For a new corporate headquarters in San Francisco's Mission Bay district, SHoP Architects designed a semi-conditioned atrium space inside the external glazed envelope known as “the commons.” To ventilate this space, the facade contractor developed a transparent, steel-framed glazing system featuring motorized bi-fold units. These operable window units, 3.0 m wide by 4.4 m tall, are computer-controlled in conjunction with operable skylights to maintain comfortable temperatures in the atrium and significantly reduce the need for mechanical ventilation. This paper describes the engineering and design development of this dynamic facade. A number of kinematic options were studied, resulting in the final configuration whereby window operation is guided in the visible upper and lower rails. In the upper rails, the self-weight is resolved via newly developed double vertical hybrid rollers. The necessary lateral stability is provided by scissor arms mounted on both sides at the top and bottom, which prevent the window from moving sideways on one side over the entire opening stroke. The windows are operated via two of the currently largest chain drives on the market (lift/pull force 1500 N). In addition, a linear locking drive moves a push rod with three double claws, which additionally keep the window in the “closed” position. The chain drives are synchronized and internally controlled with the linear locking drive. All components are integrated and concealed in the central mullion. A complete opening takes approximately 155 seconds. Safety strips integrated circumferentially in the frame protect the device, while closing, from objects that are accidentally left in the window rebate and will switch off the device automatically. The units are constructed according to the European Machinery Directive 2006/42/EG, and a risk analysis was carried out in advance. The drive technology and all electrical components are UL-certified.