100 McAllister Street, San Francisco

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The historic 100 McAllister Street building is undergoing a seismic upgrade and interior improvements to further expand campus housing and academic and instructional opportunities for Bay Area institutions of higher education. The project demonstrates UC Law’s commitment to its strategic vision, the implementation of a multi-institutional Academic Village, and an affirmation of its confidence in the revitalization of San Francisco’s Civic Center, Tenderloin, and Mid-Market neighborhoods.

On the heels of the grand opening of UC Law’s new 656-unit campus housing project at 198 McAllister, work has begun on the first of two phases to seismically upgrade and modernize 100 McAllister. Upon completion of both phases in 2027, the renovated and structurally strengthened 355,000 square foot building will add 80 residential units (one to six bedrooms each). The intention to is to provide a range of rent price point options in the building to accommodate students, faculty, and early career professionals, some of whom may share their unit with their partner or family. The lower podium floors of the building are slated to accommodate an academic institutional partner to UC Law and will include potential classroom, seminar, office, and meeting spaces. The building will also preserve and enhance historic gathering spaces including the street level lobby, two ballroom-type spaces, and the Sky Room Lounge on the 24th floor which has sweeping views of the entire city. At the lower level, the basketball court will be converted into a multi-use sports court which will be flanked by a series of spaces for gym equipment and wellness rooms of various sizes and uses. The most compelling historic space is the Great Hall, a former religious sanctuary, which is accessible by an exterior monumental stair through three archways at street level. This high-volume space is edged with a series of mezzanines and clerestory windows and could be used as an academic institutional partner space, performance space, and/or public assembly space.

The Academic Village is UC Law’s strategic concept for spurring interdisciplinary engagement among individuals and across institutions on a single urban campus. Student residents of the Academic Village have shared access to all amenities including the library, food services, study areas, and recreational spaces on a campus-wide basis.

The Bay Area’s housing crisis has intensified the need for affordable student housing, particularly considering increasing rent burdens and longer commutes. San Francisco’s limited housing supply and high costs place significant financial pressure on students.

San Francisco’s recovery from the COVID pandemic and its adverse social impacts has been uneven. Most of the City has regained pre-pandemic activity levels with commercial space performing well, supporting robust sidewalk-level activity. However, not all neighborhoods enjoy the same level of resurgence; the Civic Center, Tenderloin, and Mid-Market neighborhoods continue to lag.

100 McAllister Street is an Art Deco and Gothic Revival high-rise constructed in 1930 with a unique and storied history. Located at the crossroads of the Tenderloin, Civic Center, and Mid-Market neighborhoods, 100 McAllister is important to the development and character of San Francisco. Known as “The Tower,” it has been both witness and contributor to the patterns of history that make these three neighborhoods significant. See Timeline.

This historic building has good bones. For a building of this age, it is in great condition with little observed structural deterioration, a testament to UC Law’s deferred maintenance program. The existing structure of 100 McAllister is designed with stepped massing that defines a mid-rise podium, high-rise, and penthouse levels. The lower fourteen floors of the building are contained in an L shaped podium with the balance of the structure rising above the podium in the form of a slender tower. The primary structural system consists of reinforced concrete floors supported on concrete-encased riveted steel beams and columns. The existing seismic load resisting system for the building relies on historic steel wind-brace brackets at beam-column joints; thin, lightly reinforced concrete infill walls; and the non-structural unreinforced masonry (clay brick) infill making up the exterior facade.

The historic foundations are supported on dense to very dense sands of the Colma Formation and therefore are at low risk of liquefaction-induced settlement, as determined by the geotechnical engineers at Langan. The top of the Colma Formation below the basement slab slopes downward from north to south 5 to 15 feet. The building foundations also extend to the same depths below the basement slab for support in the Colma Formation. However, the basement slab is supported on loose to medium dense fill which is below the water level (water level is maintained with pumps beneath the basement slab).

Due to the dual function of the building’s original use as both a church and a hotel, the many different programmatic requirements on the lower floors of the building forced the original engineers to design a highly discontinuous structural system, with many transfer girders. Very little of the seismic force resisting system is continuous to the foundation which creates a significant seismic vulnerability in the building, particularly for the tower and the rear facades of the podium.

Seismic Strengthening Goals

UC Law’s goals for the project are:

Despite its location in a high seismic region, the building has only been subjected to one notable seismic event over its life: the 1989 Loma Prieta Earthquake. The building did not sustain a significant amount of damage during this event because the energy content of the Loma Prieta event was relatively low for this type of building.

By modern standards, the historic building contains many undesirable characteristics which can lead to poor seismic performance, such as a discontinuous seismic force-resisting system supported on transfer beams and non-ductile concrete walls. Additionally, the damageability of the unreinforced masonry and terracotta facade is of interest particularly in the tower portion of the building. The original engineers for the building omitted the steel “wind bracing” elements above the 20th floor which means the tower portion of the building is susceptible to greater seismic damage.

Despite the building’s weaknesses, the building has a highly redundant seismic force-resisting system that provides good resistance for smaller seismic events. In larger seismic events the building is not expected to perform well. Fortunately, engineering and construction techniques have improved tremendously in the past 100 years and these more modern standards can be applied to the retrofit of this building.

According to the USGS, there is a 51% likelihood of a magnitude 7 earthquake striking the San Francisco Bay Area within the next 30 years. By retrofitting this building, UC Law will meet the seismic safety standards required for state-owned structures under Section 317 of the California Existing Building Code (CEBC). This retrofit of 100 McAllister ensures that future generations can continue to benefit from this building for years to come.

As structural engineers with an expertise in preservation, Holmes understands the importance of quantifying a building’s inherent strengths and vulnerabilities prior to entertaining any strengthening or retrofit measures. The best, and likely the only, way to do this on a building as complex as this one is through nonlinear performance-based time history analysis. There is no other analysis methodology available to engineers that can lead to a more accurate accounting of seismic performance.

Langan developed site-specific response spectra and spectrally compatible time series for the seismic evaluation and design of the retrofit of the existing tower. The seismic evaluation was performed in accordance with ASCE 41-17, Section 317 of the 2022 California Existing Building Code (CEBC), and the UC Seismic Safety Policy. Horizontal site-specific spectra were developed for two levels of shaking, Basic Safety Earthquake (BSE) BSE-C and BSE-R, which correspond to a 5 and 20 percent probability of exceedance in 50 years in the maximum direction, respectively. In addition, eleven pairs of amplitude-scaled time series to both BSE-C and BSE-R were developed.

Based on the subsurface conditions, the site is classified as a very dense soil and soft rock profile, Site Class C. Langan used subsurface information, including shear wave velocity measurements for the development of site-specific spectra. A probabilistic seismic hazard analysis (PSHA) for 5 and 20 percent probability of exceedance in 50 years, respectively was performed to develop the BSE-C and BSE-R spectra. NGA West 2 ground motion models were used to estimate the level of shaking. Scaling factors presented in Shahi and Baker (2014) for ratios of SaRotD100/SaGMRotI50 were used to modify the average results to the maximum direction. The average directivity factors for the site were estimated using the NHR3 directivity based PSHA tool and applied to the PSHA results.

Amplitude scaling (single scalar) was selected by the design team to develop the time series. Section 2.4.3 of ASCE 41-17 requires the development of the ground motion acceleration histories be performed per Section 16.2 of ASCE 7, which requires the average of the maximum direction spectra (ROTD100) from eleven ground motions not fall below 90 percent of the target response spectrum over the period range of interest. Langan developed two suites (11 pairs each) of time series for each hazard level. The selection of the records for each suite was based on calculating the sum of the squared error (SSE) between the target spectrum (BSE-C or BSE-R) and the average of the scaled ROTD100 for each pair of time series. The scaling factors and proposed time series were selected generally based on the least SSE.

ASCE 41-17 defines near-field as sites being 15 km or less from the surface projection of active fault capable of generating a moment magnitude greater than or equal to 7 and requires that the motions be rotated in the fault normal and parallel directions. Studies by Watson-Lamprey and Boore (2007), Huang et al. (2008), and Shahi and Baker (2012) have shown that for sites less than 5 km from a fault that there is strong polarization of the ground motion in the fault normal and fault parallel directions and that the spectral accelerations in fault normal direction are larger than the median value for periods longer than 0.5 second. Beyond 5 km this effect appears to be random, i.e. fault normal is not always the largest. This effect has also been further investigated by Golesorkhi and Gouchon (2023) in a white paper published as part of their involvement with BSSC. Because the site is approximately 13 km from the San Andreas fault, Langan recommended that the selected, scaled motions be applied randomly to the structure. This method was approved by the peer review team.

Through non-linear time-history analysis, Holmes has mitigated the key vulnerabilities in the building by considering each carefully. First, the weak tower portion of the building is strengthened by adding a new concrete core that extends from the roof to a new concrete mat foundation located just below the existing slab level. This new core is in-turn stiffened by new steel outriggers and buckling restrained braces. The new concrete core and outriggers maximize structural strength and stiffness and minimize impacts to key historic areas. In addition, the new core has the benefit of providing code compliant egress stairs and a modern elevator bank which the students will certainly enjoy in years to come. The retrofit of the tower proved to be the most challenging aspect of the project given the small floor plate which is exacerbated by the wedding cake architecture of the tower. In addition to the new core, key transfer beams that support discontinuous infill walls are stiffened and strengthened as part of the retrofit. The podium portion of the building is strengthened with reinforced concrete overlay walls on the inside of the historic facades. A new 6-foot-thick mat foundation will be constructed to support gravity and anticipated seismic loads.

The fill soil that will remain beneath the new mat foundation is liquefiable and cannot support the anticipated seismic loads. Because of limited access, the high groundwater level, and almost complete coverage of the tower mat area by existing footings, permeation grouting (using vertically aligned overlapping bulbs of soil grouted with microfine cement) was selected to improve the fill beneath the new mat foundation and transfer the seismic loads to the competent Colma Formation. Permeation grouting was also used to construct shoring walls for the soil excavations. Installation of permeation grouting columns was based on the results of a field test program that investigated various design parameters (bulb size, bulb plan layout, bulb overlaps, etc.) to arrive at an optimal solution.

Strategic reinforcement of select building elements achieves the required seismic performance standards set by CEBC and UC Law, while preserving historic features. By enhancing structural rigidity and strength where most needed, this targeted approach optimizes construction costs and maintains the building's historic integrity.

The project has adopted Method B from the CEBC using non-linear time history analysis to demonstrate compliance with state mandated performance objectives. As such, the project is subject to review by UC Law’s Seismic Review Committee consisting of Rutherford + Chekene and Forell Elsesser for structural issues and Engeo for geotechnical issues.

Working closely with Page & Turnbull, areas of significant historical value were identified such that the retrofit is designed to avoid or minimize impact to the historic fabric itself as well as the experience of individuals within the spaces. Special care was taken to eliminate structural options that would interfere with any of these historic resources: terracotta facade, lobby, Walnut Room, Ladies Lounge, Dining Room, window locations, and the basketball/athletic basement area.

The seismic vulnerabilities of the tall slender tower make it exceptionally difficult to be respectful of these key historic features. With critical input from Page & Turnbull to avoid impacts to historic features, important design decisions on elevators and egress from Perkins & Will, and constructability input from Plant Construction, an elegant structural solution was developed. The structural solution was truly a full team effort.

One of the joys of working on old buildings are the lessons we learn from studying historic drawings and construction techniques. For this project the team had the benefit of beautifully detailed drawings from the original structural engineer; Trygve Rønneberg was also engineer of record for other notable buildings in San Francisco such as the Hobart Building and the Pacific Bell Building (140 New Montgomery) which Holmes and Perkins & Will retrofitted previously. A brief biography of Terres Ronneberg can be found at
https://ronneberg.org/blog/trygve-ronneberg/.

The multi-generational use and upkeep of 100 McAllister is more sustainable than new construction. Extending the building's life has a lower carbon footprint than constructing a new building for equivalent functions, even when adding significant strengthening in a high-seismic region.

In collaboration with Perkins & Will, Holmes has completed an in-depth Life Cycle Assessment (LCA) of the project. Holmes has focused on the structural components and Perkins & Will on the balance of the project. When comparing the reused and new building materials at the scale of a 28-story tower, one can grasp the significant volumes of demolition waste that would have to be transported to landfills, and new building materials that must be extracted and manufactured to replace historic elements, it becomes evident that this rehabilitation project honors sustainability goals, especially when calculating the massive amounts of energy (and fossil fuels) needed for transport, manufacturing, construction, and demolition.

Given the complexity of the building and a myriad of design constraints imposed on the project, Holmes was engaged to provide construction means and methods support to Plant Construction. Several key construction challenges were part of this project, the most significant of which is the construction of the new concrete core up the center of the tower. To build the core, Plant Construction will use self-climbing formwork within the existing building to expedite construction and reduce construction costs. To accommodate the core and the self-climbing formwork, several existing transfer beams need to be removed or cut and re-supported. The removal of the existing transfer beams requires careful consideration of construction sequencing including unloading and reloading of existing gravity loads in the structure and foundations. The new floor penetration for the new core requires strategic temporary shoring and strengthening of the existing floors and historic seismic force-resisting system during construction.

Other significant construction challenges where Holmes is providing means and methods support include the design of the tower crane foundation and tie in, and the design of the construction personnel hoist supports.

Construction began in early 2025 and is scheduled to wrap up in the summer of 2027 in time for the start of the 2027-2028 academic year. ■