Pankow Builders' hybrid framing system reaches new heights for precast in high seismic zone

A 39-story high-rise currently under construction in San Francisco has been designed to defy the odds for precast construction in the highest-risk seismic Zone 4. The $87 million 680 Mission Apartments project is proceeding under Charles Pankow Builders and a recently launched affiliate, Mid-State Precast. The structure will become the tallest concrete building in a Zone 4 setting, surpassing the 32-story 3900 West Alameda Tower in Burbank, Calif. Completed in 1989, that cast-in-place development was the previous height record holder for a commercial concrete structure in a Zone 4 region.

Prior to California's January 1994 Northridge earthquake, which severed welded connections in about 150 steel-frame high-rises, steel moment-resistant frames were considered the best safeguard against seismic activity. Since that time, the more rigorous testing required to validate the load carrying capacity of steel-frame special-moment connections - in addition to the lead time in obtaining structural steel - has had the effect of making precast more competitive for high-rise construction in Zone 4 regions, notes Dave Fanella, manager, Buildings and Special Structures of the Portland Cement Association.

Design innovations The latest technology for multi-story construction in areas of moderate or high seismic activity yields the Hybrid Precast Concrete Moment Resistant Frame. The system was developed by Charles Pankow Builders Ltd., Altadena, Calif., in conjunction with academic and government interests (note sidebar, page 20). It offers the benefits of seismic resistance, including safety, protection of property, and minimal disruption pursuant to seismic events, plus the efficiencies of precast construction.

Before the Pankow Hybrid Frame became available, enhanced performance solutions had been expensive and so complex technologically that implementing them was nearly impossible in all but the most sophisticated construction markets, notes Pankow President Tom Verti. Now, he adds, a system that is simple to build requiring only common materials and construction technologies already in use throughout the world provides a "hybrid" precast frame - a hybrid of mild reinforcing steel and post-tensioned cables - that can withstand a major quake with little structural damage other than some to grouted sections.

The precast concrete moment-resistant frame absorbs seismic energy in a manner that is independent of the integrity of the structural members. The post-elastic performance is concentrated in the connection rather than a structural member, representing a paradigm shift away from design that requires structures to absorb seismic energy through inelastic response of the framing members. The frame relies upon connections of its precast columns and beams utilizing both standard reinforcing steel and high-strength post-tensioning steel cables.

This combination facilitates the isolation and separation of the strength and energy absorption components within the joint. High strength post-tensioned steel gives the joint strength to resist seismic loads; mild steel utilized across the joint serves as an energy dissipater by yielding under seismic loading. In keeping the initial post-tensioning force low relative to the strand's ultimate strength, the high-strength steel never approaches its yield strength, thereby staying in the elastic range with a large reserve capacity for deformation. A restoring force is thus provided that allows the structure to self-center and not suffer any permanent displacement after an earthquake. Overall, a synergy of elastic forces that absorb energy through movement of the joint and inelastic forces providing shear and moment resistance to hold the joint together accommodates seismic ground motion by allowing the joint to open and close up much like a spring-loaded door. Thus, the frame provides seismic resistance in both the longitudinal and transverse directions bringing the structure back into alignment after the deformation energy of an earthquake is dissipated in the beam-to-column connection region.

In construction of the 485-unit 680 Mission building, Dywidag Ductile Connectors will also be used to bolt together precast beams and columns. Somewhat curved in shape, the 425-ft. high-rise will be tied together by horizontal post-tensioning along contiguous bays, with the Dywidags in use for single bays as needed. Though Pankow considers the proprietary system from Dywidag Systems International USA, Bolingbrook, Ill., more expensive to build with - and repair, if damaged - both systems (at a cost of $145 per sq. ft.) will provide a savings of $4 million over steel frame construction.

As reported by Pankow representatives to the Precast/Prestressed Concrete Institute membership, the 680 Mission Apartments will also see the first application of the elbow sleeve system for post-tensioning two frames connecting at a corner. Rather than post-tensioning each side separately, creating two sides that must be stressed and anchored in the same space, a system was devised involving 90-degree elbow sleeves that permit the strands to turn a corner and continue along the adjoining side. Such a configuration allows post-tensioning to occur in only two locations per floor, directly opposite to each other, thereby serving the corners of a tube frame uninterrupted around the perimeter of the building - one of the features making the significant increase in height possible for a project like 680 Mission Apartments.

Fabrication Located midway between Los Angeles and San Francisco on a 20-acre site in the heart of the San Joaquin Valley in Corcoran, Calif., Mid-State Precast, L.P., was launched 18 months ago to serve these two major markets. Pankow officials emphasize that in supplying precast for the 680 Mission Apartments, the proximity of the plant site facilitates producing the highest quality material at the lowest cost within a time frame allowing for rapid construction. As elements of the precast moment frame provide the entire exterior skin - along with glass and glazing - and the exterior structure of the building, costs are reduced, construction schedules are accelerated, and quality is built into the final product, notes Al Fink, who oversees the Mid-State operations.

Serving a dual purpose, precast concrete structural load-resisting members provide facade cladding as well. Accordingly, structural components were cast and finished as architectural precast concrete pieces incorporating two finish colors and a sandblasted surface. Using architecturally finished precast to save money on exterior cladding while producing a seismically resistant frame gets the nod from William Witte, principal for the building's developer, Irvine-based Related Companies of California.

In addition to savings realized by the multiple functions of the moment frame - thus providing the entire exterior finish plus vertical and lateral requirements for wind and earthquake loading - the allocation of plant set-up costs for Mid-State over many projects minimizes expense through economies of scale. At the same time, the increased efficiency of a plant production environment facilitates quality workmanship.

Innovative production techniques provide a quality finished product, Fink adds, and ensure an exact fit of all precast pieces during erection. The casting of the moment frame beams and columns requires the precise installation of specialty tie reinforcing, regular reinforcing, as well as ducts and sleeves for the ductile steel bars and post-tensioning cables. Careful planning and detailing of these elements is essential to a successful outcome, project officials contend.

Aggressive schedules have been achieved by producing precast components at Mid-State at the same time that excavation and foundation work has proceeded at the job site. Precast is delivered on an "as needed" basis to facilitate erection at the tight San Francisco.

Pankow's design arose from a need to enhance building systems currently in use in high seismic regions. Conforming to codes that have been written to prevent collapse during catastrophic events, structures typically dissipate seismic energy through yielding of steel reinforcement, thereby inflicting damage to the surrounding concrete during post-elastic response. The toll in damages thus sustained during major seismic occurrences has been enormous, as in the wake of the Kobe, Japan, and Northridge, Calif., earthquakes. Such disasters have provided the incentive to develop techniques that would advance construction practices beyond achieving life safety to the preservation of structural integrity. Seeking a competitive edge as well for precast in areas of high seismicity, Charles Pankow Builders, Ltd. was motivated to develop a precast system to rival structural steel in high-rise construction.

Birth of a new concept At a 1990 workshop sponsored by the National Science Foundation under the joint U.S.-Japan Precast Seismic Structural Systems (PRESSS) program, Pankow learned of research conducted in 1987 by the National Institute of Standards and Technology (NIST) that included an experimental testing program of precast beam-column connections subjected to cyclic inelastic loading at the Gaithersburg, Md., laboratory. A subsequent meeting of University of Washington (UW) and Pankow representatives in San Diego generated the proposal of a "hybrid" frame combining high-strength and mild steel to provide inelastic action for resistive strength and elastic action for energy dissipation.

Consortium formation Pankow approached NIST and proposed funding of the development of the precast hybrid frame with the goal of putting the new concept into commercial practice. NIST decided to join the consortium matching the funding "in kind" to pursue the testing and analysis of the concept as an extension of previous institute studies. The investigation would include moment capacity, shear capacity, energy absorption characteristics, and shear friction for the vertical load carrying mechanism.

Lacking experience in research dealing with mechanical properties and design procedures, Pankow decided to expand the consortium by funding the study through the Concrete Research and Education Foundation (ConREF) of the American Concrete Institute (ACI). It was agreed that ConREF's Concrete Research Council (CRC), represented by a task group of nine leading experts from industry and academia, would provide ongoing technical oversight.

As research proceeded (see Concrete Products, April 1997, System Research, p. 42), Consortium members participated in a variety of capacities to ensure success of the program from concept to implementation. To develop a realistic prototype structure and shear criteria for use in one-third scale testing by NIST, Pankow developed floor plans for marketable 12-, 22-, and 32-story office towers. From these plans, John A. Martin Associates of Los Angeles produced maximum design level forces for moment and shear in addition to frame beam and column element sizes for seismic Zone 4 and 2b locations. In turn, NIST and UW detailed frame column and beam reinforcement, which was reviewed for code compliance by Martin's staff. Connection concepts were jointly developed by NIST, UW and Pankow. While NIST and UW viewed the design from a performance standpoint, Pankow focused primarily on economic feasibility and constructibility in a precast plant.

To help solve the multitude of constructibility issues and obtain product for test specimens, construction material manufacturers were added to the consortium. The Los Angeles structural engineering firm of Robert Englekirk, Inc. provided assistance to NIST in formulating realistic and useful design procedures able to be understood and implemented by the design community. Englekirk also undertook processing the innovation through ICBO evaluation procedures.

Implementation At Pankow's prodding, ACI established an Innovative Task Group to evaluate the technology and draft the required code language changes necessary for accommodation of the innovation by ACI 318-Building Code Requirements for Reinforced Concrete. The group was patterned after the committee of experts utilized to evaluate innovations in Japan as documented in a report "Transferring Research into Practice: Lessons from Japan's Construction Industry."

In early 1995, the new technology was first applied by Pankow in the construction of the first of three precast concrete parking structures containing 1.7 million sq. ft.. The structures were part of a $150 million capital improvement project at Roosevelt Field Mall, Long Island, N.Y. A 25 percent savings was realized in building the 550,000-sq.-ft., 1,719-stall North Deck parking structure due to delivery in nine months versus 12 months for a comparable conventional structure. Subsequently, the Hybrid Frame technology was deployed in a 160,000-sq.-ft. Eugene, Ore., parking structure and a 480,000-sq.-ft. parking structure in Palo Alto, Calif.

Further extending the application of the hybrid frame, Pankow's Mid-State Precast entity has completed the casting of the 1,500-car parking structure at Stanford Shopping Center and a four-story, 156,000-sq.-ft. office building with a 539-stall below-grade parking structure at Westside Media Center in West Los Angeles. Later this year, casting for phase two of the Media Center will begin.

Concurrent with construction of the 680 Mission Apartments in downtown San Francisco, casting of architectural and structural precast elements has commenced for the 384,000-sq.-ft. Pacific Plaza Office Building in Daly City, which includes an 822-stall garage. Phase two casting for the project will be initiated later this year. "The significance of this project is that Daly City sits directly on the San Andreas fault line," notes Joe Sanders, director of engineering for Pankow. "This project is as close to ground zero in an earthquake as one can get - a real test for how the system performs."

Despite advantages in constructibility, scheduling and cost savings demonstrated with each completed project, the Pankow Hybrid System has met resistance. Deeply entrenched traditional practices, both codified and personal, are now yielding just gradually under the weight of evidence demonstrating the system's viability for high-seismic zone construction.