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April 17, 2003
Seattle’s new City Hall is a response to the demand for a more accessible facility that provides a public forum and encourages participation in government.
At the same time, City Hall delivers an environmentally sensitive and secure public building. Despite these simple goals, the design and construction of City Hall provided the project team with a series of complexities that resulted in bold solutions and innovations.
Complexities and innovations
One of the main challenges in designing City Hall was its constrained location and phased construction. In some areas, the new City Hall and the existing Municipal Building are within an inch of each other. Before City Hall is fully constructed, the occupants of the Municipal Building will move into their new space at City Hall. After the move, the Municipal Building will be demolished, thus allowing for the construction of City Hall to be completed.
Following demolition, portions of City Hall will extend further out, hanging over the footprint of the former Municipal Building. The design team worked closely with the general contractor, Hoffman Construction, to coordinate the challenging design and construction sequence.
The north side of the building is an office tower housing the Council and mayor’s offices. The south side contains the main Council chamber and a larger public reception room below it that allows more citizens to attend city proceedings. Because of these two distinct areas, City Hall functions as two separate buildings connected by a green roof, various passageways, and the main public lobby.
The exterior and public spaces of the building are ringed by tall, slender steel columns. The design is innovative because steel columns are usually fireproofed with a spray-on foaming paint that removes the steel look of the column and can degrade its aesthetic quality. With their steel appearance left intact, the designers were able to incorporate the columns as architectural features.
For seismic stability, the designers used eccentric braced frames that dampen, or absorb, the energy generated by an earthquake.
City Hall incorporates numerous techniques in sustainable design.
The building is located on the site of the former municipal garage. Portions of the existing garage basement walls were used to replace additional shoring of the building and enabled the re-use of existing materials.
A green roof atop City Hall acts as a visual connection between the buildings. In designing the green roof, the team had to consider the landscape features, which carry a live load of 75 pounds per square foot, three times what is usually considered in the design of normal roofs. The green roof significantly lowers rooftop heat generation when compared with typical buildings.
Rainwater also plays a key role in another of City Hall’s sustainable elements. A massive concrete tank, which will be located in the west plaza (currently the site of the existing Municipal Building), will be used for harvesting rainwater. The massive 30,000-cubic-foot tank collects rainwater during the peak rainy seasons for use during dry periods for toilet flushing and irrigation. The intent of the rainwater harvesting system is to maximize water efficiency in the building and reduce the burden on the municipal water supply and wastewater systems.
When the Municipal Building is demolished, the basement of the building will be retained. Using the existing basement eliminated the need for the design and construction of a new tieback wall system, thereby adding another element of sustainability to the project.
One of the goals of the design team, led by Bassetti Architects/Bohlin Cywinski Jackson, was to keep City Hall inviting and accessible to the public. The intent is to provide a facility that encourages public participation in government and the policy making process.
The key consideration in designing the public spaces was to maintain accessibility to the facility while keeping the occupants, visitors and building secure.
Constructing a fortress, which would have provided security, was not a feasible option because this type of structure is not inviting or easily accessible.
Instead, several innovative security features were incorporated into the project. The building employs structural redundancy and structural hardening features (strengthening the building so it is not a source of problems after an attack) to increase the safety of the building’s occupants and visitors.
Structural redundancy (the number of structural components beyond what is required for stability in the building) is achieved through load distribution (the amount of weight a structure bears) — that is, incorporating multiple columns.
City Hall’s pedestrian-friendly site encourages pedestrian activities while hindering vehicle intrusions to the building. One method of increasing security at the site is to install energy-absorbing bollards that resemble 4-foot-high steel tubes. In addition, terraced walls along Fourth Avenue between Cherry and James streets form natural security barriers.
Seattle’s living room
City Hall is a landmark structure that serves as the heart of our city and a symbol of our civic pride. The design focuses on public amenities that encourage involvement in our local government and process. City Hall is the community’s ‘living room,’ a place for citizens to converge and share our diverse perspectives.
Paul Diedrich, an associate with KPFF Consulting Engineers, is the project manager for Seattle City Hall.