With the number of tall buildings in the United States and around the world increasing to an awe-inspiring race for height; they pose challenges to innovative structural engineering design, enhanced performance objectives, analyses, construction materials, and construction techniques. The structural innovations required to create this record-setting objective are expected to be scrutinized to provide requisite dynamic behavior and performances during extreme events, such as strong winds and shaking caused by earthquakes that originate from near and far seismic sources.
At 1070 ft tall, Salesforce Tower, formerly known as the Transbay Tower, is one such 61 storey super-tall building that advances the state-of-the-art of high-rise seismic design through the implementation of a number of first-ever design and analysis methods that push limits and set new industry benchmarks.
In 2007, an international development competition was held by the TJPA (Transbay Joint Powers Authority) to envision and create a new transit center and an iconic tower. It was won by the collaborative teams of Hines and Pelli Clarke Pelli Architects. The project laid dormat from 2008 to 2010 due to the recession, while it increased momentum from 2010 when Boston Properties purchaced a 95% stake of the project and started working together with Hines.
The team envisioned an iconic 61-story, high-density, sustainable office tower totaling 1.5 million square feet, where above the 26th floor, each elevation of Tower curves and tapers away from the street creating a narrow, slender top finish with a sculptural crown.
The site was 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 a weak bedrock. These geotechnical conditions are subject to potential liquefaction, lateral spreading, excessive settlement, and inadequate foundation support. Given the poor soils and the sheer weight of Salesforce Tower, supporting the building on anything but bedrock was not feasible.
Until the construction of Salesforce Tower, buildings constructed in San Francisco were generally founded on 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. However, the sheer size and weight of Salesforce Tower made this project different. A new approach, one that had never before been done in San Francisco, was required. Gravity loading and overturning demands at the foundation level from MCE shaking dictated a piled-mat solution.
Two foundation systems were considered during the design process: 8-foot-diameter drilled shafts and 5.0- x 10.5-foot Load-Bearing Elements (LBEs). As the depth to rock from existing grade was approximately 250 feet, and socketing into the rock would require drilling even deeper, the limits of available drilling equipment would be tested for a drilled shaft foundation. The alternate LBE, or “barrette” foundations, were not subject to the same depth limitations as the equipment used to excavate the shafts was a combination of a line-supported clam shell and hydrofraise. Ultimately, the LBE foundation system was selected as the most appropriate for the project.
An extensive analysis of the LBEs considering extreme seismic demands was performed. Reinforcing detailing was incorporated to resist the high tensile, flexural, and shear stresses imposed on the LBEs by MCE ground shaking. Confinement reinforcement was also specified in the upper zones of the LBEs where compressive demands were the highest. As this was the first time LBEs would be used to support a tall building in San Francisco, an extensive review was conducted by the independent peer review panel. Also, two full-scale Osterberg Load Cells tests were conducted to confirm that the design parameters for the skin friction on the LBEs were appropriate. Through this test program, it was confirmed that the skin friction values were time sensitive (as expected) due to the build-up of filter caking of the bentonite on the side walls of the shafts, requiring time limits to be placed on the overall installation of each shaft.
Installation of the foundation system included first the construction of guide walls to control the location and excavation of the LBEs. Excavation then proceeded using a combination of a line-mounted clam shell in the sands and clay, switching to a hydrofraise when denser rock material was encountered. Excavation stability was maintained throughout the process using a recycling bentonite slurry system. After the excavation was completed, full-length, pre-tied reinforcing steel cages were lowered into the bentonite-filled holes, and concrete was placed using dual tremie pipes. All 42 LBEs were constructed from existing grade, rather than the bottom of the 60-foot excavation. A temporary internal bracing system would be required to support the open excavation given the limitation of the adjacent Transbay Transit Center’s simultaneously open excavation, limiting the ability for heavy equipment to work at the bottom of the hole. As such, concrete placement in the LBEs continued through elevation of the future mat foundation, and the remainder of the shaft was filled with a lean mix for ease of later excavation.
The final foundation configuration for Salesforce Tower includes 42 LBEs interconnected by a thick mat foundation to form a solid base for the tower (Fig 1). The mat varies in thickness from 14 feet at the core to 5 feet at the perimeter. LBEs extend into the underlying Franciscan bedrock, some reaching more than 310 feet below existing grade with rock-sockets of up to 70 feet. The design and construction of this foundation system set new standards for the support of tall buildings in San Francisco’s unique geotechnical and seismic conditions.
The Structural System
Since 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 tower could not rely on outrigger trusses, belt trusses, exterior bracing of any kind, or the use of damping systems. 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 was 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. (Fig. 2)
The design of a concrete shear-wall building reaching 1070 ft tall falls well outside of the 240 ft height limit prescribed by the city's building code. In order to obtain permissions to exceed this limit and to do so by such a wide margin a performance-based seismic design (PBD) approach had to be adopted (Refer the article on Performance-Based Design). Though there were numerous towers taller than 240 ft already designed using similar approaches; given the scale of Salesforce Tower and the number of building occupants to be exceeding the building code threshold of 5,000 people it triggered the building’s consideration under Occupancy [or Risk] Category III. Category III buildings require additional safety for wind and seismic demands, thus prompting new challenges for the engineering team.
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 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 fact, in the context of the historical seismic design philosophy of ductility and energy absorption, enhanced strength may be detrimental to building performance. Stronger buildings resist more force rather than absorbing the energy of the ground’s shaking. In resisting these higher forces, shear stresses and foundation demands increase to undesirable levels and building performance can be compromised. Instead, the supply side of the design equation was adjusted by adopting stricter acceptance criteria for all the design parameters, as shown in the table below.
In total, 22 unique horizontal pairs (X and Y) of seismic ground motion records were input to a detailed non-linear 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 structures performance while recognizing that tall buildings tend to respond significantly in the higher modes of vibration.
The structural design of Salesforce Tower was subjected to more stringent Acceptance Criteria for MCE shaking to achieve better performance objectives, including the following: Reduced story drift, Reduced coupling beam rotations, Reduced tensile/compressive strains in shear walls, Reduced shear demands on shear walls, Risk Category II acceptance criterion was typically modified to be more stringent by applying a factor of 0.8.
Although wind-loading conditions for the building are not trivial, wind tunnel testing confirmed that demand levels fell below seismic demands, and that occupant comfort standards would be met as judged against international standards. The lateral design of Salesforce Tower was driven by seismic loading conditions for three levels of ground shaking:
Elastic performance targeted for service-level shaking (with a mean recurrence interval of 43 years).
Moderate structural damage expected for design-level shaking (taken as two-thirds of code-defined MCE shaking).
Collapse prevention, with a reduced probability of collapse consistent with Occupancy Category III, targeted for MCE shaking.
Soil Structure Interaction
An additional challenge in the design phase was the proximity of the Salesforce Tower to the rising Salesforce Transit Centre (STC). It is uncommon that two adjacent structures are constructed together and even less common is for them to be in direct contact with each other. Therefore, there was an agreement between the TJPA and the developer, that the Tower would in no way negatively impact the seismic performance of the Transit Center, as the TJPA has defined the performance goals of the Transit Centre to be in an operational condition even after a major earthquake.
To explicitly demonstrate this outcome, a structure-soil-structure interaction (SSSI) analysis that had never been executed for real-world buildings had to be carried out. The geotechnical engineering team of Arup along with the Structural Engineering team of Magnusson Klemencic Associates, developed a complex model of the underlying soil strata extending to bedrock and inserted the Transit Center, Salesforce Tower, and one other tower of relevance (Millennium Tower), including the predicted dynamic properties of the structures. These extremely complex assessment involving extensive nonlinear computer models were developed in CSI-Perform and LS-DYNA with considering multiple seismic ground motions.
The results of the model clearly indicated stress concentrations in the transit center near the corners of the Salesforce Tower substructure. Fortunately, since the transit center was still under construction, it was possible to add reinforcement to the ground-level diaphragm of the transit center in those areas. This SSSI analysis was the first time that potential impacts of one building on a neighboring building during strong seismic ground shaking had been considered.
Construction Animation of the Salesforce Tower
Among the many innovative elements that make up Salesforce Tower, perhaps the most impressive is the new structural system created for the tower. The Structural Engineers designed a system that eliminates the need to use the exterior columns for anything other than carrying the weight of the floors to the foundation. The building uses a High-Performance Core that encloses all the elevators, emergency stairs, restrooms, and mechanical systems in high-strength concrete and creates an extremely strong structural spine. This eliminated the need for dense structure at the exterior window line and allowed for the use of only three columns on each side of the building to create panoramic views for occupants. Salesforce Tower is the tallest building in the world using only its central core to resist wind and earthquake forces, an impressive feat given the tower sits on complex soil strata and is located approximately 8 miles from the San Andreas Fault.
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.
“Transformative tower, Civil Engineering Magazine” by Klemencic, R., Valley, M., and Hooper, J. 2018.
“Salesforce Tower: New benchmarks in high-rise seismic safety, ASCE Structure Magazine” in STRUCTURE Magazine by Klemencic, R., Valley, M. T., and Hooper, J. D., 2017.
“Civil+Structural Engineer Magazine” article in April 2019.
Video Source: Engineering News-Record.