Seismic Evaluation of an Asymmetric Three-Story Woodframe Building

Several wood-frame multi-story buildings constructed in the 1960s and 1970s were built with tuck-under parking at the ground level. Many of these buildings experienced ground story collapses in the 1994 Northridge, California earthquake. A commonly recommended approach for seismic retrofit of such structures is to provide a moment resisting steel frame around the garage openings to assist in resisting the lateral forces caused by earthquakes. However, no large-scale experiments have been conducted to verify the effectiveness of this approach.

The presented research focuses on the large-scale dynamic testing of a three-story high, woodframe residential building, that incorporates features representative of those used in California construction during the 1960s and 1970s.

Some of the main conclusions of the study include:
a) Exterior stucco and interior gypsum boards reduced the maximum story drift in the open front by a factor of 2.3 while the corresponding story shear increased by a factor of 1.2.
b) Moment resisting steel frame retrofit in the open front reduced the maximum story drift in the open front of the finished building by a factor of 2.0 and the corresponding story shear decreased by a factor of 1.2.
c) The maximum rotation of the second level (floor above the garage) about the vertical axis was reduced by a factor of 2.2 due to the effect of finish materials and by a factor of 1.7 due to the retrofit of the open front of the finished building.
d) Although the building was nearly symmetric about the transverse axis (with the exception of the asymmetric openings in the first story back wall), the distribution of damage in the two end walls was very different. These differences in damage distribution led to a complex change of the system behavior of each of the three phases of testing. This asymmetric behavior was clarified through computational modeling, which clearly demonstrated the higher demands in one of the transverse walls due to the combined effect of the three components of ground motion.