South Napa Earthquake: First-hand investigations and reconnaissance
Reprinted with permission from Degenkolb Engineers
Degenkolb Engineers conducted first-hand investigations and reconnaissance of the South Napa Earthquake that occurred on August 24, 2014 at 3:20AM.
Field note contributors: David Bonneville, Ariel Creagh, Mahmoud Hachem, Roger Parra and Taka Tamiya.
The magnitude 6.0 earthquake caused many residents around the San Francisco Bay area to jolt out of bed, but fortunately, there has only been one reported fatality. Earthquakes of this scale tend to have a relatively localized ground shaking around the epicenter, not providing the broad range of high ground shaking intensity that people experienced in the Loma Prieta earthquake in 1989.
For more information on the earthquake seismicity, refer to the USGS ShakeMap found at the following link: http://earthquake.usgs.gov/earthquakes/eventpage/nc72282711#shakemap.
Downtown Napa is home to many historic masonry buildings. The performance of unreinforced masonry buildings was of particular interest due to their vulnerability to seismic activity. More modern buildings are found in the surrounding areas. Naturally, the performance of modern buildings designed to recent codes is always of interest to compare our progress in design standards. In addition, the impact to the wine industry provided an interesting case study.
Unreinforced Masonry Structures
Most unreinforced masonry (URM) structures, which had undergone a seismic retrofit, survived the earthquake with no vital structural damage and minimal nonstructural damage. This substantially reduces the financial impact due to building repairs and business closures.
In non-retrofitted URM structures, building damage was primarily found in the unreinforced masonry façade, where bricks and stones had fallen into the streets and sidewalks. Fortunately, the earthquake occurred during the middle of the night, significantly reducing the chances that such failures resulted in injury or loss of life. As seen in the pictures below, typical damage consists of fallen walls, diagonal cracks indicating shear failure originating from corners of window/door openings, as well as failure of masonry at building corners.
Most of the new construction performed well. Damage was primarily found in nonstructural elements such as X cracks in stud walls, spalling of adhered stone cladding, and broken windows.
Notable nonstructural damage occurred in a downtown building with a metal stud framed exterior wall, which had broken from its slab connection, separating by approximately a foot from the structural frame.
A Target store with a unique tower feature had some structural damage. The tower appears to have displaced from its ground connection and shifted backwards slightly into the building. The store currently has shores in place and the area is blocked off, but is otherwise fully operational.
Minimizing Economic Loss
The Target store is an interesting example of a building owner who apparently had an earthquake action plan in place with a retained engineer and contractor who were able to quickly assess, contain, and repair the damages, allowing them to reopen quickly.
The city has a responsibility to inform the public of buildings unsafe to be occupied as well as reassure that others are safe to enter. This is done through the ATC-20 system of red, yellow, and green tags placed on the door of the building by an inspector. As sometimes happens in the aftermath of an earthquake, there were a number of tagging inconsistencies. In some cases, we were unable to determine reasoning for the choice of tags. There were also a number of safe buildings red-tagged due to their adjacency to a building with serious damage despite otherwise posing a limited threat to occupants. This may have resulted in business closures for companies otherwise ready to reopen with little to no down time.
The wine industry, centered in Napa Valley, created a unique opportunity to evaluate the effects of an earthquake on wine making equipment and storage. Structural damage to the winery facilities was limited due mostly to the fact that the majority of the wineries were North of the high shaking intensity and have relatively modern construction; however, considerable damage was reported at a wide number of facilities in the form of buckled tanks and toppled wine barrel stacks. Wine barrels are often stacked six barrels high and in many cases have no tie downs or anchorage for restraint. The damage observed reinforces the need for proper seismic consideration for large nonstructural elements both for safety and for the reduction of economic loss.