{"id":3051,"date":"2013-08-20T07:33:36","date_gmt":"2013-08-20T11:33:36","guid":{"rendered":"http:\/\/www.alexblock.net\/?p=3051"},"modified":"2013-12-14T13:46:18","modified_gmt":"2013-12-14T18:46:18","slug":"a-visual-survey-of-selected-elevated-rail-viaducts-part-1","status":"publish","type":"post","link":"https:\/\/www.alexblock.net\/blog\/2013\/08\/20\/a-visual-survey-of-selected-elevated-rail-viaducts-part-1\/","title":{"rendered":"A visual survey of selected elevated rail viaducts: part 1 – the universe of post-tensioned pre-cast concrete"},"content":{"rendered":"

For background, see the prologue<\/a> for this series.<\/em><\/p>\n

With phase I of WMATA’s Silver Line through Tysons Corner nearing completion, we now have a better sense of the visual impact of the elevated guideways on the cityscape of Tysons Corner. Elevated rail in Tysons, given the widths of the roads it runs over\/along, makes perfect sense. However, there are other examples of urban rail viaducts with more visual appeal and urban design sense than the Silver Line guideways.<\/p>\n

Tunnels, all else being equal, would be preferable. Given the costs of tunneling (even with the promise of large diameter TBMs<\/a>, Spanish-level construction costs<\/a>,\u00a0and other tunneling practices that could get American subway costs under control<\/a>) and the reality of costs and land values means that most potential Metro expansions outside of the core will need to consider elevated rail.<\/p>\n

Like the roads in Tysons, many potential rights of way feature plenty of room for elevated rail – if it is done well. While elevated rail in Tysons makes sense, the execution of the guideways could’ve featured better design with less visual obstruction. Jarrett Walker discusses the pro\/con of elevated rail here<\/a>, noting that rapid transit requires full grade separation.<\/p>\n

For comprehensive visual documentation of the Phase I construction, I recommend Sand Box John Cambron’s blog<\/a>.<\/p>\n

Through Tysons, the elevated guideway is aligned in the center of the Route 7 roadway<\/a> and alongside the Route 123 roadway<\/a>. The guideways use segmented pre-cast post-tensioned box girder spans, with one box girder for each track supported by a variety of piers. Large portions of the guideway use a single pier with a large hammerhead cap to support both tracks.<\/p>\n

\"Metro<\/a>

Metro guideway in Tysons Corner, VA. Image from John Cambron.<\/p><\/div>\n

\"SBJ<\/a>

Center-running guideway with hammerhead pier caps in Tysons Corner. Image from John Cambron.<\/p><\/div>\n

\"SBJ<\/a>

Center-running guideway showing single pier supporting both tracks. Image from John Cambron.<\/p><\/div>\n

\"SBJ<\/a>

Center-running elevated rail guideway in Tysons Corner. Image from John Cambron.<\/p><\/div>\n

Using hammerhead pier caps increases the visual bulk of the elevated structure. A few columns integrate the pier into the guideway’s structure, providing a slimmer profile for the guideway:<\/p>\n

\"Support<\/a>

Support piers integrated into guideway, reducing bulk in Tysons Corner, intersection of Route 7 and Westpark Dr. Image from John Cambron<\/p><\/div>\n

Other aerial examples:\u00a0<\/strong>This isn’t meant to be an exhaustive survey, but a look at a few illustrative examples of what aesthetic alternatives are available for elevated rail.<\/p>\n

These examples are primarly from light rail and rapid transit systems relatively recently constructed; they do not represent the legacy elevated systems of Chicago, New York, and so on.<\/p>\n

WMATA examples:\u00a0<\/strong>Green Line, southern extension to Branch Ave. This extension of the Green line makes use of several segmented pre-cast concrete elevated structures, similar to the kind of guideway used through Tysons Corner. While the majority of the guideway crosses the green environment of Suitland Parkway<\/a>, this concrete guideway has the advantage of carrying both tracks in a single structure, both minimizing the bulk of the guideway and the support piers.<\/p>\n

\"WMATA<\/a>

WMATA Green Line guideway over Suitland Parkway. Image from Google Streetview.<\/p><\/div>\n

Near the Branch Avenue station, as the tracks separate for the station’s island platform, each track with its own structure. North of the Branch Ave station, the two guideways are able to share a common pier without a large hammerhead cap.<\/p>\n

\"WMATA<\/a>

WMATA guideways near Branch Ave station. Image from Google Streetview.<\/p><\/div>\n

South of the Branch Ave station, each of the guideways feature their own piers.<\/p>\n

\"WMATA<\/a>

WMATA Branch Ave station, looking towards Southern Ave station. Image from Google Streetview.<\/p><\/div>\n

Seattle Link light rail:<\/strong> Sound Transit’s Link light rail could be called a pre-metro<\/a>, thanks to extensive grade separation\u00a0combined with the repurposing of Seattle’s downtown bus tunnel. It features a large amount of elevated rail (with the requisite views along the way<\/a>) also making use of pre-cast concrete segmental bridges<\/a> used in Tysons<\/a>.<\/p>\n

\"Sound<\/a>

Sound Transit elevated rail. Image from Google Streetview.<\/p><\/div>\n

Support piers feature more detailing than in other examples<\/a>, with the shape of the pier caps matching the profile of the pre-cast box girder segments. Longer spans introduce subtle arches<\/a> to the guideway, adding a bit of elegance to the concrete structures. The guideway also makes use of metal railings rather than soundwalls next to the track, reducing the visual bulk of the structure.<\/p>\n

\"Sound<\/a>

Sound Transit elevated rail over Duwamish Waterway. Image from Google Streetview.<\/p><\/div>\n

\"View<\/a>

View of elevated guideway along arterial street. Image from Google Streetview.<\/p><\/div>\n

\"Seattle's<\/a>

Seattle’s light rail pier in roadway. Image from Google Streetview.<\/p><\/div>\n

On lower traffic roads, Seattle’s light rail includes several examples of dropping a pier in the middle of a roadway, rather than using a bigger straddle bent<\/a>.<\/p>\n

Bay Area:\u00a0<\/strong>BART’s elevated guideways don’t appear to use the same construction methods as WMATA, but have the same concrete aesthetic. In this case, the guideway runs adjacent to a residential street, while the area under the guideway is used for greenspace and a biking\/walking trail.<\/p>\n

\"BART<\/a>

BART viaduct, with greenway underneath. Image from Google Streetview.<\/p><\/div>\n

San Jose:\u00a0<\/strong>VTA light rail features several grade separations. VTA isn’t exactly<\/a> the kind of agency you’d want to emulate<\/a>\u00a0(good discussion here<\/a>\u00a0from Cap’n Transit). However, the basic geometry of their elevated track segments shows what kind of visual impact you can have with center-running elevated rail along wide roads. In this example, center-running light rail turns into an elevated alignment down the center of a wide arterial street:<\/p>\n

\"VTA<\/a>

VTA light rail elevated track above the center of an arterial street. Image from Google Streetview.<\/p><\/div>\n

\"VTA<\/a>

VTA light rail aerial station in the center of the roadway, with pedestrian access via normal sidewalk and crosswalk. Image from Google Streetview.<\/p><\/div>\n

Since VTA uses proof of payment, faregates aren’t necessary and allows for a minimal ‘mezzanine’ area for fare control. Contrast that to the visual bulk of the rather large mezzanines<\/a> in the Tysons Corner WMATA stations.<\/p>\n

\"VTA<\/a>

Aerial view of the same VTA station. Image from Google Maps.<\/p><\/div>\n

Any other examples to consider?<\/p>\n

Table of contents:<\/em><\/p>\n