The State of Utah recently embarked on a complete historic preservation and base isolation retrofit to their historic capitol. To protect this massive, yet ornate building in an earthquake, the building was strengthened using 256 state-of-the-art seismic isolators under the building. VSL provided supply and installation of a bonded post-tensioning system as part of the seismic isolation of the dome.
One of the main challenges retrofitting any historic building with base isolation is temporary support of the existing structure while inserting isolators and creating an integrated final support system, which in the case of this project, was intensified by the 7,000 kip dead load carried by each of the four large piers/footings that support the main dome. For safety and economy of construction, the designers developed a method of permanently re-supporting the rotunda of the capitol with a unique circumferential post-tensioned concrete load transfer scheme that necessitated the need for temporary support of the existing footings, eliminated the risk of settlement, minimized the need for demolition, and avoided cutting or damaging the existing footing reinforcement.
The rotunda load-transfer and isolation system consists of large post-tensioned concrete beams and girders that wrap completely around the "stem" of the existing pier footing. The beams form a single composite section with the existing footing and span 51 feet between girders. The supporting girders are each supported on four 45-inch diameter lead rubber isolation bearings founded on new footings, and placed outside the footprint of the original rotunda footing. At each rotunda footing, the new composite beam section supports the design loads through the use of 16 37-0.6" bonded tendons. These tendons are both vertically and horizontally draped on a skewed plane so that the tendons simultaneously "hug" the existing concrete tower base and balance the high vertical loads.
Due to the high strength and stiffness afforded by bonded multistrand post-tensioning, the design team produced an innovative, safe and economical load transfer system that protects the historic fabric of the rotunda of the newly renovated capitol.