The Resonance Trap: Kinematic Collapse of R/C Frames Triggered by Shallow Top-Down Liquefaction in Near-Fault Zones

Abstract

The conventional fixed-base seismic design paradigm suffers from fundamental limitations in predicting the response of structures situated on liquefaction-prone deposits in near-fault zones. Contrary to the common assumption that post-liquefaction soil softening acts beneficially as a natural base isolator, this study reveals a lethal anomaly known as a \"double resonance trap.\" Employing a non-linear coupled Soil-Structure Interaction (SSI) numerical modeling approach—utilizing the Beam on Nonlinear Winkler Foundation (BNWF) framework and the PM4Sand bounding-surface plasticity material model—computational evaluations were conducted on specific stratigraphic profiles subjected to near-fault earthquake excitation. Simulation results demonstrate that the initiation of shallow (top-down) liquefaction at a depth of 5 meters triggers a period elongation phenomenon. Mechanically, this process shifts and concentrates inertial energy into extreme resonance spikes (reaching 7.5g to 9.0g) within the medium-to-long period range (T ? 1.1–1.3 s). Simultaneously, the loss of vertical bearing capacity induces asymmetric settlement, generating massive secondary P-Delta moments. This destructive interaction overwhelms the axial-flexural capacity of column base elements, causes premature plastic hinge failure, and drives the frame structure toward global kinematic collapse (with roof drift exceeding 6.0%). These analytical findings conclusively demonstrate that the generic design spectra of SNI 1726:2019 can lead to fatal under-design, underscoring the urgent need for integrated geotechnical-structural, site-specific design.

Introduction

The text investigates how near-fault earthquakes and soil liquefaction can interact to cause severe structural damage, focusing on the Palu-Koro fault system and the site of RSUD Anutapura. The study challenges the conventional assumption that soil liquefaction may reduce earthquake forces by acting like a natural base isolator.

The research emphasizes two important near-fault effects: forward rupture directivity, which produces strong long-period velocity pulses, and fling-step, which causes permanent ground displacement. These effects can impose large demands on structures, particularly when combined with liquefaction-induced loss of foundation capacity.

Methodology

The study uses a coupled numerical finite-element approach to model soil–structure interaction and structural response. The main components are:

• PM4Sand is used to model the cyclic and liquefaction behavior of sandy soil.

• Direct Simple Shear (DSS) simulations are used to calibrate the liquefiable soil parameters and reproduce the development of excess pore-water pressure.

• A Beam on Nonlinear Winkler Foundation (BNWF) model represents the interaction between the soil and foundation through nonlinear springs for lateral resistance, shaft friction, and end-bearing capacity.

• The reinforced-concrete structure is modeled using fiber-based elements, with nonlinear material models for concrete and reinforcing steel.

• P-Delta effects are included to capture additional moments caused by lateral deformation and differential foundation settlement.

• Near-fault earthquake records are used for nonlinear time-history analysis and scaled to a PGA of 0.4g.

Main Findings

The numerical analysis indicates that shallow, top-down liquefaction is a major factor in the modeled failure mechanism at the RSUD Anutapura site. As liquefaction develops, the soil loses stiffness and bearing capacity, producing significant changes in the site's dynamic

Conclusion

A coupled computational investigation focused on the specific site of Anutapura Regional General Hospital (RSUD Anutapura) has definitively exposed a fatal flaw in the fixed-base seismic design paradigm. The study validates that soil softening resulting from shallow, superficial liquefaction does not always benefit the structure by acting as a natural base isolator; instead, it can manifest as a lethal \"double resonance trap.\" First, a spectral trap is created when the liquefaction of subsurface soil at a depth of 5 meters triggers a period-elongation anomaly, mechanically shifting and focusing near-fault seismic inertial energy into an extreme resonance spike within the medium-to-long period range ( seconds). Second, a gravity trap is simultaneously activated when the loss of vertical load-bearing capacity triggers asymmetric subsidence, generating a massive secondary P-Delta moment that prematurely ruptures the plastic hinges at the base of the columns. This destructive combination of coupled kinematic collapse utterly invalidates the protective capacity of the generic design spectrum envelope in SNI 1726:2019. Conventional probabilistic design is shown to suffer from a fatal degree of under-design, as it fails to anticipate extreme spectral demands and the axial-flexural kinematic effects resulting from geotechnical support deformation. While these findings provide an essential mechanistic synthesis regarding soil-structure interaction anomalies, the evaluation framework in this study is limited to a deterministic assessment using a single peak excitation intensity of 0.4g. As a roadmap for future research, this assessment framework will be upgraded to a probabilistic evaluation through the execution of Incremental Dynamic Analysis (IDA). This iterative dynamic analysis will be used to comprehensively construct seismic fragility curves and to evaluate the effectiveness of numerical retrofitting interventions—such as deep pile foundation substitution—in reducing site-specific structural vulnerability.

References

A coupled computational investigation focused on the specific site of Anutapura Regional General Hospital (RSUD Anutapura) has definitively exposed a fatal flaw in the fixed-base seismic design paradigm. The study validates that soil softening resulting from shallow, superficial liquefaction does not always benefit the structure by acting as a natural base isolator; instead, it can manifest as a lethal \"double resonance trap.\" First, a spectral trap is created when the liquefaction of subsurface soil at a depth of 5 meters triggers a period-elongation anomaly, mechanically shifting and focusing near-fault seismic inertial energy into an extreme resonance spike within the medium-to-long period range ( seconds). Second, a gravity trap is simultaneously activated when the loss of vertical load-bearing capacity triggers asymmetric subsidence, generating a massive secondary P-Delta moment that prematurely ruptures the plastic hinges at the base of the columns. This destructive combination of coupled kinematic collapse utterly invalidates the protective capacity of the generic design spectrum envelope in SNI 1726:2019. Conventional probabilistic design is shown to suffer from a fatal degree of under-design, as it fails to anticipate extreme spectral demands and the axial-flexural kinematic effects resulting from geotechnical support deformation. While these findings provide an essential mechanistic synthesis regarding soil-structure interaction anomalies, the evaluation framework in this study is limited to a deterministic assessment using a single peak excitation intensity of 0.4g. As a roadmap for future research, this assessment framework will be upgraded to a probabilistic evaluation through the execution of Incremental Dynamic Analysis (IDA). This iterative dynamic analysis will be used to comprehensively construct seismic fragility curves and to evaluate the effectiveness of numerical retrofitting interventions—such as deep pile foundation substitution—in reducing site-specific structural vulnerability.

Copyright

Copyright © 2026 Arzal M. Zain, Andi Arham Adam, Anwar Dolu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.