The work arrives at a pivotal moment. The second generation of Eurocode 8, the European seismic design standard, is shifting masonry verification toward displacement-based methods. In this framework, engineers no longer simply check whether wall stresses stay below allowable limits; instead they build a capacity curve for the entire building, tracing how lateral force evolves with increasing roof displacement. The accuracy of that curve depends directly on how well the drift capacity of each individual shear wall is known. Yet the codified approaches available until now assign drift limits according to predefined failure modes, separating walls into shear-dominated or flexure-dominated categories, a distinction that laboratory evidence increasingly shows to be artificial.

Experiments over the past two decades have demonstrated that drift capacity is governed by the coupled effects of axial load ratio, wall slenderness and the distribution of bending moment along the wall height. These factors interact continuously, producing gradual transitions between shear, hybrid and flexural response mechanisms. When a design code forces every wall into one of two boxes, the predicted drift capacity can jump discontinuously as a wall crosses an arbitrary classification boundary, even though the physical behavior changes smoothly. The Aachen team set out to replace this step-function logic with a mechanically transparent formulation that reflects the true, continuous nature of the transition.

The foundation of the study is a newly compiled European database of in-plane cyclic shear wall tests, the Modern Masonry Shear-wall DataBase, or MoMaS-DB. It builds on earlier datasets published by researchers at EPFL and the University of Pavia, which together supplied 225 tests used to derive the drift limits in the second-generation Eurocode, and expands them with additional campaigns on vertically perforated clay units, calcium silicate units, lightweight aggregate concrete units and concrete blocks. In total, the database contains 353 cyclic shear wall tests spanning hollow clay, solid clay, calcium silicate bricks and blocks, autoclaved aerated concrete, lightweight aggregate concrete and concrete block masonry, with thin-layer mortar emerging as the predominant joint type. Each test record captures material properties, wall geometry, loading conditions, observed failure modes and the key points of the experimental load-displacement curve.

Because even a database of this size cannot fill the entire parameter space uniformly, the researchers complemented the experiments with an extensive numerical simulation campaign. They built a simplified micro-model of a hollow clay masonry wall in LS-DYNA, representing the units as nonlinear solid elements and the mortar joints as contact interfaces governed by a stress-based failure criterion with cohesive softening. The model was calibrated against small-scale component tests and validated against 13 wall tests drawn from the database, reproducing maximum horizontal forces with good agreement and capturing the governing failure modes reliably. With the validated model in hand, the team systematically varied wall length from 1.00 to 4.00 meters, axial load ratio from 3 to 30 percent of the characteristic masonry compressive strength, and moment distribution factor from 0.5 to 1.5, generating 300 simulation variants that would have been prohibitively expensive to test physically.

From this simulation matrix, the researchers distilled the study’s central innovation: a normalized shear slenderness parameter that combines wall geometry, boundary conditions and vertical load level into a single quantity. Low values of this parameter correspond to heavily compressed, squat walls with restrained rotations, where shear-controlled behavior dominates and drift capacity is limited. High values indicate slender walls under low axial load, where rocking and flexural deformation become significant and drift capacity grows. Plotting the simulation results against this parameter revealed a clear physical picture: below a critical value of roughly 5, the response is shear-dominated; beyond it, hybrid mechanisms emerge and eventually give way to flexural failure, while the load-bearing capacity asymptotically approaches the theoretical rocking limit derived from rigid-body equilibrium.

The proposed drift capacity formulation follows directly from this observation. It consists of a constant range for shear-dominated behavior, where the drift at the Significant Damage limit state takes a fixed value, and a linear range with a calibrated slope that captures the progressive increase in drift capacity as hybrid and flexural mechanisms take over. The slope was determined through an unbiased lognormal regression procedure in linear space, minimizing the deviation of the mean predicted-to-observed ratio from unity without imposing penalty terms or artificial regularization on the scatter. This purely data-driven calibration yielded a slope of 0.053 for the simulation data, and the same procedure was subsequently applied to the filtered experimental database, which retained 210 representative tests after excluding short piers, walls with drift capacities above 2 percent associated with rocking, eccentrically loaded specimens and pseudo-dynamic tests.

The comparison with the failure-mode-dependent procedure of the draft Eurocode 8 part 1-2 is revealing. For the simulated hollow clay walls, the new approach reduced the mean absolute error at the Significant Damage limit state from 0.36 to 0.17, halving the scatter while eliminating the systematic overestimation of drift capacity for flexural and hybrid failure modes. On the experimental data, the two methods performed more comparably once the standard’s conservative upper drift limits were applied, but the new approach achieved this accuracy without needing empirical caps, because it explicitly accounts for the parameters that drive the response. Selected wall tests underscored the difference: for four-meter-long hollow clay walls, the normative approach overestimated drift capacities by a factor of roughly 2.2 to 3.0, while the new approach overestimated them by only 1.2 to 1.6, largely because the normative method neglects wall slenderness for realistic moment distributions that often exceed unity in actual buildings.

The practical significance of the work was tested at the building scale. Applying both approaches to a three-storey masonry building with autoclaved aerated concrete walls showed that the failure-mode-dependent method produced drift capacities strongly constrained by its predefined limits, with the flexural cap of 0.6 percent governing most shear-failing walls. The new approach, by contrast, captured the higher drift capacity of short walls and walls under low axial stress, following established mechanical principles rather than empirical restrictions. The method has already been incorporated as an alternative procedure in the final draft of FprEN 1998-1-2, giving European designers a validated option for deformation-based masonry design, with partial safety factors derived from the statistical scatter of the database to maintain reliability at the structural level.

The authors are candid about the limitations that remain. The experimental database is unevenly distributed across wall slenderness, moment distribution factors and masonry types, so the linear part of the formulation partly relies on extrapolation at high normalized shear slenderness values, and some masonry types still lack sufficient tests for material-specific calibration. Future work, they note, should expand the database with additional cyclic tests, particularly for large slenderness values, moment distribution factors greater than one, and underrepresented materials, and should address hybrid failure modes, cyclic degradation and wall-to-slab interaction at the building level. Even so, the study marks a decisive step away from the brittle logic of failure-mode classification toward a continuous, physics-based description of how masonry walls deform, promising more realistic capacity curves and, ultimately, safer masonry buildings in earthquake-prone regions.

Subject of Research: Seismic drift capacity estimation of unreinforced masonry shear walls using a normalized shear slenderness parameter

Article Title: A novel approach for the estimation of seismic drift capacity in unreinforced masonry (URM) shear walls based on governing structural parameters

Article References: Mukattash, N., Butenweg, C., Kubalski, T., & Klinkel, S. (2026). A novel approach for the estimation of seismic drift capacity in unreinforced masonry (URM) shear walls based on governing structural parameters. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02697-1

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02697-1

Keywords: unreinforced masonry, shear walls, drift capacity, Eurocode 8, displacement-based design, normalized shear slenderness, axial load ratio, moment distribution, cyclic shear wall tests, MoMaS-DB, nonlinear simulation, earthquake engineering

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

The study, published in Case Studies in Construction Materials, was led by Yi Ding, with co-authors Xinru Wang, Man Xu, and Huiwen Tian. Their work addresses a deceptively simple question: how do you calculate the shear strength of a squat wall when part of it is made not of ordinary concrete, but of ultra-high performance concrete, or UHPC — a fiber-reinforced cementitious material that can reach compressive strengths above 130 megapascals, roughly double or triple that of conventional structural concrete? The answer, they found, requires rewriting one of the key equations that engineers use to size these walls, because the new material fundamentally changes how forces flow through the structure.

UHPC is not just stronger concrete in a loose sense. It is engineered through optimized particle packing — arranging fine sand, cement, and admixtures so that almost every void is filled — and through the addition of steel fibers that bridge cracks and hold the material together after it fractures. In the team’s earlier laboratory tests, shear walls in which UHPC was cast only in the boundary elements, the heavily loaded vertical zones at the edges of the wall, showed remarkable improvements. Compared with identical walls made entirely of normal-strength concrete, the UHPC-reinforced versions carried 6.79 to 20.64 percent higher peak loads, showed 2.54 to 24.11 percent greater ductility, and dissipated 44.88 to 57.65 percent more cumulative energy under simulated earthquake loading. Just as striking, the steel fibers restrained crack growth so effectively that the walls avoided the catastrophic spalling, bar buckling, and longitudinal bar fracture that rendered the conventional walls essentially unrepairable after testing.

But predicting how strong such a hybrid wall should be turned out to be a genuine theoretical challenge. The researchers first benchmarked three established calculation methods against their experimental data from thirteen full-scale wall specimens, which included rectangular and barbell-shaped cross-sections, UHPC distributed at various heights and widths, and axial load ratios of 0.25 and 0.45. The Chinese standard JGJ 3-2010 overestimated strength by an average factor of 1.51, the American standard ACI 318-19 by 1.56, and even the best-performing option — the softened strut-and-tie model developed by Hwang and colleagues — overshot by 1.38 on average. None of these methods was designed with a material as strong and as crack-resistant as UHPC in mind, and none accounted for how locally concentrating that material in the boundary elements redistributes the internal load paths.

To understand the failure mechanism directly, the team built highly refined three-dimensional finite element models in the LS-DYNA simulation environment. Concrete and UHPC were represented with the Karagozian and Case material model, a sophisticated constitutive framework that captures strength surfaces, damage evolution, and failure, with parameters specially calibrated for UHPC’s much higher strength. Steel reinforcement was modeled explicitly with beam elements, and the interface between UHPC and normal concrete was treated with shared nodes, reflecting the experimental observation that the two materials remained fully bonded without separation. After validating the modeling approach against independent low-rise wall tests by other researchers — achieving peak load prediction errors within 5 percent — the team turned the simulation loose on squat walls with an aspect ratio of 1, the regime where shear dominates.

The simulations revealed the failure sequence in vivid detail. At first cracking, tiny macroscopic fractures appeared precisely at the horizontal interface where the UHPC boundary element meets the normal-strength concrete web, marking the point where tensile strain first exceeded the material’s limit. As lateral drift increased, the outermost longitudinal bars in the boundary elements reached their yield stress — around 509 megapascals on the tension side and 501 on the compression side — while diagonal cracks spread through the web. By peak load, these diagonal cracks had carved the web into a lattice of small zones, and the concrete between them consolidated into a set of diagonal compression struts. The wall ultimately failed by crushing along its principal diagonal strut, exactly the mechanism that the strut-and-tie theory describes: steel bars acting as tension ties, concrete acting as compression struts, and the two working together in a self-equilibrating truss embedded within the wall.

That confirmation opened the door to the model’s central innovation. In the classical strut-and-tie framework, the depth of the diagonal compression strut is tied to the height of the compression zone at the base of the wall, and existing formulas for that height were derived from elastic theory for ordinary reinforced concrete columns. The researchers found that casting UHPC into the boundary elements shifts the compression zone significantly — but because squat walls are disturbed regions where the familiar assumption that plane sections remain plane breaks down, they could not simply adjust the flexural theory. Instead, they ran sixty parametric simulations, sweeping aspect ratios from 0.5 to 1.5, concrete strengths from C30 to C80, UHPC strengths from 120 to 180 megapascals, UHPC widths from 10 to 40 percent of the wall width, UHPC heights from 30 to 100 percent of the wall height, and axial load ratios from 0.05 to 0.45.

The parametric results distilled into a remarkably clean relationship. The height of the compression zone increases linearly with the normalized axial load — heavier vertical compression pushes the crushing zone deeper into the wall — but decreases with the width of the UHPC region following a power law, because the stronger material concentrates compression into a narrower band. The team encoded both effects in a single equation, bounded by sensible limits, and embedded it in a five-step design procedure that retains the familiar machinery of the softened strut-and-tie model: determine the geometry, select the dominant tie mechanism based on the strut’s inclination angle, compute the strut-and-tie coefficient that captures how distributed reinforcement enhances capacity, evaluate the softened concrete strength along the strut, and finally project the diagonal crushing capacity into the horizontal shear strength. Notably, the team deliberately kept the concrete softening coefficient conservative rather than increasing it for UHPC, even though studies of full-section UHPC walls suggest the material’s tensile toughness could justify a higher value — a choice made to avoid overestimating strength in a safety-critical application.

The validation was the most convincing part. Across sixty-eight walls — fifty-nine from the new simulations and nine from independent experimental programs by three other research groups — the proposed model achieved an average predicted-to-actual strength ratio of 1.03 with a standard deviation of 0.20. The existing methods fared considerably worse: JGJ 3-2010 averaged 1.19 with a standard deviation of 0.27, ACI 318-19 averaged 1.11 with 0.31, and the original Hwang model averaged 1.14 with 0.22. In other words, the new model is both nearly unbiased and the most consistent, which is precisely the combination a design engineer wants. The model did show characteristic blind spots: it underestimated precast walls in which UHPC also filled lap-splice zones and vertical columns, because those extra contributions lie outside its scope, and it overestimated two specimens that failed not by strut crushing but by fracture of longitudinal bars at a weak post-cast interface — a different failure mode entirely.

The authors are candid about these limits, noting that the model applies to cast-in-place walls where UHPC sits primarily in the boundary elements and works in reliable composite action with the surrounding normal concrete, and that further validation is needed for other UHPC configurations and interface conditions. Even so, the significance is hard to overstate. Squat shear walls are among the least forgiving elements in seismic design, and the ability to exploit UHPC’s extraordinary strength and crack-bridging fibers in exactly the zones where they matter most — without paying the cost of casting an entire wall in the premium material — could reshape how engineers protect tall buildings in earthquake-prone regions. A formula that turns laboratory performance into trustworthy numbers is the bridge between a promising material and a safer built environment, and this study has just laid a carefully calibrated plank across it.

Subject of Research: Shear strength prediction of squat UHPC-reinforced concrete shear walls under seismic loading

Article Title: A strut-and-tie model for predicting shear strength of squat shear walls reinforced with UHPC under seismic loading

Article References: Ding, Y., Wang, X., Xu, M., & Tian, H. (2026). A strut-and-tie model for predicting shear strength of squat shear walls reinforced with UHPC under seismic loading. Case Studies in Construction Materials, 25, Article e06575. https://doi.org/10.1016/j.cscm.2026.e06575

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06575

Keywords: UHPC, shear walls, strut-and-tie model, seismic design, finite element analysis, squat walls, concrete structures, earthquake engineering, boundary elements, shear strength, structural engineering, LS-DYNA