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At a Glance:
Since 1900, there have been 54 recorded earthquakes in California with a magnitude of 6.0 or greater. Sixteen of the 54 have had a magnitude of 7.0 or greater, with the 1906 San Francisco Earthquake topping the list at a magnitude of 7.8. In the first three months of 2009, 29 earthquakes measuring greater than a magnitude of 3.0 are documented. Are we ready for the Big One?
The big transition came after the 1933 Long Beach Earthquake, which made unreinforced masonry in high seismic areas obsolete. Since then, we have refined code requirements based on masonry performance in subsequent earthquakes.
The 1971 Sylmar-San Fernando earthquake showed that wall-to-roof connections were inadequate, causing a number of roof collapses. Fortunately, this event occurred at 6 a.m., outside of normal work hours, keeping the death and injury toll low. Stringent connection requirements were implemented in the code and tested in the 1994 Northridge
The Northridge event measured unanticipated ground accelerations. Instrumentation at the Cedar Hill Nursery in Tarzana measured an east-west ground acceleration of 1.78g, far in excess of anticipated accelerations. Northridge also revealed a vertical component, or upward thrust, to earthquakes. When comparing the measured ground movements to engineering design criteria, one would expect an inordinate amount of catastrophic damage. That did not happen, since masonry has a high factor of safety and redundancies within the system.
While damage was widespread in the Northridge earthquake, it is significant to note that of the 57 fatalities, not one was attributed to masonry. Was that good planning, or luck? Perhaps it was a little of both.
One must understand that the primary purpose of the Building Code (IBC Section 101.3) is life safety. That means that during any catastrophe, including earthquakes, hurricanes or tornados (to name a few), the occupants should be able to exit or find a safe haven within the building to avoid death and injury. When this happens, the code has successfully served its purpose.
Today’s building codes and reference documents offer a balanced approach of cost versus performance. The West Coast is seismically active and must take that reality into consideration. The marriage of deformed reinforcement into a grouted masonry system is an accepted code reality. Designers are aware of the requirement and prepare project documents accordingly.
The quality of masonry installation can vary widely. While most contractors are quality conscious, occasionally, a poor representation of installed masonry may exist. That is, someone has taken unacceptable shortcuts in building the project.
Chapter 17 of the International Building Code and the reference standard, Building Code Requirements for Masonry Structures (ACI 530-05/ASCE 5-05/TMS 402-05) and Specification for Masonry Structures (ACI 530.1-05/ASCE 6-05/TMS 602-05) address this issue with inspection requirements. Essential facilities, such as hospitals and police stations, located in high-seismic areas, must be inspected by an independent observer to assure that the installation will satisfy the structural integrity requirements. This may include continuous inspection of the masonry work, or be periodic observation at critical points in the construction process.
Non-essential facilities within high-seismic regions will also require inspection by an independent observer, but at a reduced frequency. This may be limited to verification of the grade, size and location of reinforcement and the grouting process. This is the most critical issue relative to the seismic performance of reinforced masonry.
The masonry design process has proven to be adequate as evidenced by the large inventory of structural masonry buildings on the West Coast and the frequency of the ground movement from seismic activity. The code requires that we satisfy the design with quality construction, and the independent inspection process is a practical manner that assures superior installation quality, providing structural integrity for decades.
|Last Updated on Sunday, 31 May 2009 23:15|