Steel must be used as the primary structural material with special emphasis and understanding of a steel bracing system exposing the material’s slenderness, grace and transparency. Design proposals must contain at least one space that requires long-span steel structure, with special emphasis placed on innovation in steel design and an awareness of the bracing system incorporated in the design proposal.
For a proposed steel building, there are many different correct designs for a structure to carry gravity loads to the foundation or resist lateral loads. Understanding limitations and potentialities of steel as a material can improve the architectural design transforming weaknesses into strengths. Among many steel structural systems designed to resist lateral loads in steel structures, steel-braced frames are one of the most widely used in multistoried buildings. These frames are primarily placed in vertically aligned spans and allow obtaining a great increase of stiffness with a minimal added weight. Additionally, bracings can provide an increase in stability of the structure with lateral loading and help reduce lateral displacement significantly.
Traditionally, steel braces have been used in a concentric braced frame (CBFs) configuration, consisting of diagonal braces located in the plane of the frame. This system is the most common for medium-rise structures because it provides the same strength in both directions. In fact, both ends of the brace join at the end points of other framing members to form a truss, creating a stiff frame. CBFs may be arranged in several different configurations – such as X, V, inverted V or one-directional diagonal bracing, and single or multi-level bracing – and the bracing members may be designed to act in tension or compression or both. However, braces can interfere with architectural features and many architects have been experimenting with different results (IBM Building, Pittsburgh. Architect: Curtis and Davis; Hancock Building, Chicago. Architect: Fazlur Rahman Kahn, Bruce Graham; Hearst Tower, NYC. Architect: Norman Foster (Foster and Partners).
One solution to the architectural constraints is offered by eccentrically braced frames (EBFs). They provide the same advantages as CBFs but greater flexibility with architectural openings. Eccentric bracing consists of diagonal braces located in the plane of the frame where one or both ends of the brace do not join at the end points of other framing members. The system essentially combines the features of a moment frame and a concentrically braced frame, while providing excellent ductility for high-seismic regions of the country. The eccentric connection to the frame means an eccentric brace transfers lateral forces via shear either to another brace or to a vertical column. Architects have designed several eccentrically braced frames for prominent buildings in the United States from the 1960’s through the 1980’s (i.e, Office of LeMessurier Consultants). For these structures, designed prior to modern seismic provisions, eccentric bracing was the most efficient means to developing a stiff lateral system that accommodated the architectural program (http://www.lemessurier.com/work). However, more innovative designs have recently fully utilized the EBFs potentials (i.e. Fujisawa project, Tokyo, Japan, Architect: Richard Rogers). Among the many, the Sierra Bonita Affordable Housing project in West Hollywood, California by TIGHE Architecture represents a good example of how EBFs can be implemented in the architectural design process. This five story mixed-use affordable housing project features a courtyard with an eccentric brace frame core as a fundamental structural design component.
These newly evolving EBFs systems should be investigated more in terms of their structural and economic feasibility. The challenge for the next generation of buildings will be to make EBFs an adaptive structural system that responds to possible changes in occupancy levels and demands of site conditions. The complexities involved in the design should justify the continued construction within the constraint of limited resources providing innovative design solutions. Whether using EBFs or another structural system, the most compelling proposals will inevitably integrate the use of steel into the design of the project at multiple levels, from primary structure to building envelope and tectonic details. The project should be developed with an integrative approach to the innovative use of building materials and systems—spatial, structural, environmental and enclosure.