In order to differentiate the structure from the verticality of Union Station and the monumental arches of the portico, the transit center maintains a low gentle profile, "bending" the ground plane to create a shallow vault. Vaulting glazed surfaces meet the concrete buttresses rather than "walls," allowing lines of sight to flow over the structure. As in Burnham's design of shallow utilitarian vault over the train room, the bicycle center similarly celebrates the shift in our transportation paradigm with its distinct but modest shallow vaulted glazed arches.
This "non-building," without walls and roof, per se, is transparent and distinctly different than the granite-clad walls of the station. In fact, the glazed panels are more like a semi-transparent lens that allowsing the station to be seen through it as one move around the plaza. At the same time it discretely showcases bicycles and potentially other alternative modes of transit for visitors and commuters.
Reflecting the structural elegance of a bicycle in the center's design was an ongoing challenge. The BTC solves the challenge of vaulting in a similar manner similar to a rim and spoke wheel. The length of the structure is spanned by longitudinal steel tube arches. They, in turn, are stabilized by a series of transverse tension members wrapping the vaults and carrying loads to the perimeter of slab.
The tubing "rim and spoke wheel" approach maximizes lightness and efficiency by responding to specific conditions. The rim, in pure compression, balanced and stabilized by the spokes, in pure tension, creates an enormously efficient and elegant structure. In a similar manner, the vaults are tied together longitudinally by the slab, minimizing loads transferred to the roof of the metro station below. Inherently stable and acting as a shell or "helmet," the structure provides a continuous and open flexible space accommodating the changing needs of the BTC as it evolves over time.
The BTC could be described as something between a canopy and a building. The entire structure partially open, takes advantage of passive airflow when possible. The temperature of the parking area is typically is moderated by passive means, with or minimal mechanical ventilation utilized only, but only during seasonal temperature extremes. Because of the more demanding requirements of the retail segment, it is possible to seal the enclosure and mechanically heart or cool the space.
The design of the building's skin takes a number of environmental factors into account. The east and west exposures are differentiated. Like an eye, the BTC opens to Union Station to the east with transparent glazing. To the west — the "eye lid" is more opaque and protected. The east orientation has minimal solar exposure, due to the adjacent west portico, where it is a series of rotated and warped glazed planes acting as horizontal louvers allowing airflow but protecting from the elements. The west orientation, exposed to direct sun as well as the elements, is a single warped plane. It contains is openings at the top and bottom to take advantage of the chimney effect to promote air movement through the structure. Vertical louvers help to shade the late afternoon sun. Additionally, low e-coated single glazing limits heat gain but allows visibility. Rotating west to east, the coating progressively diminishes in each "louver" allowing full transparency on the east- most surfaces.
To minimize mechanical venting, cooling occurs throughis accomplished with a staged environmental control system which initially uses automated vents maximizing natural convection. If required, fans provide two air changes per minute, equalizing temperatures with the exterior. If further cooling is required in the double glazed retail area, the automated vents close, sealing the retail area for effective mechanical cooling. In winter the automated vents close to take advantage of solar heating as is required.
Solar heat gain provides benefits as well as a challenge for maintaining a comfortable environment. Ceramic frit ands well as a low-e film allow for a reasonable balance between thermal solar and visible light. Double layered ceramic frit oriented to minimize the high midday solar gain, while allowing horizontal visibility, helps to maintain a reasonable amount of transparency at the pedestrian level.
Donald Paine is a principal, RA, LEED AP, with KGP Design Studio.