By Damian Andreani, PE, Stefanie Chamorro, PE, and Mike DeRubeis, PE, SE
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As introduced in Part 1 of this three-part series, the Oregon State Capitol Renovation project goal is to seismically retrofit and renovate multiple structures utilizing ASCE 41 (see the March 2024 issue). The importance of preserving the historical Capitol increased after losing the original and second state Capitol buildings to fires. While the original design of the 1938 capitol was fire resistant, structural damage incurred during the 1993 Scotts Mills earthquake prompted the Oregon Legislature to enhance the seismic resiliency of the entire Capitol complex.
The seismic improvement comprised of two main strategies. The original 1938 building and the 1977 Connector Building with infill courtyards were designed to be base-isolated structures, while the 1977 legislative wings would remain fixed-base structures (Figure 1). The 1977 legislative wing buildings’ original lateral system consisted of reinforced concrete shear walls with insufficient strength and columns with insufficient ductility. The retrofit approach included fiber-reinforced polymer (FRP) systems to strengthen the existing concrete shear walls and confine the columns.
Taking advantage of the seismic upgrades, the 2009 Master Plan proposed several program changes to improve the operational efficiency of the Legislature. The most significant program change requested was for larger hearing rooms while keeping most of the current programing as-is. Adding square footage to the Capitol complex was required to fulfill this desire. Since base-isolating the structure required completely transferring load off the original foundation onto the isolation system, this was a perfect time to modify the basement floor level to create the additional program spaces needed at the Capitol.
This article describes the designs and implications of creating a new basement story and preparing the structure for base isolation.
1938 Base Isolation Seismic Upgrade
The seismically isolated portion of the project consists of the existing 1938 building, the existing 1977 Connector structure, and the new one-story Courtyard Infill structures that occur from the concourse level to Level 1 (Figure 1). The four structures will be tied together into one structure supported on triple friction pendulum seismic isolators on top of a mat foundation base and will be surrounded by a 2-foot minimum seismic joint that allows the entire complex to move independently in any direction from the surrounding earth during a seismic event.
One of the main architectural goals of the renovation is to create more functional program areas, such as large hearing rooms, at the concourse (basement) level. To provide the larger column-free spaces, existing column loads need to be transferred using post-tensioned (PT) concrete transfer beams at Level 1.
dditionally, new concourse level framing and a new foundation underneath the existing Capitol building is required to create the new plane of isolation and support the building (Figure 2). Given the desire to increase the height of the basement, the need to install isolators, a new framing level above the isolators, and the required transfer beams, significant excavation beneath the building is required. However, the depth of excavation encroaches on the water table on site. To minimize hydrostatic pressure design requirements and reduce the need for water-tight detailing of the foundation, a permanent dewatering system is being installed, further increasing the excavation depth.
The structural and architectural project goals generated the need to excavate 20+ feet beneath the Capitol building. Each of the primary design elements, as well as the construction sequence, had its own unique challenges which will be discussed in the following sections.
Transfer Beams
Due to the short column grid spacing at the existing basement level, there was a desire from the architect to reduce the number of columns in the new concourse level to allow for more open interior spaces. This is accomplished with a series of north-south transfer beams just below the existing first floor framing that support the four rows of interior columns above and reduce to just two interior columns below.
The transfer beams are post-tensioned concrete beams that occur at each north-south column line to the east and west of the rotunda area, span the full north-south dimension of the building, and are supported on new concrete pilasters at both the north and south beam ends as shown in Figure 3. The transfer beams are made of 8 ksi self-consolidating concrete (SCC) that was pumped from the bottom of beam formwork due to the low clearance to the top side of the forms for placement. Steel transfer beams were investigated, but the size and lengths required posed constructability issues. The PT transfer beams have the additional advantage of ‘self-jacking’ the structure during the load transfer to minimize deflections for the existing concrete structure.
The PT transfer system is also designed and used as part of the temporary shoring system to allow the deep excavation below the historic building to occur. Using permanent structure in the temporary condition is an efficient use of structural elements, which doubled as a project cost savings. Using the PT transfer in the temporary construction configuration created a large platform that the temporary shoring towers can use to jack load into the shoring tower system, thus providing the first transfer of load off the original existing foundations and onto the temporary shoring system. The locations of the towers were coordinated with the PT transfer system to minimize the effects that PT tendon drapes and induced moments have on the temporary condition while maintaining the design for final conditions as seen in Figure 4.
The construction and load transfer sequence required the PT transfer beams to be built around and encompass the existing columns. PT threaded rods were installed at the existing columns and PT transfer beams to facilitate load transfer from the existing columns into the PT transfer beams.
Excavation and Shoring of the Building
The excavation extents accommodate depth to provide the new transfer girders below Level 1, increased head-height in the new concourse level, new framing supporting the concourse level, a subgrade level crawlspace, which houses the isolators and their support plinths, the new mat foundation, and compacted gravel and the dewatering system (Figure 5).
The extensive excavation below the existing foundation level created a need for temporary gravity and lateral shoring of the 1938 building throughout construction. Temporary lateral bracing criteria were provided in the structural notes and specifications to ensure the temporarily shored building condition performed the same or better than the existing building’s lateral performance. The temporary shoring system designed by the contractor is primarily micropile towers with lateral bracing (Figure 6). Special care was taken to decouple the micropiles from the mat by cleaning and wrapping each micropile with a bond breaker before placement of the foundation concrete to ensure that the complete building load will transfer to the isolator system.
Retaining Walls and Building Separation
One of the challenges of adding a subgrade level and increasing the height of the basement level is the need to retain the surrounding soils while also accommodating 2 feet of building seismic movement for isolation. In this project, the retaining strategies varied to address each unique condition around the 1938 original building. The east and west of the building are flanked by the previously constructed underground utility vaults (Figure 1). These vaults were built lower than the original 1938 building foundation and therefore had their own secant pile retaining structures. During excavation, the backs of these piles were exposed leaving behind walls not retaining soil on either side of them. Along the south, the excavation undermines the 1977 building wings and connector foundations by several feet. Due to limited access for piling machinery, the contractor designed temporary micropiles/shotcrete retaining walls over which a permanent propped cantilevered retaining wall was designed by the structural team.
Along the north, although there was no building adjacent to the structure, the design called for two additional types of retaining walls. Early in design, the north side of the building was identified as the optimal location to place two access ramps into the newly excavated basement. At these access ramps the design team provided a cantilevered concrete retaining wall design using the building mat foundation as the footing. Where there were no access ramps, the contractor designed permanent micropile and temporary lagging retaining walls on which the design team provided a permanent shotcrete lagging system (Figure 7).
The perimeter retaining structures were placed 2 feet away from any part of the isolated building to allow for differential movement between the ground and the building during a seismic event. This created a “moat” around the entire building. Where the moat will be accessible to the public and/or weather, the design team is providing moat covers. The moat covers are designed to hinge, slide, compress, or otherwise move out of the way when differential movement is experienced. The covers were heavily coordinated between the structural engineers, architects, landscape architects, and cover manufacturer. Some of the most complex areas to coordinate were those that needed to move in multiple directions such as corners and changes in elevation. Extensive coordination was critical to ensure that no force could be transferred between the ground and the building, which would change the lateral response of the building. Additionally, once installed, visitors to the Capitol should not be able to tell that they are crossing a moat as the building is re-assembled to maintain its historic nature.
Mat Foundation
A pile cap foundation system using the same micropiles that shore the building was explored during the initial phases of the project. However, due to numerous challenges with this method, a mat foundation below the 1938 building’s existing footprint was chosen as the best solution for supporting the Capitol and its new isolation system.
One of the benefits of using a mat foundation is providing a continuous, stiff base below the building, which reduces differential settlement and distributes bearing pressure across the building footprint. This is especially important because the magnitude of loads vary throughout the building footprint, such as at the Rotunda, which has a significantly higher weight in a concentrated area. Allowable bearing pressures are 5 kips per square foot (ksf) for gravity loads and 10 ksf for total load, including seismic cases. The mat foundation was modeled using finite element software to understand the distribution of pressures and potential expected settlement. A consistent modulus of subgrade reaction was used across the entire mat. The maximum settlement expected of the foundation is less than a ½ inch. Unlike typical construction projects where loads to the soil slowly build up as construction progresses, the settlement of the mat will occur nearly immediately when the weight of the Capitol is transferred from the shoring towers to the foundation.
The isolators are supported by 3-foot tall, square concrete plinths atop the mat foundation (Figure 8). At the beginning of the design, it was crucial to understand how loads are imparted from the isolators to the mat and how best to model them. The primary building model of the Capitol outputs axial and shear reactions at each isolator location. However, the axial reactions can occur up to 2 feet in any direction from the center of the plinth due to isolator movement, resulting in many possibilities of axial and moment demands on the mat (Figure 8). The shear reaction from the building imparted at the top of the plinth produces an additive moment on the mat foundation.
Once the demands were understood, the load cases were input into the foundation model. Modeling these components simplified the analysis and provided enveloped flexural and shear demands across the mat. Typical top and bottom mat reinforcing consisted of #8 @8” on-center (o.c.) and #8 @6” o.c., respectively. Additional #9 reinforcement was specified in areas of higher flexural demand. The design also implemented two different thicknesses of the mat foundation; some areas of 40-inch thickness and others of 52 inch. Rebar congestion in the mat foundation was a concern during design. The thickness of the mat was strategically chosen to limit additional vertical shear reinforcement to only those locations with highly loaded isolators. Most of the mat’s area was designed to resist shear demands with only its own concrete strength.
Specific attention was given to the effects of the temporary shoring micropiles on the mat foundation detailing. A bond breaker is specified between the micropiles and the new foundation to ensure complete load transfer of the building loads occurs onto the mat and no load remains on the piles since the plan is to demolish them after load transfer. Additionally, the micropiles affect both the shear and flexural strength of the mat by interrupting reinforcement and reducing the area of shear failure planes. The shoring towers are concentrated around each isolator pedestal. Loss of shear strength of the mat foundation was considered, assuming that micropiles would be located at the perimeter and potentially within the punching shear failure plane of the isolator support plinths. Where piles interrupt typical mat reinforcement, additional reinforcement will be added to either side of the micropile in a new layer of reinforcement to recuperate the lost flexural capacity.
The thickness of the mat and size of concrete pour areas triggered initial concerns about how elevated concrete temperature due to heat of hydration during curing may affect the quality and strength of the mat foundation. Mass concrete requirements set by ACI (American Concrete Institute) limit the maximum surface temperature of the concrete and the differential temperature between the surface and core of the pour. During construction, the contractor may use curing blankets to reduce temperature differentials of the concrete. Additionally, to reduce the heat of hydration for the mat foundation, the design team provided criteria for a concrete mix design to reduce cement content by partially replacing it with cementitious substitutes, which a) lowers the strength of the concrete and b) slows the development of the concrete strength and stiffness. Therefore, the design team allowed the contractor to design a concrete mix that would achieve the minimum design compressive strength in 56 days instead of the standard 28-day. This increases the waiting period before the mat foundation can be loaded, but is accommodated by the construction schedule and sequence. The contractor is continually monitoring the mat temperatures during curing using surface and embedded temperature sensors and thus far, these mass concrete mitigation strategies are successful and both the surface and differential temperature limitations have been met.
Construction is ongoing at the Oregon State Capitol and continues into 2025. This article is the second of three articles presented in STRUCTURE magazine that showcases the new retrofit of the Oregon State Capitol buildings. The final article in this series will focus on the details of the seismic base-isolated retrofit design and construction. ■
About the Authors
Damian Andreani, PE (damian@catenaengineers.com) is an engineer with catena consulting engineers in Portland, Oregon.
Stefanie Chamorro, PE (stefanie@catenaengineers.com) is an engineer with catena consulting engineers in Portland, Oregon.
Mike DeRubeis, PE, SE (mike.derubeis@forell.com) is a senior engineer with Forell Elsesser Engineers in San Francisco.