DHC is proud to have been awarded the National Council of Structural Engineers Associations 2015 Excellence in Structural Engineering Award, for work on the Ballard Drive Bridge, in Seattle, WA.
The Ballard Drive Bridge is one of the many historic bascule bridges located in the Pacific Northwest. The application of engineering design was necessary to overcome the challenges associated with providing access platforms and containment for blasting and repainting the structure, while allowing the bridge to perform its normal operations. These operations not only include supporting vehicular traffic, but raising 250 to 500 times a month to allow for vessels to pass under. The complex loading of this process presented extreme design challenges beyond that of a traditional horizontal platform. Such items included intense wind loading on the underside of the platform, stabilization during raising, analysis of all bridge framing from irregular loading, and counterweight restrictions.
Creativity of Structural Design/Complexity of Criteria or Unique Problems:
The structural design utilized a flexible Safespan platform to minimize the increased dead weight on the bridge as opposed to typical scaffold or other rigid suspended platforms. This was extremely important, as every pound of platform meant additional counterweight was necessary to balance the loading due to the limited capacity of the bridge’s original motors. Precisely calculating the counterweight was extremely important for the analysis of all bridge members because the increased counterweight wanted to overload the bridge similar to having two sumo wrestlers on a child’s seesaw. However, although this light system meant less demand on the existing bridge, the flexibility allowed it to act as a sail in the wind during lifting events. To prevent the exposed face of the Safespan platform from slamming into the underside of the bridge under high winds as well as sliding under its own weight, a series of rigid anti-uplift and horizontal bracing members were installed to hold the deck firmly in place while strategically transferring the load into the stronger framing members. Horizontal and vertical suspension cables were also installed to allow the platform to shift orientation and eliminate any sag or sway of the deck toward the water. The creativity of this structural design was not only necessary to overcome the complex challenges of the dynamic system, but also allowed the general contractor to use existing/typical material and installation techniques, resulting in an efficient project execution.
Innovative Application of Existing Materials and Techniques:
Typical Safespan platforms consist of longitudinal wire rope cables supporting metal corrugated decking, intermediate wire rope tie up cables, and various attachment assemblies that are compatible with different bridge members. All components in this design utilized existing Safespan assemblies with the exception of the rigid anti-uplift bracing. However, these members were devised out of scaffold tube and clamps, which are cheap and readily available. This device was used to choke around existing bridge members and push tight against the decking to stop unwanted movement at the tie up cable locations. To further stabilize the platform, typical tie up cables were horizontally installed, giving resistance to sliding under gravity while the bridge was lifting. The distinct quality of this design was its ability to take advantage of existing equipment without the use of specialty-prefabricated pieces, which can be both costly and time consuming to produce. D.H. Charles Engineering used all the parts and pieces available to construct multiple phasing models and employed a solution that was both technically sound and realistic for the contractor to install.
Ingenuity of Design for Efficient Use of Material and Labor:
Safespan provided the equipment for this platform; however, Purcell Paintings and Coatings performed the installation. Although the design process was extremely extensive due to the complexity of the dynamic bridge structure, it was safely erected using typical installation procedures for horizontal Safespan platforms. The only caveat was that counterweight had to be added to the existing structure at each installation phase. However, this is a small price to pay when looking at the overall scope and ease of accessing such a complicated structure without impeding its functionality.
Exceeding Client/Owner’s Needs or Expectations:
Careful consideration and evaluation of all the potential risks, as well as close collaboration between the contractor, engineers and city representatives, were critical to the completion of this venture. The final product functioned perfectly throughout the duration of the project. The entire system proved safe and stable, requiring very little modification or maintenance as the job proceeded. Even with rain on 70% of the workdays, there were no breeches of the containment system, and the job finished on schedule and within budget. Given that some said a Safespan platform could not be successfully installed and function properly on this bridge under the dynamic conditions, the incredible success of the design proved to exceed the expectations of not only the client, but also the entire industry.
Suitability of the Structure for its Environment:
The nature of the project not only required complete containment of blasting operations for environmental purposes, but also demanded a fully functional bridge. Given both of these items were achieved, it is evident the design had an extremely low impact on the bridge and the surrounding environment. The structure came and went, leaving the bridge in better shape than before and with no signs of its existence. This is the ideal result of a structure perfectly designed to fit its environment.