Seismic Performance of a Full-Scale Moment-Frame Housing System Constructed with Recycled Tetra Pak (Thermo-Stiffened Polymeric Aluminum Composite)

Abstract

To address the growing need for sustainable and resilient building materials, the seismic performance of a full-scale moment-frame housing system constructed entirely from recycled Tetra Pak panels (thermo-stiffened polymeric aluminum or TSPA) was evaluated. The study presents an innovative approach to utilizing waste materials for structural applications, emphasizing the lightweight and modular nature of the system. The methodology included material characterization, finite element modeling (FEM), gravitational loading tests, and biaxial shake table tests. Seismic tests applied ground motions corresponding to 31-, 225-, 475-, and 2500-year return periods. Drift profiles and acceleration responses confirmed the elastic behavior of the system, with no residual deformation or structural damage observed, even under simultaneous peak ground accelerations of 0.37 g (x-direction) and 0.52 g (y-direction). Notably, the structure accelerations were amplified to 1.10 g in the y-direction (at the top of the structure), exceeding the design spectrum acceleration of 0.7 g without compromising stiffness or resistance. These results underscore the robust seismic performance of the system. The finite element model of the housing module was validated with the experimental results which predicted the structural response, including natural periods, accelerations, and drift profiles (up to 89% accuracy). The novelty of this research is that it is one of the first to perform shaking table seismic testing on a full-scale housing module made of recycled materials (Tetra Pak), specifically under biaxial motions, providing a unique evaluation of its performance under multidirectional seismic demands. This research also highlights the potential of recycled Tetra Pak materials for sustainable construction, providing an adaptable solution for earthquake-prone regions. The modular design allows for rapid assembly and disassembly, supporting scalability and the circular economy principle.

1. Introduction and Background

Sustainability is a critical consideration in modern construction practices, driven by the urgent need to minimize environmental impact and carbon footprints. The global construction industry faces significant challenges due to its substantial contribution to environmental degradation and resource depletion. Traditional construction practices heavily rely on finite natural resources, such as sand, gravel, and limestone, while producing vast amounts of waste, including plastics and composite materials. According to the United Nations Environment Program, the building and construction sector is undoubtedly the largest emitter of greenhouse gases, accounting for 37% of global emissions. The production and use of materials such as cement, steel, and aluminum have a significant carbon footprint [1]. Sustainable construction begins at the project design stage, where decisions about recyclable materials for façades, walls, windows, floors, roofs, structural elements, and ducts are made. This can involve reducing the use of non-recyclable materials or adopting technologies for natural energy harvesting and efficient water use (e.g., rainwater recycling and low-consumption sanitary systems). Despite these advancements, materials traditionally used for structural purposes (such as steel, cement, and masonry) continue to dominate decision-making processes due to their established mechanical performance, especially in seismic regions where stiffness and toughness are paramount [2,3]. However, traditional construction materials heavily rely on finite natural resources and are major contributors to global greenhouse gas emissions. Simultaneously, the accumulation of waste—particularly plastics and multilayer composites like Tetra Pak—has reached alarming levels, intensifying ecological crises. Tackling these interconnected challenges necessitates innovative and sustainable approaches, including the adoption of recycled materials in construction practices.

In seismic zones, construction authorities and engineers often hesitate to adopt alternative materials due to uncertainties regarding their performance under seismic loads. Nonetheless, this reluctance should not deter researchers and practitioners from pursuing innovative solutions. Polymeric and recycled plastic materials, for instance, have demonstrated significant potential as viable alternatives, forming the foundation for various structural solutions that are increasingly becoming part of the global housing market [4]. Similarly, renewable materials such as bamboo and wood offer low-carbon, sustainable options, providing outstanding examples of environmentally friendly construction practices [5,6].

Recycling post-consumer plastics, such as high-density polyethylene (HDPE), polyethylene terephthalate (PET), and polypropylene (PP), into construction materials presents significant opportunities for addressing environmental challenges while fulfilling housing demands. For example, bricks manufactured with HDPE and PP demonstrate compressive strengths exceeding 12 MPa while reducing weight by 20% compared to traditional clay bricks, making them a promising alternative for sustainable construction [7]. PET bottles have been reused as structural and decorative components, demonstrating compressive strengths of 8–12 MPa and thermal conductivities of 0.2–0.3 W/m·K, making them well-suited for energy-efficient buildings [8]. Similarly, panels produced from recycled Tetra Pak demonstrate flexural strengths of up to 15 MPa and water absorption rates below 1%, rendering them ideal for applications such as interior walls and roofing [2,9].

Tetra Pak cartons represent an underutilized resource with substantial potential for structural applications. For instance, reference [10] conducted a comprehensive review on the integration of Tetra Pak waste into construction materials, highlighting its multifunctional advantages, including enhanced mechanical properties and reduced environmental pollution, without the need for extensive separation or recycling processes. Similarly, the study reported in reference [11] serves as a cornerstone in this field, demonstrating the mechanical feasibility of hollow structural elements made from recycled Tetra Pak-based boards (RTPBBs). The findings presented in [11] revealed that RTPBB elements exhibited mechanical behavior comparable to commercial plywood, with the added benefit of reducing carbon emissions by approximately 20% during production. The study reported in reference [11] also identified challenges such as brittle failure mechanisms under compression and local buckling in unstiffened sections, underscoring the importance of design optimizations, including the addition of intermediate stiffeners. The authors of [11] also investigated the environmental benefits of recycled Tetra Pak-based boards (RTPBBs), demonstrating significant reductions in water and energy consumption compared to traditional materials. These findings align with those of [12], who highlighted the critical role of life-cycle assessments in quantifying the broader environmental advantages of recycled materials. Similarly, reference [13] reported that Tetra Pak composites not only deliver satisfactory mechanical properties but also contribute significantly to environmental conservation.

The scalability of these materials for full housing applications has demonstrated both their practicality and benefits. For instance, the authors of reference [14] evaluated the feasibility of using recycled plastic lumber as a primary material for constructing low-cost, sustainable housing. Their study highlighted significant advantages, including lower density compared to concrete and brick, enhanced thermal insulation, and substantial cost savings over traditional masonry. In Colombia (South America), over 3 million recycled Tetra Pak containers were used to construct a sustainable house, resulting in a 3000 kg reduction in carbon emissions and illustrating the scalability of such solutions for broader housing initiatives [15]. Similarly, in Benin, housing units constructed with locally sourced plastic waste achieved compressive wall strengths of 20 MPa while reducing costs by up to 60% [16]. In Southeast Asia, prefabricated systems made from recycled Tetra Pak achieved thermal conductivity values as low as 0.12 W/m·K, offering exceptional insulation suitable for tropical climates [17]. Additionally, the authors of reference [18] investigated a novel circular economy model in Benin, transforming plastic waste into construction bricks. This model integrated waste sorting, transportation, and the employment of local communities for waste collection and material processing, providing a sustainable and socially inclusive framework.

Building on the pioneering work presented in [11], subsequent studies have significantly advanced the understanding of recycled materials in construction. The initial applications of recycled Tetra Pak in construction focused on non-structural composite panels made from Tetra Pak and polyethylene waste [19]. These panels demonstrated excellent humidity resistance and thermal stability, making them suitable for tropical climates. The research also showed that mechanical properties varied with composition, with the highest modulus of rupture (15.5 MPa) observed in samples containing 40% Tetra Pak and 60% polyethylene-based particles. Although increasing Tetra Pak content reduced strength and dimensional stability, all samples met European (EN) standards for particleboard, highlighting the material’s potential for sustainable, adhesive-free panel production. Similarly, the authors of reference [20] evaluated the mechanical properties of composites manufactured entirely from recycled Tetra Pak containers. Using a hot-pressing method, the study produced samples with tensile strengths reaching up to 37.4 MPa, underscoring their potential for applications within the construction industry. A research study conducted in Indonesia expanded on these findings by developing prefabricated housing modules utilizing recycled Tetra Pak layers. This study demonstrated the feasibility of using Tetra Pak for structural applications, emphasizing its potential for reducing environmental impact while improving thermal insulation and construction efficiency. The prefabricated nature of the modules facilitated rapid assembly and cost-effective housing solutions, particularly in low-income regions, aligning with sustainability goals [17]. Meanwhile, Ref. [21] examined wood–plastic composites made from recycled polyethylene and Tetra Pak boxes. The study revealed that increasing the Tetra Pak content enhanced the flexural modulus but resulted in reduced tensile and flexural strengths, highlighting a trade-off between stiffness and strength. Such innovations underscore the versatility of Tetra Pak-based materials in addressing diverse construction needs. For instance, Ref. [22] explored the production of plastic bricks using high-density polyethylene (HDPE) waste. These bricks achieved a mean compressive strength of 24 MPa, demonstrating both structural and environmental benefits.

The environmental benefits of recycled materials extend well beyond waste reduction. For example, Ref. [23] explored the use of plastic waste in various construction applications to address environmental challenges and enhance sustainability. Their study demonstrated how plastic waste can be effectively incorporated as aggregates, fibers, and binders in cementitious composites. Replacing natural aggregates with plastic waste in concrete has been shown to reduce density by 10% and improve thermal insulation by 20%, making these materials highly suitable for lightweight and energy-efficient applications [24]. Further advancements include the integration of recycled plastic box waste particles as partial replacements for fine aggregates in concrete mixtures, which improved compressive strength by up to 30.2% when used at a 5% replacement level [25]. Similarly, Ref. [26] investigated the production of green-efficient masonry bricks using scrap plastic waste, achieving compressive strengths of up to 38 MPa. Life-cycle assessments of HDPE-enhanced compressed earth blocks also revealed compressive strengths of 6–8 MPa, alongside a 35% reduction in carbon emissions compared to traditional concrete bricks [12]. Research has further extended to earthen vernacular materials with plastic additives. For instance, Ref. [27] examined the incorporation of high-density polyethylene (HDPE) waste into cement-stabilized rammed earth. Their findings showed that replacing natural aggregates with HDPE improved compressive strength to 9.66 MPa, addressing both engineering and sustainability challenges. In a practical application, 200 tons of plastic waste were transformed into bricks for 42 housing units in Guapi, Colombia, at a cost of just USD 1.50 per unit, demonstrating a successful integration of circular economy principles into construction [16].

The structural performance of recycled materials under extreme loads has been the subject of research, particularly for applications in disaster-prone areas. For example, plastic lumber walls have demonstrated ductility values exceeding 3 and energy dissipation capacities of 25%, confirming their effectiveness in earthquake-prone regions [5]. Similarly, PET-based materials have achieved compressive strengths of 7 MPa while exhibiting resilience to cyclonic forces, making them suitable for refugee housing in disaster-affected areas [28]. Composite materials tested in high-seismic-risk zones in India have shown durability under cyclic loading, further underscoring their potential for disaster-resilient construction [29]. Moreover, Ref. [30] highlights the development of plastic houses constructed entirely from recycled plastic waste, specifically designed to withstand earthquakes, offering an innovative and sustainable solution for vulnerable regions. Lightweight prefabricated systems represent another domain where recycled materials demonstrate exceptional potential. Ref. [9] showcased the feasibility of using recycled polylaminate materials for insulating panels, offering excellent thermal performance while adhering to circular economy principles. Similarly, Ref. [5] examined the seismic behavior of recycled plastic lumber walls, highlighting their energy dissipation capabilities and suitability for moderate seismic applications. These findings align with the broader objectives of developing affordable, sustainable, and resilient housing solutions.

Despite their promise, challenges remain in scaling recycled materials for widespread construction use. Fire resistance is a critical limitation, with many recycled materials classified as class E under the EN 13501 standard, restricting their application in high-risk environments [3]. Additionally, variability in the composition of recycled materials can result in inconsistent mechanical properties, underscoring the importance of standardizing manufacturing processes and implementing robust quality control measures. Advancements in the integration of recycled materials have prioritized not only sustainability but also resilience. Housing systems made from Tetra Pak and plastic composites have demonstrated lifespans of up to 100 years in tropical climates, requiring minimal maintenance [2,17]. The scalability of these materials has been further validated through initiatives like the construction of modular units in Mexico, which combined recycled Tetra Pak and PET to create cost-effective, durable housing solutions [31].

The Tetra Pak recycling process starts by picking the empty boxes right out from their source. Sometimes it is simple to take pristine boxes, but in most cases the boxes need an earlier rinsing activity. Once the boxes are all clean, a machine shreds them into small pieces, delivering a material that can be easily cleaned and allowing all the traces of cardboard and paper to be washed out. The result is the base material made of polymer and aluminum which is arranged for mechanical and thermal compression. Once the machine compresses the loose pieces of shredded material, the board receives a shot of freezing water sealing the surface, which creates a superficial protective layer against humidity and water.

The novelty of the present research lies in its comprehensive exploration of both the mechanical and dynamic behavior of a structural system constructed entirely from flat boards derived from Tetra Pak recycling. This innovative system stands out for its lightweight nature compared to traditional materials such as concrete or steel, making it an option for sustainable construction. The structural elements, assembled using bolts, nuts, and washers, introduce modularity and allow for easy construction and deconstruction by users. This modularity also enables the conceptualization of a novel metric: the cost-efficiency index, calculated as price per square meter per number of uses, adding a new dimension to evaluating sustainable housing solutions.

A key novelty of this study is the introduction of biaxial shaking table testing to analyze the seismic performance of full-scale housing modules made from recycled Tetra Pak material. Unlike previous studies that focused on component-level assessments or static analyses, this research provides groundbreaking insights into the multidirectional behavior of integrated recycled-material systems under real-world seismic conditions. By surpassing traditional unidirectional testing, the research delivers a holistic understanding of the material’s performance under complex stress scenarios. Another critical novelty is the demonstration of scalability for recycled Tetra Pak in constructing disaster-resilient housing units, making them ideal for rapid, cost-effective deployment in disaster-prone and economically disadvantaged regions.

This research further contributes to the field by integrating circular economy principles into large-scale structural applications. It showcases how waste materials like Tetra Pak can effectively replace traditional resources without compromising safety, resilience, or affordability. A particularly innovative aspect is the creation of hybrid composites that leverage the synergies between recycled Tetra Pak and other materials, resulting in enhanced flexural strength, ductility, and energy dissipation. Finally, this research aligns with global efforts to meet the United Nations Sustainable Development Goals (SDGs), particularly SDG 11 (Sustainable Cities and Communities), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action).

2. Methodology

The methodology adopted for this research was structured into five interconnected phases, each designed to address critical aspects of the study (Figure 1). The goal was to explore the mechanical and dynamic performance of a full-scale structural system constructed entirely from thermo-stiffened polymeric aluminum (TSPA), derived from recycled Tetra Pak materials:

• Phase 1—This phase involved an examination of some mechanical properties of the material (thermo-stiffened polymeric aluminum) and its connections. Tensile strength and elasticity modulus were determined through tensile tests, providing the foundational mechanical properties for the structural system. Additionally, the performance of bolted beam–column connections, critical for shear transfer between components, was analyzed through experimental setups based on [32].
• Phase 2—A finite element model (FEM) was developed using the material properties derived from Phase 1. The FEM simulated all structural components, including beams, girders, columns, and the flooring system. Steel bolts were modeled as linear frame elements to replicate their role in shear transfer between hollow structural elements. The FEM incorporated boundary conditions and loading protocols representative of gravitational and seismic forces. Validation of the model was achieved by comparing its predictions to simplified analytical calculations and, later, experimental results from full-scale testing.
• Phase 3—In this phase, the full-scale prototype of the structural system was subjected to gravitational loading using cement sacks in compliance with standards. Displacement sensors and strain gauges were installed at critical points across the girders, beams, and flooring system to monitor deflections and strains during the loading process. The load was gradually increased to a maximum value of approximately 1.8 kN/m² and maintained for 24 h to simulate standard conditions. However, for this study, the load was held for 100 h to observe long-term effects on the structural components. After the test, detailed observations of structural pathology were supplemented with photographic evidence.
• Phase 4: The prototype was subjected to biaxial shaking table testing to simulate seismic forces. Ground motion records representing return periods of operational level (OL), damage limitation (DL), life safety (LS), and collapse prevention (CP) were applied to the structure. Key measurements, including natural frequencies, peak accelerations, and drift profiles, were recorded to evaluate the dynamic performance of the system. The seismic testing provided important data on the behavior of thermo-stiffened polymeric aluminum materials and connections under multidirectional stress scenarios.
• Phase 5: This phase involved a comprehensive analysis of the experimental and numerical results obtained from the previous phases. Comparisons between the FEM simulations and experimental data highlighted the consistency and reliability of the numerical approach. Key findings emphasized the lightweight and sustainable nature of thermo-stiffened polymeric aluminum materials, alongside their satisfactory mechanical and seismic performance. Conclusions were drawn to highlight the feasibility of thermo-stiffened polymeric aluminum materials systems for sustainable and resilient earthquake-resistant housing. Finally, recommendations for future research are included.

3. Structural Description

The housing structures depicted in Figure 2a represent an innovative application of structural systems made entirely from recycled Tetra Pak materials.

Each house features a robust frame that integrates four columns, two primary girders (V-01), two stiffening girders (V-02), and four joists per level (VT-01) (Figure 2b,c). These structural components are joined through moment connections (Figure 2d), ensuring continuity between columns and beams, which provides the required structural integrity. The façade can allow any floating or lightweight solution as the wood-framing system which is a classical solution in North America (Figure 2a).

The frame spans 3.3 m (130 in) between columns, measured center-to-center, with column heights of approximately 2.60 m (102 in). The structural elements are hollow, with beams and columns having a cross-sectional dimension of 0.25 m × 0.25 m (9.8 in × 9.8 in) and a wall thickness of 20 mm (0.8 in). Figure 2b presents the structural elevation and plan drawings, showing the distribution of columns, beams, and other key structural elements.

To ensure stability, the structural members are bolted together using steel screws and threaded bars made of Grade 8 ASTM steel. The threaded bars have a diameter of 12 mm (0.5 in), while the bolts measure 10 mm (0.4 in) in diameter. These connections are reinforced with nuts and washers, and prestressed washers are strategically placed to create friction between the components, enhancing joint stability.

The base of each column is anchored to square reinforced concrete footings measuring 0.50 m (19.7 in) per side and 0.50 m (19.7 in) in thickness. These footings, constructed with concrete having a compressive strength of 21 MPa (3000 psi), efficiently transfer loads to the underlying soil. Due to the lightweight nature of the structure, which weighs approximately 2000 kg (4409 pounds), the foundation footprint is minimal and can be adapted based on local geotechnical conditions.

The flooring system consists of customized TSPA boards arranged in rectangular and square shapes. The joists measure 0.10 m × 0.25 m (3.9 in × 9.8 in) in cross-section and are spaced at 0.6 m (23.6 inch) intervals perpendicular to the beams. These elements create a stiffened floor system supported by girders and beams. Figure 2c provides a 3D model of the entire housing unit, offering a comprehensive visualization of the structural configuration and assembly process.

A crucial detail of the structural design is the beam–column joint. Each joint features a cruciform design measuring 0.90 m (35.4 in) in height, allowing for effective perpendicular intersections of the structural elements. This configuration elevates the flooring system above ground level, enhancing serviceability and providing functional benefits for occupants. Figure 2d presents a 3D schematic of a beam–column connection, illustrating its components and assembly sequence.

The roof is composed of thermo-acoustic UPVC sheets, and the walls are assembled using OSB panels, maintaining the lightweight and sustainable design approach. The structural system effectively combines recycled materials with a modular construction strategy, ensuring sustainability while maintaining sufficient strength and resilience to support various loads.

4. Mechanical Properties of TSPA and Connection Strength

The tensile strength of the TSPA material was evaluated using 12 specimens, yielding an average strength of 7.10 MPa with a coefficient of variation (CoV) of 7.7%. Additionally, the elastic modulus of the material, based on the same 12 specimens, was of 789 MPa with a CoV of 32%. This variability is attributed to the nature of the material, which originates from the recycling of Tetra Pak containers. The manufacturing process of this recycled material inherently induces the observed levels of variation.

The mechanical properties obtained in this study are consistent with previous research on recycled Tetra Pak-based materials. The tensile strength and the elastic modulus fall within the range reported by [11], where tensile strengths between 5.6 MPa and 6.6 MPa and an elastic modulus of 747 MPa were documented. However, the results are slightly lower than those reported in [20], where a minimum tensile strength of 9.5 MPa was observed. The variations in mechanical properties across studies can be attributed to differences in the composition of recycled Tetra Pak materials, manufacturing processes, and testing methodologies. Despite these discrepancies, the results confirm that the material exhibits sufficient strength and stiffness for structural applications, particularly in lightweight and modular construction systems.

According to [32], the strength of beam-to-column connections in the structural system was assessed through pseudo-static tests. These tests utilized 15 mm thick recycled Tetra Pak plates connected with 16 mm diameter steel bolts, as illustrated in Figure 3. The results indicated that the average gravitational (downward) resistance of the beam–column connection was 38.51 kN, with a CoV of 8.8%. Conversely, the average upward resistance (opposite to gravitational acceleration) was 26.13 kN, with a CoV of 10.9% [32]. The overall average resistance was 32.3 kN, which is sufficient to withstand the imposed loads from live load, dead load, and seismic forces. Results indicated that the experimentally observed stress concentrations were consistent with those predicted by finite element modeling of the beam–column connection performed in SAP 2000. The finite element model results closely matched the laboratory observations (stress concentration zones and strains), confirming the reliability of the numerical simulations. Failure in the connection followed a defined progression: initially, ovality (the applied load caused ovalization of the bolt holes), shear block, and tension developed in the upper perforations, followed by the appearance of tensile cracks. Ultimately, collapse occurred due to a shear block mechanism, which involved the combined effects of shear and tensile forces on the bolts at the lower perforations of the joint (Figure 3), just the same as first reported for shear connections at the University of Arizona in 1984 [33] and later by other researchers in the case of cold-formed steel bolted connections [34]. These findings underscore the robustness and reliability of the connection for use in structural systems made from recycled Tetra Pak materials.

5. Finite Element Modeling to Evaluate Seismic Performance

The finite element model (FEM) of the TSPA housing module was meticulously developed using SAP 2000, incorporating advanced modeling techniques to replicate the structural behavior of the system under seismic loads. An image of the model is presented in Figure 4a. The FEM comprised approximately 117,000 nodes, over 400 frame elements representing the steel bolts used in the connections, and nearly 118,000 shell elements to simulate the Tetra Pak material. This high level of discretization allowed for a detailed representation of the geometry and mechanical behavior of the structure, enabling an accurate assessment under various loading conditions. A linear elastic analysis was conducted to evaluate the structure’s behavior under gravitational and dynamic loads, ensuring that the system’s response remained within the elastic range, as observed in experimental testing. Material properties for the FEM were derived from laboratory tests performed on Tetra Pak specimens and structural connections, effectively capturing the material’s inherent variability.

Frame elements simulating the bolts were assigned material properties corresponding to Grade 8 steel, ensuring an accurate representation of the connection stiffness and strength. This comprehensive modeling approach provided a reliable foundation for analyzing the structure’s performance under seismic demands and gravitational loading. Boundary conditions were applied to replicate the experimental setup (for the dynamic tests), with the base of the structure fully constrained to simulate the fixed anchorage provided by the foundation during shaking table tests. The dynamic characteristics were obtained by a modal analysis (eigenvectors and eigenvalues) to establish the structure’s natural vibration modes and periods. These findings were used to validate the FEM’s ability to replicate the static and dynamic properties of the structure. In addition, the seismic signals defined for the experimental protocol, representing ground motions with return periods of 31, 225, 475, and 2500 years, were incorporated into the model for a linear time-history analysis with a damping ratio of 5% of the critical damping. This approach allowed for the application of the full protocol of seismic excitations, enabling a detailed evaluation of the structure’s response under realistic multidirectional seismic demands. The FEM results included drift profiles, displacements, acceleration responses, and modal information that were consistent with experimental observations as will be presented in the following sections.

Special attention was given to modeling the structural connections, which were analyzed independently from the rest of the structural system due to their critical role in the overall response. An example of an isolated finite element model (FEM) representation of the beam-to-column connection is shown in Figure 4b. These connections primarily rely on friction mechanisms, with load transfer occurring through shear in the bolts. The bolts are arranged in pairs, forming a thickness-dependent, bearing-resistant load path that redistributes applied moments into in-plane stress fields within the TSPA wall of the connection. To accurately represent this behavior in the FEM, a set of pure axial elements—with no mass and only axial capacity—were introduced around the frame elements representing the steel bolts. This configuration effectively simulated a pure shear interaction between the bolts and the TSPA walls, ensuring a realistic representation of fastener–panel behavior. A small gap was included between these elements, becoming active only when contact occurred between surfaces. This interaction was modeled using linear springs, whose stiffness values were determined from experimental tests measuring the out-of-plane deformation capacity of the TSPA shell elements under midspan loading conditions. Additionally, the interaction between walls at the connection was simulated using hexagonal four-node finite elements, which accounted for bending resistance and stress redistribution. This modeling approach was particularly important in regions vulnerable to tension field effects, allowing the FEM to capture stress concentrations and potential failure mechanisms with improved accuracy.

6. Vertical Load Testing

Upon completion of the prototype construction, the two girders, two beams, and four flooring beams were selected as key monitoring points for midspan displacement. To capture the structural response, a set of sensors, including linear variable differential transformers (LVDTs) and strain gauges, were strategically placed at critical locations, ensuring continuous data acquisition throughout the loading test. To ensure an even distribution of the applied gravitational loads, the flooring system was divided into specific loading zones, as illustrated in Figure 5a. The test protocol classified the structure into three distinct zone types based on its geometric and structural configuration, allowing for systematic and controlled load application. As shown in Figure 5b, the prototype was fully instrumented with displacement sensors and a data acquisition system (DAQ), enabling real-time monitoring of deflections and potential cracking during the test.

The gravitational load—applied using cement sacks—was progressively introduced onto the flooring system in a series of discrete increments, following the predefined protocol detailed in

Table 1. Loading protocol for the vertical loading test.

Load Step	Details	Load (kN)	Distr. Equivalent (kN/m2)	% of Service Load
1	50 kg in each zone type	4.41	0.417	28.2
2	+50 kg in each zone type	8.82	0.834	56.3
3	+25 kg in each zone type	11.03	1.044	70.4
4	+25 kg in each zone type	13.24	1.253	84.5
5	+25 kg in each zone type	15.44	1.462	98.6
6	Final load of 22.3 kg in Zone Type 3.	15.67	1.483	100.0

. The loading sequence consisted of six steps, gradually increasing the applied weight until reaching the target service load of 1.48 kN/m² (Figure 5c). In accordance with ASTM E196-06 [35], the standard protocol recommends maintaining the full-service load for 24 h to evaluate damage acceptability under sustained gravitational demands. However, in this study, the load was maintained for 100 h (approximately four days) to assess long-term deformation behavior under prolonged loading conditions. Following this extended loading period, a structural damage assessment was conducted using photographic documentation to identify potential deterioration in the flooring system. As observed in Figure 5d, visible midspan deflections were recorded, where tensile forces were most pronounced. Most of the observed damage was attributed to stress concentrations at discontinuities within the built-up hollow structural boards, particularly near the screws at midspan, where the highest tensile demands were transmitted. Despite these deformations, no structural failure or material degradation was detected, confirming the load-bearing capacity and serviceability of the TSPA-based flooring system under sustained gravitational loads.

The results presented in Figure 6 illustrate the experimental average deflections recorded at the midspan as the applied load progressively increased to 100% of the service magnitude. The graph demonstrates a clear stepwise increase in deflection, corresponding to each incremental load step detailed in Table 1. The recorded deflections exhibit a non-linear trend in the initial loading phases, likely due to minor seating adjustments in the structural components and connections. However, as the loading progressed, the deflection pattern stabilized, displaying a consistent and predictable increase without signs of sudden stiffness degradation. The maximum recorded deflection reached approximately −35 mm, exceeding the serviceability limit of −13.54 mm (L/240) prescribed by ASCE 7-22 for full live load action [36]. However, the structure demonstrated complete recovery upon load removal, confirming its purely elastic behavior. These results confirm that the TSPA-based flooring system successfully accommodated the imposed service loads without experiencing permanent deformations.

7. Biaxial Shaking Table Tests

7.1. Ground Motions for Biaxial Shaking Table Tests

The seismic ground motions used in the biaxial shaking table tests were based on a seismic hazard assessment with a probabilistic approach conducted for Bogotá’s Piedmont zone (as defined in the city’s seismic microzonation study). Different intensity levels were established, corresponding to return periods of 475, 225, and 31 years, which are associated with exceedance probabilities of 10%, 20%, and 80% over a 50-year period, respectively. These return periods align with widely recognized structural performance objectives in seismic design: operational level (OL, 31-year return period), damage limitation (DL, 225-year return period), and life safety (LS, 475-year return period). A seismic hazard disaggregation analysis was carried out, focusing on structural natural periods in the following range: 0.3~0.7 s. The expected dynamic response of the evaluated structural system is included in this range. The analysis confirmed that the dominant hazard affecting the DL and LS intensity levels is primarily governed by shallow crustal earthquakes. The moment magnitude range of these earthquakes is 6.0~7.6. In addition, the fault distance range of the earthquakes is 50~80 km. The described methodology is similar to the one established in reference [37]. According to this analysis, real ground motions were selected to replicate the expected seismic demand on the structure. Specifically, for the DL and LS levels (where LS corresponds to the design earthquake), the selected ground motions were the acceleration time histories recorded during the earthquake of 1989 that occurred in Loma Prieta (Mw 6.9). In particular, the present research used the registers recorded at the airport of San Francisco located 59 km from the fault. To achieve the desired intensity levels while preserving the frequency content of the original earthquake, a moderate scaling procedure was applied. For the OL level, the ground motion records from the earthquake near the municipality of “Puente Quetame” (Mw 5.7) were utilized. Specifically, the records obtained 55 km from the fault at the Vitelma water tank station in Bogotá were selected. The seismic loading protocol was designed to progressively increase the intensity of ground motions in a systematic and controlled manner. Additionally, to evaluate the structural performance under extreme seismic conditions, a collapse prevention (CP) ground motion was included in the testing program. This CP motion, representing a 2500-year return period (exceedance probability of 2% over a 50-year period), was derived from the Loma Prieta earthquake record and further scaled to exceed the LS level, allowing an assessment of the structural system’s response beyond conventional design expectations.

As shown in Figure 7, the peak ground accelerations ($PGAs$) vary significantly between the two directions, with the y-direction exhibiting higher intensity levels across all return periods. The maximum $PGA$ values in the x-direction range from $0.11 \mathrm{g}$ to $0.37 \mathrm{g}$, whereas the y-direction accelerations are consistently higher, ranging from $0.14 \mathrm{g}$ to $0.52 \mathrm{g}$. These differences represent the asymmetry in ground motion intensities and emphasize the importance of evaluating bidirectional structural responses. Additionally, the temporal characteristics of the seismic ground motions, as shown in the acceleration time histories for both directions, reveal distinct peak accelerations occurring within the first 10 to 15 s of the ground motion. This concentration of seismic energy during the early stages of the event is consistent across all return periods and aligns with the typical behavior of crustal earthquakes.

The pseudo-acceleration response spectra at 5% damping, also presented in Figure 7, provide further insights into the frequency-dependent behavior of the ground motions. For both directions, the spectral acceleration increases with the structural period, reaching peak values in the range of $0.1$ to $0.5$ s. However, the y-direction spectra consistently display higher spectral accelerations compared to the x-direction, underscoring the dominant role of the y-direction motion in influencing the dynamic response of the tested structure. This observation is critical for interpreting the results of the shaking table tests, as the increased energy demand in the y-direction could lead to higher strains and stresses in the structural system.

7.2. Test Setup

The biaxial shaking table tests were conducted on a full-scale structural prototype made from recycled Tetra Pak panels, designed to evaluate its seismic performance. The MTS biaxial shaking table consisted of a rigid steel base capable of simulating complex ground motion patterns thanks to its dynamic actuators. This MTS shaking table features a payload capacity of 100 kN, a peak acceleration of $\pm 10.0 \mathrm{g}$ in both x- and y-directions, and maximum displacements of $\pm 0.25 \mathrm{m}$. The table operates with a frequency range from 0.1 to 50 Hz, allowing for precise replication of ground motions typical of seismic events. The movements in the y-direction were programmed to be of higher intensity compared to those in the x-direction, reflecting real seismic disaggregation analyses conducted. The primary structural system was installed on the shaking table, comprising four columns, primary girders, secondary beams, and a flooring system.

The structure was precisely instrumented to measure acceleration and displacement responses under seismic excitation in both x- and y-directions, enabling a comprehensive assessment of its dynamic behavior. The acceleration data were collected using eight uniaxial ICP accelerometers (model 393B04), each with a sensitivity of $1000 \mathrm{m}\mathrm{V}/\mathrm{g}$ ($102 \mathrm{m}\mathrm{V}/(\mathrm{m}/{\mathrm{s}}^2$)) and a measurement range of $\pm 5 \mathrm{g}$. These accelerometers were installed at four strategic points on the roof of the structure (A, B, C, and D), with two sensors per point to measure accelerations in both the x- and y-directions. Figure 8 shows the location of the 8 accelerometers. Their broadband resolution (0.000003 g RMS) and hermetically sealed titanium casing ensured precise and reliable data collection throughout the tests. In this way, each point measured total seismic accelerations in both x- and y-directions, allowing for detailed analysis of the structure’s performance under multidirectional loads.

In addition to the accelerometers, four laser displacement sensors (Banner LTF24UC2LDW/30) were employed to monitor inter-story displacements and derive drift ratios. The sensors, mounted at positions J, K, L, and M (Figure 8), have a range of 50 mm to 24,000 mm and an analog output of 0–10 V, ensuring high-resolution displacement tracking. Sensors L and M were strategically positioned at the foundation to monitor base displacements, with sensor M capturing movements in the x-direction and sensor L recording displacements in the y-direction. Simultaneously, sensors J and K were mounted at the roof level to capture roof displacements, with laser sensor J capturing displacements in the x-direction and laser sensor K recording displacements in the y-direction. These sensors recorded total displacements, which were used to calculate inter-story drift ratios. The data acquisition system (DAQ system) was carefully calibrated to ensure high accuracy and reliability of the collected data with a measurement frequency of 512 Hz.

The structural prototype was prepared with non-structural façade elements affixed to simulate realistic housing conditions. These façades were mounted on the frame prior to testing. To replicate real-world seismic conditions, a series of preprogrammed seismic excitations were applied following a protocol based on previous studies and seismic disaggregation analyses for Bogotá (return periods of 31, 225, 475, and 2500 years). The biaxial shaking table system allowed for synchronized and controlled application of seismic inputs in orthogonal directions.

The final configuration of the housing prototype is shown in Figure 9a, depicting the structure fully assembled on the testing platform. Additionally, Figure 9b presents a 3D schematic representation of the experimental setup, illustrating the configuration of the housing module and its positioning on the biaxial MTS shaking table with its two dynamic actuators. This comprehensive instrumentation and test configuration ensured robust and detailed data collection, forming the basis for a thorough evaluation of the innovative housing system’s seismic performance. This experiment was conducted at the Structural Laboratory of Pontificia Universidad Javeriana in Bogotá, Colombia, one of the leading research facilities in Latin America with substantial investments in advanced materials and structural testing equipment.

7.3. Acceleration and Drifts Records

As shown in Figure 10, the acceleration time histories recorded at point C during the biaxial shaking table tests exhibit differences in response across the x- and y-directions, reflecting the dynamic characteristics of the full-scale Tetra Pak housing system. For the x-direction, peak ground accelerations (PGAs) ranged from 0.11 g to 0.37 g, while in the y-direction, they varied from 0.14 g to 0.52 g, indicating a stronger seismic input in the y-direction. This trend is consistent with the applied seismic protocol, where ground motions in the y-direction were intentionally scaled to higher intensities. The response accelerations in both directions exhibit amplification relative to the input PGAs. For instance, in the x-direction, the maximum recorded response acceleration for the highest intensity level ($PG{A}_x=0.37 \mathrm{g}$) reached 0.94 g, while in the y-direction, the response peaked at 1.10 g for $PG{A}_y=0.52 \mathrm{g}$. This amplification highlights the dynamic effects driven by the natural frequencies of the structure. Comparing the x- and y-direction responses also highlights the role of structural stiffness distribution in governing the seismic behavior of the Tetra Pak housing system. The larger accelerations in the y-direction are attributed to the increased input energy and the relative flexibility of the structure in this direction.

The Fourier spectrum of the white noise excitation, recorded at the conclusion of each seismic motion protocol, was analyzed to determine the fundamental frequencies (or natural periods) of the structure. The fast Fourier transform (FFT) results for the x-direction are presented in Figure 11. The results obtained from the fast Fourier transform (FFT) analysis demonstrated a strong correlation between the experimentally recorded data and the finite element model (FEM), validating the numerical representation of the system. Given this agreement, the FEM can be reliably used for further analysis of the structural response. However, future refinements in modal identification could benefit from advanced computational techniques, such as the fully automated operational modal identification approach described in [38]. While originally developed for steel structures, this method presents a promising framework for improving modal parameter estimation, which could be adapted to optimize the numerical calibration process for alternative materials like TSPA. The analysis confirms that the structural period remained constant across all induced seismic motions, indicating that the structure remained predominantly within the elastic range throughout the testing process. The dominant period of the structure was identified as 0.52 s in the x-direction, demonstrating strong consistency within the experimental data. Additionally, the finite element model (FEM) predicted a fundamental period of 0.51 s, showing remarkable agreement with the measured values. This close correlation validates the accuracy of the numerical model, confirming its effectiveness in capturing the structure’s dynamic characteristics.

Figure 11 demonstrates a stable frequency response across all analyzed motions, with no significant period elongation in either direction. This consistency indicates that the structure maintained its elastic properties throughout the seismic testing, as evidenced by the well-defined and stable Fourier amplitude peaks, which showed no noticeable shift or dispersion that would suggest structural softening or damage. Additionally, the system exhibited amplification effects, where ground accelerations exceeding 0.5 g resulted in structural accelerations reaching 1.1 g. Despite this substantial amplification, the structure retained its stiffness and resistance, as confirmed by the absence of period elongation and the preservation of elastic behavior under all seismic intensities.

Additionally, considering that the seismic demand spectrum for the studied region [39] features a design acceleration plateau of approximately 0.7 g, the recorded peak acceleration of 1.1 g in the y-direction indicates a substantial safety margin for the structural system. This result underscores the robustness and superior performance of the recycled Tetra Pak structural system, as it successfully endured ground motions exceeding design-level seismic demands. The system maintained its stiffness and structural integrity throughout the tests, demonstrating its capacity to withstand extreme seismic excitations without signs of degradation.

The drift time histories in the x-direction, based on the second-floor displacements presented in Figure 12, provide critical insights into the structural performance under varying ground motion intensities applied by the biaxial MTS shaking table. Notably, the absence of permanent drifts confirms that the system remained predominantly within the elastic range throughout the testing protocol, exhibiting minimal plastic deformations. This finding is particularly significant given the high drift levels recorded, with peak values approaching 2% during the most intense ground motions ($PG{A}_x=0.37 \mathrm{g}$). These results underscore the structural system’s robust performance, as it effectively sustained the imposed seismic demands without developing residual deformations or signs of structural degradation.

Furthermore, the system’s integrity was preserved even at these significant drift levels, as no structural damage or connection failures were observed upon completion of the seismic protocol. The ability to accommodate substantial deformations without compromising functionality highlights the adaptability and resilience of the moment-frame system. Additionally, the absence of brittle partition walls and the strategic use of adequately separated non-structural elements ensured that these components remained unaffected by the drifts, further enhancing the overall seismic resilience of the structure.

7.4. Acceleration and Drift Profiles Compared to Results of the FEM

The comparison between the maximum drift profiles and acceleration profiles obtained from experimental measurements and numerical simulations using the FEM (SAP2000) is presented in Figure 13 and Figure 14. These figures provide a comprehensive understanding of the structural response of the recycled Tetra Pak house under seismic loading conditions. From Figure 13, it is evident that the drift profiles recorded experimentally align closely with the numerical predictions for both x- and y-directions across all ground motion intensities. The drifts remain within a consistent range, showing only slight deviations between the numerical and recorded data (the average accuracy is 89.7% with a CoV of 0.15). The most significant divergence is observed at higher intensities (e.g., $PG{A}_x=0.37 \mathrm{g}$), where the recorded drifts tend to exceed the numerical predictions (2% during the test, and 1.5% according to the numerical model, for a 75% accuracy). The observed discrepancy between the maximum drift measured experimentally and that estimated by the finite element model can be attributed to some simplifications in the numerical model, including idealized assumptions about rigid connections. Additionally, the recycled Tetra Pak material used in the construction exhibits variability in its mechanical properties due to the recycling and manufacturing process. This variability, particularly under high-intensity seismic demands, is challenging to accurately represent in the FEM. However, the difference is not substantial enough to suggest a misrepresentation of the system’s behavior in the FEM. These results indicate that the numerical model effectively captures the overall displacement trends and the response of the structure, particularly for the first three return periods, 31 years~225 years~475 years, with the latter being the design return period according to Colombian seismic code standards.

Similarly, the acceleration profiles shown in Figure 14 demonstrate a high degree of agreement between experimental and numerical data for both directions of motion. The profiles indicate that the FEM accurately replicates the distribution of accelerations across the structure’s height, particularly for the higher intensities where dynamic amplifications are most pronounced. Minor discrepancies between the experimentally recorded accelerations and those predicted by the model were observed; however, these differences are negligible and remain within acceptable limits for experimental validation. Together, these observations validate the fidelity of the FEM in representing the structural behavior of the Tetra Pak house. The numerical model captures critical parameters such as drift and acceleration distribution, both of which are essential for evaluating seismic performance. Although non-linear data are not yet available at this stage of the research, the recorded drift profiles demonstrate a significant lateral displacement capacity while remaining within the linear range of the base structural material. This suggests a level of seismic resilience, as indicated by existing models [40]. Additionally, considering that the primary deformation mechanism at the material level is governed by a tension field effect in the polymeric-based TSPA panels, this behavior may exhibit similarities to that of a steel moment frame. However, a comprehensive assessment of this comparison requires further investigation.

8. Conclusions

The investigation of a full-scale moment-frame housing system constructed entirely from recycled Tetra Pak panels represents a significant advancement in sustainable and resilient construction solutions. This study thoroughly examined the system’s seismic performance under a protocol of ground motions, offering valuable insights into the feasibility of utilizing recycled materials in structural applications. By employing biaxial shaking table tests and numerical modeling, the research provided a comprehensive evaluation of the system’s dynamic behavior and structural integrity. The findings not only validate the potential of recycled Tetra Pak as a viable construction material but also underscore its alignment with global sustainability goals. Below, the key conclusions drawn from this research are presented:

Through biaxial shaking table tests, the structure maintained elastic behavior throughout the seismic protocol, with no residual deformations or structural damage observed. Even at high seismic demands, such as a peak ground acceleration of 0.52 g in the y-direction, the system effectively resisted forces while amplifying accelerations to 1.10 g without compromising stiffness or resistance. Furthermore, the ability of the structural system to withstand high acceleration levels without cracking or experiencing plastic damage highlights the material’s toughness and its capacity to dissipate seismic energy through controlled deformation. However, this behavior introduces two important considerations: (i) the selection of non-structural materials that must match the flexibility and deformation capacity of the structural system to prevent premature façade damage due to differential movements, and (ii) the implications of scaling this system to two- or three-story configurations, where inter-story drifts and acceleration amplification effects may impose additional design challenges. These aspects warrant further research to ensure the feasibility of the system for multistory applications while maintaining its seismic resilience.

Drift measurements, with maximum values nearing 2%, confirmed the structure’s capacity to accommodate significant deformations without impacting structural integrity. The absence of brittle partition walls and the use of adequately separated non-structural components further ensured the system’s resilience, even under intense seismic loading. The results also demonstrated that the structural configuration effectively protected both structural and non-structural elements.

The finite element model (FEM) developed for the structure accurately predicted its dynamic response, including natural vibration periods, drift profiles, and acceleration demands (with an accuracy close to 90%). The slight discrepancies between the FEM and experimental results were attributed to inherent simplifications in the numerical model and the variability of the recycled Tetra Pak material. Despite these differences, the FEM proved to be a reliable tool for assessing the seismic behavior of this novel construction system.

The modular nature of the Tetra Pak-based structural system enables rapid assembly and deconstruction, aligning with circular economy principles. This adaptability makes the system particularly suitable for deployment in disaster-prone areas, where quick construction of resilient and sustainable housing is critical. Additionally, the system’s lightweight properties significantly reduce foundation requirements, enhancing its practicality and cost-effectiveness.

The research highlights the potential of recycled Tetra Pak materials as a sustainable alternative for construction in seismic-prone regions. By integrating waste materials into load-bearing systems, the study aligns with global efforts to reduce environmental impact and promote resource efficiency. The findings emphasize the feasibility of scaling this technology to broader housing applications while maintaining structural and environmental performance.

9. Patents

The present research is covered under the patent disclosure in Colombia, register NC 2019/0012797, and with patent disclosure in Chile under register No 69.063.

10. Future Works

The future of this research extends beyond laboratory testing into real-world implementation through the establishment of a spin-off initiative. This venture is actively driving the transition of the proposed structural system from experimental validation to practical applications, addressing the urgent need for sustainable and resilient housing. As part of this effort, full-scale housing units have already been constructed using the recycled Tetra Pak-based structural system, demonstrating its feasibility and scalability in real-world conditions (Figure 15). While this marks a significant milestone, further research remains necessary to refine the system and expand its applicability. A key area of ongoing investigation is the long-term performance of Tetra Pak-based structural components under environmental exposure. Given that the material originates from postconsumer waste, its durability under varying conditions such as moisture, temperature fluctuations, and UV radiation must be thoroughly assessed. These studies will provide critical insights into the necessary maintenance strategies and potential protective treatments to enhance the system’s longevity.

Future research will focus on integrating non-linear material behavior into finite element models to enhance the accuracy of structural response predictions under extreme seismic loading. Understanding the postyield performance of the material is crucial for optimizing its application in earthquake-resistant designs. Additionally, fire resistance assessment will be a key research priority, ensuring the structural system’s stability under high temperatures for broader acceptance and regulatory compliance. To further support the scalability and economic feasibility of the system, life-cycle assessments (LCAs) and cost–benefit analyses will be conducted to quantify its environmental impact and financial viability, particularly for disaster-prone and low-income regions. Investigating the inelastic response of the TSPA structural system will also be essential to accurately quantify its seismic resilience. Comparative analyses with conventional steel moment frames will help identify similarities and differences in deformation mechanisms and energy dissipation capacity. Finally, future studies will explore the integration of other recycled materials for applications such as roofing systems, further advancing the system as a comprehensive and sustainable construction solution.