What is a U-shaped grade separation?
|U-shape bridge cross section, showing the benefits of
reduced track elevation
While the peninsula rail corridor is not a new metro system, these U-shaped structures could still prove useful in a major push to grade-separate the 40 grade crossings that remain, enabling higher speeds and more train traffic while relieving road congestion and improving east-west access across the tracks.
What are the advantages of U-shaped grade separations?
U-shaped grade separations combine several attractive features that make them ideally suited for developed areas along the peninsula rail corridor, and certainly much better than the massive hollow core concrete box girder bridges considered standard issue by the HSR project as shown in the graphical comparison above.
- Lower track elevation. The U shape minimizes the depth of the structure (measured from the underside of the bridge span to the top of the rails) to 3 feet or less. This allows the standard 16-foot road clearance to be provided by raising the tracks just 19 feet above the road surface, about 8 feet less than the large elevated concrete box-girder viaducts that were proposed during the 2010 Analysis of Alternatives for peninsula HSR. The rails are lowered thanks to the U shape, which places the structural support of the bridge to the sides, rather than under the trains.
- Lower visual impacts. When the tracks don't need to rise as much, the rail approaches to a grade separation become correspondingly shorter and less obtrusive, impacting fewer views. The structures above rail level, such as overhead electrification poles, are also lowered. This reduces the so-called "Berlin Wall" effect of a grade separation structure.
- Lower train noise. The side beams function as natural sound walls, trapping rail noise before it has a chance to escape into adjacent neighborhoods. They are especially effective because they are thick and quite close to the train. This obviates the need to add sound walls on top of the bridge, making the finished structure less visually obtrusive.
- Better earthquake resistance. The lower profile of the bridge structure reduces bending moments applied to the piers and foundations, whether by earthquake forces or train braking and acceleration or wind loads. This makes the bridge piers less massive and integrates them better into the built environment.
- Better station integration. Where stations must be located on an elevated section, structures are simplified thanks to the lower profile of the track, which reduces the reach of stairs, ramps, escalators or elevators, making for a more passenger-friendly environment. The side beams of a U-shaped viaduct have their top flange at the same height as the train floor and form the actual platform interface, 50 inches above the rail and 72 inches from the track center line, allowing the U-shaped structure to continue uninterrupted through the station.
- Better safety in case of derailment. The side beams are close to the train. In case of a derailment, train cars will be guided by the structure and will not topple off the bridge. This feature is known as "derailment containment."
- Lower construction cost. U-shaped elements can be prefabricated off-site and assembled with minimal disruption compared to traditional cast-in-place construction methods. Using standardized elements throughout the corridor, in dozens of locations, provides economies of scale. The decreased profile changes for both rail and road (whether the U-shaped bridge is elevated or at-grade with the road sunk underneath) require less excavation or fill.