Seismic Resilience and Historic Preservation in the Tenderloin

By Dan Bech, SE and Katy Faix

UC Law San Francisco’s goals for the 100 McAllister project are as follows:

1. Increase the seismic safety of the building to meet the standards required by UC Law SF’s Seismic Safety Policy.

2. Preserve the historic character of the building which makes it special.

3. Modernize the residential and amenity offerings to faculty, student, and staff residents.

4. Further the development of the UC Law SF’s Academic Village in the heart of San Francisco.

By investing in this historic landmark, UC Law SF promotes the revitalization of the Civic Center, Mid-Market, and Tenderloin neighborhoods. By extending the life of this landmark building, UC Law SF reduces its carbon footprint while embellishing the character of the neighborhood and the historic building in these new campus residences.

Seismic Vulnerabilities & Strengths

The building has been subjected to at least one notable seismic event over its life, the 1989 Loma Prieta Earthquake. The building did not sustain a significant amount damage during this event; however, 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 fa?ade 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.

California's resilience against natural disasters is being tested with increasing frequency, underscoring the importance of proactive measures to mitigate known risks such as buildings vulnerable to seismic damage. 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 SF 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.

Existing Conditions

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 SF’s deferred maintenance program. The existing structure of 100 McAllister is 28 stories tall and was originally constructed in 1930 as the Temple Methodist Church and the William Taylor Hotel. The lower 14 floors of the building are contained in an L-shaped podium with the balance of the structure rising above the podium in the form 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 “wind-braces,” unreinforced masonry infill, and concrete infill walls.

The historic foundations are constructed on the Colma Formation and are at low risk of liquefaction induced settlement as determined by the geotechnical engineers at Langan Engineering. However, the basement slab is supported on fill which extends from the bottom of the historic foundation up to the basement level. This fill layer can be up to 17 feet thick in places and does require mitigation measures of soil grouting to be undertaken as part of the seismic improvements.

Due to the dual function of the building's original use, both a church and a hotel, the many different programmatic requirements on the lower floors of the building forced the engineers to design a highly discontinuous structural system. Very little of the historic 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.

Approach to Strengthening

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.

Through nonlinear 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 the foundation. 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 conventional reinforced concrete overlay walls on the non-historic facades.

Strategic reinforcement of select building elements achieve the required seismic performance standards set by CEBC and UC Law SF, 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 third party peer review which is being performed by Rutherford + Chekene, Forell Elsesser, and Engeo for the geotechnical portions.

Preserve Historic Areas

Working closely with Page and 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 fa?ade, lobby, Walnut Room, Ladies Lounge, dining room, window locations, and the basketball/athletic basement area.

One of the more challenging, and important, aspects of this retrofit is preserving the historic fabric in the Lobby area, the Walnut Room, and the Great Hall. 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 on these 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 we had the benefit of beautifully detailed drawings from the original structural engineer, Trygve R?nneberg. R?nneberg 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 retrofit previously. A brief biography of Trygve R?nneberg can be found here.

Construction Engineering

Given the complexity of the building and the myriad of design constraints imposed on the project, Holmes was engaged to provide construction means and methods support to Plant Construction. There have been several key construction challenges that are 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 cost. 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.

Sustainability — Preserving Existing Structures

Rehabilitation of the 100 McAllister building goes beyond preservation of the historic fabric and increasing seismic resilience, it is an inherently sustainable practice. Its multi-generational use and upkeep is more sustainable than new construction. This reinforces the narrative that these buildings deserve to be protected. The upgrades undertaken demonstrate that extending the building's life has a lower carbon footprint over 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 we compare the reused and new building materials at the scale of a 28-story tower, we can grasp the significant volumes of demolition waste that would have to be transported to landfills and the new building materials that must be extracted and manufactured to build a replacement, and finally calculate the massive amounts of energy (and fossil fuels) needed for transport, manufacturing, construction, and demolition.

A thorough writeup outlining the details of the LCA and the sustainability goals of the project will be presented as a future article for this project.