Helix Pedestrian Bridge

Overview

Dynamic Structures was responsible for detailing, fabricating and erecting the steel superstructure of a unique “Helix”-shaped pedestrian overpass in Seattle for the world's largest biotechnology company, AMGEN . This bridge spanned 400 feet and featured an architectural design consisting of three shallow arches configured to evoke the shape of a helical DNA molecule. This structural scheme required the use of extensive temporary support to the arches during erection.

Challenge

Our erectors were faced with extremely challenging site access limitations. A procedure had to be engineered that would permit us to assemble and erect the structure over numerous active railway lines where only minimal interruption of railway operations was permitted.

The project specifications also required the bridge components be set to very precise locations in order to satisfy architectural requirements. The two smaller side arches were designed with an incline or “lean”, so that the force of gravity tends to pull the arches not only downwards but sideways as well. Traditionally, bridges are designed with a built-in “camber”, or crown, so that gravity pulls the bridge flat without giving the appearance of sagging. The Helix bridge was designed with deformed geometry in both the horizontal and vertical directions. The amount of this deformation or camber was specifically calculated to ensure that gravity would pull the bridge into the correct architectural shape. The accuracy of these calculations was important in achieving the precise positions expected of the final bridge configuration.

Solution

The limited access conditions forced our team to develop a very innovative solution for temporary support of the bridge during erection. The Dynamic Structures team designed an erection procedure with several methods of adjustment to be able to deliver a bridge with the correct geometry.

The deck members were designed with heavy walled built-up plate sections requiring extensive welding. The shop used carefully planned welding sequences and presets to avoid the distortion problems that occur when large amounts of heat are applied as part of the welding process. The arch sections were designed using high strength (70ksi) steel pipe which posed problems for forming the pipe into curved segments. Individual arch segments were trial fit at the Dynamic Structures shop to form the complete arch. An accurate survey was performed for each segment and a least squares fitting procedure was used to accurately position the arch segments next to each other. This fitting procedure ensured that the built-up arch could be as close as possible to the target geometry.

"I have appreciated the very real intellectual interest demonstrated... Our relationship has been the most positive that we have had with any outside fabricator"

Nick Scoville, Director, Owens Valley Radio Observatory