U.S. Post-Tensioned Seismic Systems Not Yet Tested by a Quake
Tipping Mar's vertically post-tensioned concrete shear walls in the 164-ft-tall San Francisco Public Utilities Commission Headquarters is the most ambitious—and tallest—example to date of a family of lateral-load-resisting structures designed to minimize damage in a major earthquake and allow immediate reoccupancy. U.S. structural practitioners and researchers have been developing the self-centering systems, modeled after PT bridge construction, for about a decade.
Self-centering structures, when designed in structural steel, have become known as "rocking frames." In addition to concrete systems, there are examples of self-centering PT structures in precast concrete and in composite steel-and-precast structures.
There has been research, including physical testing, on the systems, which are not yet included in U.S. building codes. In 2009, a steel rocking frame was tested at Stanford University (ENR 9/14/09 p. 90). Tests at Lehigh University on a PT reinforced-concrete shear wall, planned for last year, have been delayed (ENR 3/28/11 p. 20).
PT systems are not only designed to return a building to its plumb prequake position, they also are expected to limit seismic drift to about 1%, which is more than building codes mandate for life safety. The idea is to minimize quake damage to all building systems in an effort to protect the investment in the building and limit repair work.
Tipping Mar (TM) has used the system in five buildings to date. Three of them are still under construction. None of the PT systems in the U.S. has yet been tested by a major quake.
For PT concrete, "it's a little bit of a balancing act" between the right amount of prestressing so the concrete is not overstressed and the need to get enough recentering capacity, says Stephen Mahin, director of the Pacific Earthquake Engineering Center at the University of California, Berkeley, and one of two peer reviewers on TM's performance-based seismic design for the public utilities commission. "I think it's a good system. The next earthquake will show us," says Mahin.
