Here is a feature from the May issue of Modern Steel Construction: “Fifty years after being built, the Belmont Blue Line Station is no longer a stylistic afterthought, but rather has been transformed into one of the most recognizable stations in the Chicago Transit Authority’s (CTA) vast rail system.
As part of the ‘Your New Blue’ program, CTA is re-envisioning and improving 14 Blue Line ‘L’ train stations. The Belmont Station had not been significantly modified since its construction in 1970, so upgrading the entrance provided an opportunity to improve the station’s visual presence and create a community focal point within Chicago’s Avondale neighborhood. The design, inspired by a waterfall from the bygone Olson Park, becomes animated when it rains as water cascades down the sloping canopy.
The canopy structure is formed by five petal-shaped, architecturally exposed structural steel (AESS) frames that cantilever 68 ft over the station’s plaza and 28 ft in the other direction. AESS was chosen as a way to emphasize the overlapping outlines of the petals without adding unnecessary cladding. The primary framing members that form the outline of the petals are built-up rectangular tube sections that support hollow structural section (HSS) purlins that connect to the canopy’s blue polycarbonate panels. The petals frame into a horizontal spine at the low point of the slope with custom castings that are supported on three 38-in. steel-encased concrete pipe columns concealing the drainage downspouts within the concrete.
The canopy’s geometry required the primary structural framing members to be curved in one dimension, sloped in a second dimension, and tapered in the third dimension. Through creative design and by evaluating how the geometry influenced the strength requirements, the project team was able to simplify fabrication. The primary HSS framing members efficiently resist biaxial and torsional stresses…
The overall structural system includes the cantilevered canopy supported on three columns, and over 90% of the project’s total weight (approximately 162 tons) is located in the canopies. A net-work of primary framing members provides stiffness to control deformations and is anchored to the ground with large concrete filled steel columns that act as “tree trunks” to support the overlap-ping steel “branches” of the canopy.
To facilitate the connection at the geometrically complex regions where the petal loops intersect, built-up plate nodes were prefabricated to join the members and provide the desired aesthetic of member cleanly passing through each other. Prefabrication simplified erection and field welding and provided the means to achieve the AESS Category 3 requirements of visually seamless joints….
This design-build project provided many opportunities for the design, casting, and fabrication teams to collaborate. Bringing these parties together early allowed the project team to work together to develop creative, efficient, and successful solutions. The design team used 3D and structural analysis models to coordinate and evaluate this complex structure, as well as to help facilitate information sharing. Architectural models developed in Rhino 3D were incorporated in the structural analysis model while customized software packages were used to automate portions of the analysis and evaluate numerous iterations and structural variables: the size of the AESS framing, considering the varying cantilever lengths; framing plans (e.g., numbers of columns, petals, and intersecting petal nodes); plate thickness for the primary petal framing; column diameters and thicknesses; and the amount of welding required. The automated parametric structural analyses enabled the design team to analyze stresses and defections for each combination of options and achieve the aesthetic goals while helping to minimize fabrication and erection costs…”
Read more in the May 2020 issue of Modern Steel Construction.