ASCE 7 R-Values in Soft-Story Building Retrofits

This paper presents the SEAONC Existing Buildings Committee (EBC) efforts to determine whether retrofits using the most stringent (lowest) R-value in parallel lateral force resisting systems (LFRS) is necessary to protect life safety. Case study buildings were analyzed representative of buildings in the City of San Francisco’s soft-story program. It was found that retrofits designed with R-values associated with wood structural panels (WSP) and steel momentresisting frames (MF) on independent lines-of-resistance have performance at least equal to that of a code-conforming benchmark retrofit consisting entirely of WSP. Hence, retrofits using the most stringent R-value may be considered over-conservative in the context of minimum code requirements. The findings support allowing retrofits using parallel mixed systems each designed to their respective code-specified R-value. At the request of San Francisco, the EBC provided language allowing this design approach in San Francisco’s Administrative Bulletin AB-107. Introduction Programs to retrofit wood-frame buildings having so-called soft-stories are underway in California. A popular design approach is based on the IEBC Chapter A4, Earthquake Risk Reduction in Wood-Frame Residential Buildings with Soft, Weak or Open-Front Walls (ICC 2012). The design force is based on ASCE 7-10 that includes a response modification coefficient (R-value) which depends on the seismic forceresisting system (ASCE 2010). The term “soft-story” as defined in the building code indicates a stiffness discontinuity, while the term “weakstory” denotes a strength discontinuity (Table 12.3-2 Vertical Structural Irregularities in ASCE 7-10 (2010)). In practice, buildings with either or both of these irregularities are often referred to as “soft-story” buildings, and that is how the term is used herein. For many buildings, a practical retrofit uses a combination of wood structural panel shear walls (WSP) and steel momentresisting frames (MF) in the transverse direction at the first story. The MF allows automobile access and parking space that would otherwise be blocked by WSP. WSP and MF have different R-values, and for buildings greater than two stories, ASCE 7-10 requires using the most stringent (lowest) R-value in parallel systems (Section 12.2.3). A retrofit consisting of both WSP and OMF is required to have the WSP nearly twice as strong as that in a retrofit consisting entirely of WSP. Many engineers question the necessity of this restriction, which often creates problems with the design and placement of stronger WSP, holdowns, and foundations within the building envelope. This paper presents the SEAONC Existing Buildings Committee (EBC) efforts in determining whether using the lowest R-value in parallel systems is necessary to protect life safety. San Francisco Buildings Many San Francisco neighborhoods constructed in circa 1920 have a signature architectural style with many relatively narrow midblock buildings (Figure 1a). They are typically residential buildings having large open areas on the ground floor for automobile parking and common areas (Figure 1b). To provide access for automobiles, there are large openings on the transverse wall facing the adjacent street. Since the residential units have numerous transverse partition walls compared to the parking level, the ground floors are usually weaker than the upper stories, and hence, many such buildings are soft-story candidates in their transverse directions. In the longitudinal direction, they have long exterior walls with few openings thus providing adequate length for any WSP retrofit that might be necessary. ASCE 7-10 Table 12.2-1 defines three types of steel moment-resisting frames: Special Moment Frames (SMF, R = 8), Intermediate Moment Frames (IMF, 4.5), and Ordinary Moment Frames (OMF, 3.5). WSP has R = 6.5. 2017 SEAOC CONVENTION PROCEEDINGS