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• • • UPDATE! • • •
The City of Los Angeles recently passed an ordinance to mandate retrofitting of non-ductile concrete buildings within the city limits. The time period to complete the retrofit is 25 years; however, design of the retrofit (or a demo permit) must be submitted within 10 years.
A copy of the ordinance can be found via the link below (scroll to page 9 amendment regarding non-ductile concrete buildings):
SEISMIC RETROFIT [ Non-Ductile Concrete ]
In the last several years, the Los Angeles Times has reported on non-ductile concrete buildings, and the risk they pose in the event of an earthquake. Numerous articles discuss the most recent earthquakes in Kobe, Japan and Christ Church, New Zealand and how through research following the earthquakes, information is gained on how non-ductile buildings perform during an event. With the latest research and understanding, the engineering, government, real estate, and construction industries can work together to improve the buildings in our local communities, in an effort to avoid the tragic casualties experienced by others.
In 1976, Tangshan China experienced what is believed to be the largest earthquake of the 20th century by death toll. Although the death toll and number of people injured varies depending on reports (and political influence) and geographic areas included in the reports, the current death toll is around 250,000. Tangshan experienced a 7.8 magnitude earthquake, followed by a 7.1 magnitude aftershock roughly 16 hours later. A major component to the loss of life was the quality of building construction in the area. The area was believed to be a relatively low risk for earthquakes, so very few buildings had been built to sustain a significant earthquake. In particular, "non-ductile" concrete buildings were prominent in the area and suffered numerous collapses, trapping people and causing more deaths.
Non-Ductile Concrete Buildings are those concrete buildings built until around 1978, and are known for having low amounts of reinforcing steel. Typical of the era, the seismic resisting systems are marked by weak lateral resisting elements and with low ductility detailing of the reinforcing steel. Without sufficient steel reinforcement, the concrete building is relatively weak, and brittle, making the building susceptible to breaking, or collapse as a result of a major earthquake.
Retrofits for these buildings can include all or some of the following; carbon fiber wrapped around the columns, adding new concrete shearwalls to provide an uninterrupted load path, and strengthening existing walls with shotcrete and fiberwrap. The foundation retrofit typically includes micropiles with a pile cap (footing).
It is generally more challenging to retrofit these buildings, as the buildings are heavily occupied with residential or office environments, and have existing finishes prohibiting access to the areas of retrofit work. Minimizing the construction impact to the tenants takes creativity, planning, and coordination.