When it was completed in December 2006, the Penobscot Narrows Bridge was hailed for its beautiful design, speed of construction and engineering innovation. Located on scenic Route 1 near Bucksport, Maine, the bridge sits 135 feet above the Penobscot River as it empties into Penobscot Bay and the Atlantic. Constructed at a cost of $85 million, the bridge is 2120’ in length and rises to 447’ at the top of its two massive, support towers. The longest span of the bridge measures over 1160’. The bridge also boasts the tallest bridge observatory in the world contained within one of the supporting granite towers. The State of Maine Department of Transportation (MDOT) also mandated a five to seven-year inspection cycle by the bridge designers. This inspection would be used to monitor movements or changes in the bridge. To meet the MDOT’s precisional requirements, a unique survey approach was needed.
Figg Bridge Engineers, concrete segmental bridge specialists and the original bridge designer, turned to Sebago Technics to help with the inspection. For the Penobscot Narrows assignment, the biggest challenge would be how to best capture the correct field dimensions of the existing 420-foot concrete pylons, bridge deck and steel support cables. This would be particularly challenging as no provisions were made in the original bridge design for survey access or targeting. Compounding the challenges of the assignment was the limited number of vantage points on the bridge and land and the short time windows permitted for traffic closures. Sebago determined that the use of High Definition Scanning (HDS) technology would be required.
The HDS solution addressed all of the needs and challenges of the project. HDS allowed Sebago to document the current state of the two pylons, deck and cable-stay by employing a controlled, long-range, three dimensional (3D) laser scanner. A total of fourteen (14) different scan stations averaging twenty-one minutes per setup were needed to capture the visible components of the bridge. Sebago collected 18 million points at an interval of approximately two inches by two inches within or exceeding the precisional tolerances established by the MDOT. The minimal scan time for bridge deck scan locations also worked well within the short window provided for traffic closures.
The 3D scan data collected during the inspection survey of the Penobscot Narrows Bridge will be used to provide Figg and the MDOT an as-built three dimensional CAD Model of the pylons, deck and cables. The data will serve as the baseline in their study and analysis of the bridge’s movement over time.