ScanStation laser scanner to capture areas under the deck and surrounding areas of the bridge. These early successes set the stage to complete the visionary project conceived by Aguilar--a detailed, survey-accurate digital as-built model of the entire Bay Bridge, an effort that would come to be known by the District 4 survey team as "The Erskine Project".²

"A key part of the survey project was to ensure that the structures designed by consultants and built by the contractor were done per design and would behave as expected in a major seismic event," Aguilar says.

A Tight Timeline
Since traffic was still using the original east span structure, the new east span was easy to scan. A planned closure of the bridge over Labor Day weekend in 2013 would provide the best opportunity to scan the west span decks. To manage the work, Caltrans looked to C.J. Vandegrift, PLS, senior transportation surveyor and West Bay Branch chief, who had managed the Caltrans survey crews for the east span project.

The planned closure was from Wednesday night at 10 pm to early Tuesday morning following Labor Day. On Monday late morning, an aerial photogrammetry team flew the bridge site to gather LIDAR data from the air. Ground crews performed mobile scanning with a Trimble MX-8 on Saturday morning, Sunday evening and Monday on both spans. The Caltrans crews used three Leica ScanStation C10 laser scanners to scan the lower deck of the western span, the underside of both spans, and the self-anchored suspension (SAS) bridge upper deck through the lower YBI tunnel.

All three scanners ran continuously for at least 12 hours a day. Meticulous planning allowed much of the work to proceed smoothly, but there was one challenge the team hadn't foreseen: extreme vibration. "We anticipated some thermal expansion and vibration, but nothing like what we experienced," says Vandegrift. "Even without vehicles on the bridge, the atmospheric vibration and subsequent deflections were significant."

To achieve maximum redundancy, the team added total stations and GPS. Multiple GPS measurements were taken at night on the decks at the center spans where multipath/interference with the cables was not observed. GPS measurements were also taken at the north and south corners on the top of each SFOBB tower, stable portions of the structure that were used for control.

Despite the challenging conditions, the data from the stationary terrestrial laser scanners met survey-grade accuracy requirements, and the project was completed on schedule. In fact, work was finished faster than anticipated, allowing Caltrans to open the new east span of the bridge to traffic about seven hours earlier than planned.

Powerful 3D Information
For Vandegrift and the other professionals on the team, the project provided crucial insights on the practices and procedures required to ensure a successful outcome with laser scanning.

"The most important take-away, especially on a project this size, is the need to set control," Vandegrift says. "Many contractors think they can hop off with a receiver and come up with a CORS-- that won't work on a project like this, and scanning ups the ante. Contractors must calibrate off established control or they won't have the same values. With scanning, if a project team uses the wrong control, they will gather data that is exponentially inaccurate."

Surveyors and contractors must understand that to create the model from a scan is simple--but for it to be accurate and calibrated to local horizontal and vertical datums, they must look deeper into the surveying methodology and associated errors. They must understand the scaling from the grid reference system back to the ground, particularly in an area like San Francisco, where the earth shifts continuously.

"I'm extraordinarily proud of this project," says Vandegrift. "When it comes to scanning, if you get the control tight, you get the best product with the least amount of field time.

"On many projects, there's scope creep and designs change, so the contractor ends up needing more data," she adds. "Surveyors often have to deal with more trips to the field or datum changes. But with scanning, we know once we have a model, the folks back in the office can mine data from it without more field work. We could not have created this data intensive, high accuracy model in such a short amount of time without cutting edge technology such as 3D laser scanning. We can achieve detail that conventional surveying methods simply can't create with a shot at a time. We have raw data right down to the bolts and rivets. That's powerful."

Editor's note: This article has been shortened from its original format to fit space constraints. To download the complete article as a PDF, visit www.leica-geosystems.us.

Footnotes:
¹Hiremagalur, Jagannath; Yen, Kin S.; Akin, Kevin; Bui, Triet; Lasky, Ty A.; and Ravani, Bahram, "Creating Standards and Specifications for the Use of Laser Scanning in Caltrans Projects," AHMCT Research Report, June 30, 2007.

²The Caltrans surveying project on the San Francisco Oakland Bay Bridge was named The Erskine Project in recognition of Ric Erskine, the primary Caltrans survey party chief on the eastern span, who died a few weeks prior to the Labor Day opening of the bridge after a year-long battle with lung cancer.

Vicki Speed is a freelance writer based in Colorado who specializes in the architecture, engineering and construction space.

Sidebar:
The UC Davis Trial and White Paper

Prior to purchasing its stationary terrestrial laser scanners, Caltrans funded a study by UC Davis on laser scanner accuracy with the goal of producing standards and specifications for eventual deployment of the technology. Of the four vendors that participated, Leica Geosystems, Trimble and Optech completed the entire array of real-world active roadway and scan resolution trials.

During the Leica ScanStation trial (with the air temperature hovering around 108ºF), Leica Geosystems employees Tony Grissim and Hendrik Bartel became concerned when the 3D coordinates generated for one of the Caltrans-provided control points was substantially different by many feet. After checking their work several times, they kept getting the same (apparently) wrong answer. Finally they approached Caltrans surveyor Kevin Akin, who had established the control, and shared their results. Akin quickly discovered that he had made a simple transposition of control point numbers; once these were corrected, the Leica ScanStation coordinates were shown to match within a few millimeters. The robot scanner had discovered the human error.

The peer-reviewed UC Davis white paper on laser scanner accuracy resulting from this study has become an important reference document for judges, prosecutors and private forensic reconstructionists who use Leica Geosystems 3D laser scanning for crime and crash investigation, accidents and police use-of-force investigations. In 2013, Grissim was an expert witness in an Albuquerque police shooting trial where a U.S. Federal Court judge relied upon the UC Davis study and other similar documentation to issue a federal Daubert Ruling stating that data from the Leica ScanStation is admissible as scientific evidence. The Daubert Standard governs rules of evidence regarding the admissibility of expert witness testimony in federal trials. The Leica system remains the only scanner to have ever received a ruling of this nature.

A 1.399Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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