A pedestrian staircase was part of a project to transform an old parking garage into an inviting entrance to Renton’s new City Hall. Photo by James Wiley

The new access ramp at Renton City Hall transformed an old parking garage into an inviting entrance to the new City Hall. The ramp was part of the city’s retrofit of a downtown office building with attached parking for the new City Hall.

The city decided it would be safer for the public to access the facility from South Grady Way rather than using the existing, less visible, access from Main Avenue South. This change required an aerial ramp to allow traffic from the ground floor to access the second and third floors of the parking structure. The project included adding parking, an elevator and a pedestrian staircase.

Environmental constraints made ramp construction challenging. Almost all of downtown Renton is constructed on an old lake bed containing bog material that is susceptible to liquefaction during earthquakes, plus the existing parking garage had undergone extensive long-term settlement.

Another challenge was the project’s design called for a massive underground stormwater retention facility that was positioned close to the finished grade.

High-strength concrete, 5,000 psi, was used for all structural and flatwork elements. Cast-in-place concrete was used for the vehicular access ramp and barrier wall, stairwell, elevator shaft, walkways, courtyard and parking lot.

To protect shallow underground utilities and stormwater detention piping from heavy trucks, the main access road to the ramp was designed in concrete.

For areas where large fills were required, a system of reinforced concrete over structural Styrofoam was used. The lightweight system was favored because of the soft, compressible areas below and long-term settlement issues.

During the Feb. 28 earthquake, the access ramp had no apparent settlement due to liquefaction. The rest of the project also faired well, with only a few trim tiles coming loose from around the elevator.