SDOT Bascules

Fremont, Ballard, and University Bridge are double-leaf bascule bridges located in Seattle, WA. All three bridges operate frequently, with Fremont in particular being one of the most frequently opened moveable drawbridges in the US. The bridges also have significant historical and aesthetic significant to Seattle, and are all listed on the National Register of Historic Places.

BDI was engaged by Seattle Department of Transportation for performance testing of the bridge bascules. The primary objective was to assess counterweight balance and trunnion friction for each leaf. A secondary goal was to gain insights into the mechanical operations of the bascules and identify opportunities for improvement. Testing involved measuring pinion shaft torque, leaf rotation, and motor current on each leaf. These measurements were taken during both normal and slow-speed operations using BDI’s Structural Testing System (STS) hardware and software. This allowed for a comprehensive analysis of various operational performance conditions. After data collection, BDI engineers evaluated the results to determine center-of-gravity angles, toe reactions, imbalances, and maximum friction values. Based on these findings, BDI recommended balance adjustments for certain bascules. Additionally, the team identified operational practices that were contributing to elevated stresses during operations. To reduce stress and potentially extend service life, BDI proposed several optimizations, including limiting downward torque as the leaf approaches the closed position, graduating the lift speed during initial acceleration, and releasing brakes after locks are applied at the end of each lower.