Retrofitting PG&E’s New Oakland HQ

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At 28 stories and 1.06 million square feet, 300 Lakeside is the largest office building in Oakland, California. At the time of its completion in 1960, the project, previously known as the Kaiser Center, also claimed the title of tallest building in the city. Following a significant renovation completed in 2023, 300 Lakeside has added a new accolade as it is one of the largest office buildings ever seismically retrofitted. With 272 viscous dampers weighing half a ton each across 23 stories, 300 Lakeside is reborn and ready to withstand another 60-plus years in a region with heightened earthquake risks.

Prominently located on the northwest arm of Lake Merritt, the historic office is instantly recognizable by its long, curved shape (Figs. 1-2). TMG Partners purchased 300 Lakeside in 2020 and subsequently entered a long-term lease with a purchase option with Pacific Gas and Electric Company (PG&E), a California-based, investor-owned utility. TMG began a major tenant improvement renovation that same year to deliver the spaces to PG&E’s workplace standards, including seismic risk reduction goals. Although the planned renovations did not trigger a seismic retrofit under Oakland’s building code, PG&E requested TMG increase the building’s seismic performance, as 300 Lakeside was to serve as PG&E’s headquarters following the company’s move from San Francisco. Magnusson Klemencic Associates (MKA) was selected as the structural engineer for the massive voluntary seismic upgrade in 2020. Coordination began immediately to find the right solution with the constricts of the preexisting renovation schedule.
While 300 Lakeside escaped relatively unscathed in the 1989 Loma Prieta earthquake, which caused significant damage in downtown Oakland, how the building might fare in the future was unknown. Working with TMG Partners, PG&E, lead architect Gensler, and Plant Construction, MKA vetted multiple options before settling on viscous dampers as the suggested upgrade solution—as they most effectively balanced the need to maintain interior and exterior appearance, cost-efficiency, and minimization of additional strengthening to protect 300 Lakeside’s historic stature while achieving the safety and performance necessary without significantly affecting the timeline for occupancy.

Before selecting viscous dampers, MKA evaluated the existing structure to determine performance targets based on PG&E’s goals as an owner/operator, the complexity of the existing moment frame system, budget constraints, and the proximity of the Hayward Fault Zone running through Oakland. Notably, the upgrade needed to address several critical considerations:

Renovation efforts could not affect the existing foundation and basement levels to avoid disrupting building operations or significantly impacting cost and schedule.
The retrofit construction had to be tightly coordinated with the move-out and move-in schedules of various tenants throughout the building.

It was desired to avoid work on the top five floors to prevent displacing the existing tenants.
While a complete set of original construction documents was available for this building, the details for the critical connections (beam-column and column splices) were sometimes ambiguous and allowed for more than one connector (e.g., bolts versus rivets). They did not list all the specific material properties and were not dimensionally complete. Given this uncertainty, MKA developed a detailed on-site investigation and material testing program, which was later implemented by Plant Construction. The investigation involved removing the existing concrete encasement around several beam-column connections and column splices and studying the materials and geometry used to make them. Each component was thoroughly documented, allowing the project team to model the components to evaluate their behavior more accurately.
The steel moment frame at 300 Lakeside is unique in that unlike most “Pre-Northridge” moment frames, 300 Lakeside’s frame features bolted and riveted connections that utilize “double split tees” (Figs. 3-4).

During the testing program, MKA found the bolts would not experience brittle bolt shear failure and snap off as was anticipated. Instead, the split tee stem allows the frame to yield, indicating more ductility available in the frame than expected.

Since simplified linear analyses were insufficient to accurately inform building performance, MKA subjected a nonlinear model of the existing building to ground motions representative of the regional seismicity. With earthquake time histories conditioned to the natural periods of the structure, analyses of the building subject to a ground motion level consistent with a 225-year earthquake—a Mean Recurrence Interval (MRI) of 225 years/BSE-1E per ASCE 41— combined with the results of the investigation and testing program showed moment frame beam rotations exceeding those permitted by ASCE 41 for Life Safety acceptance criteria. This demonstrated that the building is susceptible to significant damage and potential life loss even under moderate ground motions.

MKA explored various retrofit options using the nonlinear dynamic procedure per ASCE 41-17. After several iterations of seismic evaluation and the development of retrofit strategies, the seismic performance objective for the voluntary retrofit was selected as Collapse Prevention for a 475-year seismic hazard (10 percent chance of exceedance in 50 years). Given the somewhat unique seismicity in the Oakland area—more frequent earthquakes are not much smaller than rarer, larger earthquakes due to the city’s proximity to the Hayward fault—this seismic hazard is fairly consistent with 75 percent of the seismic forces for new building design under ASCE 7-16, the conditions that would be required were this a code-triggered retrofit.

The various options MKA considered to achieve Collapse Prevention using ASCE 41 included:
Adding braced-frame or shear wall cores. While structurally efficient solutions, these cores were quickly eliminated from consideration because any placement of new cores would have significant impacts on either interior space or exterior appearance depending on if either new interior or exterior cores were added—an issue for the historical building wanting to preserve as much original integrity as possible. Furthermore, and most importantly, additional foundations or foundation strengthening would be required.

Adding distributed Buckling Restrained Braces (BRBs). This approach would maintain the historical appearance, and BRBs are well-known as a cost-effective seismic bracing approach. However, the increased stiffness and corresponding shorter period would increase global demands, requiring considerable column and foundation strengthening.

Using distributed supplemental damping to reduce drift and deformation. This approach features a minimal increase in stiffness, keeping global forces approximately the same, with energy dissipation in these added elements reducing drifts and plastic rotations in the moment frame members. MKA identified it as the ideal solution early on.

Once the retrofit approach was determined, MKA evaluated two types of dampers: friction and viscous.

Friction dampers provide significant energy dissipation due to the nearly rectangular hysteretic loops with negligible stiffness increase once slip occurs in the dampers. However, the force in friction dampers is in-phase with the force in the moment frames, meaning that the damper forces are additive to the column forces resulting from overturning in the frames. This increases specific column and foundation loads despite the global reduction in seismic demands, leading to increased column and column splice strengthening and potential foundation impacts.

Viscous dampers provide similar benefits as friction dampers in that the global stiffness increase is negligible after small movements occur, and significant energy dissipation reduces building movements. The key difference is that viscous damper forces are out-of-phase with the moment frame forces, so despite slightly less efficient hysteretic behavior, increases in column and foundation loads are minimal.

Ultimately, adding viscous dampers, the selected retrofit approach, was the best balance between interior and historical appearance impacts, cost-efficiency, and minimization of additional strengthening (Fig. 5). Most importantly, this approach allowed the retrofit to move forward without needing to access the basement levels. Also, damper sizing and layout could be designed without requiring work on the top five floors.

The dampers were laid out to permit better control of force concentration in columns, minimizing column and column splice strengthening and preventing significant increases in local foundation loads (Fig. 6). MKA worked with Gensler, the tenant improvement architect, to locate and place the dampers strategically, maximizing space usage without disrupting existing mechanical and plumbing risers or compromising structural efficiency. The final number, size, and distribution of the dampers were determined via iterative nonlinear time-history analyses. In all, 272 viscous dampers were installed throughout all 23 seismically upgraded floors. Given the colossal size of each damper, weighing approximately half a ton each, in addition to the HSS extension pieces, cranes had to lift each damper into place versus using the manlift—a significant effort that required careful coordination with Plant Construction and their subs.

Another complication of construction was that the occupied floors were not vacated from the bottom of the building upward as the structural upgrades naturally should occur. Given this, damper installation was taking place above some floors that had yet to be retrofitted. To prevent portions of the frame without retrofit being subjected to increased loads from dampers installed above, it was determined that only a limited number of dampers above columns pending retrofit could be engaged. To address this, MKA designed damper-end connections with temporary slotted holes, allowing the contractor to install the dampers and complete most of the work while keeping the dampers from engaging before access was available to floors below. The contractor then returned and “locked off” the dampers by installing the permanent bolts once the necessary column strengthening below was complete. This significantly compressed the construction schedule compared to requiring the damper installation to progress bottom-to-top, allowing the retrofit work to be performed within the TI schedule.

In the end, the strengthening of the existing moment frame was very limited. Less than three percent of the building’s columns required strengthening, which consisted of adding side plates across the flanges of the columns. These plates were added only to the accessible sides of the perimeter columns to avoid having to remove additional cladding for access, as the columns were too close to the façade to access otherwise. Less than 20 percent of the building’s column splices required strengthening, and the approach varied based on the failure mechanism. In many cases, simply welding the existing splice plates was sufficient because the bolts were the controlling element. In other locations, the bolts were removed, and the splice plate was replaced and welded to the column flanges. For one-sided access, different designs were provided at each flange so that the plates could be placed from the same side (one on the outside of the flange and a pair of plates on either side of the web on the inside of the flange, Fig. 7). Lastly, kickers were added at fewer than 2.5 percent of the building’s beams to brace the bottom flange to prevent buckling before the development of plastic hinging at the beam ends.

The immense effort at 300 Lakeside was successfully completed in September 2022, with PG&E completing its move into the space in March 2023. Despite being a retrofit of immense scale, navigating an occupied building, and working within the schedule constraints for separate tenant improvement modifications, MKA’s addition of viscous dampers to the moment frame system achieved each performance goal within the established retrofit budget and without extending the TI schedule. This solution controls drifts and framing deformations without significantly increasing column and foundation loads. The project team’s thorough preconstruction investigation and planning, as well as its collaboration with TMG, Gensler, and Plant Construction, ensured that the damper installation and structural upgrade construction went smoothly and were seamlessly integrated into the overall tenant build-out project without impacting the building’s exterior, the overall project timeline, or building operations (Fig. 8). PG&E already occupies all but three floors at 300 Lakeside and exercised its option to purchase the building with planned transaction close mid-2025, enjoying residence in a renovated and seismically improved building that maintains its historic integrity while providing the safety and performance needed in California’s earthquake-prone territory.

About the Authors

Peter Somers, PE, SE, is a Principal and Jeremy Hasselbauer, PE, SE, is a Senior Associate at Magnusson Klemencic Associates (Seattle).