November 20, 2003
Building a highrise on the fault line
By ROBERT BOURDAGES
Poggemeyer Design Group
Parts of Southern California are known for their strong and frequent earthquakes. So, what does it take to design a modern building in quake country? The answers can be found in a project under construction in San Diego.
The project, Acqua Vista, is a 387-unit residential building located in the Little Italy district of San Diego. It is a cast-in-place concrete building with twin towers extending 18 stories above grade and two levels of underground parking. The towers are connected up to level eight, where an open courtyard and pool level are located. The building footprint is 200 feet by 300 feet.
Acqua Vista's site is situated within 2 kilometers of an active fault and is directly over an ancient fault line that has not been active for more than 125,000 years.
The dominant tectonic system in the local area is the Rose Canyon Fault Zone, a fault zone associated with the general San Andreas Fault system. These faults are identified with the Pacific-North American Plate margin in Southern California. The western side of the San Andreas Fault moves northward relative to the eastern side at an estimated rate of 50 millimeters a year. Earthquakes here tend to be shallow, strong, and frequent with short duration. Aftershocks are common.
The Seattle tectonic setting, by comparison, is characterized by subduction, where the heavier Pacific Plate moves westward and under the North American Plate. Earthquakes here generally are deep seated, moderate in strength, and are longer in duration. Aftershocks are less common than the Southern California quakes.
For this project, the lateral load resisting system was required to resist seismic forces prescribed in the California Building Code: Zone 4, Downtown Special Fault Zone. Wind loading was specified at 70 mph (basic wind speed), exposure C. Wind loading did not control the lateral design.
The underlying soils were classified as Bay Pointe Formation, beginning about 15 feet below the existing grade. These materials are good bearing materials for conventional spread footings and have an allowable bearing pressure of 6,000 pounds per square foot. The soils were identified as severely corrosive. The ancient fault running through the site was determined to be inert (classified as potentially active) and therefore no setbacks or other mitigating measures were required.
An 84-inch-diameter sewer interceptor buried 50 feet below grade passes through a portion of the site. There was about 22 feet of clearance between the top of the pipe and the bottom of the mat foundation. The city of San Diego required that the building not adversely affect the deteriorating concrete sewer main, and it was necessary to prove that a pipe failure from degradation would not unravel the overburden materials and undermine the footings.
Temporary earth shoring within the city right of way was required to be removable.
The 200-by-300-foot site was shored with removable soil nailing. Seams of loose sand were encountered during the soil nailing installation, which added a degree of difficulty to the stabilization of the excavation. Typical nail spacing was 6 feet vertical, except at the sand layers where the spacing was narrowed down to as little as 2 feet. Some movement was detected at the north wall, requiring additional soil reinforcing.
Under a portion of the north tower, drilled concrete piers were used to support a large mat footing that spans over the sewer interceptor pipe. The south tower was supported by another large mat footing, and balance of the structure is supported by conventional spread footings.
The lateral resisting system was provided by a building frame system with concrete shear walls founded on mat footings. Tower shear walls were located at elevator cores, stairwells and demising walls. Shear walls were typically 18 inches thick with heavy boundary reinforcing. Number 18 bars were used at shear wall boundary elements at the lower levels. Rebar congestion was evaluated in the design phase and coordinated with the concrete subcontractor prior to construction. The execution of rebar placement and formwork went very smoothly.
The floor system and roof is cast-in-place post-tensioned flat plates supported by concrete columns. Typical bay spacing ranges from 26 to 34 feet. Slab thicknesses were typically 8.5 inches. Each floor and roof level was designed using finite-element post-tensioning software that provides for faithful modeling of actual geometry, including floor openings and changes in slab thicknesses. Slab closure pours were used at large slab areas to minimize the effects of concrete shrinkage.
Portions of the level 18 penthouse floor were offset from columns below, such that the slab was suspended with tube steel tension elements to the roof.
The building is expected to be finished next spring.
Poggemeyer Design Group provided the preliminary and final structural design and construction administration services. Matt Schmitter was the project engineer.
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