In the United States, seismic design standards are crucial in classifying buildings into Risk Categories I to IV. These categories are based on the buildings' occupancy type and the potential risk they pose to public safety, the protection of human life, and the socioeconomic consequences of structural collapse in the event of an earthquake. As the risk category increases, a higher seismic importance factor and more stringent drift limits are imposed on the respective building. This results in enhanced lateral strength and stiffness of the seismic force-resisting system. This study, which compares the seismic demands of special moment frame buildings assigned to high-risk categories, focusing on static system overstrength, ductility, and collapse risk, provides practical insights for structural engineers and architects. For this purpose, nonlinear static and dynamic analyses are performed to quantify the seismic demands of 18 steel frame buildings assigned to Risk Categories II, III, and IV. The findings indicate that buildings in Risk Category II do not meet the target collapse risk of 1% in 50 years, as specified in ASCE/SEI 7. For buildings in higher risk categories, the equivalent lateral force method for estimating seismic base shear is deemed more effective in ensuring adequate seismic performance.
There have been meaningful changes in column stirrup spacing by KDS 41 20 00 in 2022, which is to decrease one of the spacing limits from the minimum section dimension to half of the minimum section dimension. Decreased column stirrup spacing increases the seismic shear resistance of columns and the seismic performance of the entire building. Among the effects of the column stirrup spacing change, this study focused on deformation compatibility in the seismic design of building frame system buildings with ordinary shear walls for seismic design category D. The beams and columns in building frame systems shall satisfy moment and shear strength, or deformation capability induced by seismic design displacement for satisfaction of the deformation compatibility. The commentary of KDS 41 17 00 describes that the deformation compatibility check can be ignored if the members in moment frames are upgraded to intermediate section details. The study showed that the deformation compatibility of columns was satisfied without additional consideration if the building frame systems were designed by the decreased column spacing in KDS 41 20 00. However, beams adjacent to walls needed further consideration, such as the recommendation of commentary in the code.
Due to the limited experimental data on the seismic performance of concrete-encased steel columns, standardized guidelines for nonlinear modeling parameters and acceptance criteria have not yet been developed. This study utilized analytical and numerical methods to predict the nonlinear behavior of concrete-encased steel columns with H-shaped steel sections. The findings of this study have direct and practical implications for the design and evaluation of concrete-encased steel columns. For instance, for concrete-encased steel columns constructed with normal-strength concrete and subjected to low-to-moderate axial load ratios, the yield rotation angle can be determined through fiber-based section analysis and analytical equations, and the nonlinear modeling parameter can be evaluated based on section analysis and the proposed empirical equation. For concrete-encased steel columns with high-strength concrete or high axial load ratios, inconsistencies between section analyses and experimental results are observed. Accordingly, the nonlinear modeling parameter a can be evaluated using the proposed empirical equation. The empirical equation was conservatively developed based on the modeling parameter criteria for reinforced concrete columns in ASCE 41-13.
In apartment buildings in Korea, irregular walls, such as T-, L-, and U-shaped walls, are commonly used. However, in practical design, the geometric irregularities of walls are often neglected when determining the length of the lateral confinement region. Further, although earthquake loads apply from various directions, the lateral confinement region is typically determined for the in-plane direction of the web. Thus, using finite element analysis, this study investigated the structural performance of irregular walls subjected to various loading directions. As the design parameters, wall shape, cross-sectional aspect ratio, and loading direction were addressed. According to the parametric analysis results, as the length of flange in tension increased, the lateral confinement region should be evaluated with consideration of the geometric irregularity. Further, for the L- and U-shaped walls, it is recommended to evaluate the lateral confinement region for various loading directions. Based on these results, a design method to determine the lateral confinement region of irregular walls was suggested.