2008 Issue

www.utahengineerscouncil.org 14 The performance of a base isolated build- ing is enhanced when the superstructure above the isolators can be made as stiff as possible. This reduces the inter-story drifts and concentrates the primary dis- tortions at the base isolation system itself. For the Utah State Capitol, real estate occupied by an original ventilation system was harvested for new concrete shearwalls. The venti- lation shafts flanked existing building columns at the building perimeter and were also present at various struc- tural gridlines within the building footprint. Walls bounding new stair and elevator shafts at back-of-house locations were also utilized as new concrete shearwalls while other new walls were located within the hollow spaces of the massive rotunda piers. Careful coordination of new shearwall loca- tions enabled the placement of new shearwalls in a virtually inconspicuous manner. Unless occupants know exactly where to look, the additional walls are non-apparent. The dome and drum represented an interesting challenge with regard to the seis- mic renovation. Seismic forces amplify with increased height in a building. The concrete of the dome and drumwas found to be among the worst in the entire building. The net result was a high likelihood of failure for the dome and drum with an increased potential for partial or full collapse above that of the base structure. To address these issues a multi-fac- eted approach was developed. First, the base isolation system and shearwalls were tuned to create the maximum potential reduction of seismic amplification of the dome and drum. Second, the mass of the dome and drum was significantly reduced by replacing the massive solid concrete colonnade (consisting of 24 individual columns) with a much lighter tube steel frame clad in terra cotta. The addition of terra cotta actually enabled the realization of an original cladding concept abandoned during original construction due to lack of funds. The much lighter system also means a significant reduction in total seismic forces on the dome and drum structure. Third, the badly deteriorated concrete was prepared and reinforced with a new layer of reinforced con- crete at the interior surface. This provides the added strength needed to compensate for the deficient concrete mentioned previously. Other seismic strengthening measures included the installation of steel frames to brace the massive unreinforced masonry para- pets. Other steel frames were added within the void spaces of the attic and pediments to provide supplementary strength to themassive Mr. Johnson is a principal in Reaveley Engineers & Associates UTAH STATE CAPITOL — continued pediments located above each of the primary entrances. The granite columns at the south, east and west entrances were also stabilized with an innovative yet totally inconspicu- ous method of epoxy reinforcement thereby increasing their strength against lateral buck- ling. Strengthening of nonstructural elements included bracing of hollow clay tile partitions, adding seismic anchorages to stone ornamentation and stabilizing inte- rior baluster assemblies precariously located at balcony edges of the third and fourth flours. As opposed to the bleak diag- nosis of the building following the historic structures report (and other reports), the current projections regarding expected seismic perfor- mance are excellent. Though no degree of work can render a building completely invulnerable to a quake, the work incorporated at the Utah State Capitol improves the expected outcome to such a degree that the building occupants and the building itself are expected to survive a major quake with rela- tively little apparent injury or damage.