Seismic retrofit of a historic city hall in California
Reinforcement of the Pasadena City Hall with DYWIDAG Prestressing Systems, Pasadena, CA, USA
Built in 1927 in Spanish Mission Style, the Pasadena City Hall is one of the most beautiful and historically most valuable buildings in Pasadena. So far, the building has survived the region’s many earthquakes. However, accor - ding to recent studies, the structure would no longer withstand a major earthquake of magnitude 7 or 8.
As a result, the Pasadena City Council decided on an extensive seismic retrofit and the execution of further reinforcement and restoration measures. The construction project also included the repair of damage caused by previous earthquakes and restoration of the architecturally and historically valuable façade.
The most important part of the reconstruction was the installation of a base isolation system designed to protect the 63 m high structure against seismic activities. To this end, a concrete "moat" was first built around the building. Subsequently, wide strips of ground under the existing basement were excavated and new grade beams were installed under the old slab elevation.
Using more than 1,000 36 mm Ø grade 150 DYWIDAG Threadbars, clamping beams were installed and connected to the existing columns and walls. Next, a total of 44 concrete transfer beams, incorporating two 9x0.6" MA DYWIDAG Multistrand Tendons each, were built and connected to the clamping beams.
The stressing operations presented a special challenge, since the DYWIDAG Multistrand Tendons in the 44 transfer beams had to be stressed simultaneously in a confined space. DSI USA designed a special trolley to fit in the new moat that could be pushed from one beam to the next. In this way, all beams could be easily stressed and grouted from outside the building.
A total of 240 isolators were placed between the new lower level grade beams and the new posttensioned transfer beams. Then, the bases of the existing columns and walls were demolished, and the entire weight of the building was transferred onto the isolators.
These isolators now totally support Pasadena City Hall and will dampen the movement of the building during a seismic event.
DSI USA is proud to have made a significant contribution to the long-term reinforcement of this historically important building with the utilization of versatile DYWIDAG Threadbar and Multistrand Tensioning Systems. During the one-year construction period, the post-tensioning strand tendons and threadbars were delivered just in time to the construction site by the nearby DSI facility in Long Beach, CA.