For example, the tests showed prevailing winds come across the face of the Hancock Building and hit the Raffles tower directly perpendicular on its northern facade, the Stuart Street side, before pushing downward. The design decision to create a curved profile to the building aided the wind performance by forming an airfoil effect, deflecting wind pressures around its perimeter and reducing some of the high-pressure critical zones. In conjunction with the color and reflectivity of the glass itself, the curvilinear profile also offered a strong point of formal differentiation from the adjacent Hancock. Another realization from the studies was the Raffles tower needing an entrance canopy to project over the sidewalk for a significant distance to prevent the wind energy from moving straight downwards and onto pedestrians.
Instead of using curved glass to form the building’s rounded profile, the project team decided to create smaller individual facade modules with gently faceted mullions, which created the same profile when assembled. This generated a narrow, tall shape for the modules—one bay wide but three lites high—to accommodate the spandrel glass covering the deck edge, the vision glass, and a band of operable windows at the residences. For those, the project team selected awning-type windows, which have certain benefits from a performance and weather perspective. Since the units are hinged at the top and the bottom pushes out as the windows open, the top is sealed and the air comes in from the bottom, acting as a roof and shedding any water over the face of the window.
Curtain wall-building structure interface
Wind and related environmental conditions, including potential seismic forces, also influenced the curtain wall design, the connections between the facade system, and the building structure.
For this as well, structural wind load modeling and other environmental testing, including seismic racking tests, revealed to the design team how much and what types of movement they should expect from the building, including forward, backward, and twisting or torsion forces. An especially critical measure and indicator is the interstory drift ratio (IDR), which describes the relative translational displacement between two consecutive building floors, divided by the height of a given story. Having an accurate IDR is critical for designing the right amount of movement into the curtain wall and anchoring system, and for understanding the range of conditions in which the curtain wall and building structure will need to perform.
For the Raffles project and other sites with similar conditions, a worst-case scenario might call for 25.4 mm (1 in.) of movement, whereas a typical condition would be closer to 12.7 mm (0.5 in.). In both extreme and normal conditions, facade flexibility is key, and this is another advantage of a unitized curtain wall approach, which offers inherent flexibility from panel to panel because it is a system of individual units attached to the building and they overlap at gasketed connection points. Additionally, there are horizontal “stack joints” designed to accommodate differential movement between the building stories.
