Recent Recorded California Earthquake
Within the past 24 hours there have been seven earthquakes in Southern California, each of which with a magnitude of 2.2 or below (earthquaketrack.com). Although earthquakes of this magnitude are not even considered “minor” on the Richter scale and are generally not felt, their occurrences and closeness to one another are unsettling nonetheless. The last 30 Days has seen 190 earthquakes in the last thirty days in Southern California, the largest of which actually occurred less than 48 hours ago. Isla Vista experienced a “slight” 4.1 magnitude quake at 9:42 pm, though no reports of damage were reported. While earthquakes of a relatively small magnitude do not literally “shake” us and therefore may be easy to ignore, it is important to remember that at one point or another, an earthquake of larger magnitude will occur; it is just a matter of when.
For years, there has been extensive discussion about when “the big” earthquake will hit. It has been suggested that Southern California is “overdue” for a big earthquake, which could hit at any time. Although many people would prefer to simply ignore Southern California’s precarious position in relation to the San Andreas Fault. According to the San Andreas Fault website, “the big one”, although “hypothetical”, is expected to have a magnitude of 8 or greater. Proper steps must be taken to ensure that the negative impacts of this earthquake are as minimal as possible. This is of particular importance in urban areas, due to larger populations and the types of buildings found in these particular places. It is evident that there has been a greater awareness of the threat of these quakes when considering the recent mandatory seismic retrofit ordinances that have been passed in Southern California as well as other areas of California. The recent discourse around the somewhat small but frequent earthquakes that have been occurring, in addition to the imminent threat of a much larger one, has led to a greater understanding the need and importance of seismic retrofitting. The implementation of mandatory seismic retrofit ordinances for possibly vulnerable buildings will benefit and serve thousands of buildings, but most importantly save the lives of those who live in buildings that may be severely impacted by a large and devastating earthquake.
What are the differences between “Ordinary Moment Frames” and “Special Moment Frames”?
Due to the latest seismic ordinances passed in Santa Monica, there has been a lot of talk around different seismic solutions and options, moment frames being one of those options. There are three different types of moment frames: ordinary, intermediate, and special. After reading this short article, it will only take a moment for you to understand the fundamental differences between Ordinary Moment Frames and Special Moment Frames.
To begin, a standard moment frame is defined as a system of columns and beams which are connected by either fully or partially restrained moment connections. As one might expect based on its name, “ordinary” moment frames, one of the three types of moment frames, are generally found in areas described as non/low-seismic regions. The expectation is that ordinary moment frames are able to endure limited inelastic deformations caused by lateral forces. Ordinary moment frames only require onsite welding, which differs from the requirements for special moment frames. Special Moment Frames, another type of moment frame, are “special” in that they are expected to endure more significant inelastic deformation than ordinary moment frames. In addition, Special Moment Frames utilize different connections than Ordinary Moment Frames and they do not require field welding. Generally even with on-site welding the cost of Ordinary Moment Frames won’t go over that of a Special Moment Frame. These connections must be prequalified, as stipulated by the American Institute of Steel Construction. Since Special Moment Frames can endure more deformation, they are often found in areas described as mid to high-seismic regions. Seismic retrofits constructed utilizing moment frames can sometimes be more costly and time consuming because moment frames often require more welding on site than other frames do. Traditionally, special moment frames are more expensive than ordinary moment frames because these types of frames are often found in mid/high-seismic regions. Your engineer may decide whether or not moment frames are right for your particular project but they are preferred under certain circumstances. If your building includes large open areas within your structure, such as a tuck-under parking garage, moment frames would often be the building solution that is preferred. Considering the latest ordinances regarding soft story seismic retrofits, the Santa Monica area will be seeing moment frames of all types over the course of the next five years.
For more helpful information on seismic retrofit services that will fit your individual projects needs, keep an eye out for more blog postings on our website- www.penhall.com/seismic-retrofit/
References inlcude: http://csengineermag.com/article/steel-moment-frames-101-what-to-consider-when-creating-wide-open-spaces/ (by Dylan Richard and Walter Moore)
Seismic Retrofit Service in Santa Monica – Industry News
In Santa Monica, many property owners had retrofits completed in 1994 after the Northridge quake. However, many of those retrofits were only partially completed, i.e. parapet only, diaphragm anchorage, full or partial wall bracing, etc. It is at the discretion of the owner to what level of resiliency they desire for their building. At minimum, the seismic retrofit ordinances aim to make buildings safe enough for tenants to exit the building safely in the event of a large earthquake. This does not guarantee that the building will be habitable after an earthquake. The seismic retrofit notice that owners will receive would inform them of the additional work that needs to be done to be up to code. For other buildings that would in fact need to be in operation after a large quake, i.e. hospitals, these buildings must be kept to a higher standard of resiliency.
One such rating system for evaluating how well a building will hold up in an earthquake is known as the United States Resiliency Council Building Rating System. Per the USRC website, “The USRC Building Rating System assigns one to five stars for three performance measures – Safety, Damage expressed as repair cost, and Recovery expressed as time to regain basic function.”
Every city should be concerned first and foremost about the safety of its residents, but secondly, they should want to make sure that the City would still be able to thrive after an earthquake. Would the residents be able to return to their homes and/or jobs after a quake, or will they move to an area of less seismic activity? Per www.fema.gov/building-codes, “If you live or work in retrofitted structures, you’re less likely to be injured during an earthquake. After the earthquake, you’re also more likely to have a home and a job to which you can quickly return. Businesses that use retrofitted buildings are more likely to survive damaging earthquakes and to sustain shorter business interruptions and fewer inventory losses.”
May 18, 2017