Seattle Public Library - Detailing the Skin
To help meet the required performance level, approximately half of the insulated glazing panels were fabricated with airspaces containing krypton gas and newly developed high-performance low-E coatings. In addition, to combat increased solar heat gain experienced during the summer months, an aluminum expanded metal mesh interlayer was chosen for the glass panels receiving the most sun. The mesh’s mini-louvers provide shielding of direct sun as well as views to the exterior through the mesh. Ideal for shading, the microdiamond pattern of the metal mesh also mimics the larger diamond pattern of the curtain-wall mullions.
Originally envisioned as laminated between sheets of glass, the metal mesh floats within a 2-millimeter (0.08-inch) airspace in the final design. Clear low-iron glass is used in front of the mesh to brighten its appearance when viewed from the exterior.
To waterproof the building in Seattle’s rainy climate, Seele designed three levels of defense into the system: the exterior aluminum mullion cap with premolded silicone gaskets; insulated glazing panels with flexible butyl tape applied along every joint between panels; and silicone gaskets molded to fit the interior surface of the mullion body. While the mullion cap, seals, and glass keep most of the water from entering the system, the next two layers channel any remaining moisture to weep holes along the lower edge of each facade.
Even standing water caught at the lower vertex of each diamond is addressed by tooling a slight ramp in the sealant joints of adjacent mullions allowing for the water’s release. To verify these measures actually worked as designed, a portion of the building was built as a full-scale mock-up and forced to undergo extreme weather testing in controlled conditions.
The complicated geometry of the facade facets, including four that are nonplanar, meant increased scrutiny where adjacent surfaces meet to form a seam. Insulated aluminum closure panels were designed to transition across each seam while maintaining angle alignment with the adjacent planes of glass.
Because the geometry for each facet needed to be precise, the position of the seismic steel was adjusted to the curtain wall’s position in order to guarantee tight-fitting seams. A typical process would have the steel located first with the curtain wall layered over it.
Each of the building’s four general street elevations has a diamond module continuous from top to bottom, bending at each horizontal seam; because of differing geometries, the diamond module is not contiguous around corners. This creates four vertical corner conditions, one at each elevation’s edge.
Corner panels having an inward profile accentuate the discontinuity of the diamond pattern. All other panels have an in-plane alignment from facet to facet where diamond patterns align. This geometry was tracked throughout the course of the project by use of an unfolded elevation diagram depicting the best overall view of the curtain wall’s complexity.
Special attention was given to facets where water collection and snow retention were causes for concern. Gutters sized for the required capacity were incorporated as a thin trough across the lower edge of each facet. Since horizontal joints allowing for thermal movement could not be incorporated into the mullion’s diamond pattern, expansion space was provided at the leading edge of each gutter.
Three stainless-steel snow fences, 1/4 inch (6.4 millimeters) thick, are located in the gutter openings where necessary. Used to prevent snow and ice from sliding off the building’s sloped surfaces, the fences are shaped to appear as if unfolded out of the adjacent gutter void. Steel armatures that penetrate the gutter’s waterproofing layer and connect back to the structural steel support each fence.
To clean the expanses of exterior glass, a process inspired by mountain climbing is used. Traditional outriggers at the roof of the building allow window washers to descend each elevation from top to bottom on bosun chairs. While the skylight and vertical facades are scaled more easily, the underslung surfaces require additional measures to reach the glass.
Stainless-steel eyebolts, which protrude through the mullion body and cap, are spaced at close intervals. Each eyebolt is attached back to seismic steel members, providing load-carrying capacity. Window washers use carabiners to connect to these eyebolts in order to pull themselves within reach of the glass. These same eyebolts can also be utilized to support small work platforms should it be necessary to replace any underslung glass panels.
Fabricated entirely in Germany and shipped overseas to Seattle for installation, the library’s curtain-wall system made a long journey both physically and metaphorically. From sketches to shop drawings, the collaborative design and engineering effort produced a working envelope that fulfills stringent functional requirements while still expressing its earliest conceptual intentions.
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