Credit: TMG Partners
Fig. 8. Viscous dampers, show here in an interior photo, reduce drift and deformation.
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.
Moment Frame Strengthening 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, 50 STRUCTURE magazine
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.
Dampers Deliver 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 buildout 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. Peter Somers, PE, SE, is a Principal and Jeremy Hasselbauer, PE, SE, is a Senior Associate at Magnusson Klemencic Associates (Seattle).