Henley Street bridge, originally completed in 1932,  spans over 1800 feet across the Tennessee River in Knoxville, Tennessee. It was constructed out of cast-in-place concrete and was designed with 4 approach spans and 6 concrete arches anchored at the base of 7 piers.  A total of 170 spandrel columns rose from the arches to support the roadway. Much of the structure did not conform to the original plans, however, Wilson & Associates, P.C. was tasked with providing data to build back the Henley Street bridge as it was originally constructed.  Therefore, detailed as-builts would be necessary to calculate the required dimensions.  Traditional survey methods could not achieve the accuracy needed for reconstruction and thus Wilson & Associates decided to use 3D-laser scanning for the as-builts.

This 3D-laser scanning process required setting an extremely accurate control network across the river and in the confines of a city environment.  Once again, traditional survey methods would not achieve this requirement. Instead this precise control network was established using simultaneous occupations of 8 control points with Trimble GPS static receivers.  These occupations were performed in the morning and evening and the results were post processed in Trimble Business Center.  This gave us a reference network that was field verified with a maximum distance error of 1/8.

Once the reference network was established, we targeted the bridge with 24 3D-laser scanning targets.  The targets were set at the base and top of each pier on the east and west side.  The targeting involved 2 days of maneuvering with cranes on barges and john boats.  The targets, comprised of 6″ diameter globes, were anchored into the concrete with red head concrete anchors.

A Leica C10 Scan Station was used to conducted a home scan of all 24 targets and 5 of the 8 control network points.  Leica Cloudworx software performed a least-squared analysis of the home scan, allowing us to calculate the results to less than 1/8 for the entire network.  Producing a home scan allows us to work in remote sections of the bridge utilizing only a few targets.

Over the next month, we scanned all sides of the arch, base and top of each pier and base of each spandrel bent.  This produced a point cloud of over 21 million points of data that contained X,Y,Z values.  Leica Cloudworx software was used to model the point cloud data and the model was transferred to Microstation for reduction to planimetric dimensions and elevations.