The distinction between ductile and limited-ductility reinforced concrete frames lies at the heart of why this line of research matters for earthquake engineering practice. Ductile frames are detailed with close transverse reinforcement spacing, robust confinement of column cores, and capacity-design principles that steer inelastic action into flexural plastic hinges where energy can be dissipated stably through many cycles of deformation. Limited-ductility frames, by contrast, are typically designed and constructed with minimal seismic detailing, often in regions of moderate seismicity or in older building stocks designed under earlier code generations such as the Uniform Building Code provisions referenced in the study. Because their members receive less transverse confinement, these structures are prone to shear-critical behavior, in which diagonal cracking and degradation of the concrete compression zone can precipitate brittle failure before the substantial deformation capacity associated with flexural yielding is ever mobilized. Capturing this behavior in analysis therefore requires models that couple flexural and shear response rather than treating them independently.

The flexure–shear interaction modeling approach adopted in the OpenSees numerical models reflects a significant evolution in nonlinear simulation capability. Earlier generations of frame models relied on lumped or distributed plastic hinges governed purely by moment–rotation relationships, which implicitly assumed that shear strength remained intact throughout the analysis. For shear-critical members, that assumption can be dangerously unconservative, because shear capacity degrades as displacement ductility demand increases, and axial load carrying capacity can follow. Modeling frameworks that track the interaction between flexural yielding and shear degradation, drawing on the deformation capacity formulations developed for members with continuous or lap-spliced reinforcement, allow the analyst to reproduce the gradual loss of lateral strength and the eventual axial failure modes observed in laboratory tests of poorly detailed columns. For limited-ductility frames, this modeling fidelity is not a refinement but a necessity, since the governing failure mechanism is precisely the one such models are designed to capture.

Masonry infill walls introduce a further layer of complexity that has been studied extensively for frames of intermediate and high ductility but far less thoroughly for limited-ductility systems. Infills interact with their surrounding frames through a diagonal compression strut mechanism once the wall-frame interface separates and cracking localizes the contact region. At low levels of ground shaking, infills stiffen the structure substantially, shortening the fundamental period and raising the lateral strength of the building. This stiffening effect can be beneficial, reducing displacement demands, but it also redistributes internal forces, potentially inducing shear failures in columns trapped between stiff walls, a phenomenon widely documented in past earthquake reconnaissance. Equivalent strut macromodels, which represent the infill as one or more diagonal compression elements with nonlinear cyclic behavior, have become the standard practical approach for capturing these effects in frame analysis, and their state of development has been reviewed extensively in the literature on infilled frame modeling.

The finding that infills raise seismic capacity by up to fifty percent at lower performance levels but contribute progressively less near collapse is consistent with the known mechanics of infill behavior. At minor and moderate damage states, the walls act as effective lateral load-resisting elements, sharing demand with the frame and limiting drift. As shaking intensifies, however, the infill panels crack, lose their diagonal compression capacity, and may fail in out-of-plane modes or slide along bed joints, effectively removing themselves from the lateral system. What remains is the bare frame, now potentially compromised by the earlier interaction effects, such as local column shear damage caused by the strut action of the walls. Near collapse, therefore, the structure behaves increasingly like a bare frame with accumulated damage, which explains why the beneficial contribution of the infills diminishes precisely where it would matter most for life safety.

Soil–structure interaction adds the third coupled phenomenon examined in the study. When a structure is founded on flexible soil rather than a rigid base, the foundation itself deforms, introducing rocking and translational flexibility that lengthens the effective period of the combined soil-foundation-structure system and adds radiation damping as well as hysteretic energy dissipation in the soil. The classical expectation, rooted in period lengthening, has often been that SSI is beneficial, reducing the spectral accelerations experienced by flexible structures. However, the roughly twenty percent reduction in seismic capacity reported here, together with amplified displacement demands under both mainshock and aftershock excitation, illustrates the counterintuitive side of SSI for this class of buildings. Period lengthening can move a structure closer to a site period where spectral amplification is greater, and the additional flexibility concentrates displacement demand at the foundation level, increasing drifts in the superstructure and potentially aggravating P-delta effects. Nonlinear constitutive models for the soil, including the widely used material formulations for shallow foundation response developed through the Pacific Earthquake Engineering Research Center, allow these effects to be captured rather than idealized away through a fixed-base assumption.

The treatment of mainshock–aftershock sequences addresses a dimension of seismic hazard that conventional design practice largely ignores. Design codes typically assess structures against a single design-level event, yet real earthquake sequences frequently deliver multiple strong shocks, and damaged structures must withstand the aftershocks in a degraded state. Research on inelastic displacement ratios under repeated earthquakes has shown that residual deformations, accumulated damage, and the reduced stiffness of a structure after a mainshock can all conspire to produce aftershock demands that exceed what an intact structure would experience under the same level of shaking. For limited-ductility frames, whose members degrade with each cycle of deformation, the vulnerability to aftershocks is particularly acute because the shear-critical failure modes are progressive in nature: cracking and strength loss from the mainshock lower the threshold at which brittle failure can be triggered by subsequent shaking.

The quantitative relationships reported between mainshock and aftershock demands offer a useful empirical picture of this degraded-state response. On average, roof displacement demands during aftershocks reach approximately half of the corresponding mainshock values, while inter-story drift demands reach sixty-five to seventy-five percent, indicating that the structure does not simply re-experience its mainshock response. The asymmetry between these two ratios is itself informative: roof displacement reflects the global, first-mode-dominated response of the damaged system, whereas inter-story drift is more sensitive to local stiffness concentrations and damage patterns. The observation that several ground-motion sequences produced aftershock drift demands exceeding the corresponding mainshock demands by up to one hundred seven percent is a striking reminder that aftershocks are not merely weaker copies of the mainshock. Depending on the frequency content, duration, and phasing of the aftershock relative to the damaged configuration of the structure, the second shock can be more damaging than the first.

The fragility methodology underlying the study connects these nonlinear response results to probabilistic statements about damage. Incremental Dynamic Analysis scales each of the twenty selected mainshock–aftershock ground-motion sequences across a range of intensities, producing a population of response trajectories from which the probability of exceeding defined damage states can be estimated. Mapping structural response quantities onto the HAZUS damage state definitions provides a standardized vocabulary for damage, aligned with the loss-estimation framework widely used in earthquake consequence modeling. Fragility curves constructed in this way express, for any given ground-motion intensity, the likelihood that a building will reach or exceed each damage state, from minor cracking through complete structural damage. This probabilistic framing, which builds on the performance assessment philosophy articulated in foundational work on seismic performance evaluation, is what allows the relative influence of infills, SSI, and sequence effects to be compared on a common scale.

The choice of two-, and four-story archetype buildings designed under the Uniform Building Code gives the results a concrete anchor in a recognizable class of existing construction. Low- and mid-rise frames of this type are ubiquitous in seismically active regions worldwide, and many were designed before modern capacity-design and ductility detailing requirements took hold. The two-story configuration represents the short-period end of the building population, where infill stiffening effects are proportionally largest and where period lengthening from SSI can shift the response into more demanding spectral regions. The four-story configuration introduces greater flexibility and higher drift demands, making it more sensitive to P-delta amplification and to the concentration of drift in lower stories where infill interaction and foundation flexibility effects converge. Studying both heights therefore brackets a meaningful range of behavior for the limited-ductility building stock.

The implications for performance-based assessment and retrofit are considerable. If fragility estimates for infilled, flexibly based, limited-ductility frames are made using fixed-base bare-frame models, the analysis may simultaneously overestimate capacity by crediting a rigid base that does not exist and underestimate the stiffening benefit of infills at lower damage states, while missing the near-collapse degradation of both. Retrofit strategies that add strength without addressing shear-critical member behavior, or that stiffen the structure without accounting for foundation flexibility, could fail to deliver the intended improvement in reliability. The study’s demonstration that the coupled effects of infills, nonlinear SSI, and aftershock sequences significantly alter the computed vulnerability profile argues for assessment workflows that represent all three phenomena explicitly, particularly for the large inventory of limited-ductility RC buildings whose seismic safety remains governed by brittle failure modes.

More broadly, the work sits within a growing body of research that treats seismic vulnerability as an emergent property of interacting subsystems rather than a property of the frame alone. Recent studies of infilled frames under mainshock–aftershock sequences, of SSI effects on the damage states of infilled structures considering both in-plane and out-of-plane wall behavior, and of modeling uncertainties in residual drift assessment under sequences all point toward the same conclusion: the most consequential aspects of seismic performance arise at the interfaces between structural, non-structural, and geotechnical components. For limited-ductility construction, where the margin between serviceable response and brittle failure is inherently narrow, resolving those interfaces in analysis is not an academic exercise but a prerequisite for credible risk assessment and for prioritizing the retrofit investments that follow from it.

Subject of Research: Probabilistic seismic fragility assessment of limited-ductility infilled RC frames with nonlinear soil–structure interaction under mainshock–aftershock sequences

Article Title: Probabilistic seismic fragility assessment of flexibly based infilled limited-ductility RC frames under earthquake sequences

Article References: Probabilistic seismic fragility assessment of flexibly based infilled limited-ductility RC frames under earthquake sequences. (n.d.). https://doi.org/10.1007/s10518-026-02716-1

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02716-1

Keywords: seismic fragility, limited-ductility RC frames, masonry infill walls, soil–structure interaction, mainshock–aftershock sequences, incremental dynamic analysis, OpenSees, HAZUS damage states, flexure–shear interaction, performance-based seismic assessment, earthquake engineering, retrofit

The research team focused on three configurations of a five-storey reinforced concrete building. The first was a regular building standing on flat land, serving as the baseline for comparison. The second was a step-back building, in which floors are progressively shortened on one side as the structure climbs the slope, creating a vertically irregular profile. The third was a step-back setback building, which adds a further geometric complication: not only do floors shorten toward the hill, but columns at the downhill side are of vastly different heights, a configuration common in towns built along Himalayan and other mountain slopes. These three systems allowed the researchers to isolate how each type of geometric irregularity alters seismic demand.

At the heart of the study lies a demanding computational campaign. The authors subjected their building models to nonlinear time-history analyses, meaning the structures were simulated through the full recorded shaking of real earthquakes, with the model free to crack, yield, and redistribute forces just as a real reinforced concrete frame would. Twenty far-field ground motions drawn from the publicly available PEER NGA database supplied the excitation. Each record was applied first in a single horizontal direction and then in both horizontal directions simultaneously, reflecting the reality that earthquakes almost never shake the ground along a single clean axis. The team also varied the record scale factor, systematically scaling the intensity of each motion to probe how demand grows as shaking strengthens.

The headline metric was the maximum inter-storey drift ratio, essentially how far one floor displaces sideways relative to the floor beneath it. Drift is a trusted proxy for structural damage: excessive drift means cracked columns, yielded reinforcement, and, in the worst cases, collapse of individual storeys. When the buildings were shaken in one direction only, the flat-land building produced maximum drift ratios ranging from 0.39 to 1.35 percent across the scaled records. The step-back setback building ranged from 0.48 to 1.67 percent, and the step-back building, the most geometrically compromised of the three, ranged from 0.74 to 2.23 percent, already signaling its heightened vulnerability.

The striking result emerged when shaking was applied in two directions at once. Under bidirectional excitation, the drift demands climbed sharply: the flat-land building’s range rose to 0.70 to 2.13 percent, the step-back setback building to 0.86 to 2.33 percent, and the step-back building to 0.93 to 2.79 percent. In other words, simply accounting for the second horizontal component of ground motion, something many simplified analyses still ignore, pushed every configuration toward substantially larger deformations. The physical explanation lies in bidirectional interaction: as columns yield in one direction, their capacity to resist forces in the perpendicular direction degrades, so the two components of shaking effectively erode each other’s resistance and demand accumulates in ways a unidirectional analysis cannot capture.

But the study did not stop there. The direction in which a ground-motion record is oriented relative to the building, known as the incidence angle, was varied systematically. This matters because earthquake records contain two horizontal components, and rotating the record changes which component aligns with the building’s strong and weak axes. For regular flat-land buildings, this rotational sensitivity exists but is comparatively modest. For the hillside configurations, the effect was dramatic. At critical incidence angles, the normalized drift demand of the hillside buildings reached roughly 2.0 to 2.8 times the response of the flat-land building. The step-back building proved especially direction-sensitive, its worst-case angles exploiting the mismatch between the short, stiff columns on the uphill side and the tall, flexible columns at the bottom of the slope.

To convert these deterministic results into a probabilistic picture of risk, the team turned to incremental dynamic analysis, a method in which each ground motion is scaled through a series of increasing intensities until the structure approaches its limits. From this family of analyses they constructed fragility curves, which express the probability that a building will exceed a given damage state as a function of ground-motion intensity, here measured by peak ground acceleration. The damage thresholds were drawn from the drift limits specified in FEMA 356:2000, the widely used American prestandard for seismic rehabilitation, allowing the curves to distinguish between levels of damage from minor cracking through to more severe structural compromise.

The fragility analysis delivered a nuanced verdict. Across the board, the step-back configuration generally emerged as the most seismically vulnerable of the three, consistent with its larger drift demands under both unidirectional and bidirectional shaking. Yet the researchers found something more unsettling: the relative ranking among the flat-land, step-back, and step-back setback buildings is not fixed. It shifts with the seismic incidence angle. A building that appears safer than its neighbor when the ground motion strikes from one orientation can appear more dangerous when the same record is rotated. For code writers and practitioners, this undermines any single-scenario assessment and suggests that hillside structures should be evaluated across a spectrum of incidence angles rather than at one assumed orientation.

For a rapidly urbanizing mountain world, the implications are hard to ignore. Cities such as those sprawling across the Indian Himalayas, the Andes, the Mediterranean hillsides of Turkey and Italy, and countless other sloping terrains host millions of reinforced concrete frames whose geometry resembles the step-back and step-back setback prototypes studied here. Previous research has already documented elevated collapse risk for hillside buildings in the Himalayan belt, and recent studies have proposed new descriptors of the coupled irregularities these structures exhibit. The present study adds a critical layer: even a thorough fragility assessment may significantly understate risk if it considers only one direction of shaking or one angle of incidence. The authors’ demonstration that record-to-record variability, illustrated by plotting mean responses together with mean plus and minus one standard deviation for selected cases, further complicates the picture, reminding engineers that individual earthquakes can deviate substantially from average predictions. The practical message is sobering but actionable: as hillside construction accelerates, seismic evaluation of buildings on slopes should routinely incorporate bidirectional excitation and sweep across incidence angles, because on sloping ground, the direction an earthquake comes from may matter almost as much as its strength.

Subject of Research: Seismic response and fragility of reinforced concrete hillside buildings under unidirectional and bidirectional ground motions

Article Title: Seismic response of hillside buildings under unidirectional and bidirectional ground motions with varying incidence angles

Article References: Ansari, M. G., Dutta, S. C., & Kumar, S. (2026). Seismic response of hillside buildings under unidirectional and bidirectional ground motions with varying incidence angles. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02720-5

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02720-5

Keywords: hillside buildings, seismic response, bidirectional ground motion, incidence angle, inter-storey drift, fragility analysis, incremental dynamic analysis, nonlinear time-history analysis, vertical irregularity, step-back setback, reinforced concrete frames, earthquake engineering

The research, published in the Bulletin of Earthquake Engineering, was carried out by Mustafa Al-Mashaykhi and Pathmanathan Rajeev of Swinburne University of Technology in Melbourne, together with Kushan Wijesundara and Sahan Bandara of the University of Peradeniya in Sri Lanka. Their focus was a workhorse of steel construction: the concentrically braced frame, or CBF, a structural system in which diagonal steel braces carry lateral forces through axial tension and compression. CBFs are popular because they are stiff, economical and efficient at controlling drift, yet their braces are notoriously vulnerable to the kind of repeated inelastic cycling that long-duration earthquakes deliver.

The problem with drift-based assessment, the authors explain, is that it captures the peak deformation a frame experiences but says nothing about how that deformation was reached or how many times the structure yielded along the way. When a brace buckles in compression and stretches back in tension, cycle after cycle, the steel accumulates low-cycle fatigue damage even if the maximum drift never changes. Two ground motions can produce identical peak drifts while leaving the structure in radically different states — one largely intact, the other with fractured braces and a frame on the verge of collapse. Classic post-earthquake surveys, including studies of the 1994 Northridge and 1995 Hyogo-ken Nanbu earthquakes, documented exactly this kind of cumulative damage in steel buildings, motivating engineers to look beyond single-deformation measures.

To capture that cumulative effect, the team turned to an energy-based damage index, building on earlier work by Rajeev and Wijesundara that applied the continuous wavelet transform to steel braced frames. The wavelet approach, rooted in mathematical techniques first formalised by Dennis Gabor in 1946 and developed extensively since, allows the seismic energy absorbed by the structure to be computed as a function of time. Instead of treating the ground motion as a single peak event, the method decomposes the response record into localised wave components, making it possible to track how hysteretic energy — the energy dissipated through plastic deformation — accumulates over the full duration of shaking. The resulting damage index blends the maximum deformation demand with the total dissipated energy, echoing the influential Park and Ang damage model of 1985 and later energy-based formulations by researchers such as Bojorquez, Teran-Gilmore and Fajfar, who explicitly linked ductility demands to low-cycle fatigue.

The mechanics of the study were ambitious in scale. The team designed eight different CBF steel structures, varying both building height and bracing configuration — the geometric arrangements of diagonals, such as diagonal and chevron patterns, that determine how forces flow through the frame. Each frame was then subjected to thirty different ground motion records spanning a wide range of seismic intensities and hazard levels, drawn from European strong-motion databases. Using the open-source simulation platform OpenSees, with brace models capable of capturing cyclic inelastic buckling as characterised by Uriz, Filippou and Mahin, the researchers ran Incremental Dynamic Analysis, the method introduced by Vamvatsikos and Cornell in 2002, in which each record is scaled to progressively higher intensities until the structure reaches defined limit states.

From this mountain of nonlinear simulations, the team built probabilistic seismic demand models — statistical relationships between ground-motion intensity and structural demand — for both the conventional drift measure and the new energy-based damage index. These models then fed into fragility curves, the standard tool of seismic risk assessment, which express the probability that a structure will exceed a given damage state at a given level of shaking. Fragility curves are what engineers and insurers ultimately rely on to estimate expected losses and to decide where retrofit money should be spent, so their statistical quality matters enormously.

The headline finding is that the energy-based approach produces fragility estimates with lower statistical dispersion. In practical terms, the record-to-record variability — the scatter that arises when different ground motions, even at the same intensity, push a structure to very different levels of damage — is reduced when damage is measured by accumulated hysteretic energy rather than peak drift. Lower dispersion means tighter, more confident fragility curves, and tighter curves mean more reliable estimates of collapse risk. For a profession that must make safety decisions under deep uncertainty, reducing this scatter is not a cosmetic improvement; it directly changes the width of the confidence intervals within which real buildings are judged safe or unsafe.

Equally significant is where the two approaches agree and where they diverge. At lower damage levels, corresponding to the Immediate Occupancy performance state — the condition in which a building remains safe to enter and use immediately after a quake — the drift-based and energy-based methods yield broadly comparable fragility predictions. This makes sense: at light damage, the structure has barely yielded, little hysteretic energy has been dissipated, and peak deformation is a reasonable proxy for the state of the frame. But at the high damage end, at the Collapse Prevention limit state, where the structure is barely standing and braces have undergone many large inelastic cycles, the two methods diverge substantially. The energy-based index, sensitive to those cumulative inelastic hysteretic cycles, paints a different — and, the authors argue, more physically faithful — picture of how close a CBF really is to collapse under severe, long-duration shaking.

The implications reach beyond braced steel frames. Energy-based damage indices have been gaining traction across earthquake engineering, with recent work applying energy concepts to mainshock-damaged buildings, to integrated displacement-and-energy design procedures, and to the plastic design of steel moment frames that explicitly account for cumulative damage. The new study adds an important probabilistic leg to that edifice: it demonstrates, quantitatively and across a family of real frame designs, that an energy-based damage measure delivers a more stable estimate of seismic performance precisely where it matters most — near collapse. For code writers, that suggests energy demands deserve a place alongside drift limits in performance-based design frameworks, particularly for structural systems whose components, like steel braces, degrade through repeated buckling and yielding.

There are, of course, practical hurdles before energy-based fragility becomes routine practice. Computing hysteretic energy requires nonlinear time-history analysis and the wavelet-based signal processing that underpins the damage index, tools that are more demanding than the elastic drift checks familiar from everyday design offices. The authors note that all data, models and code supporting the study are available from the corresponding author on reasonable request, which should help other researchers replicate and extend the method to other structural systems, longer-duration motions and site classes not covered here. But the direction of travel is clear. As earthquake engineers push toward more reliable, performance-based assessment of the buildings people depend on, measuring the total energy a quake pumps into a structure — not just how far it sways — may become the difference between a fragility curve that looks reassuring and one that tells the truth about collapse risk.

Subject of Research: Energy-based seismic fragility assessment of concentrically braced steel frames

Article Title: Energy-based seismic fragility for concentrically braced steel frames

Article References: Energy-based seismic fragility for concentrically braced steel frames. (n.d.). https://doi.org/10.1007/s10518-026-02717-0

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02717-0

Keywords: seismic fragility, concentrically braced frames, energy-based damage index, inter-storey drift, incremental dynamic analysis, wavelet transform, hysteretic energy, collapse prevention, probabilistic seismic demand, steel structures, low-cycle fatigue, earthquake engineering

The research, led by Azlan Adnan of Universiti Teknologi Malaysia together with Yuyun Tajunnisa and colleagues at Institut Teknologi Sepuluh Nopember in Surabaya, Indonesia, combines full-scale laboratory testing with advanced computer simulation to trace the entire chain of seismic damage, from the first hairline crack in a joint to the collapse of an entire frame. The work was funded by the Indonesian Endowment Fund for Education under the EQUITY Program and supported by Malaysia’s Public Works Department, which supplied technical data for the connection details studied.

At the heart of the study is a deceptively simple structural element. In many precast industrial buildings, particularly in Southeast Asia, beams are not cast monolithically with columns. Instead, they are supported on nibs, short cantilevered projections that carry the beam’s reaction through a combination of bearing pressure, dowel action of any projecting reinforcement, and friction. This arrangement speeds up construction enormously, but it also creates a discontinuity in the load path precisely where seismic forces concentrate. When a frame sways during an earthquake, the beam-to-column interface must transfer alternating moments and shears through a joint that was never designed as a continuous monolithic element.

To understand how such a joint actually deforms, the team built a full-scale specimen of a nib connection and subjected it to cyclic loading, the laboratory equivalent of the back-and-forth shaking an earthquake delivers. The test followed internationally recognized protocols for cyclic seismic testing of components, applying progressively larger displacement cycles while measuring the force resisted by the joint. The resulting load-deformation loops, known as hysteresis curves, revealed how stiffness degrades and energy is dissipated as cracking, bond slip, and concrete crushing accumulate at the nib. These experimental curves are the gold standard for characterizing connection behavior, but they describe only a single joint, not a whole building.

That is where the hybrid modeling strategy of the study becomes significant. The researchers first constructed a detailed three-dimensional nonlinear finite element model of the tested joint in ABAQUS, using plastic-damage constitutive models for concrete that capture the material’s degradation under cyclic loading. By calibrating this micro-scale model against the full-scale test data, they ensured the simulation reproduced the real joint’s stiffness, strength, and hysteretic shape. From the validated model, they extracted a moment-rotation relationship for the connection, a curve describing how much rotational resistance the joint provides as it is progressively damaged. Because this curve was synthesized from both experiment and simulation, the authors describe it as a hybrid moment-rotation curve, and it forms the critical bridge between component-level physics and building-level analysis.

The hybrid curve was then imported into SAP2000, a structural analysis platform widely used in engineering practice, as a custom nonlinear hinge placed at each beam-to-column interface of a complete precast frame. This allowed the global model to reproduce the complex, pinching hysteretic behavior observed in the laboratory rather than assuming an idealized elastic or perfectly plastic joint. With the frame model in place, the team ran two complementary forms of nonlinear analysis. The first, nonlinear static pushover analysis, pushes the structure laterally with a monotonically increasing force pattern to identify the sequence of damage states and the governing collapse mechanism. The second, incremental dynamic analysis, subjects the frame to a suite of earthquake ground motion records scaled to ever-higher intensities, tracking how drift demands and damage accumulate as shaking intensifies.

The output of these analyses is a set of fragility curves, probabilistic functions that express the likelihood of reaching a defined damage state at a given level of ground shaking, here quantified by peak ground acceleration. Fragility curves are the workhorses of modern seismic risk assessment: they allow engineers, insurers, and code writers to translate a hazard forecast into an expected probability of damage. For the precast frame with nib connections resting on rock sites, the results are encouraging. The probability of first cracking, corresponding to the Immediate Occupancy performance level at which a building remains safe to use without significant repair, begins to rise at a peak ground acceleration of about 0.20 g. More strikingly, the probability of reaching the Collapse Prevention threshold, the point beyond which the structure is on the verge of partial or total collapse, remains a negligible one percent even at 0.30 g, a level of shaking that would be destructive in many regions.

Those favorable numbers, however, come with an important caveat that the authors emphasize through parametric variation. When the same frame is assumed to stand on softer soils, local site amplification magnifies the ground motion transmitted to the structure, and the damage probabilities climb substantially. The finding underscores that soil-structure interaction is not a secondary refinement but a first-order control on seismic risk for precast buildings. Two identical frames on the same street can face materially different fragilities depending on what lies beneath their foundations, a fact with direct consequences for site selection, foundation design, and retrofit prioritization in moderate-seismicity zones.

The study’s broader significance lies in its methodology as much as its results. In regions of low to moderate seismicity, such as much of Malaysia and Indonesia outside the great subduction zones, large destructive earthquakes are rare enough that extensive experimental databases do not exist, yet frequent enough that vulnerability assessment matters for schools, hospitals, and industrial facilities built with precast technology. By anchoring a numerical model to a full-scale test and then propagating that validated connection behavior through pushover and incremental dynamic analyses to fragility curves, the researchers have demonstrated a transferable framework. Other connection types, from dapped-end beams to grouted duct and post-tensioned joints, could be assessed with the same pipeline, filling a persistent gap between idealized code assumptions and the idiosyncratic behavior of real precast interfaces.

For the precast industry, the message is cautiously optimistic. Nib connections, when properly detailed and founded on competent ground, appear capable of delivering adequate seismic safety in moderate seismic zones, with cracking initiating only at shaking levels that would already alarm occupants and collapse probabilities remaining vanishingly small at 0.30 g on rock. But the same analysis shows that the margin evaporates as soil softens, and that the connection, not the member, is the component that governs the whole frame’s fate. As cities across Southeast Asia and beyond continue to expand with factory-built construction, studies of this kind provide the quantitative evidence base that codes and practitioners need to ensure that speed of assembly never comes at the cost of resilience when the ground finally moves.

Subject of Research: Seismic fragility assessment of precast concrete frames with nib beam-to-column connections

Article Title: Seismic performance and fragility analysis of precast concrete frames with nib connections

Article References: Adnan, A., Tajunnisa, Y., Ramli, M. Z., Falahi Abdul Halim, N. H., Ab Kadir, M. A., Nurhadi, H., Yudoprasetyo, K., & Hariyanto, I. R. (2026). Seismic performance and fragility analysis of precast concrete frames with nib connections. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02700-9

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02700-9

Keywords: precast concrete, nib connections, seismic fragility, cyclic testing, incremental dynamic analysis, pushover analysis, soil-structure interaction, finite element modeling, earthquake engineering, beam-to-column joints, moment-rotation curves, ABAQUS

The research, carried out by Vahid Mohsenian of the University of Science and Culture in Tehran and Luigi Di-Sarno of the University of Naples Federico II, is the first life-cycle cost analysis ever performed for this structural system. Life-cycle cost analysis, or LCC, is an economic evaluation framework that adds up every cost a building will generate over its entire service life: the initial construction price, plus the expected losses from earthquake damage, injuries, fatalities, relocation, lost rental income, and downtime, all discounted to present value. Instead of asking only whether a building prevents collapse under the design earthquake, LCC asks a harder question—which design choice minimizes the total bill that nature and time will present over decades?

The tunnel-form system itself is a product of industrialized construction. Derived from load-bearing wall construction, it involves casting both the walls and the floor slabs of a building simultaneously, in a single pour, using reusable steel tunnel-shaped formwork. The technique offers shorter construction times, lower costs, better quality control, and improved worker safety, which explains its popularity in large-scale housing projects. Structurally, the result is a box-like assembly of cross walls and slabs with considerable inherent lateral strength and stiffness. Field observations after recent strong earthquakes, including the devastating 2023 Kahramanmaraş earthquake sequence in Turkey, have repeatedly described the performance of tunnel-form buildings as exemplary, yet the system still has no dedicated seismic design guideline of its own and is treated in codes as a subset of the conventional reinforced concrete load-bearing wall system.

That classification matters, because the two systems behave differently, and because the latest edition of the Iranian Seismic Design Code mandates special ductility detailing for reinforced concrete shear walls. Meeting those requirements in tunnel-form construction is practically difficult given the constraints of the casting process, and many existing tunnel-form buildings simply do not comply. This raises a pressing question: are the special ductility provisions actually necessary for this system, or are they a conservative inheritance from a different structural family? Answering that question was one of the study’s central objectives.

To do so, the researchers designed two residential tunnel-form buildings—five and ten stories tall—for a highly seismic site with a design peak ground acceleration of 0.35 g and stiff soil conditions. Crucially, the walls were designed with only intermediate ductility provisions, deliberately omitting the special ductility requirements. The coupling beams connecting the walls were modeled as hybrid elements equipped with replaceable steel seismic fuses, a configuration intended to concentrate damage in sacrificial components. The buildings were then subjected to incremental dynamic analysis using twenty far-field ground motion records selected from the PEER database, scaled in steps of 0.05 g, with peak ground acceleration as the intensity measure and both horizontal components applied simultaneously.

The structural results were striking. The average peak ground acceleration corresponding to the onset of slight damage in the structural walls came out at approximately 0.6 g—well above both the design-basis earthquake of 0.35 g and the maximum-considered earthquake of 0.52 g. In probabilistic terms, the chance of the walls even reaching the slight damage state under the design-basis earthquake was estimated at less than ten percent, and the same held true for the moderate damage state under the maximum-considered earthquake. In other words, even without special ductility detailing, the tunnel-form walls showed such high strength and stiffness that the code’s special ductility mandate appears, in the authors’ assessment, to be conservative for this system.

But strength alone does not settle the economic question, and this is where the study’s accounting framework becomes important. The researchers divided the buildings into three component categories: structural elements, acceleration-sensitive non-structural components (things like ceilings, mechanical equipment, and piping that break when shaken), and displacement-sensitive non-structural components (partitions, facades, and finishes that crack when the building sways). For each category and each of four damage states—slight, moderate, extensive, and complete—they combined fragility curves derived from the dynamic analyses with the site hazard curve to compute the probability of exceeding each state over the building’s life. Those probabilities were then multiplied by detailed unit costs drawn from Iranian regulations and surveys: replacement costs per square meter, content costs of roughly 1,600 dollars per square meter, temporary accommodation at 20 dollars per person per day, lost rental income, and legal compensation values for minor injuries, major injuries, and fatalities.

The headline economic finding is remarkably consistent. Regardless of building height, and regardless of whether the losses originated from structural or non-structural damage, the total expected earthquake-induced loss for the tunnel-form system remained below one percent of the initial construction cost. For the five-story building, the expected losses attributable to structural, acceleration-sensitive non-structural, and displacement-sensitive non-structural components amounted to just 0.59, 0.94, and 0.55 percent of construction cost respectively; for the ten-story building, the figures were 0.46, 0.51, and 0.30 percent. Perhaps the most consequential detail, however, is which components dominate the bill: acceleration-sensitive non-structural elements generated the largest share of losses in both buildings, a result consistent with a growing body of research showing that in well-designed structures it is the contents and equipment, not the frame, that drive earthquake economics.

Height mattered in a subtler way. When structural components were treated as the primary source of injury, fatality, and downtime losses, the probable loss in the shorter building was roughly 48 percent lower than in the taller one; for non-structural components the gap was about 26 percent. The explanation lies in code mechanics: for shorter buildings, minimum code requirements tend to govern the design of structural elements, inflating their overstrength and seismic capacity well beyond what taller, more demand-driven designs achieve. The practical implication is that shorter tunnel-form buildings are consistently safer and cheaper over their lifetimes—a finding the authors suggest could guide developers, regulators, and insurance companies in selecting optimal building heights for industrialized housing programs.

The study’s implications reach beyond the tunnel-form system itself. Because no current seismic design guideline incorporates life-cycle cost assessment, the authors argue that codes should evolve in one of two directions: either by embedding LCC analysis directly into the design process, or by steering designers toward structural systems that inherently minimize lifetime losses—which their results suggest tunnel-form construction does. They also recommend that the system be recognized as an independent structural category with its own seismic parameters, and they caution that their conclusions are bounded by the models and assumptions used: near-fault motions, vertical ground motion components, aftershocks, soil-structure interaction, corrosion, aging, and multi-hazard scenarios all remain open questions. Still, for a construction technique already favored for its speed and economy, the new analysis adds a powerful third argument. In the world’s most earthquake-prone regions, the humble tunnel of cast concrete may be one of the cheapest insurance policies a city can build.

Subject of Research: Life-cycle cost and seismic performance assessment of concrete tunnel-form building systems in earthquake-prone regions

Article Title: Tunnel-form system as an ideal solution for seismic regions: a numerical study based on life-cycle cost analysis

Article References: Mohsenian, V., & Di-Sarno, L. (2026). Tunnel-form system as an ideal solution for seismic regions: a numerical study based on life-cycle cost analysis. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02632-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02632-4

Keywords: tunnel-form system, life-cycle cost analysis, earthquake engineering, seismic fragility, incremental dynamic analysis, shear walls, non-structural components, ductility provisions, seismic design codes, downtime, reinforced concrete, seismic hazard