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Using this delivery method, the design team, a joint venture between architect Office for Metropolitan Architecture (OMA) and LMN Architects, hoped to gain greater control over the system’s final outcome by having technical expertise available during the design process. The bid was awarded to the German firm Seele GmbH under a design-build contract.

This collaboration between architect and manufacturer helped the curtain-wall system develop from the initial design concepts into a constructible reality with little lost in translation. The team reviewed the design of the system and its components from many aspects, including aesthetics, structural capacity, thermal performance, weatherproofing, maintenance, and constructibility.

In order to understand the construction of the curtain-wall system, some understanding of the building’s structural system is necessary. In general the structure is composed of both load-bearing and seismic systems. The load-bearing system, in the form of columns and beams, supports the elevated platforms containing program spaces.

The seismic structure — I-beam steel arranged in latticelike geometry — connects platform to platform, providing bracing during a seismic event. While both systems exist to ensure the building’s stability, they each create different conditions for the support of the curtain wall.

The predominant mullion system, sloping in both an overslung (skylight) and underslung (reverse skylight) orientation, became the basis for many design development studies. The final design incorporates a diamond module that marries the most efficient use of nonstandard glass panel shapes with adequate steel spanning capacity.

Since it is directly supported on seismic steel, this particular system incorporates a thinner overall depth with a thickened aluminum section to allow for greater spanning ability and fewer support connections. As a result, the engineered mullion system relies on only two connection points per diamond.

While adding efficiency to the curtain-wall system, the utilization of seismic steel for direct support also added much complexity. Due to its high cost, architectural grade steel could not be specified for the seismic structure; therefore, erection tolerance differences between the steel and curtain-wall system are considerable. To allow for these different tolerances, Seele developed setting blocks of various depths with hidden slots routed into them. The slots provide multiple planes of adjustment, allowing holes connecting steel and aluminum to align.

The mullion system’s multiple components are attached using a screw-within-a-screw attachment, allowing for sequenced installation of each component while minimizing the number of connection points needed. To emphasize the component construction of the curtain-wall system, each of these stainless-steel screw attachments is exposed to view.

A different mullion system had to be developed for the vertical glass facades. Unlike the sloping facades, seismic steel is not needed for structural support. For these areas the team designed a deeper aluminum mullion that would withstand lateral loading while spanning from floor to floor along a diagonal.

When viewed from the exterior, the appearance of both the vertical and sloped mullion systems match. On the interior however, the deeper aluminum profile of the vertical mullion body adds sufficient strength to sustain lateral curtain-wall loads between floors.

Originally designed as a rectangle profile, the mullion was later shaped to mimic the I-beam profile of the sloping seismic steel. A hollow void within the mullion section allows an aluminum splice plate to join mullions at tall facade sections. The void also provides space for aluminum plates to snap in at each floor edge, closing the gap between slab edge and glass.

The vertical weight of each facet is supported from below on armatures connected back to the structure. Lateral bracing is transferred through threaded rod attachments located along the edge of each floor. The threaded rods provide tolerance adjustment and are sized to flex when responding to thermal movement of the curtain-wall system.

Integral to the design development of the curtain wall was the envelope’s thermal performance. Except for the roof, louvers, and exposed concrete foundation walls, the exterior envelope is comprised entirely of vision glass. This posed a considerable challenge to the design team given the current energy codes and the level of thermal performance the curtain-wall system would be required to achieve. The entire envelope, including all of its component parts, was included in the energy calculations.   >>>

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Steve DelFraino is an architect at LMN Architects in Seattle, Washington.

Christine Killory is an architect and principal of Davids Killory Architecture in Berkeley, California. René Davids is an architect, a professor of architecture at the University of California, Berkeley, and a principal of Davids Killory Architecture.

This article is excerpted from Details in Contemporary Architecture, copyright © 2007, edited by Christine Killory & René Davids, available from Princeton Architectural Press and at Amazon.com.

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