A team of engineers—including The Crosby Group, under the direction of Degenkolb Engineers—is making sure that the District 4 headquarters of the California Department of Transportation (Caltrans) is functional in the event of a major earthquake. The building is home to critical San Francisco Bay Area transportation operations, including emergency response and traffic management and reporting. It is located just five miles from the Hayward Fault in Oakland.
"We are looking forward to having a safer building for our employees, especially those who operate the 24-hour Traffic Management Center, which is the heartbeat of traffic operations in the nine Bay Area counties. Just as we are making our highways and bridges seismically safe, this seismic upgrade will allow us to keep these critical operations functioning when the Bay Area needs us most, in the event of an earthquake," said Bijan Sartipi, Caltrans District 4 director.
Traffic Management Center personnel activate metering lights and freeway message signs, monitor traffic flow, and dispatch personnel and equipment to clear traffic obstructions as quickly as possible. This day-to-day function becomes critical during incidents such as last year’s collapse and rebuild of the Macarthur Maze following a gas tanker overturn and high-temperature fire.
Constructed in 1991, the building consists of a 15-story steel moment-frame structure that meets the 1988 Uniform Building Code. It 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 also has a large atrium above the parking levels. Full-height moment frames are located along the perimeter frame lines, as well as two interior transverse moment frames adjacent to the atrium on either side.
Poor building performance of steel moment frames in the 1994 Northridge Earthquake caused the state of California to commission a study of Caltrans District 4’s structural system and laboratory testing on moment connections similar to those in the existing building. This study verified that the building’s existing structural system might also suffer high levels of damage during future seismic events similar to that experienced during the Northridge Earthquake. The building was subsequently categorized as Risk Level V. According to the scale used by the California Department of General Services, buildings identified within this risk level have a substantial risk to life safety, a total disruption of systems, substantial structural damage, and a likely risk of partial collapse.
The Degenkolb-led team was tasked to seismically upgrade the building structure to 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. A series of four full-scale tests were conducted in order to evaluate the performance of the proposed rehabilitation schemes by FEMA guidelines 351 and 356.
"Our approach was very thorough," said Jim Malley, senior principal at Degenkolb Engineers who led the design of the retrofit. "The testing indicated more work was required than was initially assumed. We came up with a solution that greatly decreases the threat to life and increases the operational ability of the building."
To meet the requirements under FEMA 351 and 356, Degenkolb Engineers developed two phases of analysis to assess the rehabilitation design. To estimate the necessary connection strengthening and damping, multi-mode, two-dimensional nonlinear pushover analyses were performed along with single-degree-of-freedom nonlinear dynamic time-history analyses. The simplified analysis substantially underestimated the drift in the lower stories and overestimated the drift in the upper stories. As a result, the initial analyses called into question the ability of presently available simplified design procedures to predict the performance of existing moment-frame structures with highly, non-linear existing connections.
The second round of analyses was intended to refine the scheme and perform final checks. Nonlinear time history analyses of building frames were used for this portion of the work.
The Degenkolb team developed a retrofit scheme that included a combination of moment connection strengthening and the addition of viscous dampers. Compared to an alternate scheme without dampers, the proposed strengthening reduced the number of work locations by 35 percent with associated reductions in staff displacement and construction duration. Currently, construction is underway at Caltrans District 4, with little disruption to the agency’s operations due to the extensive planning that the team underwent with both Caltrans and DGS to establish an orderly phasing program to orchestrate the work.
The structural design for the project was a collaboration between Degenkolb and The Crosby Group. The two firms had previously worked together on a similar project, the East Bay Municipal Utility District Administration Building in downtown Oakland, Calif. Also on the team were YEI Engineers from Oakland and The Ratcliff Architects from Emeryville, Calif. Specialty consultant for the dampers is SIE, Inc., of Oakland.