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Transformative Tower

By Ron Klemencic, P.E., S.E., Hon.AIA, F.ASCE , Mike Valley, P.E., S.E., M.ASCE, and John Hooper, P.E., S.E., F.ASCE

The tallest structure in San Francisco, Salesforce Tower, has transformed the city's skyline. And by being the first skyscraper to extend deep  foundations into the city's bedrock, the first to extend performance-based seismic design to account for increased capacity, and the first to employ a structure-soil-structure interaction analysis in a real-world project, it may transform structural engineering in the Bay Area as well.

Its skyline as iconic as any around the world, San Francisco has been identified by the silhouette of the Transamerica Pyramid and the Golden Gate Bridge for decades. Transforming this skyline to reflect the city's position as a global leader in technology and civic progress would be no small feat; yet this is precisely what city planners set out to accomplish more than a decade ago.

Fast-forward to 2018, and the recently completed $1.2-billion Salesforce Tower has accomplished this transformation with a design that pushes the envelope of tall-tower design in San Francisco and serves as a symbol of the city's progressive leadership in the world.

In 2001, the City and County of San Francisco, the Alameda-Contra Costa Transit District, the Peninsula Corridor Joint Powers Board, the California High-Speed Rail Authority, and the California Department of Transportation (Caltrans) established the Transbay Joint Powers Authority (TJPA) to design, build, operate, and maintain an intermodal terminal and rail extension and to collaborate with the San Francisco Redevelopment Agency and city departments to create an adjacent new transit-oriented neighborhood. The TJPA represents a variety of modes of public transit, from bus systems to commuter rail. Its ambition was lofty—to essentially create the Grand Central Station of the West and a brand-new neighborhood around it. The proceeds of land sales adjacent to the transit center contributed to the funds required to construct it.

In 2007 the TJPA hosted an international development competition to envision and create a new transit center and an iconic tower that would mark the transit center's position in the city on land amassed by the authority. The land had been occupied by the dilapidated vintage-1939 Transbay Terminal and the Embarcadero Freeway, which had been dismantled in the wake of damage sustained during the 1989 Loma Prieta earthquake. Additional surrounding parcels were acquired by the TJPA to create a large transit district.

A team led by Houston-based developer Hines in collaboration with Pelli Clarke Pelli Architects, based in New Haven, Connecticut, submitted a design that fulfilled the ambitions of the TJPA. The Hines/Pelli Clarke Pelli team was selected from a list of esteemed international competitors. The winning proposal included an iconic office tower and multimodal transit station as well as an added public amenity, a 5.4-acre park atop the transit center. (See accompanying article.) Shortly after Hines/Pelli Clarke Pelli was selected as the winning team, the U.S. economy fell into recession. From 2008 to 2010, the project lay essentially dormant, with only the developer, Hines, remaining steadfast in the belief that the tower would be a success in the long run. Hines's patience and tenacity paid off.

In 2010, Boston Properties, of Boston, purchased a 95 percent stake in the project. With Hines and Boston Properties working together, momentum increased. Design began in earnest in late 2010; the developers' goal was to break ground in 2012. The design and construction team members were: 

* A joint venture of Clark Construction, of Bethesda, Maryland, and Hathaway Dinwiddie, of San Francisco, general contractor
* Kendall/Heaton Associates, of Houston, architect of record
* Magnusson Klemencic Associates (MKA), of Seattle, structural engineer
* WSP USA, of New York City, mechanical, electrical, and plumbing engineer
* Arup, of London, geotechnical engineer and seismic ground motions consultant
* PWP Landscape Architecture, of Berkeley, California, landscape architect

At 1,070 ft tall, Salesforce Tower is the tallest commercial office building west of Chicago. With 61 floors and a total of 1.5 million sq ft of space, it is a mammoth building by any measure. Designing and constructing such an ambitious project required new ways of thinking and the application of technologies that had never been tried before—an approach consistent with the culture of the San Francisco Bay Area.

San Francisco is underlain by a complex mix of soil types ranging from loose marine sand and bay mud at the top to, in descending order, a densely packed sand known as the Colma formation, Old Bay clay, valley deposits, and the Franciscan formation, which serves as bedrock. (See the figure at left.) These layers vary in thickness; bedrock is near the surface atop the seven famous hills of San Francisco but is as much as 300 ft below the ground level in other locations. Unfortunately, the site of Salesforce Tower is situated in one of the locations where bedrock is deepest.

Located on Mission Street, between First and Fremont Streets, the site of Salesforce Tower was once the approximate shoreline of the San Francisco Bay. After the 1906 earthquake and subsequent fire, much of the city's debris was pushed into the bay in this location, creating new land.  This helped the city's rebuilding effort but created a variety of shallow soil conditions.

Until the construction of Salesforce Tower, buildings constructed on similar soil profiles in San Francisco were generally founded in the dense Colma formation sand located beneath the shallow fill. This sand layer typically offers bearing pressures upward of 10,000 psf for the support of shallow and mat foundations and good friction values (2,000 to 3,000 psf) for the support of piles. Numerous high-rise buildings have been successfully constructed atop the Colma sands.

The sheer size and weight of Salesforce Tower, however, made this project different. The pressures exerted by such a massive structure on the Colma sands could potentially overtax the weak Old Bay clay below. With the applied pressures predicted to exceed the plastic limit of the clay, significant building settlements were of great concern.

A new approach, one that had never before been done in San Francisco, was required. Large-diameter (6 to 8 ft) drilled shafts and load-bearing elements (LBEs), also known as barrettes, were considered appropriate alternate foundation types to reach the bedrock, which was located 300 ft below the ground surface. At the time of design, these depths approached the limits of the technology for drilled-shaft installation, and LBEs had not previously been attempted in San Francisco.

With either foundation type, a detailed testing program would be required to confirm the design assumptions. While commonly referred to as bedrock, the Franciscan formation is actually a mélange of soft, shale-like layers interbedded with very hard sandstone boulders. This is very unusual and much softer than the hard granite bedrock commonly found in such skyscraper-filled cities as Chicago and New York.

The original foundation design called for the use of 42 8 ft diameter drilled shafts and was considered highly constructible because multiple contractors possessed the equipment to conduct such an installation. During the bidding process, however, two separate contractors proposed the use of LBEs in lieu of the drilled shafts, highlighting the virtually unlimited depth to which the LBEs could extend. Ultimately, a design was selected that included 42 LBEs, each 5 by 10.5 ft, extending 250 to 310 ft belowground.

The capacity of the LBEs was ultimately confirmed through two full-scale Osterberg Cell load tests. These tests, capable of exerting up to 24,000 kips of force in opposing directions at a specific design depth on the LBEs, provided the design and construction team with the data necessary to complete the foundation confidently.

Because the Salesforce Transit Center (STC) was already under construction immediately adjacent to the tower's site, complexities with the temporary excavation support system demanded that the LBEs be installed from the existing grade. Once all 42 LBEs were installed, excavation and shoring could begin, and this required the demolition of the uppermost 60 ft of the LBEs to the elevation of the toe of a 14 ft thick mat foundation, which interconnects the LBEs to create a solid base for the tower. 

Throughout the construction of Salesforce Tower, building movements, including those of the foundation, were monitored. Predictions suggested vertical movements at the foundation of approximately 1.5 in., primarily due to the elastic shortening of the LBEs as loads were applied from the building above. Actual measurements throughout construction confirmed these predictions; the vertical movements were roughly 1.25 in. at the completion of the tower.

Foundation design in San Francisco has been transformed since the completion of the tower. With Salesforce Tower serving as the first to extend deep foundations into bedrock, there have since been three other towers following suit, and more are planned.

Hines had requested a 1,000 ft tall tower that would not need to rely on outrigger trusses, belt trusses, exterior bracing of any kind, or the use of damping systems. The developer did not want any structural elements to encumber valuable, leasable space or block any of the amazing views of the San Francisco Bay.

The architects and engineers eagerly agreed to take on this challenge. As the requirements for vertical transportation, occupant egress, mechanical and electrical systems, and restrooms became defined, it became evident that with an efficient arrangement of these elements surrounded by reinforced-concrete walls in a central core, sufficient structural strength and stiffness could be provided to brace the tower against wind and seismic demands. With plan dimensions of 83 by 89 ft, the aspect ratio (height divided by width) of the core was approximately 12, registering as slender—but not unreasonably so.

The resulting structural design is simple and elegant. Reinforced-concrete walls designed to resist wind and seismic forces are arranged efficiently around the core of the building, leaving the entire perimeter open and flexible for use by the building's occupants. With a mere three columns per side and column-free corners, the commanding views of the entire Bay Area have been preserved. (See the floor plan) Pelli Clarke Pelli has called the design a "quiet" structure because the structural elements all but disappear from view.

The design of a concrete shear-wall building reaching 1,070 ft tall falls well outside of the 240 ft height limit prescribed by the city's building code. In order to obtain permission to exceed this limit, and to do so by such a wide margin, a performance-based seismic design (PBSD) approach was adopted. While numerous towers taller than 240 ft had already been realized using similar approaches, Salesforce Tower would be the first to be designed to more stringent performance criteria.

The city's building code defines typical commercial office buildings as belonging to risk category II, for which standard safety factors and performance objectives (for example, those relating to tower sway) are required. The code defines larger buildings that will house more than 5,000 occupants as belonging to risk category III, which takes into account the greater potential threat to occupants for which higher safety factors and more stringent performance objectives must apply.

How PBSD could be transformed to meet the goals of risk category III became the fundamental question. In simple engineering terms, there is the "demand side" and the "capacity side" to any engineering design equation. Prescriptive code requirements suggest that to achieve the enhanced safety and performance objectives for a risk category III building, the demand side of the design equation should be multiplied by 1.25. In doing so, the strength of the building will surely be enhanced but not necessarily the overall building performance.

In the context of PBSD, increasing the demand side of the design equation for this project made little sense; the parameters associated with seismic ground shaking in the Bay Area are capped as a deterministic maximum based on the tectonics of the San Andreas Fault. Arbitrarily applying a 1.25 factor to the demand side of the design equation would not result in the desired enhanced performance levels.

Instead, the supply side of the design equation was adjusted by adopting more strict acceptance criteria for all the design parameters, as shown in the table. With this approach, seismic ground-shaking inputs are consistent with well-established parameters and defined deterministic maximums while the structural design is enhanced to meet more strict performance objectives. This is completely consistent with the underlying objectives of the building code.

In total, 22 unique horizontal pairs (X and Y) of seismic ground motion records were input to a detailed nonlinear computer model of the building to test the performance of the tower using a scenario spectra approach. Eleven pairs of ground motions were conditioned around the first mode of vibration of the tower, and 11 additional unique pairs of ground motions were conditioned for the higher modes of vibration. The purpose of this approach is to more rigorously test the structure's performance while recognizing that tall buildings tend to respond significantly in the higher modes of vibration. The resulting design meets or exceeds all design objectives.

On January 4, 2018, a small tremor centered approximately 9 mi away, in Berkeley, California, shook the Bay Area. A seismic monitoring system had been installed throughout the height of the tower to enable the collection of performance data to confirm the design. As expected, the tower's primarily response was in the second mode of vibration, displayed in the figure on page 51. The fundamental periods of vibration of the tower were measured in each direction as approximately five seconds as compared to the calculated value of six seconds. Not surprising for this low level of ground shaking, the nonstructural elements (the exterior walls, for example) contributed to the overall tower response, inherently providing added stiffness to the tower. 

With this newly defined capacity-side adjustment to the PBSD of Salesforce Tower, PBSD was transformed to produce designs of enhanced safety and performance. Following suit, towers all along the West Coast—from Los Angeles to Seattle—have adopted similar approaches to seismic design.

Designing and constructing a tall tower in a dense, urban environment is commonplace. What is unique about the Salesforce Tower is its proximity to the rising STC. Not often are two adjacent structures constructed at the same time. Even less common is for those structures to be in direct contact with each other.

The agreement between the TJPA and the developer specified that the construction of the new tower should in no way adversely affect the seismic performance of the transit center. To explicitly demonstrate this outcome, an analysis that had never been done before was conducted. The concept of a structure-soil-structure interaction (SSSI) analysis had been the focus of some academic studies but had never been executed for real-world commercial buildings.

Arup developed a complex model of the underlying soil strata extending to bedrock. Into this model were inserted the transit center, Salesforce Tower, and one other tower of relevance (the Millennium Tower at 301 Mission Street), including the predicted dynamic properties of the structures.

Working collaboratively, MKA and Arup studied the results of the model, which clearly indicated stress concentrations in the transit center near the corners of the Salesforce Tower substructure. Fortunately, the transit center was stil under construction, so it was possible to add reinforcement to the ground-level diaphragm of the transit center in those areas.

The SSSI analysis was the first of its kind to assess the impacts of one building on its neighbor during an earthquake. Subsequently, several other buildings constructed adjacent to the transit center have followed suit, transforming the state of practice of earthquake engineering in San Francisco.

Crowning Salesforce Tower is a 150 ft tall, open latticework lantern. The design of the lantern faced conflicting demands. Capping the height of the tower at 1,070 ft was essential to city planners but in direct opposition to the developer's view of market demands for office space. Groups concerned about shadows that would be cast by an opaque structure of this height also challenged the design team, as did wind loads generated by the gusty San Francisco Bay climate.

The solution is the lantern, which is clad with a porous metal skin that allows light and wind to pass through, reducing the impacts on the structure and its surroundings while maintaining the desired height of the tower without compounding the developer's risk.

Designing this unique structure, essentially a 15-story building resting on top of a 61-story office tower, posed additional engineering challenges. Detailed wind tunnel investigations were conducted to better understand the demands on this structure and what benefits a porous metal skin could provide. A 10 percent reduction in the global wind overturning moments was realized and shadows cast on the city, most importantly on Union Square, were minimized.

The metal skin also provides a unique opportunity for the tower to serve as a canvas for public art. Acclaimed artist James Campbell was retained by the developer to create a design that would be unique and could be enjoyed by all from any vantage point from which the tower is visible. The resulting installation includes 11,000 light-emitting diodes that create animated images on the tower top at night. The dynamic images are taken from cameras strategically located around the city of San Francisco to capture daily activities and then abstractly reflect them in the nightly tower-top show. It is an amazing and inspirational sight to see.

The tower also features the largest on-site water recycling system in a commercial high-rise building in the United States and is seeking the highest-level rating, platinum, from the the U.S Green Building Council's Leadership in Energy and Environmental Design (LEED) program. (See accompanying article.)

The tower was completed on January 8 and opened shortly after, on January 19, named for Salesforce, the cloud computing company that leases 60 percent of its space.

It is rare, perhaps just once in a lifetime, that a project fundamentally transforms an already iconic skyline while at the same time introducing new technologies and methodologies to the world through its creation. The transformative effect of Salesforce Tower on the skyline of San Francisco, and on the practice of structural engineering, is reflective of and consistent with the impacts that the San Francisco Bay area has on the world through technological and social advancements.

Ron Klemencic, P.E., S.E., Hon.AIA, F.ASCE, is the chairman and chief executive officer of Magnusson Klemencic Associates (MKA), of Seattle. Mike Valley, P.E., S.E., M.ASCE, is a principal and John Hooper, P.E., S.E., F.ASCE, is the director of earthquake engineering for the firm.

PROJECT CREDITS 

Developer Hines, of Houston, and Boston Properties, of Boston
Architect Pelli Clarke Pelli, of New Haven, Connecticut, with Kendall/Heaton Associates, of Houston, architect of record 
Structural engineer Magnusson Klemencic Associates, of Seattle 
General contractor Joint venture of Clark Construction, of Bethesda, Maryland, and Hathaway Dinwiddie, of San Francisco
Geotechnical engineer and seismic ground motions consultant Arup, of London 
Mechanical, electrical, and plumbing engineer WSP USA, of New York City 
Leadership in Energy and Environmental Design (LEED) consultant stok, of San Francisco 
Landscape architect PWP Landscape Architecture, of Berkeley, California

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Transit Center Propels San Francisco

*   Update: Just after press time, a crack was discovered in two of the transit center facility's beams, which span Fremont Street. The authors have informed   Civil Engineering   that this has since been determined to be a localized issue, that the area has been stabilized, and that an investigation is ongoing to determine the cause and the proper remediation.  -Eds.

The Salesforce Transit Center (STC) in downtown San Francisco features an iconic architectural design by Pelli Clarke Pelli Architects that was selected through a juried competition. The STC anchors a new transit-oriented neighborhood and has facilities, capabilities and amenities—including a public rooftop park—unimaginable for its predecessor, a 1939 terminal that was built for electric trains but served buses only over its final five decades. Developer Transbay Joint Powers Authority (TJPA) worked with the San Francisco Redevelopment Agency and city departments related to local development to create an innovative structure that would transform the hub into a true public amenity. The changes in the neighborhood since the center was completed on August 10 have been dramatic, with new construction on numerous lots adjacent to the STC, including the 61-story Salesforce Tower and the 56-story, mixed-use 181 Fremont tower.

When fully operational, likely in 2028, the STC will host 11 transportation operators connecting Bay Area counties and other parts of California through several transportation systems, including AC Transit, the San Francisco Municipal Railway, the Bay Area Rapid Transit (BART) system, Caltrain, and California's future high-speed rail, which is expected to connect San Francisco to Los Angeles.

Although not officially a lifeline infrastructure system, the TJPA specified a performance goal that the STC should be able to support bus operations after a major earthquake in the Bay Area. This was a key factor in many of the design decisions made for the facility.

At almost 1,600 ft long, the STC covers three city blocks. A reinforced-concrete, below-grade "train box" extends beneath the city streets and includes concourse and train platform levels. Above-grade, steel-framed construction includes two retail levels, a bus deck, and the roof park level. While the train box is structurally continuous, the superstructure consists of three discrete pods separated by seismic joints to allow independent movements during earthquakes. 

The center's structural framing is highly visible, forming an integral part of the architecture. Inclined circular columns on each side of the building form eccentric braced frames in the longitudinal direction. The intersections of the inclined circular columns with other frame members resulted in complex geometric conditions, so these connection nodes were studied as both weldments and as castings. Ultimately the nodes were built as exposed steel castings, some weighing up to 46,000 lb. The castings were studied for design loads, including seismic load combinations, which typically governed, using highly advanced finite-element analysis tools usually not required in building design.

Casting procurement was schedule-critical for developing the shop assemblies and for steel erection in general. This required close collaboration between the structural engineer (Thornton Tomasetti Inc.'s San Francisco office), the casting detailer (Cast Connex Corp., of Toronto), and the foundry (Bradken's Kansas City, Missouri, office). Three-dimensional software platforms were used for the efficient review and coordination of these highly complex shapes.

The building superstructure incorporates special moment frames in the transverse direction. The performance of the longitudinal and transverse systems was verified through advanced analytical studies and full-scale component testing performed at the University of California, San Diego.

The STC sits on a mat foundation at the base of the train box, anchored with 10 in. diameter micropile tiedowns to resist buoyancy. Buoyancy was determined on the basis of factored loads and the possibility that the weight of the soil on the park, which averages 3 ft in depth across the roof, may be reduced or redistributed during construction and future renovations.

Because seismic performance and resilience were critical, the structural systems that were selected to suit the architectural design intent and the  building functionality went beyond current building code limits, so a performance-based seismic design approach was followed. This required more rigorous analytical procedures than in typical buildings and a peer review in addition to standard city plan-check approvals.

The seismic design approach typically  used for buildings relies on members and details sized for forces based on a design earthquake (DE), reduced in anticipation of ductile behavior. The intent is to achieve life safety for a DE event and an acceptably low probability of collapse in a larger, maximum considered earthquake (MCE). In downtown San Francisco, the MCE roughly corresponds to a magnitude 8 earthquake along the San Andreas Fault. Buildings that meet these safety-based performance objectives could still be unusable for many months after the event until costly repairs could be performed.

To keep bus operations uninterrupted even after very large earthquakes, the STC was instead designed for a higher seismic performance objective than for a typical building. Meeting that higher performance level meant including structural and nonstructural systems in the analyses. The structural seismic load-resisting elements were evaluated using explicit nonlinear modeling and realistic seismic time histories rather than ductility assumptions. Operations-related mechanical, electrical, plumbing, and fire protection systems, as well as architectural elements such as partition walls and cladding, were hardened to meet or exceed those required for California hospitals.

With the incorporation of enhanced structural and nonstructural seismic performance targets, the STC is likely one of the most seismically resilient buildings in California. 

Transit bus operators have already started to use the building as their San Francisco terminus, accessing the elevated bus deck from the San FranciscoOakland Bay Bridge via an elevated bus ramp and a cable-stayed bridge. Use of the STC train box awaits future construction of connector tunnels to bring rail service into the facility.

Residents and visitors have also begun to use the 5.4-acre rooftop park, which includes an open-air amphitheater, gardens, trails, open grassy areas, children's play space, and a restaurant and cafe. Some of the high-rise buildings adjacent to the STC will offer their tenants direct park access via bridges, and a gondola system that is under construction will move people, as will elevators and escalators, from the ground level to the roof park.

With its dramatic, beautiful, and functional spaces, the STC offers San Francisco a welcoming and innovative destination.

—By Bruce Gibbons,  P.E., S.E., LEED AP , a Managing Principal, and C. Kerem Gulec, Ph.D., P.E., a Vice President, of Thornton Tomasetti Inc.'s Los Angeles office

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Salesforce Tower Achieves LEED Platinum for Core and Shell

Salesforce Tower pushes boundaries in many ways, including its sustainability. Awarded 89 points in the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) rating system for the core and shell (C&S) of a building, the skyscraper achieved a platinum-level certification and is the nation's highest scoring high-rise construction project in the LEED C&S rating system, according to the council's Green Building Information Gateway.

Sustainability was a key aspect of the project from the outset. Integral to Pelli Clarke Pelli Architects' winning bid, it set the project apart in the eyes of the judges of the international competition held by the Transbay Joint Powers Authority (TJPA) for the tower's design.

The original target was earning a gold-level certification in the LEED C&S system, which would have been challenging enough for such a large structure. But the project team, with guidance from stok, the project's San Francisco-based sustainability consultant, began to consider pursuing a platinum certification in 2013. Because this was to be the first building to exceed the city's previous height limits and would therefore become an iconic part of San Francisco's skyline, the team believed Salesforce Tower should set an example of how sustainability should be integrated into the city's future high-rise projects. So an exhaustive study was commissioned to determine which areas should be the focus if the team was to push the boundaries of sustainable performance and gain the additional points needed to go from a gold to a platinum certification. The team landed on two focus areas to maximize available points: energy and water efficiency. 

The results were exemplary, with the project earning 43 of the available 47 points in the two related categories in the C&S program, energy and atmosphere, and water efficiency. To achieve such an outstanding performance, the project features myriad unique and innovative high-performance design strategies and technologies.

Energy

Speculative C&S developers can typically only affect 40 to 50 percent of the future energy use of a building, as lighting and plug-load energy usage is controlled by future tenants. The energy usage that developers can control is almost entirely related to the heating, ventilation, and air-conditioning (HVAC) systems. Knowing this, the project team decided to create a unique high-performance HVAC system. Known as a "tri-path" system, this floor-by-floor air handling system takes hot water from high-efficiency condensing boilers and chilled water from high-efficiency chillers to supply conditioning and ventilation through a combination of underfloor air distribution and overhead systems. This multiple supply-path approach allows the system to deliver a maximum amount of outside air to the floors while minimizing the cooling usage required to supply it.

Coupled with a super-efficient envelope system that incorporates solar shading and high-efficiency glass, the project was able to reduce the energy usage of the HVAC system by nearly 50 percent compared with a conventional building, preventing 2,000 metric tons of CO2 emissions every year.

Water

To maximize water efficiency, the team took a proactive approach by focusing first on specifying elements that reduce usage before moving on to considering producing water on-site. 

On the interior of the building, state-of-the-art low-flow fixtures allow the building to achieve a reduction of more than 40 percent in plumbing fixture water use versus a traditional building.

Exterior to the building, native, low-water-use plants were specified, and these are watered with a moisture-sensing irrigation system that supplies water only when needed. To further reduce potable water use, a 50,000 gal. cistern in the parking garage collects and treats rainwater from the building's roof. The system recycles 225,000 gal. of water per year that can be used to irrigate the landscaping, as well as to flush toilets and urinals. Combined, these strategies allow the project to meet 100 percent of its irrigation demand with harvested rainwater.

The most extraordinary feature of Salesforce Tower, however, is its blackwater recycling system. Once completed later this year, it will be the largest on-site water recycling system in a commercial high-rise building in the United States. First proposed by stok to achieve the platinum certification, the idea was eventually adopted and championed by the tenant. With the help of the system manufacturer Aquacell, an Australian company with U.S. headquarters in Louisville, Salesforce's project team designed the system, and it is currently being installed.

The system runs on a principle that nature has employed for billions of years: turn waste into a resource rather than a liability. It collects water from the sinks, showers, toilets, urinals, and dishwashers throughout the building, as well as wastewater from the cooling towers on the roof, and stores it in large concrete tanks located in the underground parking garage. Aquacell's membrane bioreactor technology then treats it to meet the tertiary standards established in Title 22 of California's Code of Regulations, producing upward of 30,000 gal. of usable recycled water per day.

Not only does the black-water system significantly reduce the demand for potable water by reusing water on-site, it also significantly reduces the volume of wastewater that would otherwise be sent to the heavily burdened San Francisco sewer system.

Another distinguishing aspect of a black-water system, in comparison with the more common gray-water water recycling system, is that all drainage water in the building can be collected in one set of tanks. A graywater system, on the other hand, requires sinks and shower drains carrying gray water to be separated from urinal and toilet drains carrying black water. By choosing a black-water system over a gray-water system, buildings can avoid adding an additional set of plumbing lines, which reduces the embodied carbon impact and cost associated with them.

Overall, the black-water system reduces the potable water usage of the tower by 7.8 million gal. per year—saving nearly 12 Olympic swimming pools' worth of water. Coupled with the rainwater collection and reuse system, the tower's total potable water use reduction is more than 8 million gal. per year. From the black-water storage tanks in the garage up to the 1,070 ft high view, Salesforce Tower is a truly remarkable achievement of design, engineering, and construction, with sustainability not an afterthought but a focal point.

—By Jacob Arlein, LEED AP ID+C, a partner of stok, a high-performance real estate design and project management consultant based in San Francisco

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