Seattle's "Blue Glass Passage" takes advantage of SentryGlas® Plus structural interlayer

Blue Glass Passage, Seattle City Hall.

On James Carpenter's Blue Glass Passage at Seattle's new City Hall, fully exposed edges and the lack of cumbersome fixtures give the glass bridge the look of a "transposed slice of water". DuPont™ SentryGlas® Plus interlayers enabled aluminum inserts to be laminated directly into the bridge's glass floor, giving an innovative technical solution for the aesthetic look the designers wanted.

In a unique application of DuPont™ SentryGlas® Plus, James Carpenter Design Associates (JCDA) of New York has created a striking, cobalt blue, laminated glass bridge, 20 m long, over which Seattle City Hall council members walk to enter the building's chambers.

"'New digs of cobalt blue' are a main event" – Seattle Times
So pleased was the Seattle City Council with Carpenter's Blue Glass Passage that straight after the City Hall's ribbon cutting ceremony on the office tower side of the bridge, the full Council ceremoniously walked across the astonishing blue walkway to open their first regular meeting that day.

Council president Peter Steinbrueck said: "The bridge is ethereal. Besides being an elegant walking experience it is a functional bridge to the People's Hall, where everyday decisions are made affecting people's lives". As the Seattle Times of July 8, 2003 commented: "'The new digs of cobalt blue' (referring to the walkway and the council chambers) are a main event in the new City Hall."

Seattle's City Hall, designed by architects Bohlin, Cywinski, Jackson (BCJ) in association with Basetti Architects, both of Seattle, occupies an extraordinary site that affords views onto Elliot Bay, Puget Sound and the Olympic Mountains beyond.

Phantasmagoric effect from below.

SentryGlas® Plus™: A transparent engineering material with dependable results.
Blue Glass Passage designer James Carpenter said: "In order to take advantage of, and reinforce the notion of residing above the city, we proposed a glass bridge that essentially floats above and through the City Hall's main lobby area. This was made possible only be the incorporation of new SentryGlas® Plus."

The high-strength structural interlayer enables architects to design a robust new generation of laminated glass applications that meet stringent security – or seismic – standards. SentryGlas® Plus structural interlayers bond well to a range of materials beyond glass, meaning that enhanced performance can be 'engineered in' to the overall construction.

"A bar of captured light" - Carpenter
The floor of the bridge was always conceived by Carpenter and his design associates as being in blue glass, making the bridge connection resemble a transposed slice of water – the Puget Sound being so omnipresent to Seattle residents. The bridge also includes a generous, wooden handrail that one can lean against to look out at the views. By interfacing with the adjacent floor slabs the bridge marks its own site and was intended to encourage conversations and interaction between city council individuals and members of the public attending the sessions.

JCDA's Choi says: "We were very pleased with the way the laminated glass sits so well with the steel frame. Fully exposed edges were important to us as part of the effect we wanted of 'a transposed strip of water'.

Carpenter continued: "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." While light penetrates its surface, people or objects on the bridge are seen only as shadows by anyone standing below.

"The floor panels act as a solid lateral beam" – O'Callaghan
James O'Callaghan, Senior Associate at structural engineering firm Dewhurst Macfarlane and Partners of London and New York, said: "SentryGlas® Plus as an interlayer has a fantastic potential for laminating metallic inserts into glass panels. This breakthrough on its own opens a myriad of possibilities in terms of concealed fixtures. The weaving of SentryGlas® Plus into the blue glass bridge eliminated the need for cumbersome fixtures.

"Another great feature of SentryGlas® Plus interlayer harnessed on this project is its stiffness once laminated resulting in the ability to span glass panels further with lower deflections that the same panels with PVB.

"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 7 ft 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 ft on centre. The 5 ft wide floor 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, creating a shear key and resulting in all the independent panels acting as a solid lateral beam. So the interrelationship between the glass panels is critical for lateral, seismic and gravitational loading cases. Clearly, with this level of reliance and the very public location of the structure, redundancy in the panels is a vital design feature and this is always best achieved using laminated glass technology."

"A transposed strip of water" - Choi
JCDA designer Choon Choi remarked: "The City Council and the Seattle architectural committee liked the idea of a 'transposed strip of water' for Council Members to walk across. But everyone immediately also said: "Wow! Great idea! But how will you do it?" 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.

"We are very pleased that the laminated glass sits so well with the steel structural frame. Fully exposed edges were important to us as part of the concept of the 'transposed strip of water'.

"In addition, the laminated glass balustrade works as a stiffener or truss to the bridge. All the laminated glass in this project is designed to work together as a full structural member of the building.

Aluminum inserts laminated directly into glass floor panels
"Without SentryGlas® Plus, we would have had to cut out a series of small notches in each of the glass floor panels, attach various types of aluminum inserts into these notches with extremely tight tolerances between the steel and glass, and then precisely locate each floor panel over the steel rails underneath, to align dozens of bolted connection points.

"As the number of parts continued to increase, the complexity of the proposed installation method, coupled with the rising uncertainty in the glass fabrication process, threatened to bring the project to a standstill.

"Incorporating SentryGlas® Plus proved to be infinitely simpler – and therefore cost efficient - from a fabrication standpoint. Our JCDA design team simplified the design approach by laminating the aluminum inserts directly into the floor panel. This allowed us to eliminate multiple steps in the subsequent installation process. Extensive testing when using Plus™ technology in other major projects we are working on allowed us to overcome any reservations we may have had (for example possible peeling of the lamination due to the curvature of the floor panel under a live load) about how laminated glass incorporating SentryGlas® Plus™ technology would stand the test of time. Specialists from DuPont were incredibly helpful in providing detailed stress calculations and other application data for SentryGlas® Plus."

The Blue Glass Passage consists of 12 pieces of 7.5 ft x 5 ft panels of laminated glass that appear to be "floating in the air", according to Choi. The total thickness of the glass and aluminum bar construction is 2.25 inches and its walking surface has been treated with a textured glass on the top layer to assure that it is skid proof and safe.

Phantasmagoric effect from below
Laminator Wes Depp of Depp Glass, Long Island, New York, said: "It was a challenge to achieve the thickness and colour the designers wanted and of course this was the first time that we had laminated aluminum bars into the glass itself. We used water-jet cutouts to achieve the laminated glass shapes that had been specified, which were each 2.25 inches thick. DuPont™ SentryGlas® Plus gave the laminated glass construction the stiffness and overall strength it needed. Without this technology, the Blue Glass Passage would not have been possible. The views when you're underneath the bridge and see people walking across it above are phantasmagoric, giving a mystical blue effect in that you can see the footsteps and shadows of the people through the translucent, cobalt blue glass but nothing more. It's quite beautiful."

Seattle Council president Peter Steinbreuk says: "The bridge is ethereal. Besides being an elegant walkway experience it is a functional bridge to the People's Hall where nearly every day decisions are made affecting people's lives."

City Hall: a new landmark for Seattle
Architects BCJ were briefed to design a dignified new symbol of the city and its people; a very prideful, very Northwest civic space. "The city's never had a City Hall you go to and you are excited about," said Ken Johnson, project director for the city. "We wanted to change that."

As BCJ principal Peter Bohlin put it: "Beyond a technologically capable, seismically sound and energy-efficient new home for the city government, our team took on an additional charge: to design a building that is at once welcoming and approachable, but also a memorable downtown landmark."

Rick Sundberg, an architect and former chairman of the Seattle Design Committee, which reviewed the project, said: "My hope is that the council chamber ensemble, including the Blue Glass Passage, will become more a symbol of our city than the Space Needle and the monorail." "The big thing we're going for is sustainability," added Martin Munguia, communications specialist for the city council. "We wanted the building to be something people would be proud of, and glad to be going to."