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 research team, led by Qiyu Liu of Chalmers University of Technology together with Maud Lanau, Johan Rootzén, Zhi Cao and Filip Johnsson, set out to address a long-standing blind spot in the field of dynamic material flow analysis, or dMFA. This modeling approach tracks the accumulation, use and retirement of materials over time, enabling consistent estimates of construction demand, demolition waste and in-use stocks across building cohorts. It has become a cornerstone technique for quantifying the material metabolism of the built environment, a sector that accounts for more than one-third of global final energy demand and a comparable share of energy-related carbon dioxide emissions.

The problem, the authors argue, lies in a simplifying assumption that pervades most existing studies. A review cited in the paper found that around eighty percent of published building stock analyses did not distinguish between materials held in structural and non-structural components. Buildings are typically modeled as monolithic entities governed by a single lifetime distribution, with renovation represented either as a fixed annual rate or as statistically independent renovation cycles decoupled from demolition probabilities. But real buildings are not monoliths. Structural frames can stand for a century or more, while façades, roofs and interior partitions are replaced repeatedly over the same period without ever triggering demolition.

To capture this heterogeneity, the researchers turned to an influential idea from architectural theory: the shearing layer concept, popularized by Stewart Brand in his 1995 book How Buildings Learn. Brand described buildings as several layers of longevity, from the site and structure to the skin, services and movable contents. The new framework focuses on three layers most relevant to material stock modeling: the structure, which determines when a building is demolished; the skin, meaning the building envelope; and the space layer, comprising non-structural interior elements such as partition walls, ceilings and flooring. Each layer ages on its own schedule, and each renovation targets a specific layer, so capturing their different longevities is essential for realistic modeling.

The methodological core of the study is a cohort-based, stock-driven dMFA framework implemented in Python using the Open Dynamic Material Systems Model. The first step calculates the gross floor area dynamics of the building stock from construction and demolition, applying a Weibull survival function calibrated to Swedish demolition statistics, which show that only about 0.05 percent of multi-family buildings are demolished each year. The second step runs two parallel stock-driven models for the skin and space layers, each with its own lifetime distribution, drawn from Sweden’s National Renovation Strategy: a mean of 35 years for the skin and 25 years for interior elements.

The key innovation is a lifetime convolution approach that ensures renovation probabilities are conditional on building survival. In plain terms, a demolished building cannot be renovated, and a building unlikely to survive long enough to recoup the value of a new façade is also unlikely to receive one. The framework shifts each layer’s lifetime profile by the mean renovation cycle so that a building’s expected remaining life after renovation is at least as long as the renovation interval itself. Material inflows are then calculated by multiplying renovated floor area by layer-specific and cohort-specific material intensities, reflecting the fact that buildings of different construction eras embed different material recipes per square meter.

The framework was applied to Sweden’s residential stock of 2.8 million buildings, using property registry data from the Swedish Land Survey, machine-learning-derived estimates for missing building attributes, and a refined set of material intensities disaggregated across building layers and ten construction materials. After recalculating these intensities with detailed density data and expert consultation, the team arrived at average total material intensities of 557 kilograms per square meter for multi-family buildings and 1,054 kilograms per square meter for single-family homes, figures notably lower than those of a widely used earlier Swedish database.

The results paint a striking picture of the materials locked in Swedish homes. The residential building stock held 354 million tonnes of materials in 2024, equivalent to 33 tonnes per person, with multi-family buildings accounting for 59 percent of the total. Structures dominate, comprising 81 percent of the stock, while skin layers contribute 22 percent of single-family stocks but only 4 percent of multi-family stocks. Between 2025 and 2050, single-family buildings are projected to require between 1.4 and 1.6 million tonnes of renovation materials annually, roughly triple the roughly 590 kilotonnes needed for multi-family buildings each year.

The comparison against conventional monolithic models confirmed the team’s hypothesis. Because the traditional approach lets renovation cycles run independently of demolition risk, it keeps renovating buildings that are in reality near the wrecking ball. Over the modeled period, the monolithic model overestimated renovated floor area by 12.6 percent for the skin layer and 8.8 percent for the space layer of single-family buildings, and by 7.4 and 9.5 percent respectively for multi-family buildings. Cumulative discrepancies by 2050 reached 44 million square meters for single-family and 30 million for multi-family dwellings, equivalent to 14 and 15 percent of their total floor areas. Sensitivity analyses showed the result holds under faster and slower renovation cycles, and that structural lifetime assumptions have only a minimal effect on the layered model’s output.

The implications reach well beyond Sweden. Accurate renovation forecasts underpin climate mitigation strategies, energy-efficiency retrofit programs and circular economy policies, all of which depend on knowing when and where materials will be demanded and discarded. Because different building layers carry very different circularity potentials, from as little as one to five percent for structural materials to ten to twenty-five percent for interior elements, a layered model enables far more targeted policy design. It also allows energy retrofit schemes to be mapped precisely to the layers where they occur, such as wall and window upgrades in the skin. The authors acknowledge that renovation decisions in the real world respond to economics as much as to physical decay, and that detailed renovation histories are still lacking, but they present the layer renovation model as a robust alternative wherever material intensity data permits. As mature economies shift from expanding their building stocks to transforming them, the difference between a monolith and a system of layers may determine whether material demand estimates, and the climate strategies built upon them, stand on solid ground.

Subject of Research: Layered dynamic material flow analysis of residential building renovation material demand in Sweden

Article Title: A layered dynamic material flow framework for modeling building renovations

Article References: Liu, Q., Lanau, M., Rootzén, J., Cao, Z., & Johnsson, F. (2026). A layered dynamic material flow framework for modeling building renovations. Journal of Industrial Ecology. https://doi.org/10.1007/s44498-026-00164-3

Image Credits: AI Generated

DOI: 10.1007/s44498-026-00164-3

Keywords: dynamic material flow analysis, building renovation, material stocks, shearing layers, Sweden, residential buildings, circular economy, material intensity, building lifetimes, construction materials, sustainability modeling, layered