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On Solid Ground
by Alexander Itsekson
The term “soft-story building” describes a building with a level or story that is significantly more flexible than the stories above it, which can be hazardous in the event of an earthquake. In multiunit wood-frame buildings, weakness at the ground level usually stems from large openings in perimeter walls. Examples of perimeter openings include garage doors, tucked-under parking or large windows. The garage openings are often supported by beams instead of rigid walls. During a strong earthquake, the ground levels of a soft-story building are unable to support the stiff and heavy mass of the floors and roof above as they move back and forth. When these slender walls or columns shift sideways, they break, causing the building to collapse, crushing the ground floor.
Most soft-story buildings that are of concern to the public were built before 1980 and are of wood-frame construction with large openings at the ground level. However, it is not possible to know for certain that a building is a soft-story simply by looking at it from the street. While large openings at the ground level are a strong indicator that a building may have a soft-story, many characteristics affect a building’s response to an earthquake, including the contribution of interior partition walls, strength of walls and previous retrofits completed. Only a building-specific investigation and analysis conducted by an engineer can determine if a particular building is a soft-story building.
Bay Area Soft-Story Buildings
The Bay Area currently has thousands of potentially hazardous soft-story buildings, with tens of thousands of units. Damage to soft-story buildings has caused significant economic loss to building owners and has also resulted in fatalities and serious injuries to tenants during recent California earthquakes. In the 1994 Northridge earthquake, 16 people were killed and 34,000 housing units were left uninhabitable due to soft-story collapses, while during the 1989 Loma Prieta earthquake 7 lives were lost and 7,000 housing units were rendered uninhabitable due to soft-story building damage.
San Francisco has an estimated 2,800 to 4,400 soft-story buildings, Oakland has almost 1,500, and Alameda, Berkeley and San Leandro have approximately 1,000 soft-story buildings combined. Without seismic retrofits, these buildings will remain vulnerable, and will likely be damaged in the event of a large earthquake, leaving hundreds of thousands of Bay Area residents homeless.
It is highly likely that another large earthquake will occur in the Bay Area within the next 30 years. Recognizing this potential danger, many Bay Area cities, including Berkeley, Alameda and Oakland, have passed soft-story ordinances, which require owners of identified soft-story buildings to file engineering analysis reports for their buildings. A Fremont ordinance requires mandatory retrofits of soft-story buildings, and other cities have already inventoried potential soft-story buildings and are currently developing their own strategies to address this problem. At a February Earthquake Engineering Research Institute meeting, San Francisco Mayor Gavin Newsom revealed that he is in the process of creating legislation for a mandatory retrofitting program, which will also include a financing package to mitigate retrofit costs. Newsom hopes that this legislation will be on the November 2010 ballot for voter approval of certain elements.
The City of Berkeley passed its soft-story ordinance in 2005, and began notifying building owners of ordinance requirements soon after. Owners were required to submit engineering reports within two years of notifications, but were not required to complete retrofits.
The City of Alameda has approved a soft-story ordinance similar to that of the City of Berkeley, which aims to identify soft-story buildings and encourages voluntary seismic retrofits. In addition to the engineering analysis report, the Alameda soft-story ordinance also requires building owners to install earthquake-actuated gas shutoff valves and notify tenants that their building has been classified as a soft-story building. Alameda has also set aside some funds to assist low-income building owners with financing for retrofits. At press time, Alameda had yet to begin notifying building owners that their buildings are on the inventory of soft-story buildings.
The City of Oakland, working in association with the Association for Bay Area Governments, passed its soft-story ordinance in 2009, which requires building owners of suspected soft-story buildings to hire their own registered design professional (such as an architect or engineer), licensed contractor or certified inspector to prepare an initial seismic screening report of their building within two years of notification. This initial seismic screening report requires detailed information about the building’s floor plan, elevations, ground-floor openings and calculation of Area Demand Ratios for the building’s ground floor.
Under Oakland’s ordinance, owners of buildings with ADRs exceeding ABAG recommendations will be required to hire registered engineers to prepare level 2 engineering reports for their buildings, which require an analysis of each building’s lateral force resisting system. Owners will also be required to notify current and prospective residential tenants that the building is classified as a soft-story building. Presumably, owners of buildings that are found to be seismically deficient will eventually be required to retrofit. The City of Oakland is working with banks, design and construction professionals, policy experts and RHANAC representatives to develop financial incentives so that the owners of these buildings will retrofit them.
Soft-Story Retrofits: Now or Later
Depending on your building location, completing a seismic analysis may bring your building into compliance with the current soft-story ordinance, but buildings must be retrofitted to be removed from the soft-story inventory list. To encourage owners to complete voluntary retrofits, some Bay Area cities have reduced or eliminated plan check or permit fees. For example, the City of Alameda will waive all plan review and inspection fees for soft-story retrofit projects if they are submitted within 18 months of notification.
Nonetheless, completing a seismic retrofit under the soft-story ordinance guidelines can be costly, as most cities, including Berkeley and Alameda, reference building codes that mandate the strengthening of all walls at the ground level, as opposed to strengthening only the open soft or weak wall lines. Many soft-story buildings have apartment units at the ground floor, making retrofitting even more complex, lengthy and costly due to the necessity of relocating ground-floor tenants during construction or waiting until units become vacant before completing construction. Because this requirement greatly increases construction costs, it has deterred many building owners from strengthening their buildings.
For soft-story building owners who have not yet received notification from their city, one way around this requirement is to retrofit the building before being notified. By retrofitting only the weak elevations of your building before your building’s engineering report is due, your engineer can report to the city that your building is not a soft-story building and should not be included in the inventory of soft-story buildings, excluding it from the ordinance requirements. This type of retrofit will help protect your building at a lower cost while limiting disturbance to building tenants.
There are a number of retrofit techniques, which can be used to fix a soft-story condition. Depending on the building’s type of existing construction, building size and weight, space and parking limitations, the retrofit can utilize concrete, masonry or plywood shear walls, steel moment frames, steel cantilevered columns, or prefabricated metal walls on concrete grade beams to strengthen weak walls. Each method has its own benefits and caveats. Consult an engineer to choose the most appropriate and cost-effective retrofit method for your building.
Although undertaking a seismic retrofit may seem daunting in today’s economy, strengthening your building now will save you money in the long run. According to the City of San Francisco’s Community Action Plan for Seismic Safety, the direct cost of repair of a soft-story building after an earthquake is approximately six times the cost of retrofitting. This figure does not incorporate business interruption costs, legal costs and lost rental income of an uninhabitable building, further increasing the benefits of seismic strengthening. Moreover, due to the general slowdown in the construction industry, retrofit costs are significantly lower today than a few years ago, but will rise again as the economy picks up.
Although Fremont is currently the only municipality that requires mandatory retrofits of soft-story buildings, Bay Area cities will eventually require mandatory seismic upgrades for soft-story buildings as the risk of soft-story building collapse and loss of life becomes too great to ignore. Municipalities and elected officials are concerned about the potential problems they will face after an earthquake if soft-story buildings are not retrofitted: hundreds of red-tagged buildings and thousands of homeless people on the streets. While it may take several years to pass the required legislation, mandatory retrofit ordinances are inevitable.
Moving Forward
The San Francisco Board of Supervisors recently passed a program to incentivize voluntary seismic retrofits. This program offers owners an expedited permit process and waived permitting fees if they voluntarily retrofit their buildings. Those who choose to take advantage of the program will also be exempt from any mandated retrofitting for 15 years following the construction. Owners will certainly want to take advantage of these incentives as the Mayor’s Office has put together a task force, which includes SFAA representatives, to design a future mandatory program for soft-story seismic upgrades. Code-mandated work remains eligible for a 100% capital improvement passthrough if proper paperwork is presented to and approved by the San Francisco Rent Board.
Deciding to retrofit is a difficult decision for a building owner to make, because the costs of retrofitting are clearly evident and demanding, while the benefits of a seismic retrofit lie far in the future, when an earthquake strikes. Contact a structural engineer or general contractor experienced in soft-story building strengthening to help you determine the magnitude of capital improvement costs associated with a seismic retrofit.
The opinions expressed in this article are those of the author, and do not necessarily reflect the viewpoint of the SFAA or the SF Apartment Magazine. Alexander Itsekson is the principal engineer of Enginious Structures, a Bay Area structural engineering firm specializing in seismic reports and retrofit design. Itsekson has 20 years of engineering experience and lives and works in Oakland. More information about soft-story buildings is available at www.enginious-structures.com. Copyright © 2010 by Black Point Press. All rights reserved.