Stevenson Hall Transformation

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Stevenson Hall at California State University recently underwent a major seismic upgrade and renovation, a project that blends technically rigorous structural engineering with an architectural vision rooted in openness and daylight. The 130,000-square-foot, three-story concrete and steel-framed building relied on an outdated lateral force-resisting system that no longer met current seismic standards. Upgrading it to modern standards was no small feat, considering the tight budget and the architectural need for flexible, transparent interior spaces.

Designed by the State of California’s Department of Public Works–Division of Architecture in 1964 and completed in 1967, Stevenson Hall was the first building in the Sonoma State University campus. It housed classrooms, faculty offices, and a cafeteria, and underwent minor renovations in 2002 and 2006, serving the Schools of Education, Social Sciences, and Business and Economics. Upon completion of this most recent renovation in August 2022, the retrofitted structure will provide 22 general purpose instruction spaces accommodating 24 to 224 seats, as well as collaborative gathering areas throughout the building. Rutherford + Chekene (R+C) served as the structural engineer, while the architect was EHDD Architecture.

The existing building consists of a three-story structure plus a partial basement, with plan dimensions of approximately 310 feet by 180 feet. Story heights are 14 feet at typical levels and 12 feet at the basement. The building is rectangular in shape with an internal open atrium and an east-side breezeway. A 2-inch seismic joint separates the west wing of the rest of the building above Level 1, resulting in three distinctive structural wings.

The structural system consists of steel beams and columns encased in concrete, cast-in-place concrete slabs with joists, perimeter precast columns, and concrete shear walls. Columns are typically spaced at 24 feet in both directions. The typical floor system includes a 4-inch-thick slab over 7.5-inch by 18-inch concrete joist spaced about 3 feet on center. Steel framing includes W21 girders and W12 beams, encased in concrete, supported by interior W10 columns. Perimeter concrete-encased columns are 26 by 26 inches in section and consist of W10 steel columns encased in concrete, not connected to the slab but to the girders in the north-south direction only. Exterior columns at the east and west facade of the building extend from Level 1 to the roof and are not connected to the structure at the intermediate levels. These columns are 26 by 26-inch hollow precast concrete elements. The lateral-force resisting system consists of 8-inch interior concrete shear walls. Foundations are typically spread footings interconnected with grade beams. The cladding system consists of precast wall panels connected to the edge of slab with steel inserts.

The soil at the site consists of up to 6 feet of heterogeneous fill over stiff to very stiff clay with intermediate layers of loose to very dense sand with clay to maximum explored depth of 51.5 feet. Groundwater level is about 15 feet below ground surface.

The controlling seismic source at the site is the Hayward-Rodgers Creek fault system, located approximately 3.9 km from campus. A 7.4 magnitude event is expected when the Rodgers Creek fault and the northern segment of the Hayward fault rupture together. Liquefaction and lateral spreading hazards are low. Expected settlement includes densification-induced settlement between ¼ to ¾ inches, and liquefaction-induced settlement of about ½ inch.
The site is classified as Site Class D per ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Seismic parameters were taken from CSU Seismic Requirements (2019), which provide campus-specific values.

The renovation project was delivered using a hybrid collaborative design-build process with a fixed budget, requiring continuous coordination to balance cost, schedule, and performance. This framework strengthened collaboration among the design team, general contractor, and trade partners to maintain alignment with the University’s financial constraints. As with most retrofit projects, existing-building unknowns introduced additional complexity and made cost control more challenging.

With a total project budget of $71 million, the design-build team aimed to maximize value and deliver a modern, flexible 21st-century learning environment within the project’s tight budget. The project design and construction were at the height of the COVID 19 pandemic which also challenged the design team to find new ways to collaborate and impacted the contractor’s supply chain, particularly for the glazing and new exterior panels. A preliminary assessment found that the renovation cost exceeded 25% of the replacement cost of the building, triggering the need for a seismic evaluation and retrofit per CEBC 2022 Section 317. Given the University’s desire for larger, more transparent interior spaces, retrofitting the existing interior 8-inch concrete shear walls was not feasible. Thicker walls would reduce usable space, and reinforcing the existing wall foundations would be very difficult to execute given limited access to the interior of the building. Instead, the team opted for a complete replacement of the lateral force-resisting system using new exterior reinforced concrete shear walls, allowing the removal of the interior structural walls and minimizing foundation disruptions.

The original 8-inch interior shear walls no longer met seismic performance requirements per ASCE 7-16 and conflicted with the architectural vision for openness and daylight. To meet both seismic criteria and design objectives, a new lateral system was required. The retrofit was designed under the California Existing Building Code and ASCE 41-16 Seismic Evaluation and Retrofit of Existing Buildings with performance objectives and seismic parameters based on CSU Seismic Requirements (2019). The performance objective is BSE-R (S-3) for life safety and BSE-C (S-5) for collapse prevention. The seismic parameters were defined according to prescriptive values specific to the Sonoma Campus per CSU Seismic Requirements (2019) Tables 1 and 2b for the two performance levels. These values are the following:

The resulting base shears are:

The performance objective for non-structural components, including seismic anchorage and bracing is BSE-1 per CSU Seismic Requirements, which is equivalent to a new building per ASCE 7-16. These values are:

Analysis followed the linear dynamic procedure per ASCE 41-17 and 2019 CEBC. The building was analyzed assuming semirigid diaphragms and reduced effective stiffness of the new concrete walls per ACI 318-19 Building Code Requirements for Structural Concrete. M-factors of 4 for BSE-R and 6 for BSE-C were used. Acceptance criteria were determined by ASCE 41-17.
The new lateral force-resisting system consists of 18-inch special reinforced concrete shear walls, located primarily at the building perimeter, enabling demolition of the interior concrete walls and opening the floor plates for larger, more modular classrooms and collaboration areas. Joining all three wings together was one of the critical improvements to the lateral system that using these walls provided. This also simplified the roof addition that enclosed a previously open interior courtyard, which provides a gracious open interior meeting space at the ground floor.

High-strength Grade 80 reinforcing steel and 8,000-psi concrete were used to reduce length and thickness of walls while meeting seismic demands. Walls were designed as flexure-controlled elements capable of resisting the maximum shear that can be developed in the wall at their maximum flexural capacity. Typical wall reinforcement consists of two layers of #6 and #7 horizontal bars spaced either 6 inches or 12 inches on center, #5 or #6 vertical bars spaced at 12 inches on center and 4-foot-long boundary elements reinforced with #6 vertical bars spaced at 6 inches on center and #5 confinement ties at 6 inches on center.

Interior wall demolition followed a carefully phased sequence to maintain structural integrity per ASCE 37 Design Loads on Structures During Construction construction load requirements. The design team worked in close collaboration with the contractor to allow for progressive demolition so that as new shear walls were erected old walls were removed, eliminating any temporary bracing and keeping the schedule efficient. This approach also enabled mechanical and electrical trades to begin their work in the interior spaces while the exterior work progressed.
The three building wings were tied together at all levels to form a single diaphragm to redistribute the seismic loads to the new walls through collector elements. Chord and collector elements at Levels 2 and 3 were added to drag forces from the diaphragm into the walls and to reinforce the diaphragm at the interior reentrant corners. These collector elements consist of a thickened 8-inch solid concrete slab that is doweled to the existing perimeter steel beam through a continuous top steel plate.

Existing truss bracing between second and third floor was strengthened to support increased loads on Level 2. A similar truss was added where an interior column was removed to create a larger classroom space.

Most of the precast cladding panels were removed to create a more open and transparent exterior, improving natural interior lighting. Their removal also reduced the structure’s overall seismic mass, resulting in reduced seismic demands on the newly added reinforced concrete shear walls and their supporting foundations.

Strengthening the foundation to support the new shear walls involved adding pile caps, grade beams, and long 12 ¾-inch-diameter, 80-foot-long Tubex piles, installed from outside due to limited interior headroom. The added pile caps were located between existing footings where possible. At instances where pile caps could not be perfectly aligned with the wall locations due to the existing foundation, large-section grade beams—up to 66 by 64 inches—were utilized to distribute forces from the walls to their foundation. The foundation system was designed for wall capacity. Pile ultimate capacities are 424 kips in compression and 258 kips in tension, with lateral resistance provided by passive soil pressure on the pile caps.

Courtyard foundation work required partial demolition of existing footings and temporary shoring of columns, later integrating those elements into the new wall foundations. Close collaboration between designers and the contractor was essential for proper sequencing. In the early stages of planning for the shoring, R+C found creative ways to temporarily utilize the new foundation’s Tubex piles as part of the shoring contractor’s system helping to reduce construction cost and speed the construction schedule.

Early in the project, R+C communicated to the construction team how the design was meant to work—highlighting key elements that were essential to the success of the project. During the structural construction phase R+C visited the site every 5 to 10 days to help the contractor overcome the usual challenges that retrofits involve. This close collaboration in the field was a key element in the successful completion of the project. Due to the stacking of the early construction activities, requests for information from the field were routinely answered within 24 hours to allow the construction team to maintain their momentum.

Architecturally, the vision was to transform the underutilized courtyard into the heart of the building. By connecting the building’s three seismically separated wings, the team infilled the courtyard and added a glass roof to create a stunning atrium. The atrium was enclosed with conventional steel wide flange framing, with north-south girders spanning the width of the atrium, supporting the east-west joists. A 3-inch steel deck with 3 ¼ inches of light weight concrete fill provides the roof diaphragm and walking surface. The new deck is doweled into the existing concrete slab to provide a complete seismic diaphragm.

The typical added slab at Levels 2 and 3 consists of 4-1/2 inches of normal weight concrete fill on W2 deck. This slab is supported on W18 or W21 steel beams and W21 or W24 girders, except at the long spans at the added classrooms where W40 and W42 girders are used. Columns are typically W12 steel sections on isolated spread footings.

This atrium space is now both a social hub and a structural component, efficiently distributing diaphragm forces to shear walls while providing a bright, inviting space for collaboration.

A seismic peer review was conducted by the Campus Principal Peer Reviewer according to CSU Seismic Requirements. The peer review process starts at the beginning of the project and continues until construction completion. Peer review concurrence letters were issued at completion of the Schematic Preliminary Design and Construction Document Phases and during construction on deferred submittals that have a seismic component.

Stevenson Hall is now more than code-compliant--it has been transformed into an open, flexible, and resilient academic building. It meets CSU’s Life Safety and Collapse Prevention standards while delivering modern instructional spaces and a vibrant atrium that encourages engagement. The project demonstrates how thoughtful engineering and innovative architecture can work together to create a building that both performs and inspires. ■