It is common knowledge that California is vulnerable to earthquakes. Geological records show a pattern of seismic events that highlight areas most susceptible to an earthquake and give the expected intensity. Recent experience with California earthquakes within our lifetime have demonstrated the structural vulnerabilities in building types described as “weak” or “soft-story” structures. Having identified the more vulnerable building type, we must ask what corrective measures can be taken to strengthen them in order to avoid a housing catastrophe on a similar scale to Hurricane Katrina?
Soft-story structures (also known as weak-story) are identified as buildings with inadequate stiffness (soft) and/or strength (weak) at the first floor to prevent significant damage or collapse in an earthquake. These structures have been designed for open, common space at the first floor to accommodate parking cars or for use as retail space. The first floor framing along open areas is typically post and beam construction designed to support vertical loads from the structure above but providing very little lateral support. Levels above the first floor are typically apartment spaces that have more walls dividing the living area, which provides more lateral strength and stiffness than is available in the soft/weak first story.
In an effort to increase the resiliency of the city and decrease the potentially devastating economic fallout from a significant earthquake, the City of San Francisco adopted a soft-story ordinance in 2013. With a similar goal, the City of Los Angeles adopted a comparable mandate two years later. Other jurisdictions — including the cities of Oakland, Berkeley, Fremont, Alameda, and Santa Monica — also have adopted policies to identify, evaluate, and/or strengthen vulnerable buildings.
These soft-story retrofit ordinances concern multiple-unit, multifamily wood-framed residential structures such as apartment buildings, condominiums, and residential/retail mixed use that were common construction prior to the adoption of the 1978 building code provisions. Typically the first floor space in these buildings has one or more exterior faces with very narrow wall lengths, if there are any walls at all. The lack of lateral force-resisting elements along the exterior face makes the building very susceptible to excessive damage or collapse in the event of a significant earthquake.
Scope of timeline of ordinances
There are currently 18,000+ wood frame buildings identified that will require retrofitting under their local ordinance. The LA Times reported that in the City of Los Angeles alone there are an estimated 13,500 buildings identified as soft story, all of which are required to be seismically retrofit.
The amount of time building owners have to complete the seismic upgrades varies depending on the city in which they are located, building size, and number of occupants. In Los Angeles, owners have as long as seven years to complete retrofit requirements. In San Francisco, the due date depends on the type of building (http:// sfdbi.org/softstory). Type I buildings defined as “any building containing educational, assembly, or residential care facility uses” must be retrofit by Sept. 17, 2017. Type IV buildings (“any building containing ground floor commercial use”) require permit applications to be submitted by Sept. 15, 2018 and the work to be completed by Sept. 15, 2020.
While there is not a single lateral force structural solution that works for all buildings nor, in many cases, is there a single solution that works within all locations of a particular building, there is a small group of lateral force-resisting elements that are most often considered. Where space permits, conventional plywood shear walls are often added to increase the lateral force-resisting capacity of the building for seismic loads.
For the more common conditions where long lengths of wall are not available, pre-manufactured, narrow light gage or structural steel shear wall panels may be appropriate. Given that many softstory structures are two or more stories above ground level, have narrow vertical supports, and need to protect parking spaces, Special Moment Frames (SMFs) designed to endure the “significant inelastic deformation” caused by earthquakes are frequently the best solution.
The Hardy Frame prefabricated SMF was made available in 2008 using the SidePlate SMF connection, the first to be approved by AISC’s Connection Preapproval Review Panels for inclusion in the AISC 358 Prequalified Moment Connection Standard. Connections that are included in AISC 358 have gone through a rigorous review by industry experts and academia and have been proven to meet seismic design requirements and performance expectations of the building standards.
Additionally for the Hardy Frame SMF with SidePlate Connections, AISC 358 includes the use of square and rectangular hollow structural section (HSS) beams. The inclusion of HSS sections was based on full-scale seismic testing of Hardy Frame moment frames with HSS beams included. The tests highlighted that lateral bracing is not required as it is with other SMFs using conventional wide-flange beams.
Aside from design advantages, the ability to have SMFs delivered to the jobsite completely preassembled reduces installation time and eliminates onsite welding along with the special inspection it requires. Installation can be as easy as excavating and constructing new concrete pads or grade beams for the anchorage and overturning forces associated with the new lateral force-resisting elements, installing the frame on the anchorage as a single element, and, once in place, connecting the frame to the existing building at the floor level.
DAVID LOPP is vice president of Technical Support for the Hardy Frame Shear Wall System. (http://www.hardyframe.com/softstory).