Redefining Structural Efficiency & Resilience

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With the recent boom in the biotechnology industry, the Oyster Point neighborhood of South San Francisco, California, evokes large, busy life science campuses more so than its namesake bivalve mollusks. Serving the workers on 865,000 square feet over three buildings of laboratory space on the new life science campus, the Kilroy Oyster Point Phase Two Parking Structure provides 1,961 parking spaces on a 260 feet square footprint over ten stories.

As large as the parking structure appears on the approach, drivers may be surprised to realize they have just entered on Level Three, as it is uniquely carved into a hillside to minimize its towering height. With vehicle entrances and exits distributed on three sides and on two different levels, aided by the use of reversible lanes with dynamic LED digital signage guiding traffic, the time it takes to enter and exit the parking structure is minimized.

The parking structure’s four-bay configuration has two central park-on ramps and pedestrian vertical circulation points that include three stair towers and five elevators. The elevators are arranged in a dual-bank that are strategically located to minimize the pedestrian travel time from the parking space to their final destination.

As the first impression to those entering the campus, the parking structure serves as the gateway to other buildings. The parking structure’s aesthetic picks up design cues from the buildings it serves. The exposed prefabricated precast concrete is color-integrated and accentuated with a perforated metal screen that undulates along its elevations with form, color and light.

The structural concrete elements are precast under controlled conditions at Clark Pacific’s Woodland manufacturing plant outside of Sacramento, California. The gravity system is comprised of precast double tees, 63 feet to 68 feet in span, that are supported by precast girders, which are in turn supported on the corbels of gravity columns. A thin, 3 ½-inch thick cast-in-place concrete topping slab over the double tees ties the precast gravity elements together and acts as the seismic diaphragm.

For its lateral system, the parking structure layout is a perfect candidate for precast hybrid moment frames (PHMF). With almost a square plan, Forell | Elsesser Engineers are able to efficiently locate the moment frames along the four exterior elevations of the building. The ramps are strategically located in the two interior bays of this four-bay parking structure, meaning each level of the PHMFs is completely flat and not complicated by the ramp geometry. This translates to much greater fabrication and erection efficiency. Seismic joints are provided at mid-height of each ramp such that they do not interconnect the floors and inhibit inter-story drift and therefore don’t negatively impact the performance of the moment frames.

The moment frame elements are designed to all have the same dimensions to streamline precast production. The Moment Frame Columns (MFCs) are 30 inches by 48 inches and Moment Frame Beams (MFBs) are 24 inches wide by 50 inches deep. At first glance, the MFBs appear to be very deep: in fact, the beam depth is carefully coordinated so these perimeter beams can be upturned and double as vehicle barriers and guardrails. Minimum code required guardrail height is the primary driver to the MFB depths.

Compared to the conventional cast-in-place moment frame system, the PHMF dissipates seismic energy through a similar mechanism but in well-defined locations. The precast MFCs are delivered to the site as multi-story elements, and each precast MFB is single bay, spanning between adjacent columns. The only elements that go through the beam and column joints are the unbonded post-tensioning tendons, located at the mid-depth of the MFBs, and special rebar that is descriptively termed “energy dissipating rebar,” typically located at the top and the bottom of the MFBs. At the concrete plant, the MFCs and MFBs are cast with sleeves for these jointing elements. Once the moment frames are erected at the project site, the post-tensioning tendons are stressed to a pre-determined tension, similar to bridge PT construction, and then locked, cut, and grouted along with the conduits for the energy-dissipating rebar. Since the tendons are sheathed, they are unbonded between end anchor points. The energy dissipating rebar is installed in corrugated metal sleeves through the column and into the connecting beams. The most important feature of the PHMF system is that a portion of the energy-dissipating rebar is de-bonded before the grout is injected into the full length of the sleeves. This de-bonded region of the energy-dissipating rebar serves as the designated yielding element in the lateral system.
During an earthquake, the moment frame beam-to-column joint opens and closes, which puts these energy-dissipating rebar to work by axial yielding. After the seismic event, the post-tensioning tendons, which are designed to remain elastic, will restore the frame to its original position with no residual drifts. As such, the only yielding locations of the PHMFs are limited to the beam and column joints, and near the base of the MFCs to a lesser extent.

Clark Pacific Director of Operations Randy Clark was instrumental in leading the design, fabrication, and installation of the precast structural components utilized in the Precast Seismic Structural Systems (PRESSS) research program that constructed a five-story, 60 percent scale shake table test performed at the University of California San Diego over 20 years ago. This research significantly advanced the understanding of the seismic-resistant design of PRESSS, of which the PHMF was one of the lateral systems being tested.

"As a young engineer, I didn't fully grasp the profound implications of the work we were undertaking,” Clark said. “It seemed like just another project at the time. However, in retrospect, understanding the wealth of knowledge gleaned from those tests and the transformative impact this has had on the field of building science, I feel a deep sense of pride for being part of such a seminal project."

During testing, the PHMF was subjected to high seismic story drifts and performed with flying colors. At the early stages of loading, the researchers observed low levels of damage, limited to inelastic action at the beam-to-column connections, as intended. They gradually increased the seismic loading and loaded the specimen to a drift level of 4.5 percent, which is 225 percent of the code-allowed design drift level of 2 percent and observed no significant strength loss in the structure.

The analysis of this system is similar to a conventional cast-in-place special concrete moment frame system, apart from an important difference in the establishment of the effective sections to use for the frame elements, and more accurately, the beam and column joint. Many analytical investigations were conducted in the preceding two decades, and the methodology developed by Forell | Elsesser Engineers draws on those earlier studies, as well as practical limitations of model run time and project schedule.

To study the effects of joint opening at the beam-column interface, Forell | Elsesser Engineers examined results from a detailed, component-level, nonlinear analytical study in CSI SAP2000 by Computers & Structures, Inc., using solid elements. As noted previously, only two types of elements go through the joint. The energy-dissipating rebar is modelled as nonlinear axial hinges to capture potential yielding and the corresponding energy dissipation. The post-tensioned tendons are designed to remain linear and are modelled as linearly elastic link elements. In a conventional cast-in-place concrete moment frame structure, the beam section modifier is used to account for the effective section after cracking. In the case of a PHMF structure, however, the post-tensioned beams are designed to remain elastic (no cracking), and this modifier is used to capture the effects on beam-column joint opening. A part of the topping slab can be included to the MFB section (similar to T-beams), but since the slabs are located near the middle of the MFB depth, the slab contribution was deemed not significant in the analyses. Jointing the topping slab around the MFC is not recommended due to maintenance concerns.

By calibrating the two models, Forell | Elsesser Engineers were able to distill the nonlinear behavior from the solid model as a single number to use in the linear frame model as a beam modifier. Compared to the ACI 318 recommended value for an effective beam section of 0.35, they found that a lower modifier of 0.30 is more appropriate. It is worth emphasizing that while the modifier in ACI is a result of cracking, the modifier used here is to account for the opening of the beam-to-column joints; the individual precast elements remain uncracked. While a lower value modifier means a softer structure with larger seismic drifts, this softening also lengthens the structure’s fundamental period and hence reduces the associated spectral acceleration. This results in a reduced seismic base shear for the structure. The MFCs are conventionally reinforced (not post-tensioned), hence ACI 318 recommended section modifiers are appropriate.
The design of PHMF elements is governed by the ACI Standard 550.3 Design Specification for Unbonded Post-Tensioned Precast Concrete Special Moment Frames. While it is not the intention of this article to be a primer to this ACI Standard, it is worth noting that the frame element design is governed by seismic drifts and rotation of the beam-column joints, and that it is an iterative process to satisfy multiple competing criteria.

A prime benefit of precast construction is that most of the time- and labor-intensive work has already been done at the plant. Since the project is developed as a campus with multiple buildings, having a staging area is sometimes not possible. The precast parking structure team was able to schedule truck shipment of precast elements on a just-in-time basis, thus eliminating the need for a large lay-down area.

As opposed to a typical average crew size of 53 workers for a cast-in-place project, you would typically only see a team of 24 workers at the project site for precast construction. While the total labor between the two construction types is comparable, the precast system is able to shift about 75 percent of the total labor offsite. This reduction in field labor translates to a reduction in field construction duration from about 40 weeks to just 22 weeks—a significant reduction by any measure.

With a mission to educate, advocate, and promote resilience-based design that considers the impacts of natural disasters as an essential component of long-term sustainability, the US Resiliency Council (USRC), a 510(c)(3) non-profit organization, was established in 2011 by cofounders Evan Reis and Ron Mayes as a way to educate building stakeholders and the public about the importance of resilient design to the community and the inseparable link between resiliency and sustainability. Its earthquake building performance rating system is being used by public and private owners and communities and is forming the basis of economic and financial incentives being developed by lenders and insurers to reward high-performing buildings.

Using the PHMF system, the parking structure received the prestigious Platinum Verified Earthquake Rating from the USRC. This is the highest rating awarded by the USRC and represents the pinnacle in structural earthquake performance. In order to achieve a Platinum rating, a building must meet strict performance thresholds in three categories: Safety, Damage, and Recovery. Platinum-rated buildings suffer negligible damage, with repair costs less than 5% of the building’s replacement cost. They are also expected to allow for functional recovery from immediately to within a few days of a major seismic event. Injury and blocking of building egress paths is also unlikely.

The evaluation to determine building performance is based on performance-based design philosophies and the methodology outlined in FEMA P-58 “Seismic Performance Assessment of Buildings.” The methodology is based on a probabilistic approach to risk assessment. Instead of evaluating the building performance based solely on a single maximum considered earthquake, the probabilistic approach considers a range of possible earthquake scenarios and their likelihood of occurrence, as well as the variability in building response and potential consequences of different damage states. To evaluate the performance of key components in the building, FEMA P-58 uses fragility curves that represent the probability of certain levels of damage that may occur to a building component (structural, architectural, mechanical, electrical, etc.) given a specified degree of ground motion intensity. Each curve is component-specific and is developed and built from intensive empirical testing data.

Rooted in the decade-long PRESSS research program, the PHMF system is backed by testing and aided by advanced analysis. In the third decade of implementation in the San Francisco Bay Area, PHMF structures continue to provide owners efficient structures with savings inherent with precast construction, in addition to enhanced seismic performance. With the USRC ratings program, we can now quantify the seismic performance beyond the minimum code-level life-safety performance in the face of high seismic demands and deliver a structure that is efficient, resilient, and sustainable. ■

About the Authors

Mei Kuen Liu, SE, is a Senior Associate with Forell | Elsesser Structural Engineers, San Francisco, California.

Chris Petteys, SE, is President and CEO of Forell | Elsesser Structural Engineers, San Francisco, California.