ctbuh.org/papers

Title: Sustaining a Historic High-Rise Structure

Authors: Nina Mahjoub, Project Engineer, Holmes Culley Megan Stringer, Project Engineer, Holmes Culley Bill Tremayne, Principal, Holmes Culley

Subjects: Retrofit Seismic

Keywords: Life Cycle Analysis Retrofit Seismic Structure Sustainability

Publication Date: 2015

Original Publication: CTBUH Journal, 2015 Issue I

Paper Type: 1. Book chapter/Part chapter 2. Journal paper 3. Conference proceeding 4. Unpublished conference paper 5. Magazine article 6. Unpublished

© Council on Tall Buildings and Urban Habitat / Nina Mahjoub; Megan Stringer; Bill Tremayne Retrofit / Seismic Sustaining a Historic High-Rise Structure One of the tallest seismic retrofits in North America was undertaken in the heart of San Francisco. The Pacific Telephone & Telegraph Company headquar- ters was an achievement of architecture of its day when completed in 1925, and it remains an emblem of the Art Deco movement. The building’s current owner decided to embark on the challenging endeavor of reviving the historic structure. This meant preserving the historic fabric, creating an open, flexible Nina A. Mahjoub Megan Stringer workspace, and infusing state-of-the-art technology and sustainability into all its aspects, including a voluntary full seismic structural upgrade.

Introduction capacity of the existing building system. Moreover, this approach allows the engineer Situated in the heart of downtown San to better understand how the new and Francisco, the Pacific Telephone & Telegraph existing systems behave together during a (PT&T) Company headquarters opened in seismic event, and therefore provides a Bill Tremayne 1925, reaching 132.7 meters and becoming smart, more sustainable, and less obstructive the tallest building in the city upon solution while maintaining the historic fabric Authors completion (see Figure 1). The building, now of the building. Nina A. Mahjoub, Project Engineer known as 140 New Montgomery (140NM), Megan Stringer, Project Engineer Bill Tremayne, Principal still stands as an icon of design and a Lastly, the paper discusses the environmental Holmes Culley San Francisco reminiscence of the power of the latest benefits of retrofitting versus rebuilding, and 235 Montgomery Street, Suite 235 San Francisco, CA 94104, United States technology of the time. how the sustainability objectives of the t: +1 415 693 1600 project shaped the design. f: +1 415 693 1760 e: [email protected]; The building’s current owner since 2008, [email protected]; Wilson Meany, a real estate developer, [email protected] http://www.holmesculley.com decided to embark on a challenging The Historic Building endeavor of reviving the historic structure.

Nina A. Mahjoub While it will continue to host offices, the 140NM consists of a 26-story base, with a Nina Mahjoub is a project engineer at Holmes Culley, building will now introduce state-of-the-art four-story tower above Level 27 and two and was involved in the 140 New Montgomery project from schematic design phase to construction technology in all aspects, including a basement levels, designed by Timothy completion. She holds a Bachelor’s Degree in Civil voluntary structural system upgrade, while Pflueger and Frank Miller. When completed, Engineering from UCLA and a Master of Engineering in High-performance Structures from MIT. She has maintaining the architect’s original intent. In 140NM became the tallest building in the more than five years of experience in New York City addition to preserving the building’s historic city, until its height was matched by the and California. Her projects have been focused on renovation, adaptive reuse, and seismic retrofits of features, the project team wanted to create a neighboring Russ Building two years later. existing and historic buildings. healthy, sustainable space for its tenants and targeted LEED Gold for the project. The building provided space for PT&T’s 2,000 Megan Stringer Megan Stringer is a project engineer at Holmes employees. The PT&T building was known Culley. She holds a Master’s degree in Structural This paper outlines the design goals of this nationally and internationally in the business Engineering from the University of Texas at Austin. An active member in the sustainable design community, upgrade: from the preservation of the and design communities, and was visited by she chairs the Structural Engineers Association of historic fabric to the creation of open flexible VIPs such as Winston Churchill, who in 1929 California’s Sustainable Design Committee and is a member of ASCE’s SEI Sustainability Committee. She office space, all while providing a safe and made one of the first Transatlantic phone has presented at various conferences on sustainability sustainable structure in San Francisco’s calls from the building. and the role engineers play in shaping the built environment. unforgiving seismic environment. It also discusses the strengthening scheme The building is classified by the City of San Bill Tremayne evaluated and challenges faced during the Francisco as a Category I Historic Building Bill Tremayne is a principal and technical director at Holmes Culley. With more than 13 years of experience, design. It presents details on the analysis and is eligible to register for the National he leads the firm’s practice of performance-based method of the seismic retrofit, which utilized Register of Historic Places. Some of its engineering, which has been successfully applied to numerous seismic evaluation, retrofit, and new a performance-based design. This method historic features include 2.4 hectares of building projects in the United States and New presents the engineer with the capability to terracotta façade constructed by the Zealand. Bill is an ongoing contributor to the development of the masonry and analysis provisions look past conservative building codes and Gladding McBean Company, and eight of ASCE 41. determine in a more precise way the terracotta eagles perched atop the tower

34 | Retrofit / Seismic CTBUH Journal | 2015 Issue I Some of the building’s historic features “include 2.4 hectares of terracotta façade constructed by the Gladding McBean Company, and eight terracotta eagles perched atop the tower. The entrance houses an ornate and dramatic lobby with detailed bronze doors, marble walls, and a hand- painted plaster ceiling by Mark Goodman.” now, one can quickly notice that the historic companies in the tech sector, by providing Figure 1. 140 New Montgomery, San Francisco. fabric of the structure has stayed intact them with state-of-the-art technology © Nathaniel Lindsey throughout the decades. infrastructure and flexible workspace within (see Figures 2 and 3). The entrance houses an a historic high-rise. To achieve that vision, the ornate and dramatic lobby with detailed After housing one company for over 80 developer engaged a design team in 2011 bronze doors, marble walls, and a hand- years, the building was sold in 2008 to a real that would spend the next few years painted plaster ceiling by Mark Goodman estate developer. It was the refined character following the guiding principles that would (see Figure 4). The entrance also leads to a of the historic building that would set the restore and reinvigorate this iconic structure. marble staircase. tone for the project and the vision of its new and proud owner. Some of the major work undertaken in this Since completion, the building has remained renovation includes the historic lobby relatively untouched, with a façade rehabilitation, elevator modernization (to renovation in the 1980s and parapet bracing Project Vision and Goals support destination control), and entirely installed just prior to the 1989 Loma Prieta new mechanical, electrical, and plumbing earthquake. Comparing the building then to An article in San Francisco Newsweek from systems designed with tenant controllability. 1925 described 140NM as “the new building generation, a monument to western progress, and foresight.” Eight decades later, a new developer was determined to continue this vision and honor its original inception as a modern communication hub and a center of innovation. 140NM was going to continue housing the technology of tomorrow by attracting creative entrepreneurs and

Figure 2. Terracotta façade along New Montgomery Figure 3. Eight terracotta eagles perched atop the 140 Figure 4. Historic Lobby Entrance. © Stephen Schafer Street. © Stephen Schafer NM. © Nathaniel Lindsey

CTBUH Journal | 2015 Issue I Retrofit / Seismic | 35 Figure 5. Typical interior floor after soft demolition. © Blake Marvin Figure 6. Typical existing wind brace moment-resisting connections.

The majority of the windows were replaced at Level 19, 23, and roof. The existing founda- finite element model of the building was with high-performance glazing and will tion consists of perimeter concrete walls and developed. The analysis simulated how the remain operable to promote natural ventila- concrete spread footings at each column. building would perform during a large tion. Some exterior restoration of the historic earthquake, and how a seismic strengthening terracotta and historic glazed brick façade was The existing lateral structural system consists scheme could be most effectively designed to undertaken, which included repointing of of steel beams and columns, classified as reduce damage and mitigate collapse hazards. joints and patching/repainting individual units “partially restrained moment frames.” The This provided the developer with a better as necessary. beams and girders, typically around the understanding of the value and risk associated perimeter, are connected to the columns via with the building. The design team was able But one of the most important components riveted clip-angle connections (see Figure 6) to develop a smarter and more efficient of 140NM’s refurbishment was the full seismic and, in some instances, large gusset plates, system, taking full advantage of the inherent retrofit. This retrofit was carefully designed denoted on the existing drawings as wind strength of the existing building materials and and implemented to improve the building’s braces. overall retrofitted system. safety and performance in the harsh seismic environment of San Francisco, while preserv- Performance-based engineering Rehabilitation objective and design criteria ing the building’s iconic historic fabric, The requirement to assess and enhance the In the case of 140NM, the global seismic including the exterior façade and grand lobby seismic safety of 140NM stemmed from two rehabilitation objective for the building was to (see Figure 5). main drivers: owner-desired improvement achieve the Life Safety Performance Level at and code-required triggers per the San 75% of the ASCE’s Basic Safety Earthquake 1 Francisco Building Code (SFBC). Given the (BSE-1) seismic hazard. This corresponds to a Seismic Upgrade Criteria building’s historic classification, the following return period of 310 years with a 15% code of reference was used: 2010 California probability of exceedance in 50 years. This Existing building structural system Historical Building Code (CHBC), as permitted performance objective was consistent with The existing building has an L-shaped floor by the 2010 San Francisco Building Code. For the intent and requirements of the CHBC, plan, measuring about 46 meters along New the evaluation of the building’s seismic which requires enforcing agencies to accept Montgomery Street (the north-south performance and retrofit design, ASCE 41-06, reasonable equivalent strategies to the regular direction) and about 43 meters along Minna Seismic Rehabilitation of Existing Buildings code, and permits the seismic forces used to Street (the east-west direction). Typical (ASCE 41) was primarily used. ASCE 41 uses a evaluate the structure to be limited to 75% of floor-to-floor height is about four meters. The performance-based engineering (PBE) those prescribed for new construction. structural system is made of a grid of steel approach. While traditional code-prescribed beams encased in concrete for fireproofing, approaches are often conservative, especially Analysis procedure and earthquake ground connected to steel columns encased in for existing buildings, PBE provides the motions concrete at the interior floors and in brick at framework to capitalize on the inherent In the vicinity of the project site, strong the perimeter. The infill beams are reinforced strength of the original structure. shaking can be expected from a moderate-to- concrete and the floor consists of a 102- to large earthquake. In particular, a potential 152-millimeter-thick reinforced concrete slab. Once seismic hazard and performance moment-magnitude 7.9 rupture on the The building steps back throughout its height objectives were defined, a comprehensive nearby San Francisco peninsula segment of

36 | Retrofit / Seismic CTBUH Journal | 2015 Issue I the San Andreas Fault controls the seismic ƒƒ Life Safety Record (Earthquake, year, station) Magnitude Duration (second) hazard adopted for this project. The ƒƒ the proposed Loma Prieta, 1989, Los Gatos, California 6.9 25 geotechnical engineer, Treadwell & Rollo, architectural layout of the Landers, 1992, Joshua Tree, California 7.4 44 developed the suite of earthquake time renovated space Landers, 1992, Yermo, California 7.4 44 histories used for this project (see Table 1). ƒƒ the need to maximize Kocaeli, 1999, Gebze, Turkey 7.4 28 Those earthquakes ranged from a magnitude leasable square footage of 6.9 to 7.9, with duration of 25 to 90 ƒƒ the need to minimize Kocaeli, 1999, Duzce, Turkey 7.4 27 seconds. For this project, a nonlinear impact on the existing Duzce, 1999, Duzce, Turkey 7.1 26 dynamic procedure (NDP) was adopted for historic building Denali, 2002, PS10, Alaska 7.9 90 the analysis of 140NM. elements Table 1. Earthquake time history suite ƒƒ constructability Finite element modeling the existing ƒƒ LEED Gold objective efficient and innovative structurally, and was building ƒƒ overall project cost cost-effective. The analytical model was prepared using geometry obtained from a comprehensive The brick and terracotta cladding system is The new reinforced concrete shear walls, with set of existing drawings. Grades and inherently brittle. Therefore, the seismic coupling beams, were located around the strengths of materials used in the model strengthening system needed to be new stair and service cores extending from were obtained from material testing, sufficiently stiff to limit the potential for the basement level foundation to the including strengths of the steel (at column, damage to this façade under the specified underside of Level 27. The walls over the beam, plate, angles, rivets, and reinforcing) seismic hazard. typical floors were connected to the existing and concrete. Being able to use expected adjacent steel columns to effectively provide a strengths versus more conservative New Seismic Load Resisting System (SLRS) composite column, which strengthened the code-prescribed strengths provided an Based on the evaluation of a number of existing deficient elements. These new “core” average increase in strength of 10%. schemes with respect to the previously elements provided not only a suitable location discussed selection criteria, a strengthening for new lateral load-resisting elements, but The model included the foundation, scheme utilizing new reinforced concrete also a functional separation between the concrete floors, built-up steel columns, and core walls with structural steel outrigger tenant spaces and service areas. the steel beams with the contribution of trusses was chosen. Other considered their concrete encasement. The partially schemes included propped steel-plate shear The core walls were stiffened with the restrained (PR) moment frame connections walls, internal steel-braced frames with two-story deep outrigger trusses located were explicitly modeled to evaluate their buckling restrained braces (BRBs), and below Levels 8 and 19. They were located as strength and flexibility. The contribution of perimeter shotcrete overlay walls. The high as practicable in the building without the relatively stiff existing infill brick façade proposed scheme had the smallest impact adversely impacting the usable floor space, as was also accounted for. The stiffer response on the existing building, was the most the building façade steps at Level 19. The resulted in the exterior frames attracting a greater proportion of the lateral load that would be assumed from the bare steel PR-frame alone, which consequently resulted in more severe demands on the existing steel columns.

After running the model of the existing building (pre-retrofit), it was concluded that the existing building would perform poorly, with excessive damage to the masonry façades and column failures due to high axial demands.

The strenghtening scheme In order to select a seismic strengthening scheme, a number of criteria were established by which to measure overall performance: Figure 7. BRB at outrigger truss. Figure 8. Supercolumn installation.

CTBUH Journal | 2015 Issue I Retrofit / Seismic | 37 Combined model The new SLRS and existing structure, each The Seismic Lateral Resisting System was described in the preceding sections, were combined in a single analytical model (see “capable of resisting 100% of the seismic Figure 10). This step of the analysis was crucial to verify the deformation compatibility of the demands associated with the specified seismic two systems and ensure that they work hazard. Story drifts were generally low – less effectively together during a large seismic event. than 1.2% of the total height – except at the New Structural Lateral Resisting System upper levels. Drifts at the northwest corner were Upon running and analyzing the new structural scheme only, it was concluded that also slightly higher. the new SLRS was capable of resisting 100% ” of the seismic demands associated with the outrigger truss frame included one bay of At the west and south wing façades, new specified seismic hazard. Story drifts were diagonal BRBs which form a ductile fuse to reinforced-concrete overlay walls were generally low – less than 1.2% of the total absorb and dissipate the seismic energy in the added. Those perimeter overlay walls were height – except at the upper levels. Drifts at form of heat. Figure 7 shows the BRB installed punched to maintain the existing window the northwest corner were also slightly higher. on-site. From Level 19 and below, new steel openings. supercolumns were provided at the perimeter Combined systems to transfer the load from each outrigger truss Figure 9 illustrates the new seismic load- The combined model showed that the new down to the foundation. Figure 8 shows the resisting system as implemented in the SLRS added significant strength, stiffness, and supercolumn during installation. analytical model. ductility to the existing building while maintaining deformation compatibility with the existing lateral force resisting elements. This allowed the rehabilitated building to achieve Life Safety at the specified seismic hazard.

Story drifts were generally significantly less than 1%, with larger drifts at the corners and towards the top of the building (above the upper outrigger trusses) and maximum drifts less than 1.5%. Opportunities to strengthen of some existing components were identified, including reinforcement of the existing columns. The low drifts will provide added protection to the relatively brittle façade elements.

Sustainability

In the context of the built environment, sustainable design should meet the needs of the present without compromising the ability of future generations to meet their own needs. The “triple bottom line”, a phrase coined in the mid-1990s by John Elkington, is the model for evaluating sustainability that includes environmental, economic, and social performance measures. While many are Figure 9. New seismic lateral resisting system. Figure 10. Combined model

38 | Retrofit / Seismic CTBUH Journal | 2015 Issue I starting to be concerned about impacts associated with a product or 20,000,000 environmental impacts of the built process over its life cycle with the goal of environment, a creative, effective, and truly evaluating and reducing those impacts. sustainable design should consider all three. LCA offers a powerful tool for evaluating the environmental impacts of buildings and 15,000,000 eq) Benefits of a retrofitted building strategically reducing them, considering all 2

It is often said that the most sustainable the building systems and all stages of a CO building is the one already built. As the building’s life cycle. It enables comparison annual replacement rate of buildings has between different building components 10,000,000 historically been around 2%, reusing and and system selections, and offers a greening the existing building stock offers comprehensive way to evaluate and reduce one of the greatest opportunities for the overall environmental impacts.

reducing the environmental impacts of Unit of Measure (kg 5,000,000 buildings. Reusing existing buildings requires The impacts from the retrofit versus creative problem solving, evaluation of the construction of a new building were best use of the existing structure, and compared from an LCA standpoint. Rough integrated design practices. It has material quantities were estimated for a 0 dramatically lower impacts on the new building with equivalent floor area. 140NM - New 140NM - Retro t environment by requiring not only less new Athena’s Impact Estimator was used for the Figure 11. Comparison of global warming potential by material and use of nonrenewable resources, LCA analysis. From the LCA, it can be seen assembly groups. but reducing the amount of waste produced that the retrofit has a little over a quarter of and energy required for demolition and the environmental impact of constructing landmark buildings close to their original rebuilding. an equivalent new building, per Figure 11. design, while lending them a strong, efficient, The most common metric of an LCA is and sustainable scheme for maintaining their The retrofit scheme used for 140NM allowed Global Warming Potential. This is a measure future viability. It must also be recognized for the reuse of 95% of the existing structure. of the potential to increase the temperature that, with advancing structural analysis, This has a great impact on the sustainability of the Earth’s atmosphere and oceans. historic buildings no longer need to be of the project, and was achieved because of viewed as archaic, but rather as successful the owner’s objectives and through utilizing 140NM achieved LEED Gold certification. marriages of preservation and innovation.  PBE. Preserving the building had more than The building received the maximum just environmental benefits. 140NM is a amount of credits available for building Unless otherwise noted, all photography credits historic building and preserving it is essential reuse, including an Innovation in Design in this paper are to the authors. to maintaining the city’s heritage. Historic and Regional Priority credit. buildings are typically structurally robust and can be upgraded to meet current standards References with typically minimal impacts to the Conclusion American Society of Civil Engineers. 2006. ASCE 41-06: environment. The significant reuse of the Seismic Rehabilitation of Existing Buildings. Reston: ASCE. existing structure, paired with the minimal Reviving 140NM entailed designing Chambers, J.; Tremayne, B.; Dreyer, R. & Kelly, T. E. 2008. addition of new materials to bring the building system upgrades and insertions as ”Performance Based Assessment and Retrofit of Existing existing building to Life Safety level, provided elegant and contemporary elements that Buildings: A Case Study.” Structural Engineers Association of California 2008 Convention Proceeding. Sacramento: SEAOC. not only a sustainable solution, but a smart would complement the building’s historic and necessary one. With inevitable seismic features. It required delivering a robust, Kestner, D.; Goupil, j. & Lorenz, e. (eds.) 2010. events in the coming years, this retrofit is smart, integrated, and nimble infrastructure, Sustainability Guidelines for the Structural Engineer. Reston: ASCE. expected to have increased the building’s and required a flexible architectural layout service life by another 50 to 100 years, and that could adapt over time. All those goals ASCE-SEI Carbon Working Group. 2012. “Structures and has brought back to life a historic building were achieved when the building reopened Carbon How Materials Affect the Climate.” Reston: ASCE. unoccupied for years as a revitalized, at the end of 2013. It now houses more United States Green Building Council. 2013. LEEDv4 restored, sustainable, and safer prominent than 1,000 people. Moreover, the seismic for Building Design and Construction: Ballot Version. http:// structure of San Francisco. upgrades allow many more to experience www.usgbc.org/leed/v4. Accessed on September 2014. the historic structure in future generations.. Life Cycle Assessment Analysis Life-Cycle Assessment (LCA) is a technique These future generations must used to account for all of the environmental acknowledge the significance of keeping

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