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CONTENTS
Volume 31, Number 5, November 2026
 

Abstract
In nonlinear response history analysis, Rayleigh damping is calibrated by enforcing a target damping ratio at two anchor frequencies. The first anchor period (T1) is conventionally set to the fundamental period, but the second (T2) is assigned without a consistent, physically grounded criterion—most commonly defaulted to the second-mode period (T2/T1≈0.38)—and its influence on seismic demands has not been systematically quantified. This paper presents a parametric sensitivity study of nonlinear response to T2 using the SAC 9-story Los Angeles pre-Northridge steel moment frame under 20 SAC LA 10/50 ground motions. A three-tier framework—nonlinear (Tier A), linear direct-integration (Tier B), and linear modal (Tier C) analysis—decomposes the total discrepancy into a pure damping-formulation effect (Tier B-C) and a nonlinearity-damping interaction effect (Tier A-B), across five T2/T1 ratios (0.10-0.50) and 220 cases. Peak floor acceleration (PFA) is highly sensitive to T2, varying by 41%; this arises almost entirely from the damping-formulation effect—progressive overdamping of higher modes—and is largely independent of nonlinearity. Maximum inter-story drift varies by only about 4%, its ~13% deviation from the constant-damping reference being governed by the interaction effect and independent of T2. A mass-participation criterion is proposed: T2 should keep every mode within the first 90% of cumulative lateral mass participation underdamped, yielding T2/T1<=0.23. The mechanism is corroborated on a 20-story frame, for which a higher (95%) threshold is appropriate, and the pattern is robust to the target damping ratio (2%, 3%, and 5%).

Key Words
nonlinear time-history analysis; peak floor acceleration; Rayleigh damping; second anchor period; seismic demand sensitivity; steel moment frame

Address
Department of Civil and Environmental Engineering, Kookmin University, Seoul, Korea

Abstract
This study investigates the probabilistic seismic performance of steel moment-resisting frames (MRFs) designed using performance-based plastic design (PBPD) by incorporating both epistemic and aleatory uncertainties. To this end, three target inter-story drift ratios of 1.5%, 2%, and 2.5%, and four archetype steel office MRFs of 4-, 8-, 12-, and 20-stories have been considered. Each archetype frame has been designed using the PBPD method to meet a specific target drift. The probabilistic seismic performance of PBPB-designed MRFs has been evaluated using the mean annual frequency of exceedance framework. Numerical analysis of this study reveals that under deterministic conditions, MRFs show satisfactory performance. However, when uncertainties are included, the results of probabilistic analysis demonstrate that office occupancy MRFs which are designed for target inter-story drift ratios of 1.5% and 2% have acceptable performance in terms of code-specified collapse requirements. However, mid- and high-rise MRFs designed for a target drift ratio of 2.5% have a collapse exceedance probability in 50 years of more than 1% and consequently fall short of fulfilling code requirements regarding collapse annual exceedance rates. It is also observed that incorporating epistemic uncertainty has changed the probabilistic performance of structures considerably in some cases; hence, its effects are non-negligible.

Key Words
mean annual frequency of exceedance; performance-based design; performance-based plastic design; probabilistic seismic performance assessment; steel moment-resisting frames

Address
Department of Civil Engineering, University of Mohaghegh Ardabili, Ardabil, Iran

Abstract
Performance-based earthquake engineering (PBEE) enables precise control over the functionality, damage, and seismic safety of reinforced concrete (RC) buildings. Nonetheless, there is limited practical experience of applying metaheuristic optimisation methods together with nonlinear and damage sensitive performance measurements. This paper proposes a nonlinear performance-based seismic optimization of RC moment-resisting frames based on the nonlinear-static pushover test using a combination of nonlinear time-history analysis (NLTHA) and particle-swarm optimization (PSO). The eight-story RC frame is located in a high-seismicity area and was developed in accordance with IS 456: 2000 and IS 1893 (Part 1): 2016. The demand parameters, such as roof displacement, inter-storey drift ratio, base shear capacity and plastic hinge performance conditions, are considered in accordance with ASCE 41-13, FEMA 440 guidelines. The multi-objective optimisation problem marginally increases the construction cost, structural weight, and reduces drift demand and maximises a nonlinear-response-based Performance Index which is a measure of seismic efficiency at the performance point. The optimisation is realised with the help of an entirely automated Python-based computational platform permitting systematic search over beam-column design options. The optimized design showed a significant decrease in the inter-storey drift, an improved post-analysis hinge distribution consistent with strong-column weak-beam behaviour, and marginal-increase in construction cost without sacrificing seismic safety. The results of NLTHA-validation indicate that the pushover-based optimisation is an effective tool in identifying the major trends of dynamic responses, thereby indicating the effectiveness of the proposed framework as a computationally efficient and cost-effective strategy in the cases of mid-rise RC buildings.

Key Words
particle swarm optimisation; performance based design; pushover analysis; reinforced concrete design; seismic performance; story drift; structural optimisation; time history

Address
Department of Civil Engineering, National Institute of Technology Patna, Bihar, India

Abstract
This study explores the dispersion characteristics of anti-plane shear (SH) waves in a novel dual-layered composite structure composed of two infinite elastic layers having finite but unequal thicknesses, embedded between two semi-infinite isotropic half-spaces. Unlike conventional single-layer configurations, this dual-layered system introduces a new class of stratified media with distinct physical properties. To ensure physical consistency, all interfaces are assumed to be perfectly welded, thereby guaranteeing continuity of both displacements and stresses across the boundaries. To incorporate realistic physical conditions, one of the layers is considered materially heterogeneous, while the other is subjected to a uniform initial stress, enabling the investigation of the combined effects of material inhomogeneity and pre-existing mechanical fields on wave propagation. An analytical approach is employed to derive a closed-form dispersion relation for the system, which is subsequently reduced to several special cases. A comprehensive parametric study is carried out to examine the influence of key parameters such as the heterogeneity gradient, magnitude of initial stress, thicknesses of the individual layers, and the thickness ratio between the two layers. This work not only enhances the understanding of SH wave propagation in complex media but also introduces a new composite model with potential applications in non-destructive testing (NDT), seismology, and geotechnical exploration. Importantly, the study introduces a new class of stratified structural configurations for further theoretical and experimental investigation in the field of wave mechanics and material modeling.

Key Words
heterogeneous material; initial stress; shear horizontal (SH) waves; wave dispersion; welded interface

Address
Vikas Sharma: School of Engineering and IT, Manipal Academy of Higher Education, Dubai, UAE
Shikha Deep: Unitedworld Institute of Technology, Karnavati University, Gandhinagar, Gujarat 382422, India
Gaur Singh: Department of Mathematics, Lovely Professional University, Phagwara, Punjab 144411, India

Abstract
Piloti reinforced concrete (RC) structures are highly vulnerable to soft-story failures during earthquakes. While base isolation using lead-rubber bearings (LRBs) effectively mitigates this risk, prohibitive costs often deter its implementation. This study evaluates the seismic performance of a typical Korean piloti RC structure, investigating the efficacy of full versus theoretically economical selective isolation schemes. Including a conventional fixed-base (CASE-1), full LRB isolation (CASE-2), and two selective isolation designs (CASE-3 and CASE-4), four configurations were comparatively analyzed using quasi-static analysis in SAP2000. Also, including plastic hinge formation, lateral displacement, and energy dissipation, key structural parameters are assessed. Results reveal that selective isolation schemes are inherently detrimental to structural integrity. Rather than achieving mechanical decoupling, partial isolation reduces the active lateral-load-resisting elements, amplifying the shear burden on non-isolated columns. This causes substantially higher base shear, increased cumulative energy dissipation, and premature progression of plastic hinges to the collapse state. Structural performance demonstrates that full isolation is optimal, followed by the fixed-base structure, with selective schemes performing worst. According to the findings, this study proves partial base isolation is mechanically flawed for piloti buildings, emphasizing that full isolation is necessary to ensure seismic resilience.

Key Words
lead-rubber bearing; piloti RC structure; plastic hinge; quasi-static analysis; selective isolation

Address
Mo Shi: School of Economics and Management, Ankang University, 92 Yucai Road, Hanbin, Ankang 725000, People's Republic of China
Yeol Choi: School of Architecture, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea

Abstract
This study evaluates the seismic performance of a hybrid post-tensioned precast concrete system through numerical analysis. A numerical model for the hybrid post-tensioned precast concrete system was developed using OpenSees and validated against test results from previously conducted experiments on hybrid post-tensioned precast concrete frame specimens. Based on the validated numerical model, a parametric study was conducted to comprehensively evaluate the effects of the unbonded reinforcement ratio on the structural behavior of the hybrid post-tensioned precast concrete system. Furthermore, a regression equation was proposed to represent the relationship between the equivalent damping ratio, unbonded reinforcement ratio, and story drift ratio. Considering the hysteretic characteristics of the hybrid post-tensioned precast concrete system, simplified numerical models using the Pinching-4 material in OpenSees were developed and applied to prototypes for nonlinear time history analysis. The seismic performance of the hybrid post-tensioned precast concrete system was evaluated through comparison with that of a conventional reinforced concrete system. The results showed that the hybrid post-tensioned precast concrete system exhibited lower energy dissipation capacity and larger maximum story drift ratios than the reinforced concrete system, while exhibiting smaller residual story drift ratios, consistent with its self-centering characteristics.

Key Words
nonlinear time history analysis; numerical model; OpenSees; post-tensioning method; precast concrete; seismic performance

Address
Jae Hyun Kim, Khaliunaa Darkhanbat, Sang-Hoon Lee: Department of Architectural Engineering, University of Seoul, 163 Siripdae-ro, Dongdaemun-gu, Seoul 02504, Republic of Korea
Jin-Ha Hwang: Jeonnam CCU Center, Korea Conformity Laboratories, 137 Yeosusandan-ro, Yeosu-si, Jeollanam-do 59631, Republic of Korea
Hoseong Jeong: Department of Architectural Engineering, Hankyong National University, 327 Jungang-ro, Anseong-si, Gyeonggi-do 17579, Republic of Korea
Kang Su Kim: Department of Architectural Engineering and the Smart City Interdisciplinary Major Program, University of Seoul, 163 Siripdae-ro, Dongdaemun-gu, Seoul 02504, Republic of Korea

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