[Experience Music Project]
Experience Music Project
DJC.COM Special Issue © June 15, 2000

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How to drive a monorail through the EMP

Bush, Roed & Hitchings

A new thrill awaits future Monorail passengers. For a brief moment during their journey, they will glimpse the Experience Music Project's "Canyon," an enclosed tunnel surrounding the train.

Bush, Roed & Hitchings, a Seattle-based land survey and civil engineering firm, was given the unique assignment of mapping the Monorail's motion as it approaches, transits and departs EMP, to provide close, yet safe, tolerances to the architectural design team.
Sculpting the buidling around the monorail required GPS surveying techniques, graveyard shift work and interviews with monorail drivers and mechanics. Photo courtesy of EMP.

How to map an object in motion

The solution to mapping a safe envelope through which a dynamic object travels at high speed was to attach a Global Positioning Satellite antenna to the roof of each train. By employing "on-the-fly" GPS surveying techniques, in real time kinematic mode, or RTK, the firm was able to map the precise motion of the trains as they travel along the supporting structure. Position readings were taken every tenth of a second.

Thousands of resultant data points enabled the surveyors to describe a "mean" path of travel: a "line" in space along which the train moved.

The next step was to identify the exact dimensions of the coaches, using the German train manufacturer's shop drawings. Finally, the "as-built" of the coach cross section was hung from the line at each position under study, and a digital 3-D envelope model, through which the trains move, was built along the entire Monorail support structure from the Seattle Center terminus to Virginia Street.

Monorail support

A detailed survey of the train's support structure yielded the vertical angle and cross sectional profile of the supports and revealed that no two concrete columns are identical. This survey, while performed from the ground, required that one member of the team be driven along the track in a "cage" used by the train maintenance crew.

The motion of the Monorail

It is only when one studies the motion of the train, that one begins to appreciate how much they tilt through curves, undulate vertically on the track, and even shift toward the inside of the track by an amount that varies with different loading conditions and speeds. This motion is accentuated by the unique rubber wheel system that flexes, under centrifugal force, as each train banks around a curve on its individual track.

BRH coordinated with the Seattle Monorail Services managers who oversee train operations, and conducted interviews with train mechanics and drivers regarding the motion and behavior of the trains. A particular complication was the way the coaches move into a curve. As a train hits the curve outbound from the Seattle Center terminus, the front of the lead car continues on a straight line, cantilevering over the support structure and then suddenly "snaps back" to conform to the curve of the track.

Darrell Nance was the BRH project manager, with Dave Creek, Scott Roed and Dakin Bell as project team members. This survey won an Engineering Excellence Merit Award by the Consulting Engineer's Council of Washington.

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