Analyzing the Malfunction of a Vertical-lift Drawbridge

Vertical-lift drawbridges are unique structures. To provide clearance for vessels on waterways, the entire road or rail section is lifted vertically between two towers. Counterweights, equal to the weight of the road section, reduce the amount of mechanical force needed to move the section upward or downward.

Bridge No. 21, a prime example of a vertical-lift drawbridge, is located on the Welland Canal in Port Colborne, Ontario, Canada. The 27-mile-long waterway, part of the Great Lakes St. Lawrence Seaway System, connects Lake Ontario to Lake Erie. Bridge No. 21 carries vehicular traffic on Clarence Street over the canal just before it connects to Lake Erie. The bridge was built in 1929.

As with other aging civil infrastructure, operational or structural problems may not be evident until something fails or malfunctions. During normal bridge open-close cycles, rubbing of a counterweight against a column was observed. This led to the suspicion that deformation of the tower structure had occurred, causing it to lean inward.

Surveying the complex tower structures to identify areas of deformation was not possible with conventional single-point instruments such as a total station. Laser scanning, however, can produce millions of accurate x,y,z points that can be used to build a highly accurate 3D model for analysis. An initial scanning attempt was made, but the instrument did not have sufficient range to reach the top of the towers and midsection of the bridge. Coordinate Control, a local survey firm with experience in laser scanning, was called in to make a second attempt in September of 2012.

Using Topcon’s GLS-1500, the bridge was scanned from six locations to provide data for a 360, three-dimensional model. With a range of 500 meters or 1,640 feet (long range mode), the instrument was able to capture all areas of the structure. At the same time, a panoramic image was recorded using the integrated two megapixel camera. Scanner functions and operations were controlled with a notebook computer using Topcon’s ScanMaster software.

After the field work was completed, ScanMaster was again used to register the point cloud, generate a 3D model, and measure deformation. Since there were no as-built 3D models of the towers for comparison, cross sections were taken at different heights. By analyzing these sections, the specific locations of deformations could be identified.

This story ends with a real bonus for LiDAR technology. The St. Lawrence Seaway authority was quite impressed and convinced with the findings from the scans. They have included the use of LiDAR as part of a larger RFP for the rehabilitation of all bridges on the seaway stretching through Ontario and Quebec.

Real-time availability for vehicular traffic of all bridges on the Welland Canal can be found here.

Credits: Information for this story was provided by: Dilip Lal, Director of Sales and Operations – Canada, Sokkia Corporation and Rajive Sharma, Manager – Imaging and Monitoring, Sokkia Corporation