2008 Issue

UTAH ENGINEERS COUNCIL JOURNAL 13 very elaborate in nature and are not merely decora- tion. The building itself along with the contents within comprise a singular historic collection repre- senting the culture, history and heritage of the region and of the United States. If lost, the building and its contents cannot be feasibly replaced. Indeed, for the expected seismic event the likelihood of complete loss of the building was high, a tragedy that would only be surpassed by the loss of life that it would accompany. Shortly after the knowl- edge of Utah’s seismic po- tential came to light, con- cerns regarding the seismic performance of the inventory of archaic buildings along the Wasatch Front began to rise. Chief among these was the Utah State Capitol. Real mo- mentum regarding seismic concerns of the Capitol seismic began in the mid 1980s with preliminary studies and ancillary cost models for proposed upgrade. Additional studies fol- lowed in the early 90s with a comprehensive historic structures report and seismic evalua- tion beginning in the late 1990s. Evaluations accompanying the historic structures report confirmed the fears regarding the expected seismic performance of the building. In addition, the evaluation pointed to seismic base isolation as the most viable candidate for improving expected seismic performance. Not only was this identified as the most ame- nable solution in regard to maintaining the character and historic nature of the building, it was also deemed the most cost feasible since it provided for the least amount of disruption to the structure above the foundation. In Fall 2002, design efforts began for the seismic base isolation and renovation of the Capitol. Two years later the building was officially closed while the massive undertak- ing of removing and replacing the building’s foundation began. Over the following years, a series of 265 individual base isolators were installed beneath the building. On May 7, 2007 the isolators were officially mobilized, thus marking the completion of the base isolation system. Other efforts continued through this time period and through 2007 to complete the renovation project which included the addition of a limited amount of new concrete shearwalls in the superstructure as well as the stabilization of many existing assemblies including the dome and drum. As part of the renovation, the building was returned to its original historic configuration with floor layouts and architecture consistent with historic drawings and photos. Whereas conven- tional seismic retrofits have the primary objec- tive of increasing struc- tural strength, seismic base isolation has the objective of reducing the intensity of the ground motion. For many ex- isting structures the incorporation of new seismic systems is not practical. Such systems typically consist of new concrete walls or braces, the extent of which is dependent on the nature of expected ground motions and the mass and configuration of the building in question. Such retrofitting at the Utah State Capitol would have been extensive insomuch that it would likely have altered the historic nature and con- figuration of the building. Rather than compromise the building’s character with a new seismic system, the base isolation system dramatically reduces the intensity of an earthquake thereby minimizing the amount of seismic retrofit- ting necessary to enable satisfactory performance. Base isolation filters out the most severe ground motions by physically de- coupling the building from the ground. While the ground is subject to violent horizontal shaking the isolated building literally floats above the ground in relative safety. The seismic base isolators consist of laminated plates of rubber and steel in a cylindrical configuration. The isolators are typically 20 inches in height with diameters ranging from 34 to 44 inches. Each isolator is designed for a horizontal displacement (from neutral) of 24 inches in any direction, making a total swing from one extreme to the other of 48 inches. A covered moat space around the building perimeter pro- vides an empty volume in which the building can displace. Though the base isolation can- not completely eliminate lateral accelerations from propagating from the earth and into the building, the peak ac- celerations are expected to be reduced by as much as 75 to 80 percent. This translates directly to a reduction in forces for the global structure and all of its components. continued on page 14 Shortly after the knowledge of Utah’s seismic potential came to light, concerns regarding the seismic performance of the inven- tory of archaic buildings along the Wasatch Front began to rise.