By Lori Jue, SE, Michaella DeRubeis, PE, and Geoff Bomba, SE
As introduced in Parts 1 and 2 of this three-part series, the Oregon State Capitol Building Renovation project was designed to achieve the goals outlined in a 2009 master plan, which recommended seismic repairs, life-safety improvements, and renovations to improve the Legislature’s operational efficiency. Seismic base isolation was chosen as the preferred approach given the historic nature of the 1938 Capitol structure and its associated brittle finishes and non-ductile structural elements.
A significant challenge of base isolation is altering the vertical load path for the building from its existing foundation system onto a new foundation that includes the seismic isolation bearings. Combined with implementing seismic isolation, a new structural level was added below the building to increase the square footage and add the much-needed hearing rooms and meeting spaces. Further, the adjacent 1977 Connector building was structurally tied into the 1938 building to create a single, larger, isolated building to maintain programmatic connections without large seismic joints between them and enhance the performance of the 1977 building.
The historic 1938 Capitol building is a large concrete frame structure with a central drum and rotunda over 350,000 square feet across four stories, including the new concourse level. The Senate and House chambers are two clear-story volumes flanking the rotunda symmetrically. The structure consists of non-ductile detailed cast-in-place concrete framing with limited areas of unreinforced masonry (URM) bearing walls. Floors consist of reinforced concrete pan joist slabs spanning to concrete beams supported by concrete columns. The central drum has a cylindrical shape with a concrete roof slab spanning to steel beams supported on a circular brick bearing wall. The existing lateral system is the non-ductile concrete frame infilled with red brick URM. The existing 1977 Connector building is also a non-ductile concrete building with concrete shear walls. It is a two-story building with a ground floor parking garage, hearing room spaces above, and a roof terrace.
The combined seismically base isolated structure is surrounded by a minimum 2’-0” seismic joint that allows the entire complex to move as a unit up to 2’-0” in any lateral direction. The plane of isolation is below the 1938 building’s new concourse level and courtyard infills, and at the north side of the existing 1977 Connector, transitions up to the top of the parking garage columns.
Code Provisions and Performance Objectives:
The owner’s performance goal for the seismic renovation is to have the Capitol building operational within 30 days following a seismic event so that the legislature can convene throughout the response and recovery timeframe. The retrofit uses the 2019 Oregon Structural Specialty Code (OSSC) and ASCE-41-17 – Seismic Evaluation and Retrofit of Existing Buildings as the main code standards used for the project. To meet the owner’s described performance goal and preserve the building’s historic nature, the chosen level of seismic performance is Immediate Occupancy (S-1) at the BSE-1N hazard level (475-year recurrence period) and Life Safety (S-3) at the BSE-2N hazard level (2475-year recurrence period). The chosen level of non-structural performance is Position Retention (N-B) at the BSE-1N hazard level and Life Safety (N-C) at the BSE-2N hazard level. Combining the structural and non-structural performance levels gives the overall building performance level. These objectives aim to provide a retrofit with less damage to sensitive historic elements and allow quicker re-occupancy of the building. Since the seismic base isolators provide energy dissipation and damping of the seismic movement, the superstructure exhibits significantly lower drifts and ductility demands than fixed-base structures. This results in lower m-factors meeting the more stringent selected performance criteria. For acceptance criteria in ASCE 41-17 when using seismic isolation, there’s a cap of 1.5 for m-factors used for Immediate Occupancy and Life Safety.
Although ASCE 41-17 generally does not require vertical seismic loads to be considered in the design of structural elements, there are two elements in this project where vertical seismic accelerations are considered: the post-tensioned gravity transfer system at the underside of Level 1 and the seismic isolators. To account for the vertical seismic component, ±0.2SxsW is added to the gravity component during the post-processing of nonlinear response history analysis (NLRHA) load cases.
While seismic isolation has been used in the United States for roughly 40 years, the code still requires a peer review process that specifically reviews the isolation system and its associated design. This project used a collaborative phased peer review and permitting process. The process was broken into three main parts: isolators and Level 1 transfer system, building superstructure, and non-structural related components. This allowed the construction to begin earlier, as significant time was required to shore the building, and gave the design team additional time to coordinate non-structural elements.
Material Testing
The existing structure for the 1938 building was well documented and included both drawings and specifications. As part of the initial 2014 investigations about the building in preparation for the seismic retrofit work, a material testing program verified the documentation from the original construction. This information followed ASCE 41-13 guidelines for material testing process and was extremely important to understand the existing strengths of the various elements. Two of the more unique testing types were for the red brick masonry to establish the brick material strength and the brick wall out-of-plane (OOP) strength.
The mortar used for the URM contained cement, which increased the capacity beyond typical mortar materials of that vintage. In addition to lateral resistance, the existing URM infill is relied upon for OOP bracing of the perimeter marble cladding through arching action. In-situ testing on the OOP capacity of the URM infill was performed using an “air bag” test in multiple locations. A reaction wall was built, and air bags were inflated to approximately 1.5 psi that corresponds to 216 psf (Figure 3). Results indicated that the walls had OOP capacity under this static load with no signs of damage, such as cracks, dislocations, or related spalls to the brick infill or marble cladding. Deformations were measured during the testing and determined to be negligible. The isolated building dynamic acceleration estimates were far below the tested values with large factors of safety and URM infill walls satisfied updated ASCE 41-17 prescriptive checks, which were not incorporated into the code at the time of material testing.
URM infill walls provide lateral stiffness and strength to the building through a behavior mode of compression struts and bed-joint sliding. URM was tested to determine the compressive strength (f’m) and the expected shear strength (vme) per ASCE 41-17. Despite the strength of the URM infill, new concrete shear walls were used to take the majority of the design base shear. The stiffness of the URM and its contribution to the building dynamics was considered and explicitly modeled in the analysis to capture its participation as part of the lateral system.
Non-Linear Response History Analysis
For the structural analysis, a 3D model was created that captured the existing building structural components and retrofit elements following ASCE 41 nonlinear response history (NLRHA) procedure.(Figure 4). The 158 base isolators were modeled using native link elements within ETABS analysis software with non-linear properties. As required by the code, both upper- and lower-bound properties of the isolation system are considered to determine the superstructure’s maximum forces and the isolation system’s maximum displacements. All other elements in the analysis model are represented as linear-elastic and use expected strength properties in accordance with ASCE 41-17. The linear-elastic modeling for the rest of the building is appropriate since the goal of the isolation system is to prevent damage in the structure and historic finishes. Non-structural components were not included as part of the analysis model.
For the NLRHA, the geotechnical engineer for the project, GRI, completed a site-specific hazard analysis and provided two suites of 11 spectrally matched ground motions to capture the BSE-1N and BSE-2N hazard levels. These two suites of ground motions were run for both upper and lower bound isolator properties, and all element checks were done for both sets of models. In addition to the inherent damping included in the matched ground motions, modal damping of 3% was used for all modes except the first three isolator modes. The first three modes were assigned negligible damping as they do not have additional damping from the structure. It was determined through various analysis studies that over 15,000 modes were required to fully capture the behavior of the structure due to the inherent degrees of freedom of the nonlinear model.
Seismic Base Isolation
Even though Salem, Oregon, is situated in a location susceptible to subduction zone earthquakes, it is 50 miles from a major fault, resulting in lower relative seismicity than other seismically active regions such as coastal California. As a result, lower accelerations are expected at long building periods than for other base isolated projects in more seismically active regions. In 2014, when the initial design work for the retrofit project began, several systems for base isolation were studied, including friction pendulum systems, lead rubber bearings, and a hybrid system composed of lead rubber bearings, low friction slide bearings, and seismic ball bearings. In 2021, when the project’s final phase was re-started, isolator systems had been further developed over the 7-year period. New studies were conducted, and the Triple Pendulum friction isolator provided by Earthquake Protection Systems Inc. was selected for its bearings performance when considering the variation in seismic displacement demands in the different hazard levels, the high axial loads, and the lower seismic base shear required to protect the building.
Following ASCE 41-17, a bounding analysis captured the potential for variability in the isolation system at the time of installation far into the future. Modifiers to the isolator’s friction factors by decreasing or increasing the stiffness and strength of the isolation system bounded the overall seismic design. Typically, the lower bound isolator properties produce higher displacements (the system is more flexible), whereas the upper bound isolator properties produce higher base shears (the system is stiffer and closer to a fixed base building). However, due to the site seismicity and chosen bearing design, the lower bound isolator properties of the project’s bearings resulted in higher displacements and higher base shears. This occurred because the upper bound system had higher damping and a shorter fundamental period than the lower bound system; combined, this produced a higher spectral acceleration at higher periods with low damping. After production testing of the bearings ultimately installed in the Capitol, the bearings were within the tolerances provided and used during design.
An additional design goal was to keep the isolation system within the limits of triple pendulum bearing behavior with no Phase V behavior (bearing rim contact) for any single ground motion and to keep the average of the maximum displacements for all ground motions within Phase 3 behavior (Figure 7). The Phase 3-4 transition occurs at roughly 16 inches of total displacement, and the Phase 4-5 transition occurs at approximately 19” of total displacement for the isolators used on this project. Even with no Phase V behavior, lower bound stiffness results showed higher base shear. This was further proven through the energy calculation of the hysteresis loop of the triple pendulum bearings, accounting for the effect of the radii of the surfaces and each surface having different coefficients of friction. The nominal coefficients of friction of the isolators used to achieve these design goals are 0.0085 for both the top and bottom inner slider surface, 0.033 for the bottom outer slider surface, and 0.060 for the top of the outer slider surface.
Building Behavior and Beyond
A critical part of achieving the owner’s performance goals for seismic renovation hinged upon overall building accelerations remaining low enough that many of the existing materials comprising the original lateral system, such as URM infill, met prescriptive ASCE 41-17 checks and would not require extensive modification. The use of friction pendulum isolators achieved this goal, exhibited by both new and retrofit element forces that are within a reasonable threshold for design while also keeping isolator displacements within the project performance goals.
The nearly 115,000 Kip structure sits atop 158 triple friction pendulum isolators, 63 of which are isolators with a higher bearing capacity located in areas with higher axial load. These isolators have the same dish configuration allowing the two types of isolators to move together. Maximum isolator displacement of 19 inches and average isolator displacement of 13 inches are found using lower bound isolator properties.
Overall building accelerations remained below 0.20 g from the concourse level up to the roof of the main legislative chambers (Table 1). Above the roof, up to the top of the rotunda, where a large 17 Kip bronze statue sits, accelerations were notably higher than the rest of the structure, peaking at approximately 1.15 g due to the dynamic amplification caused by the change in stiffness at the dome. A study comparing fixed base and isolated behavior of the structure found that fixed base periods in the east-west and north-south direction fell in the range of 0.5 seconds compared to the long period behavior of the isolated structure of approximately 3.5 seconds. The dramatic difference in building response of the fixed base structure results in significantly higher accelerations throughout the structure, which would not have allowed the project team to meet performance goals, thus necessitating the use of isolation.
As with any low friction isolation system, the building behavior should be evaluated for other components that can affect the movement of the building. For the Capitol building, the moat covers and nonstructural components were reviewed so that these elements did not have an unanticipated effect on the planned displacement and base shear. Nonstructural components, such as utilities crossing the isolation plane, are seismically jointed to make the movement of the heavy isolated structure unaffected. Stairs and elevators crossing the isolation plane are entirely separated and, in all cases, hung from the Level 1 isolated structure so that there is no contact with the fixed base portions of the building. For the unique case of the moat covers, the sliding surfaces created friction and “pop-up” forces that proved to be the highest level of force imparted on the movement of the building.
While isolation is undoubtedly the cornerstone of this retrofit, seismic isolators alone are not a cure-all for addressing the historic structure’s lack of strength and stiffness. It is the use of isolation combined with the additional retrofit elements, such as new reinforced concrete shear walls, critical collector elements, and fiber-reinforced polymer (FRP) strengthening, that allows this retrofit to meet and exceed the performance objectives.
With a target completion date of 2025, the Oregon State Capitol Building Project is currently under construction and supported by temporary shoring with portions of the new lower level and foundation installed. Like most voluntary seismic upgrades, this project was mainly driven by the complex interaction between structural engineering design goals and building upgrades. However, it is essential to consider even a predominantly structural renovation project within the context of the original building’s architecture and functionality to ensure success. The preservation of the existing structure of the building and the thoughtful incorporation of all new elements as part of the architectural design are crucial. The seismic retrofit, along with upgrades to accessibility and mechanical systems, has extended the life of this landmark building far into the future. This project was only made possible through the close collaboration of the architect, the structural engineer, and the general contractor from conception to construction. ■
See Parts 1 and 2 of this series in the March 2024 and April 2024 issues of STRUCTURE magazine.