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The California Department of Transportation (Caltrans) District 4 headquarters stands just five miles from the Hayward Fault. The building, which was constructed two years after the destructive Loma Prieta earthquake of 1989, houses critical transportation operations including emergency response and traffic management and reporting for nine Bay Area counties.
When the structural integrity of the building was called into question following the damage to several similarly designed buildings during the 1994 Northridge earthquake near the Los Angeles area, the California Department of General Services (DGS) sought consultation from structural engineering and design firm Degenkolb Engineers along with The Crosby Group to assess and seismically upgrade the 15-story steel moment frame structure. The team employed extensive state-of-the-art analysis techniques and full-scale connection testing to design a retrofit scheme that would protect the building following a major earthquake.
The structural composition of the building and the unique services that the building sheltered dictated many of the Degenkolb team’s design decisions. Constructed to meet the 1988 Uniform Building Code, the District 4 headquarters has one basement level, a first story lobby with public space, four levels of above-grade parking, and 10 stories of office space. The building has a large atrium above the parking levels and full-height moment frames are located along the perimeter frame lines. There are also two interior transverse moment frames adjacent to the atrium on either side.
The Traffic Management Center within activates metering lights and freeway signs, monitors traffic flow, and dispatches personnel and equipment to clear traffic obstructions as quickly as possible. Because of the vital services provided by the government agency in the building, the engineering team developed a retrofit scheme that included a combination of moment connection strengthening with the addition of viscous dampers. Compared to an alternate scheme without dampers, the proposed strengthening reduced the number of isolated connection locations by 35 percent with associated reductions in staff displacement and construction duration. Due to an orderly phasing program to orchestrate the work created by the design team along with Caltrans and DGS, construction lasted 33 months with disruption to the agency’s operations minimized to the greatest extent possible.
Preliminary evaluation
Long before the team decided to employ a retrofit scheme that included the dampers, a preliminary evaluation of the building’s structural system and laboratory testing of moment connections similar to those of the existing building were conducted. These tests were performed by the University of California, Berkeley at the Pacific Earthquake Engineering Research Center. The test results confirmed that the 1988 code-compliant connections were vulnerable to fracture and demonstrated even less rotation capacity than smaller specimens tested for the FEMA-sponsored SAC Steel Program. Initial fracture at the beam top flange occurred at approximately 0.6 percent drift, and the maximum applied load was equivalent to 50 percent of the beam plastic moment capacity. Conclusions culled from the testing also led DGS to rate the building as Risk Level V, having substantial risk to life safety, a total disruption of systems, substantial structural damage, and a likely risk of partial collapse. It was clear that seismic upgrade was necessary.
To improve the building’s rating from a Risk Level V to a Risk Level III, which will result in a minor risk to life, disruption of systems for days to months, minor structural damage, and the ability for employees to return within weeks after a disaster with minor disruptions, the design team explored four strengthening schemes before deciding on the connection strengthening plus dampers scheme. The other schemes included: an all connection strengthening scheme, a buckling restrained brace scheme, and a base isolation scheme. Each proposed scheme was designed to meet the design criteria and was compared on the basis of construction cost and associated “soft” costs. Aside from cost effectiveness, the proposed schemes were evaluated for seismic performance, schedule, and disruption to the client among a litany of other qualifications.
Testing
A series of four full-scale tests were conducted under FEMA guidelines 351 and 356 to validate the connection designs being considered as part of the overall scheme. In the early evaluation stages, the team performed multi-mode, two-dimensional nonlinear pushover analyses and single degree of freedom nonlinear dynamic time history analyses to estimate the necessary upgrade measures. Seven pairs of time histories for use in the nonlinear response history analysis were developed and scaled in accordance with FEMA requirements. All testing was completed using site-specific response spectra representing anticipated earthquakes of Richter magnitude 7.0 and 7.25 on the nearby Hayward Fault.
The moment frame models constructed for this laboratory testing were built with compound elements containing elastic and inelastic components to precisely mimic the steel moment frame of the existing headquarters. Beam elements were built from an elastic beam section and a nonlinear moment-rotation hinge for strengthened connections or a nonlinear fiber section for the existing connections. The fiber section used to model the existing connection was a user-defined cross-section in which axial-only fibers, with a chosen area and nonlinear material property, were assigned locations along a vertical axis. The cross section acted nonlinearly in tension, compression, and bending, but was considered to remain elastic in shear. The existing connection fiber model was comprised of three different types of fibers: one fiber representing the top flange, one fiber representing the bottom flange, and one fiber for each of the bolts in the shear tab (Figure 1). Since not all existing connections were to be retrofitted due to high costs, a model of the existing connections that simulates their fracture behavior needed to be developed. The testing would indicate at which locations the existing connections needed to be retrofitted.
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Connection retrofit
After determining which connection locations needed to be replaced, the next step was to determine what retrofit design was needed to replace them. Various connection upgrade schemes were considered based on previous research results. The scheme considered and tested included a single-welded haunch (WBH), a double-welded haunch (WTBH), a double haunch on one side of the column and a double gusset plate on the other, and a bolded bracket (BB). Deep column sections and large beam sizes on the existing structure led to upgrade connection test performance that differed from previous testing of both WBH and BB approaches. As a result, the WTBH scheme was selected for the connections upgrades. A final test that simulated the application of damper gusset plates into the moment confirmed that these plates could meet the project’s drift demands.
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Detailed time history analyses of the proposed scheme showed that the preliminary analyses substantially underestimated the drift in the lower stories of the actual building and overestimated the drift in the upper stories. As a result, the team implemented significant revisions to the damper and connection strengthening scheme configuration in the final design, which referred back to the non-linear response history analyses previously performed. These analyses also demonstrated the ability of the new damper elements to absorb a vast majority of the input energy from the earthquake and limit the inter-story drifts to levels well below the capacity of the existing fracture sensitive connections.
Once the multiple rounds of analysis and testing were complete, construction on the 15-story steel moment frame structure presented its own unique set of challenges. One specific challenge related to the connection of the new steel frame elements to the existing concrete basement walls and foundations.
The design team developed a scheme that included the addition of new steel plates to be connected into the existing concrete elements via drilled dowels that were epoxied in place. To accommodate interferences from items such as electrical conduit and concrete reinforcing bars, the design called for the contractor to do surveys of each location prior to drilling the holes in the steel plates. These surveys were then used by the design team to establish a final pattern of holes that would avoid the interferences, but still deliver the forces into the foundation and basement wall elements. Having these pre-construction surveys allowed the construction to proceed without significant delays to deal with unanticipated interferences to the placement of the dowels.
Conclusion
Several design iterations resulting from thorough, full-scale connection testing and advanced analysis eventually led to a finished project that allowed the design team to give the California Department of Transportation choices for the future of its District 4 Headquarters and the hub of Bay Area traffic management. The extra steps taken beyond typical engineering practices were intended to provide better assurance that the project’s performance goals would be met during the design basis seismic event. Degenkolb’s in-depth examination of moment connection strengthening schemes went beyond modal pushover analysis and employed non-linear response analysis. The team’s testing technique models the actual expected behavior of the building during previously recorded earthquakes thus advancing the practice of performance-based engineering for steel moment frame structures.
Jim Malley, P.E., S.E., has been the president of the National Council of Structural Engineers Associations since 2009, and serves as chair of the AISC’s International Seismic Steel Joint Action Group. He is a senior principal at Degenkolb Engineers and can be reached at malley@degenkolb.com.
Jim Malley would like to acknowledge the design team partners: The Crosby Group, YEI, Ratcliff Architects and SIE, Inc.; the clients DGS and Caltrans; the construction manager APSI; and the contractor Arntz Builders and Bragg Crane and Rigging, as contributors to the project.
