The two materials at the heart of the study could hardly be more different in character. Burnt clay bricks, or BCB, are the conventional workhorse of masonry construction in South Asia and much of the developing world, fired in kilns at high temperature and prized for their compressive strength. Unburnt sandcrete blocks, or USB, made from a sand-cement mix and cured without firing, represent a greener alternative because they avoid the energy intensity and emissions of brick kilns. In many rapidly urbanizing countries, both materials sit side by side in the same neighborhoods, often in the same buildings, yet their influence on how a frame behaves in an earthquake has rarely been compared head to head under controlled laboratory conditions. That gap is precisely what the Bangladeshi team set out to close.

The experimental program involved three small-scale multi-story, multi-bay reinforced concrete frames, each partially infilled with a different masonry unit, subjected to quasi-static cyclic loading that mimics the back-and-forth demands of a strong earthquake. The researchers evaluated seismic performance through hysteretic behavior, the looped force-displacement curves that encode a structure’s strength, stiffness, ductility and capacity to dissipate energy. These four quantities are the currency of earthquake engineering: strength determines the load a frame can carry, stiffness governs how much it sways, ductility measures how far it can deform before collapse, and energy dissipation reflects how effectively the structure absorbs the earthquake’s input rather than passing it on to its occupants.

The headline finding is that the type of infill masonry matters far more than many design codes assume. The frames with burnt clay brick infills delivered the highest lateral load capacity of the three specimens, confirming the material’s reputation for stiffness and strength. But that strength came at a cost. The BCB-infilled frames showed sudden degradation of their initial stiffness and a brittle post-peak response, meaning that once the infill panels began to fail, the loss of strength and stiffness was abrupt rather than gradual. In seismic terms, brittleness is dangerous because it gives occupants and structures little warning and little reserve capacity after the peak demand has passed.

The unburnt sandcrete block infills told a different and arguably more nuanced story. Although the USB-infilled frames reached a slightly lower maximum lateral capacity, they exhibited improved ductility and the highest post-peak cumulative energy dissipation of the studied frames. In other words, the greener material did not simply lose to the conventional one; it traded a modest amount of peak strength for a more forgiving failure mode, deforming further and absorbing more cumulative energy after its peak load had been exceeded. For engineers weighing sustainability against safety, the study suggests that the trade-off is not one-sided, and that softer, weaker infills may actually enhance the survivability of a frame in a severe event, provided the reduced strength is accounted for in design.

Beyond the material comparison, the study examined failure mechanisms in detail and found extensive damage concentrated in the frame members and infill panels near the exterior beam-column joints. This observation matters because exterior joints are precisely the locations where infill frames are most vulnerable in real earthquakes: the panels interact with the surrounding columns and beams, transferring forces that the bare frame was never designed to resist. The concentration of damage near these joints echoes a long-standing concern in the literature, dating back decades to pioneering work on infilled frames, that infills can create unintended load paths, short columns and local failure concentrations that compromise an entire building even when the code-designed frame itself is adequate.

The second half of the study turned from the laboratory to the computer. The team evaluated two available macro-models of masonry infilled reinforced concrete frames, assessing how reliably they could predict the measured seismic performance. Macro-models, typically built from equivalent diagonal struts that stand in for the infill panel, are the workhorses of seismic assessment because modeling every brick individually is computationally prohibitive for whole buildings. The verdict, however, was sobering. Implementing the two macro-models proved difficult, and neither reproduced the experimental behavior with acceptable accuracy. One model depended on substantial empirical calibration, effectively requiring test data to be tuned before it could predict test data, while the other tended to overestimate both stiffness and energy dissipation, painting an optimistic picture that real structures may not match.

These modeling shortcomings have practical consequences. Nonlinear static and dynamic analyses of infilled buildings underpin seismic retrofit decisions, loss estimates and code provisions worldwide, from FEMA guidelines to Eurocode 8 and national codes such as the Bangladesh National Building Code. If a macro-model overestimates energy dissipation, it will underpredict displacements and could lead engineers to conclude a building is safer than it is. If it requires extensive calibration, its predictive value for buildings that have never been tested, which is to say nearly all of them, is limited. The study’s experimental dataset, generated with support from the Housing and Building Research Institute and Chittagong University of Engineering and Technology, therefore serves a dual purpose: it documents how two common infill materials behave, and it provides a benchmark against which future models can be honestly judged.

The broader significance of the work lies in the collision between sustainability goals and seismic safety. As countries seek to phase out energy-hungry brick kilns, unburnt alternatives such as sandcrete blocks are being promoted on environmental grounds, sometimes with little attention to their structural implications. This study suggests that such a transition need not be seismically harmful and could even improve post-peak performance, but it also warns that the two materials produce measurably different frame responses in stiffness degradation, strength, ductility and energy dissipation. Codes and assessment procedures that treat all masonry infills as equivalent, or that ignore infills altogether, are missing behavior that the experiments show to be significant and, in the case of brittle burnt brick panels, potentially hazardous.

For a region that sits near the active boundaries of the Indian and Eurasian plates and carries the memory of devastating earthquakes, research of this kind is not academic. The study’s combination of physical testing and critical numerical evaluation offers engineers a clearer picture of what actually happens inside an infilled frame when the ground shakes, and a candid reminder that the computational tools used to certify safety still lag behind the messy reality of masonry-concrete interaction. As Mukhlis and his colleagues demonstrate, the humble brick in the wall may be the most important structural element nobody designed.

Subject of Research: Seismic performance of reinforced concrete frames infilled with burnt clay brick and unburnt sandcrete masonry

Article Title: Seismic performance of RC frames with burnt and unburnt brickwork masonry infills: experimental and macro-model evaluation

Article References: Seismic performance of RC frames with burnt and unburnt brickwork masonry infills: experimental and macro-model evaluation. (n.d.). https://doi.org/10.1007/s10518-026-02670-y

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02670-y

Keywords: reinforced concrete frames, masonry infill, burnt clay bricks, sandcrete blocks, seismic performance, hysteretic behavior, stiffness degradation, ductility, energy dissipation, macro-modeling, earthquake engineering, Bangladesh

The scale of the problem motivating this work is striking. According to the most recent Italian census of residential buildings, more than seventy percent of the country’s housing stock was built before 1981, when seismic design provisions were absent or only partially enforced, and horizontal earthquake forces were rarely considered in routine design. Concrete buildings erected between the 1960s and the 1990s typically lack the ductility that modern capacity design principles provide, and their unreinforced masonry infill walls interact with the frame in ways that can drastically alter seismic behaviour. The consequences have been visible in the devastating earthquakes that struck Molise in 2003, L’Aquila in 2009, Emilia in 2012, and Central Italy in 2016. At the same time, the building sector accounts for roughly forty percent of energy consumption and thirty-six percent of greenhouse gas emissions in Europe, making integrated renovation an environmental as well as a life-safety imperative.

Existing retrofit strategies each carry significant drawbacks. External steel exoskeletons allow buildings to remain occupied during construction, but they are expensive to fabricate, demand complex connections to the existing structure, and require free space around the entire perimeter for independent foundations, a luxury rarely available in dense cities. Internal solutions such as added shear walls or steel bracing avoid that problem but consume usable floor area and force occupants to endure invasive interior works. Earlier timber-based proposals have shown promise, since cross-laminated timber is lightweight, insulating, and prefabricated, yet most previous systems required removing existing infill walls or transferring substantial new loads into columns that may themselves be degraded. The e-CLT system was conceived to sidestep these limitations by mounting panels externally and letting friction dampers control how forces flow between the new panels and the old frame.

The operating logic of the system is elegantly two-tiered. Under low to moderate ground shaking, the friction dampers hold the CLT panels rigidly against the concrete frame, increasing the building’s lateral stiffness and strength, which reduces inter-storey drift and the internal forces that would otherwise damage columns and beams. When a severe earthquake strikes, the dampers activate and begin to slide, converting seismic energy into heat through friction while capping the forces transmitted to the timber. This deliberate cap keeps the CLT panels within their elastic range even under extreme excitation, meaning the panels themselves should survive a major event without replacement. A companion prefabricated envelope, called e-PANEL, wraps the assembly to deliver the thermal upgrade, so a single intervention on the façade addresses both earthquake safety and energy performance.

Getting there required fixing a subtle but critical engineering flaw. An earlier version of the friction damper had been validated under idealised laboratory conditions in which only displacements parallel to the friction plane were applied. In a real two-dimensional frame, however, the rocking of the panel during an earthquake generates parasitic forces perpendicular to the friction plane. Preliminary tests showed these out-of-plane actions severely compromised the damper, destabilising its dissipative behaviour, overloading the screws and preloaded bolts, and concentrating stresses at the edges of the concrete elements. The research team therefore redesigned the device from the ground up, decoupling the connection system along the two directions parallel and perpendicular to the friction plane so that rocking motions could no longer corrupt the friction mechanism.

The improved damper consists of two principal steel components: an anchor profile mechanically fixed to the concrete inter-storey beam, and a free profile screwed to the back of the CLT panel that is free to slide horizontally. Between them sits a stiffened alignment component housed in a box formed by three plates welded to the free profile. Energy dissipation occurs as the alignment component slides against the anchor profile across shim layers lubricated with graphite, all clamped together by three bolts tightened to a torque of 100 newton-metres, corresponding to a preload of roughly 31 kilonewtons. The geometry is carefully proportioned using capacity design principles: the friction force is deliberately kept below the resistance of both the timber and concrete fastening systems so that sliding, rather than brittle connection failure, governs, and all steel components remain elastic under the maximum expected friction force. Installation requires just three straightforward operations and no specialist equipment.

To validate the system under realistic conditions, the team built two identical full-scale, single-storey, single-bay concrete frames replicating 1970s Italian construction practice, complete with the strong-beam-weak-column behaviour and low concrete strength typical of that era. Columns measured 300 by 300 millimetres, beams 300 by 500 millimetres, with a 4-metre span and 3.2-metre storey height. Four cyclic quasi-static tests were performed at the University of Bologna’s structural laboratory: a bare frame loaded only within its elastic range, a masonry-infilled frame pushed to failure, an identical infilled frame retrofitted with the e-CLT system, and finally the retrofit system itself mounted on a deliberately damaged frame whose columns had their reinforcement cut to create a pendulum-like substructure with almost no lateral resistance. This clever arrangement allowed the dampers to be characterised within the real geometric interaction of panel, damper, and frame, rather than in isolation. A constant vertical force of 250 kilonewtons per column simulated the gravity load of a three-storey residential building.

The results were dramatic. The unstrengthened infilled frame developed an initial stiffness of 47.0 kilonewtons per millimetre and reached a peak base shear of 157 kilonewtons at a drift ratio of three percent, by which point plastic hinges had formed at the column ends and the longitudinal reinforcement had buckled. The retrofitted frame, by contrast, exhibited an initial stiffness of 70.1 kilonewtons per millimetre, a 49 percent increase, and a peak base shear of 263 kilonewtons, a 67 percent gain. The friction dampers activated at a drift ratio of about 0.5 percent and then delivered stable, repeatable hysteretic loops of the nearly rectangular shape characteristic of friction devices. Crucially, the CLT panel stayed elastic throughout, with maximum reversible diagonal elongation of only 0.6 millimetres, roughly 200 microstrain, confirming that it behaved as a rigid diaphragm protected by the capped damper forces.

The energy numbers are perhaps the most compelling headline figure. Up to 0.5 percent drift, dissipation was minimal because the dampers had not yet engaged. Beyond that threshold, the retrofitted system’s cumulative dissipated energy climbed steeply while the unstrengthened specimen’s performance deteriorated with progressive infill and column damage. At a drift level of three percent, close to the collapse-prevention limits defined in FEMA and Eurocode 8 documents, the strengthened frame dissipated approximately three times as much energy as its unstrengthened counterpart. The equivalent viscous damping ratio of the retrofitted system rose to about 58 percent at three percent drift, compared with a range of roughly 16 to 28 percent for the unstrengthened frame, and showed negligible degradation across repeated cycles. Damage to the concrete members themselves was markedly reduced: cracking remained limited to transverse cracks at column ends, with no bar buckling or concrete cover spalling, in stark contrast to the extensive degradation observed in the control specimen.

The authors conclude that e-CLT is a robust, sustainable route to upgrading seismically vulnerable concrete buildings, one whose stiffness and strength gains can be tailored by adjusting the number and size of dampers and panels, and whose prefabricated, external, lightweight installation minimises disruption to occupants. They also note honest limitations: façades with large openings may not accommodate the panels, brittle failure modes may require supplementary local interventions, and long-term effects such as bolt preload relaxation and temperature fluctuations on the friction interfaces still warrant study. Future work will include shaking-table tests on large-scale three-dimensional specimens and comprehensive cost-benefit analyses. For now, the message is clear: a renewable material, a cleverly decoupled steel damper, and a full-scale laboratory frame have together shown that Europe’s ageing concrete stock can be made dramatically safer without demolishing a single wall.

Subject of Research: Seismic retrofit of pre-code reinforced concrete frames using cross-laminated timber panels with friction dampers

Article Title: Structural performance of RC frames retrofitted with CLT panels: experimental evidence from the e-CLT system

Article References: Pozza, L., Barbagallo, F., Morganti, A., Licciardello, E., Marino, E. M., & Mazzotti, C. (2026). Structural performance of RC frames retrofitted with CLT panels: experimental evidence from the e-CLT system. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02690-8

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02690-8

Keywords: cross-laminated timber, seismic retrofit, friction damper, reinforced concrete frames, energy dissipation, quasi-static cyclic testing, e-SAFE project, masonry infill, building renovation, earthquake engineering, energy efficiency, Horizon 2020

The team focused on a configuration familiar to anyone who has walked through Mediterranean or Asian cities: the pilotis building, in which the ground storey is left open, its columns rising without the confining embrace of masonry infill walls. Buildings with this open ground floor have collapsed repeatedly in past earthquakes, including the Yushu earthquake in China, because the abrupt stiffness discontinuity between the open base and the stiff upper storeys concentrates deformation there. Yet pilotis buildings continue to be constructed because open ground floors are architecturally desirable, leaving engineers to assess the risk with tools of uncertain reliability.

To probe that risk systematically, the researchers built finite element models of reinforced concrete frames at three heights—three, six and twelve storeys—in three configurations each: bare frames, fully infilled frames, and frames with ground-floor pilotis. The plan geometry was held constant across every model, with four bays of 4.0 metres and five bays of 3.0 metres, and uniform 3.0-metre storey heights, so that only the infill arrangement and the building height varied. Reinforcement detailing followed Eurocode 2 and Eurocode 8 practice for medium-ductility frames, with longitudinal reinforcement ratios of roughly 1.7 percent in beams and 2.7 percent in columns, and confined concrete cores represented through the Mander constitutive model.

The modelling strategy balanced fidelity against computational practicality. Reinforced concrete beams and columns were represented with fibre-based force-formulated frame elements, which subdivide each cross-section into fibres whose nonlinear uniaxial stress-strain behaviour is integrated along the member, capturing the spread of inelasticity in both space and time. Masonry infills were represented through a macro-model based on Crisafulli’s double-strut formulation, in which each panel is replaced by six components: two parallel axial struts in each diagonal direction and shear springs that capture bed-joint sliding. This equivalent-strut approach accounts for four failure mechanisms—diagonal tension, sliding shear, corner compression and compression at the panel centre—without the prohibitive cost of detailed micro-modelling, making it well suited to large parametric studies.

Nonlinear time-history analyses were performed in SeismoStruct using the HHT-alpha integration algorithm with a fixed timestep of 0.05 seconds, while eigenvalue analyses employing the Lanczos algorithm established the modal properties of each frame. Three recorded earthquake motions—Landers 1992, Northridge 1994 and Kobe 1995—were deliberately selected because their response spectra peaked near the natural periods of the structures, creating challenging resonant conditions. Critically, the records were applied without scaling or spectrum matching, so the results reflect genuine recorded events, though the authors caution that the limited number of motions means the findings should be read as indicative of behavioural mechanisms rather than statistically robust design predictions.

The modal analyses confirmed that masonry infill fundamentally reshapes a building’s dynamics. Infilled frames were consistently stiffer than bare frames, with shorter fundamental periods, while pilotis frames fell between the two, slightly softer than fully infilled counterparts because of the missing ground-storey walls. As height increased, fundamental periods lengthened and the first three modes separated further apart, consistent with classical structural dynamics. Notably, the difference between fully infilled and pilotis models was small, suggesting that total infill quantity, rather than its vertical distribution, governs overall stiffness—a first hint that the notorious soft storey might not be the dominant factor engineers often assume.

The nonlinear time-history results deepened that surprise. Acceleration profiles along the height of the buildings were strikingly non-monotonic: after an initial rise, accelerations decreased and rose again in the six- and twelve-storey frames, a pattern the authors attribute to higher-mode participation shifting as stiffness degrades non-uniformly with accumulating damage. Masonry infill reduced acceleration amplification by a factor of roughly 1.5 across the model families, and the peak amplification period scaled almost linearly with building height—doubling as storey count doubled—indicating that the fundamental mode dominates response even deep into the nonlinear regime. Perhaps most counterintuitively, the twelve-storey bare frame sometimes showed lower maximum displacements than its infilled counterparts, hinting that added mass and nonlinear interaction can offset the benefits of added stiffness.

The height-dependent transition in pilotis behaviour emerged most clearly in the drift and force demands. In the three- and six-storey frames, the expected concentration of inter-storey drift at the open ground floor failed to materialise; the pilotis models behaved much like their fully infilled twins. Only in the twelve-storey building did deformation concentrate at the base, and even there, the fully infilled frame experienced comparable ground-floor drift, suggesting that infill mass rather than soft-storey flexibility alone drove the effect. Column shear forces told a parallel story: in shorter buildings, pilotis columns carried less shear than bare-frame columns, but in the twelve-storey pilotis frame, ground-floor column shear exceeded that of the fully infilled counterpart by about 50 kilonewtons, evidence that the detrimental influence of the open storey strengthens as buildings grow taller.

Comparisons across the three earthquake records revealed why the picture is so record-dependent. Under Landers, bare frames showed far higher peak spectral accelerations than pilotis frames, and the pilotis configuration occasionally proved beneficial, reducing amplification relative to the bare frame. Under Kobe and Northridge, differences between configurations shrank, and in one case the infilled frame’s peak acceleration nearly matched the bare frame’s, shifted to a longer period of about 1.0 second. The researchers traced this variability to nonlinear period elongation: as damage softens a structure, its elongated periods migrate across the frequency landscape of the input motion, sometimes sliding toward spectral peaks and sometimes away from them. Whether pilotis proves harmful therefore depends on where the resonant peak of the shaking happens to fall relative to the structure’s evolving dynamics.

The study’s most consequential finding concerns design codes. Eurocode 8 permits structures with significant infill asymmetry to be analysed as bare frames, on the assumption that this is conservative. The Bath team’s results show that such simplification can be conservative for some response quantities and configurations, but not universally so: in several cases, ignoring the infill underestimated demands or overstated benefits in ways that depended on the interplay of period elongation, spectral content and building height. The authors acknowledge limitations—a small set of unscaled records, macro-modelled infills that cannot capture local cracking, rigid diaphragm assumptions, neglected torsion and soil-structure interaction—but argue that the message stands. Assessing irregular reinforced concrete buildings accurately demands nonlinear dynamic analysis that honours infill-frame interaction, because static or linear thinking simply cannot predict where a soft storey will turn from a nuisance into a catastrophe.

Subject of Research: Nonlinear seismic response of masonry-infilled reinforced concrete frame buildings with pilotis configurations across different building heights

Article Title: Height-dependent nonlinear seismic response of masonry-infilled RC frame buildings with pilotis configurations

Article References: Height-dependent nonlinear seismic response of masonry-infilled RC frame buildings with pilotis configurations. (n.d.). https://doi.org/10.1007/s10518-026-02687-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02687-3

Keywords: earthquake engineering, reinforced concrete, masonry infill, pilotis, soft storey, nonlinear analysis, time-history analysis, inter-storey drift, period elongation, Eurocode 8, finite element analysis, building height