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Advertising supplement provided by DuPont Glass Laminating Solutions, E.I. du Pont de Nemours and Company

Seismic innovations

A passageway in Seattle’s new City Hall was envisioned as a “transposed strip of water” for City Council members to traverse. The idea was well-received although the common response was, “Great idea! But how will you do it?” according to Choon Choi, designer at James Carpenter Design Associates. “It took us three years to figure out how the structural glazing on the floor could act as a safe structural member instead of an infill member in this building in the seismically challenged town of Seattle.”

James Carpenter’s “Blue Glass Passage,” a laminated glass bridge with fully exposed edges and a striking cobalt color that member use to enter the chamber, was made possible by a structural interlayer that allowed aluminum inserts to be incorporated directly into the structure’s floor.

The 20-meter passageway links the chambers and offices and floats above the main lobby area of City Hall, which was designed by Bohlin, Cywinski, Jackson in association with Basetti Architects, both of Seattle. The floor of the bridge was always conceived as being in blue glass, as a visual association with Puget Sound.

“This bar of captured light, floating through the lobby, silhouettes and presents the activities and movements of the people within the building to the city passers-by below,” said Carpenter. While light penetrates its surface, people or objects on the bridge are seen only as shadows by anyone standing below.

James O'Callaghan, senior associate at structural engineering firm Dewhurst Macfarlane and Partners of London and New York, said that the structural interlayer allowed laminating metallic inserts into glass panels, opening up many possibilities in terms of concealed fixtures. Weaving the interlayer into the blue glass bridge eliminated the need for cumbersome fixtures, he said.

The structure of the bridge owes its integrity entirely to the action of the glass floor and its interaction with the glass guardrail. The glass floor spans seven feet between two stainless steel rails, which in turn are supported by hangers on either side. Visibility from the leaning plate side of the bridge was maximized by the subtle spacing of the hangers at every 10 feet on center. The five-feet-wide glass panels have an intermediate support via the laminated glass guardrail acting as a beam between the hanger rods. The floor panels are interlocked to one another using the continuity of the stainless steel rails and the laminated aluminum channels set in the floor glass.

“The interrelationship between the glass panels is critical for lateral, seismic and gravitational loading cases,” O’Callaghan said. “Clearly, with this level of reliance and the very public location of the structure, redundancy in the panels is a vital design feature.”

Carpenter also worked with an advanced polymer or structural interlayer in creating dome ceilings over a courtroom and atrium in the new federal courthouse in Phoenix by architect Richard Meier of Richard Meier and Partners. The courtroom won the National Design Award for 1999, and Carpenter won a design excellence award from the General Services Administration as well.

The ceilings act as a lens serving both structural and light purposes. “It gathers and redeems daylight and redistributes it throughout the building,” Carpenter said. The laminated glass ceiling also serves an acoustic future. Sound is distributed evenly under the dish-shaped ceiling, whereas traditional dome-shaped atria tend to carry sound away.

The lowest layer of the lens is hanging purely by its adhesion to the structural interlayer, which continues out beyond trapezoidal glass panes to form tabs for drilling. These are used as structural members to support the roof and act as buffers in the case of seismic loading, fulfilling safety requirements for overhead glazing and the sense of openness that Meier sought.

“We wanted to introduce some ‘softness’ into the system and a corner tab fixing detail allowed us to do just that,” said Matt King of the structural engineering firm Ove Arup, part of the courthouse team. “Instead of rigidly joining the pieces of glass together, the tabs will ‘give’ in the event of an earthquake – they quite literally act as shock absorbers.”