Date of Award
Master of Science (MS)
Light-frame wood construction is one of the most common types of construction in North America, particularly for low-rise residential dwellings and apartment buildings. Light-frame wood buildings were found to perform well during recent earthquakes. However, past earthquake events also revealed a common deficiency in many light-frame wood buildings, namely soft first-story damage, and, in some extreme cases, pancake collapse. Many buildings have a soft first-story because of an open-space floor plan used for retail or parking with minimal partition walls while the upper stories are apartment units. Typically, partition walls are considered as non-structural elements, however, they add strength to the overall lateral load resisting system. When both the structural elements (prescribed by engineers) and non-structural elements (partition walls sheathed with gypsums) are considered, vertical irregularities in strength and stiffness often occur in buildings with open floor plan in the first story. The current force-based design procedure, namely the Equivalent Lateral Force (ELF) procedure, does not explicitly consider the contribution of non-structural elements. This research (1) studied soft-story deficiency in light-frame wood buildings due to unintended stiffness and strength contributions from non-structural elements and (2) developed a strategy through the use of an adaptive displacement-based design (ADD) method in which the demand (required story shears) of the as-designed building is revised continually as the design progresses from one story to another. Nonlinear time history and incremental dynamic analyses were performed for the as-designed buildings using both ELF and ADD methods. The seismic performance in terms of (1) collapse probability at the Risk-targeted Maximum Considered Earthquake (MCER) level, and (2) peak median story drift ratios at various hazard levels were used to evaluate the overall performance of a soft-story building designed using both the ELF and the ADD procedure. It was observed that for a building designed using the ELF procedure, the collapse probability increased on the inclusion of non-structural elements in the model, signaling the detrimental effects of non-structural elements due to the inability of the ELF procedure to quantify the contribution of these elements. In contrast, the ADD procedure took into account the contribution of these elements and was able to provide a structural design for which the collapse probability actually decreased on the inclusion of nonstructural elements.
In addition, a parametric study was carried out to compare the differences in MCER collapse probabilities obtained using a 3D building model with biaxial ground motions and an equivalent 2D building model with uniaxial ground motion. The result of this parametric study was a factor that can be used to relate the MCER collapse probabilities between the 3D and 2D models, referred to as the 3D factor. The study confirmed that if the collapse results from both directions were used in calculating the overall collapse probability for a 2D building model, the 3D factor is 1.2 whether the building is designed for equal strengths or unequal strengths in its two lateral directions.
Shakya, Bikal, "Performance-based Seismic Design of Multi-Story Light-frame Wood Buildings using Adaptive Displacement-based Design Procedure" (2020). All Theses. 3478.