Experimental Assessment of the Influence of Drywall Infills on the Seismic Behaviour of RC Frame Buildings

Abstract

The use of drywall as a non-structural infill has grown significantly due to its rapid and economical installation. Despite this widespread use, a common assumption in structural design is that these elements do not significantly affect seismic performance and are often ignored in analysis. This assumption, however, is increasingly questioned. This study presents a full-scale experimental evaluation of the influence of drywall infill on the seismic response of reinforced concrete frames under cyclic loading. The results quantify how the inclusion of these non-structural elements alters the dynamic properties and structural response of the frame. The infill increased the initial lateral stiffness by approximately three times with respect to the bare frame, thus modifying the structure’s fundamental period. The infill also altered the failure mechanism, initiating with a transient compression strut action at very low drifts, which rapidly and concurrently transitioned into a dominant membrane behavior. This membrane contribution ceased abruptly at a drift of 0.89%, prior to the life-safety limits specified by Eurocode 8. The study’s findings demonstrate the necessity of incorporating the non-linear stiffness and energy dissipation of drywall into structural models to ensure reliable and accurate predictions in seismic design methodologies.

1. Introduction

Throughout history, seismic movements and their catastrophic effects have been a concern for humanity. Damage to structures and non-structural elements, human fatalities, economic losses, and the ability to return to normalcy after earthquakes are critical considerations in seismic-resistant design. Earthquakes such as the large one in Maule, Chile (2010); the moderate one in Lorca, Spain (2011), and the recent Gaziantep-Kahramanmaras event in Turkey–Syria (2023) highlight the importance of studying all aspects affecting structural performance.

In high seismicity regions, reinforced concrete framed buildings are commonly used to resist ground motions. While reinforced concrete frames have been extensively studied, leading to design methods like response spectra and pushover analysis, the interaction between these structural systems and non-structural infill elements (used for partitioning or as exterior facades) remains insufficiently addressed.

It is well established that infill walls influence structural behavior by increasing a building’s overall stiffness and reducing its fundamental period (Bertero and Brokken, 1983 [1]). Current seismic design codes, such as Eurocode 8 (2020) [2], ACI 318-19 (2019) [3], and NCSE-02 (2002) [4], are based on a nonlinear structural response philosophy that assumes a gradual reduction in stiffness accompanied by an increase in energy dissipation. In this context, recent research has emphasized the critical importance of incorporating complex boundary conditions—such as Soil–Structure Interaction (SSI) and foundation flexibility—into numerical models to accurately predict the global seismic response and displacement demand (Patrício et al., 2024 [5]; Bagheri et al., 2025 [6]). Similarly, and often with a decisive impact on the structural dynamics, non-structural infills significantly alter this expected behaviour.

Pallarés et al. (2021) [7] demonstrated this effect in a 7-story residential building, where masonry infills reduced the fundamental period by about 30% (see Figure 1) and, according to their simulations, increased seismic demand by approximately 80%. This case underscores the potential risks of neglecting infills, making their accurate simulation both complex and essential.

Numerous experimental and numerical studies have recently quantified the influence of various masonry infill types on frame structures.

Specifically, Papasotiriou et al. (2021) [8] assessed the seismic performance of 32 planar reinforced concrete frames through analytical investigation. These authors concluded that the infill effect is systematically governed by the structural system’s characteristics (i.e., the number of storeys and column strength). Their study revealed that while infills can be beneficial for low-rise frames (e.g., two-storey frames), their presence is primarily detrimental for intermediate-rise moment frames (four- and six-storeys), as they induce an increase in maximum column ductility demand.

In parallel, Baloevic et al. (2022) [9] conducted experimental research on autoclaved aerated concrete block infills. They concluded that while infills significantly augment both the structure’s mass and stiffness, their ultimate effect on resistance and safety remains complex. The authors further established that uniform infill distribution is imperative, since an irregular distribution across the elevation can be highly unfavorable in seismic regions, resulting in a substantial increase in both drift and column strains in the flexible storeys.

In general, the most studied infill materials are concrete blocks, ceramic bricks, and concrete panels. However, drywall is increasingly used due to its rapid and easy installation, yet its interaction with structural frames is still under-researched. Drywall consists of plasterboard panels screwed to a framework of cold-formed steel channel profiles anchored to the main structural system (see Figure 2).

Cold-formed steel profiles are classified as non-structural due to their lower stiffness and strength compared to structural members. Plasterboard panels are brittle, with failure mainly characterized by tearing around fasteners, buckling of panels and detachment, local crushing at panel edges and buckling of the metal studs when subjected to in-plane cyclic loads. Nevertheless, several studies have demonstrated that despite their apparent fragility, drywall systems can significantly contribute to lateral stiffness and resistance.

Initial studies demonstrated the substantial contribution of drywall infills to lateral stiffness. According to Gad et al. (1999) [10], drywall infills can significantly enhance lateral stiffness; their study on an Australian residential frame revealed that the ultimate lateral load capacity of a drywall-infilled frame was three times higher than that of a bare frame. Liew et al. (2002) [11] also reported that cold-formed steel studs contribute to lateral resistance in timber frames, observing that adding an extra stud increased resistance by 50%.

Regarding residential buildings in the United States, which are often timber-framed with drywall partitions, Hart et al. (2008) [12] compiled performance data on these partitions under cyclic loads. They described these systems as plasterboard shear walls and noted that, in many cases, they were the only system providing resistance to lateral seismic forces.

In the subsequent period, experimental research began quantifying the seismic performance of these systems using Performance-Based Earthquake Engineering (PBEE) metrics. For instance, Petrone C. et al. (2015) [13] conducted quasi-static tests on 5 m high plasterboard internal partitions typical of European industrial buildings. The authors concluded that the collapse mode is characterized by out-of-plane buckling of the cold-formed steel studs, which initiates failure in the attached plasterboard panels at the horizontal joints. Crucially, the authors developed fragility curves for three different damage states (DS1, DS2, and DS3) based on drift ratios (IDRs), providing median IDR values up to 2.05% for collapse (DS3).

However, while early studies examined timber frames and high partitions, limited research has addressed the influence of drywall on reinforced concrete or steel frames using dynamic protocols. Magliulo et al. (2012) [14] investigated the seismic performance of plasterboard partitions via shake table tests on a steel test frame, observing damage onset at approximately 0.8% inter-story drift and noting a significant increase in the damping ratio (from 1.5% to 5.7%) due to the partitions. Fiorino et al. (2019) [15] performed shaking table tests on scaled RC buildings with drywall and found the fundamental frequency increased by approximately five times, while the dynamic response was amplified by up to four times for interior partitions.

To complement these structural-level studies, recent research has provided robust characterization at the component level. Magliulo et al. (2025) [16], for instance, conducted an extensive experimental campaign specifically focused on the hysteretic behaviour and damage of screw connections between plasterboard panels and cold-formed steel studs. The authors successfully estimated the influence of numerous key parameters (e.g., board thickness and stud cross-section) on the connection’s mechanical response, generating quantitative data essential for the calibration of advanced numerical models.

Despite these advancements in component modeling, a clear need remains to characterize the overall influence of the drywall system on full-scale reinforced concrete structures during seismic events, particularly through large-scale experimental validation. This article presents the results of an experimental study on two full-scale RC frames, one bare and one infilled with drywall, subjected to horizontal cyclic and pulse loads and the results that will allow estimating the impact of plasterboard infills on the vibration periods of a frame, the change in the deformational behavior of the nodes and the type of stiffening mechanism (diagonal compression strut or membrane effect) of the infill on the structure.

2. Research Context and Motivation

2.1. Context Framework

To substantiate the assertion regarding the critical need for further research on the impact of gypsum board partition walls on structural performance under seismic loading, a historical context is required. To better contextualize the limited number of scientific publications concerning the influence of drywall systems on the structural seismic response, a timeline was developed to illustrate the historical development and global adoption of these systems (see Figure 3).

While drywall was invented in the early 20th century, becoming widespread in the United States after World War II [17], its expansion into Europe only commenced in the 1980s, followed by popularization in Western Europe and Asia in the 1990s [18]. A significant expansion in Latin America occurred in the 2000s [19], while Oceania saw widespread popularization much earlier, in the 1940s/1950s, concurrently with the USA [20]. Crucially, full-scale seismic studies on drywall infills only began to emerge in the 2010s, with a growing focus on their interaction with reinforced concrete frames. This delayed research trajectory helps to explain the current dearth of scientific articles addressing the seismic performance of drywall systems.

2.2. Bibliometric Study

The rapid worldwide increase in the use of drywall underscores the necessity for experimental and full-scale studies to accurately characterize the parameters associated with the cyclic response of the interaction between structures and drywall. Developing a robust and quantifiable database is essential to ensure accurate structural analysis of buildings and adherence to performance-based seismic design standards.

This section presents the results of a bibliometric study on the representation of drywall in the current scientific literature within the field of seismic engineering. The study also compares its presence with that of other non-structural infills, such as non-structural masonry infills. Data were obtained from Elsevier’s ScienceDirect database, considering the following parameters:

• Presence of terms such as “seismic” or “earthquake” or “cyclic”; “infill” or “infilled”; “masonry”; “drywall” or “plasterboard” or “gypsum board”; “reinforced concrete”; “experimental” and “full scale” in the title, abstract, or keywords of the publication.
• Coverage from 1989 to the present.
• Publications categorized as “Article” or “Review”.
• Subject area: “engineering”.

As of December 2024, the search combining the terms “seismic” or “earthquake” or “cyclic”; “infill” or “infilled”; “reinforced concrete,” and “experimental” produced a total of 2963 articles. Among these, 1318 are related to “masonry,” and 46 to “drywall,” “plasterboard,” or “gypsum board.” The latter accounts for approximately 1.5% of the published articles (see Figure 4).

Furthermore, analysing how many of the 46 articles on “drywall” (or “plasterboard” or “gypsum board”) address “full-scale” studies, only 15 publications were identified. This fact provides an idea about the very few papers related to drywalls when compared to the number of masonry papers, and highlights the relevance of this article, which contributes to expanding the database on the cyclic behaviour of reinforced concrete frames with drywall infills.

2.3. Research Motivation and Objectives

Despite progress in component-level and steel-frame studies, full-scale experimental data on the interaction between drywall partitions and reinforced concrete frames remains scarce. This gap is critical given the distinct stiffness degradation and joint behaviour of reinforced concrete structures compared to steel.

Consequently, this study aims to resolve specific uncertainties in the seismic design of reinforced concrete buildings with drywall infills. The experimental campaign prioritizes the following specific research objectives:

• To quantify the increase in initial lateral stiffness and its impact on the vibration period.
• To identify and describe the in-plane interaction behaviour and load-transfer mechanisms between the drywall infill and the reinforced concrete frame.
• To evaluate the influence of the infill on the damage distribution and yielding sequence of the reinforced concrete structural joints.

By addressing these points, this work aims to provide reliable data to refine seismic design methodologies for reinforced concrete buildings.

3. Experimental Program

The experimental program was conducted using two full-scale reinforced concrete frames to investigate the effect of drywall infill on the seismic response of this structural typology. The campaign comprised cyclic displacement tests and pulse test responses. The specimens are defined as follows:

• Bare Frame: This specimen consists of a reinforced concrete portal frame without any infill, serving as the control specimen to capture the inherent structural response.
• Infilled Frame: This frame incorporates an interior partition made of plasterboard panels. The infill system consists of two 15 mm thick plasterboard panels applied to each face of a cold-formed steel frame. The cold-formed steel frame is constructed using studs and channels of 70 mm depth (described in detail in Section 3.2). The spacing between the studs is maintained at 400 mm. The cold-formed steel frame is anchored directly to the beams and columns of the structural frame, ensuring interaction during the tests (Figure 5 and Figure 6). This setup (double 15 mm panels on 70 mm studs) was selected to strictly adhere to the installation specifications of the Spanish standard UNE 102043 [21], ensuring representativeness of current construction practice.

3.1. Description of the Structural Frame

The reinforced concrete frame consists of two columns and two beams, with nominal dimensions of 3 m high and 5 m wide. The general layout and test scheme, including the dimensions of the test setup, the strong floor, the strong wall, the position of the hydraulic actuator, the locations of instrumentation, and the details of the anchors and connections, are comprehensively detailed in Figure 7.

The frame was designed as a low-ductility structure, representative of intermediate floors in a building, according to the Spanish Seismic Code, NCSE-02 (2002) [4].

All reinforced concrete elements—columns and beams—have identical 300 × 300 mm cross-sections. The structural reinforcement details, including the longitudinal and transverse bar configurations, are fully depicted in Figure 8 and Figure 9. The base of the columns is rigidly connected to the strong floor, ensuring a fixed-base support condition for the test (Figure 7).

The two full-scale specimens used in the experimental campaign—the bare frame and the infilled frame with drywall partitions—are displayed in Figure 10.

3.2. Materials

The concrete used for both reinforced concrete frames was specified as HA-30/F/12/I according to Spanish Standard EHE-08 (2010) [22], which corresponds to a C30/37 concrete class according to the current Eurocode 2 [23]. To confirm the material properties, six cylindrical specimens (150 mm diameter and 300 mm height) were made for each tested frame. Compression tests were conducted using a universal testing machine (IBERTEST, Madrid, Spain) in accordance with UNE-EN 12390-1 (2013) [24], confirming that the concrete strength of both frames meets the C30/37 class requirement. Based on the design specifications for this class, the modulus of elasticity was set to 32450 MPa for all subsequent calculations, in accordance with the provisions of Eurocode 2 [23].

The reinforcement steel used was B-500-SD type. Its mechanical properties were characterized using a testing machine (ZwickRoell GmbH & Co. KG, Ulm, Germany), obtaining a tested yield strength of 558 MPa.

The plasterboard panels used as infill for the frame are 2700 mm in height, 1200 mm in length, and 15 mm in thickness. To characterize the material and verify that the mechanical specifications match those indicated in the technical datasheet, 8 plasterboard specimens were tested in bending—4 in the longitudinal direction and 4 in the transverse direction (see Figure 11)—using a universal testing machine (IBERTEST, Madrid, Spain) in accordance with UNE-EN 520:2005+A1 (2009) [25]. The average longitudinal flexural strength was 585 N, with an elastic modulus of 1860 MPa. In the transverse direction, the average flexural strength was 279 N, with an elastic modulus of 1393 MPa. The flexural strength curves in each direction are presented in Figure 12.

The plasterboard panels are screwed to the cold formed steel frame that is composed by profiles made of DX51D steel with a Zinc Z140 coating. The profiles used for the frame infill are M70/35 Z1 studs and C70/30 Z1 channels. The stud profiles have dimensions of 70 mm depth, 34–36 mm flanges, and 0.62 mm thickness, while the channel profiles have dimensions of 71.1 mm depth, 30–30 mm flanges, and 0.55 mm thickness. All profiles have a length of 2700 mm. The steel has a minimum yield strength of 140 MPa and a tensile strength between 270–500 MPa, in accordance with UNE-EN 10346 (2015) [26] and UNE-EN 14195 (2014) [27].

3.3. Instrumentation

For the pulse tests, two piezoelectric accelerometers (PCB Piezotronics, Depew, NY, USA) were positioned near the top joints, in the longitudinal direction of the frame plane (see Figure 13), to determine the vibration period of the bare and infill frames through in-plane pulse excitation. Dynamic data for the frames were recorded by a Kyowa PCD-320 data acquisition system (Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) with a sampling rate of 5000 Hz. The resulting acceleration data were analyzed using Kyowa DAS-100a software (Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) to determine frequency response characteristics.

For the cyclic tests, a comprehensive instrumentation setup was utilized to capture global deformation, localized infill behaviour, and structural element strains. Linear Variable Displacement Transducers (LVDTs) with a 300 mm range (Penny & Giles, Christchurch, UK) were placed around the perimeter of the structural frame to monitor the overall deformation (Figure 13).

To capture the infill behavior, thirty strain gauges, arranged in ten rosettes, were installed on the drywall to determine the principal strains on the surface of the gypsum boards. Additionally, Digital Image Correlation photogrammetry was utilized to capture the full-field strain distribution using GOM Correlate 2018 software (GOM GmbH, Braunschweig, Germany). A stochastic speckle pattern was applied to the reverse face of the drywall partition, as shown in Figure 14, while the front face was kept clear for visual damage inspection.

Furthermore, strain gauges were placed on the longitudinal steel reinforcement at the joints to monitor their deformation and detect the onset of yielding. Six gauges were installed per joint (see Figure 15 and Figure 16): two on the longitudinal bottom reinforcement of the beam (V-Ni-IE and V-Ni-II), two on the exterior longitudinal reinforcement of the column (P-Ni-SE and P-Ni-IE), and two on the interior longitudinal reinforcement of the column (P-Ni-SI and P-Ni-II). In the notation, “i” represents the joint number in each specimen.

The lateral load was applied using a hydraulic tension–compression actuator (HINE S.A., Olaberria, Spain) with a 250 mm stroke in both directions. The load was measured using an HBM U10M load cell (Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) with a 500 kN capacity.

3.4. Loading Protocol

The load history consists of progressively increasing imposed displacements, which strictly follow the quasi-static loading protocol recommended by FEMA 461 (2007) [28]. The specific displacement increments were defined as 0.62 mm; 1.92 mm; 3.84 mm; 5.76 mm; 7.68 mm; 11.51 mm; 17.27 mm; 25.91 mm; 30.70 mm; 46.05 mm; 61.40 mm; 76.75 mm; 90.00 mm; 100.00 mm; and 120.00 mm; with two sub-cycles for each displacement, as shown in Figure 17. The first peak of each cycle is taken with a positive sign.

The drift ratio (%) was calculated based on the relative horizontal displacement of the frame, defined as the displacement measured by the hydraulic actuator minus the horizontal sliding displacement measured by LVDTs at the base of the columns, divided by the frame height (3000 mm).

This specific protocol by FEMA 461 was selected as it is widely recognized as the standard methodology for assessing the seismic performance of non-structural components. Unlike dynamic tests, the quasi-static approach allows for a precise observation of the progressive damage evolution, enabling a direct correlation between specific inter-story drift levels and the degradation of stiffness and strength, which is essential for developing reliable fragility functions.

4. Results

In this section, the results of the experimental campaign are presented, including acceleration tests and cyclic displacement tests. These experiments aim to evaluate the dynamic properties of the frames and to analyse the influence of drywall infills on their seismic behaviour.

4.1. Vibration Frequency

Acceleration records from excitations applied to the frames were processed using the Fast Fourier Transform with a sampling frequency of 5000 Hz (see Figure 18 and Figure 19). The bare frame’s vibration frequency was 7.813 Hz, while the drywall-infilled frame’s frequency was 19.531 Hz. This represents a substantial increase of 149.92% (approximately 2.5 times), confirming that drywall partitions significantly influence the structural response by increasing the overall stiffness of the system.

These frequency increase values are consistent with external investigations into infills made with both plasterboard and masonry. Fiorino et al. (2019) [15] reported frequency increases of up to 5 times when the infills were made of plasterboard, with even greater increases observed given the low initial stiffness of the frame they were testing.

Regarding the comparison of the frequency increase with other types of infills, such as masonry infills, Pallarés et al. (2021) [7] found that stiffness increases up to 8 times, which translates to an approximately 2.8-fold increase in frequency.

This substantial increase in the natural vibration frequency, which corresponds to a reduction in the fundamental period (T = 1/f), is a key finding. A shorter period generally places the structure in a higher-demand region of the acceleration response spectrum, leading to higher seismic forces. However, it is important to note that the dynamic response of a structure depends on multiple factors, including energy dissipation and drift demand, which will be analyzed in the following sections.

Regarding design implications, the reduction in the fundamental period shifts the structure towards the lower-period region of response spectrum. This shift generally leads to an increase in spectral acceleration demand, resulting in higher design base shear forces compared to the bare frame. This indicates that neglecting the stiffness contribution of the drywall underestimates the seismic actions. For instance, in a typical 7-story building, a decrease in the vibration period of a frame of approximately 2.8 times implies a decrease in the period of the first mode of vibration of 30%, which implies an increase in the acceleration action of 42% (Pallarés et al. [7]).

From a Performance-Based Earthquake Engineering perspective, this stiffening alters the deformation capacity. The rigid drywall infill restricts the frame’s deformation, limiting the activation of ductile mechanisms in the reinforced concrete elements. Therefore, the reduction in displacement capacity is a factor to consider alongside the increase in acceleration demand.

Finally, regarding damping, although not directly quantified in this phase, previous studies such as Magliulo et al. (2012) [14] documented a substantial increase in the equivalent viscous damping ratio, rising from 1.5% for the bare frame to 5.7% after the installation of partitions. This contribution becomes relevant at the onset of non-structural damage (e.g., screw tilting or local gypsum crushing), where frictional energy dissipation mechanisms are activated, influencing the response prior to the ultimate failure of the panels.

4.2. Force–Displacement Curve and Stiffness

The force–displacement relationship is a fundamental metric for evaluating the seismic performance of structures. The results from the cyclic displacement tests, as shown in the hysteretic curves (Figure 20), reveal a significant influence of the drywall infill on this relationship.

The force–displacement curves show a clear increase in the slope for the infilled frame compared to the bare frame, indicating a significant increase in the drywall system stiffness. The infilled frame reached a maximum strength of 152.6 kN at a displacement of 26.66 mm, while the bare frame achieved a maximum strength of 99.6 kN at a much larger displacement of 116.0 mm.

The envelope curve of the hysteresis plot for the positive cycles, shown as a function of drift in Figure 21, illustrates the behaviour of both frames. The infilled frame’s peak strength was reached during cycle 10, at a drift of 0.89% (corresponding to a relative displacement of 26.67 mm), where the plasterboard exhibited out-of-plane displacements (see Figure 22). After this point, a gradual reduction in strength was observed in subsequent cycles.

The drywall partitioning experienced significant damage (see Figure 22) before reaching the drift limit imposed by Eurocode 8 (2020) [2], which ranges between 1.00% and 1.25% depending on the seismic hazard and the importance category of the structure. In contrast, the bare frame, consistent with the expected behaviour of a ductile system, reached its maximum strength at a drift of 3.17%, without experiencing any loss of strength during the test, which is consistent with the minimum drift value accepted (between 2.00% and 2.50%) before the structure experiences a significant loss of strength.

Despite the post-peak strength reduction, the infilled frame exhibited higher force values than the bare frame throughout the entire test (Figure 23). This figure illustrates the force ratio, defined as the force developed by the infilled frame divided by the force developed in the bare frame for the same displacement.

Stiffness, a key parameter in seismic design, also showed a significant increase. As shown in Figure 24, the stiffness of the infilled frame at the first displacement cycle was 19.24 kN/mm, which is a 208.2% (approximately 3 times) increase compared to the bare frame’s stiffness of 6.24 kN/mm.

A rapid stiffness degradation was observed in the infilled frame. In cycles 11 (drift 1.82%) and 12 (drift 2.46%), the stiffness values of the infilled frame were slightly higher than those of the bare frame, due to the remaining contribution of the infill system (i.e., the cold-formed steel frames). Finally, in cycle 13 (2.92% drift), the stiffness of both frames became nearly identical (Figure 24), as the cold-formed steel frames had detached from the structure, as can be observed in the final state of the infill system (Figure 25).

This analysis demonstrates the significant contribution of the drywall infill to the overall stiffness and initial strength of the structure. The differences in stiffness between the two tests were significant until cycle 10, after which it tended to decrease.

4.3. Joints Behavior

The behaviour of the reinforced concrete joints was a key focus of the experimental programme, providing critical insight into the influence of the drywall infill. Strain gauge measurements on the longitudinal steel reinforcement in the columns and beams (Figure 26, Figure 27, Figure 28 and Figure 29) allowed to identify the onset of yielding and compare the performance of the bare and infilled frames.

4.3.1. Bare Frame Behaviour

The bare frame exhibited a flexural yielding failure mode consistent with its design. Strains in the steel at joints 1 and 4, located at the bottom of the frame, indicated yielding of the beam’s bottom reinforcement. The maximum strain values were 3.24 × 10⁻³ at joint 1 (at a drift of 3.12%) and 4.21 × 10⁻³ at joint 4 (at a drift of 2.01%), both exceeding the steel’s yield strain of 2.79 × 10⁻³. In contrast, the column rebar at these joints remained within the elastic range.

At the upper joints, the longitudinal bars of the column at joint 2 yielded, with a strain value of 4.35 × 10⁻³ (at a drift of 2.78%), while the beam rebar remained elastic. At joint 3, both the column and beam rebar yielded, with strain values of 3.00 × 10⁻³ (at a drift of 1.46%) in the column and 3.44 × 10⁻³ (at a drift of 1.64%) in the beam’s bottom reinforcement.

4.3.2. Infilled Frame Behaviour

The presence of the drywall infill significantly altered the frame’s joint behaviour by delaying steel yielding. Initially, at lower drifts, the infill provided considerable stiffness, effectively preventing the free rotation of the joints. As shown by the LVDT sensors (Figure 30 and Figure 31), which compare the deformed shape of the upper beam in both frames, the plasterboard prevented significant flexural deformation in the infilled frame.

Steel yielding in the infilled frame only occurred at higher displacement levels, after the drywall system had begun to fail. In fact, throughout the initial phase of the test, the longitudinal steel rebar in the columns at joints 2 and 3 remained within the elastic range. Once the plasterboard panels began to experience out-of-plane movements, the steel rebar experienced increased strains, with the column reinforcement yielding in cycle 12. Strain values of 3.77 × 10⁻³ for joint 2 and 3.74 × 10⁻³ for joint 3 were recorded at a drift of 2.56%. The upper beam’s steel did not yield and remained within the elastic range in both joints.

4.3.3. Overall Comparison

Comparing the strain gauge data directly confirms that the rebars in the bare frame yielded at earlier drifts than the rebars in the infilled frame. This highlights the substantial contribution of the drywall infill in delaying the formation of plastic hinges within the reinforced concrete frame. The infill shifts the primary damage mechanism from the main structural elements to the non-structural partitions, a behaviour which is consistent with the observations from the force–displacement curves.

However, this modification of the internal load path introduces a trade-off. While the infill relieves flexural demands on the columns during the initial stages, the interaction—initially driven by a transient compression strut and subsequently by a dominant membrane behaviour—concentrates forces at the beam–column interfaces. The membrane mechanism transfers lateral loads through diagonal tension fields anchored into the frame boundaries, thereby increasing the local shear demand at the joints. In this study, this interaction was evidenced by the formation of minor diagonal shear cracks in the joint regions prior to the infill failure.

Crucially, this behaviour differs fundamentally from that of masonry infills, which typically develop stable, high-strength compression struts that can sustain large loads up to higher drifts, often precipitating catastrophic shear failures in columns (e.g., short-column effects or joint shear failure) [Pallarés et al., 2021 [7]]. In contrast, the drywall membrane mechanism relies on the limited bearing capacity of the gypsum at the screw connections. As evidenced by the tests, this capacity degrades rapidly, leading to system failure at relatively low drifts (0.89%). This early failure acts as a load-release mechanism: once the connections fail and the membrane action ceases, the additional shear demand on the joints vanishes, preventing the accumulation of stresses that could jeopardize the structural integrity of the reinforced concrete joints. Regarding multi-storey buildings, while this brittle failure could potentially trigger vertical irregularities (soft-story mechanisms) if partitions fail selectively, the magnitude of the force redistribution is expected to be significantly lower than that induced by heavy masonry infills. Nevertheless, detailed numerical investigations are required to quantify the exact impact of these redistribution effects on the global stability of multi-storey structures

4.4. Dissipated Energy

Energy dissipation is a key indicator of a structure’s ability to absorb seismic forces. The energy dissipated in each cycle, represented by the area enclosed within the hysteretic curves (Figure 20), and the cumulative energy dissipated throughout the tests were analysed for both frames.

As observed in Figure 32, the infilled frame dissipated a higher amount of energy compared to the bare frame throughout the test. During the initial cycles, a substantial difference in energy dissipation was observed, attributable to the friction and interaction between the panels and the frame. As the test progressed up to Cycle 10, the energy dissipation in the infilled specimen was primarily driven by the degradation of the drywall system (crushing and tearing of the panels), which acted as the main energy-dissipating mechanism while the concrete frame remained largely elastic.

After Cycle 10, once the plasterboard panels failed and moved out-of-plane, the contribution of the infill diminished. However, the infilled frame continued to dissipate a considerable amount of energy in the final cycles (e.g., Cycle 13) compared to the bare frame. This behaviour is explained by the delayed onset of damage in the structural elements: since the drywall dissipated the energy demand in the earlier stages through its own failure, the reinforced concrete frame of the infilled specimen retained capacity to dissipate energy through steel yielding in these later stages. In contrast, the bare frame, which had accumulated structural damage from the beginning of the test, was nearing exhaustion.

Figure 33 shows the cumulative energy dissipation for both frames. The infilled frame consistently dissipated more energy over the course of the test, indicating that the non-structural element absorbed a significant portion of the seismic energy demand through its damage process prior to the activation of the main structural mechanisms.

It is important to clarify that the energy dissipation quantified in this study represents the global response of the infilled system. While the progressive degradation of the gypsum panels (crushing and tearing) constitutes a visible and significant component of this dissipation, the frictional interaction between the cold-formed steel channels and the reinforced concrete frame, as well as the local deformation of the steel studs, also contribute to the total energy absorption. Therefore, the results reflect the combined damping capacity of the drywall assembly interacting with the structure. Segregating the specific fraction of energy dissipated by the gypsum material versus the steel framing friction would require detailed component-level testing, which is identified as a necessary line for future research.

4.5. Failure Mode

The failure mode of the bare frame was consistent with the expected behaviour of a low-ductility reinforced concrete frame. It exhibited minimal energy dissipation during the initial cycles. As displacements increased, the hysteresis curve approached flexural yielding, reaching a maximum load of 99.6 kN. The failure was primarily characterized by flexural yielding of the longitudinal reinforcement in the columns and lower beam, which constituted the dominant structural mechanism (Figure 34). The lower beam displayed multiple and significant cracks, particularly in the regions where plastic hinges were formed. The columns also experienced cracking and flexural yielding of the reinforcement, while no shear cracking was observed at the beam–column joints.

In contrast, the drywall-infilled frame exhibited a distinct multi-stage failure mechanism that evolved as the drift increased. At the very onset of loading (initial low-drift cycles), the system behaved linearly as the drywall engaged the reinforced concrete frame. This initial interaction was confirmed by the strain gauge rosettes (Figure 35 and Figure 36) and further corroborated by the Digital Image Correlation analysis. As shown in Figure 37, the horizontal (εx) and vertical (εy) strain maps reveal coincident zones of high compressive strain localized at the loaded corners. This concurrent biaxial compression provides experimental evidence of the formation of a diagonal compression strut. The origin of this initial mechanism is attributed to the junction treatment (gypsum compound and tape) applied between panels and the perimeter interfaces. At very low drifts, this treatment creates a temporary monolithic continuity, allowing the infill to resist lateral loads through a diagonal compression path.

This strut action, however, was highly transient. As the drift increased slightly beyond this initial phase, while the system was still responding in its elastic range, a rapid transition of the load transfer mechanism occurred. This transition was characterized by the concurrent diminishing of the localized strut action and the progressive activation of a dominant membrane behaviour.

To visualize the development of this membrane mechanism, a Digital Image Correlation analysis was performed. The analysis focused exclusively on the plasterboard panel surface, masking the RC frame to eliminate edge effects and shadows. The strain maps obtained at representative drifts of 0.46% and 0.68% (actuator pushing right-to-left) illustrate the progression of this behaviour:

• The Minor Principal Strain (ε₂) fields (Figure 38 and Figure 39) reveal a distinct, wide compression zone concentrated along the left joint region of the panel (depicted in blue), which intensifies and expands as the drift increases from 0.46% to 0.68%.
• Conversely, the Major Principal Strain (ε₁) fields (Figure 40 and Figure 41) exhibit a broad concentration of tensile strains on the opposing right side (depicted in red).

This complementary distribution of extensive compression and tension fields across the panel surface, observed consistently at both drift levels, provides evidence of the dominant membrane behaviour governing the response.

During this brief low-drift phase, both mechanisms coexisted, but the membrane behaviour quickly became the dominant mode of resistance. This membrane mechanism, which relies on the composite action of the plasterboard panels, the screws, and the cold-formed steel frame, governed the response for the majority of the test. This dominant membrane contribution remained effective until a drift value of 0.89% (cycle 10), beyond which the panels ceased to provide stiffness as they experienced a combined failure mode. The primary failure consisted of crushing at the joints and edges, followed by local buckling of the panels and tearing due to out-of-plane displacements (Figure 42, Figure 43 and Figure 44).

This non-structural failure fundamentally altered the behaviour of the reinforced concrete frame. Unlike the bare frame, the infill’s presence shifted the onset of yielding in the longitudinal reinforcement to a higher drift capacity. No cracks were observed in the columns, indicating that the infill redistributed stresses and altered the load path. However, shear cracking, characterized by 45° diagonal cracks, was observed at the interior regions of the joints up to cycle 10, stopping propagation only after the infill had failed.

In summary, the infilled frame initially behaved as a highly stiff composite system, providing substantial lateral stiffness through an initial diagonal strut action that rapidly and concurrently transitioned into a dominant membrane behaviour (which governed the response until failure). This interaction resulted in significant energy dissipation through the degradation of the non-structural element. This failure of the plasterboard panels occurred at a low drift (0.89%), prior to the life-safety drift limits specified in Eurocode 8 (1.00–1.25%). After the infill system failed, the reinforced concrete frame replicated the flexural failure mode of the bare frame, but with the onset of structural damage (steel yielding) occurring at a higher drift capacity.

4.6. Drift Limits and Design Implications

The failure of the drywall system observed at 0.89% inter-story drift provides insight into its performance relative to current seismic codes. This value aligns with the onset of damage reported by Magliulo et al. (2012) [14] at 0.80% drift in steel frames. However, a distinction emerges regarding the severity of the interaction. While prior studies on flexible steel frames reported only “light damage” at this drift level, the present study on a rigid reinforced concrete frame evidences severe degradation and a massive period reduction (consistent with the stiffening effects observed by Fiorino et al. (2019) [15]). This suggests that the stiffer boundary conditions imposed by reinforced concrete frames induce higher local stress concentrations, precipitating failure earlier than in steel systems.

In terms of code compliance, this failure at 0.89% technically satisfies the Damage Limitation (Serviceability) requirements of Eurocode 8, which typically restrict inter-story drifts to 0.5% or 0.75% to protect brittle non-structural elements. However, under severe seismic actions where drifts are expected to reach the 1.0–1.25% range (associated with the Life Safety objective), the standard partitions will have already suffered brittle failure. This creates a critical performance gap: while the reinforced concrete frame maintains its integrity and ductility reserves, the premature collapse of the infill generates safety hazards (falling debris) and economic losses well before the structure reaches its design limit.

Therefore, although the system meets serviceability criteria, the rapid transition to brittle failure at higher drifts highlights the necessity of decoupling strategies (isolation) or drift-tolerant connections in high-seismicity regions. These measures are essential not just to limit damage, but to ensure that the non-structural elements can accommodate the deformations of the primary structure during severe events without posing a risk to occupants.

5. Conclusions and Future Research

The experimental investigation demonstrates the significant influence of drywall infill on the seismic response of reinforced concrete frame structures. Based on the cyclic displacement tests on a bare and an infilled frame, the following conclusions are drawn:

• The drywall infill system substantially increases the overall lateral stiffness and modifies the dynamic properties of the structure. The infilled frame exhibited an increase of about 208.2% (approximately 3 times) in initial stiffness and 149.9% (approximately 2.5 times) in natural vibration frequency. This reduction in fundamental period may shift the structure into a higher-demand region of the acceleration response spectrum.
• Despite being a non-structural element composed of brittle materials, the drywall system (plasterboard, screws, and cold-formed steel studs) develops a complex multi-stage load transfer mechanism. It initiates with a transient diagonal strut action at very low drifts, which rapidly and concurrently transitions into a dominant membrane behaviour. This membrane contribution ceases abruptly once the panels fail at an interstory drift of 0.89%.
• The infilled frame exhibited energy dissipation capacity, primarily derived from the progressive degradation of the drywall system (crushing and tearing). Additionally, minor shear cracking was observed in the beam–column joints, which ceased to propagate once the infill failed. Thus, the observed energy absorption is strictly a consequence of the non-structural element’s failure process and its interaction with the frame joints.
• The presence of the infill fundamentally changes the failure mode of the reinforced concrete frame. The increased stiffness forces the structural elements to resist higher lateral loads for smaller displacements. The infill then fails sequentially, allowing the reinforced concrete frame to replicate the flexural failure mode of the bare frame only in later cycles, with the onset of primary structural damage (flexural yielding) occurring at a higher drift capacity, as evidenced by the absence of cracking in the columns.
• The significant influence of the drywall infill demonstrates that this non-structural element must be appropriately accounted for in seismic design, since it changes the structural response and failure mode. To ensure a satisfactory design, these systems must either be included in the structural analysis to predict their behaviour or their interaction with the structure should be isolated to allow the frame to behave as intended, consistent with a performance-based seismic design approach.
• Given the complex, nonlinear nature of the infill–frame interaction—characterized by a rapid transition from diagonal strut action to membrane behaviour—and its crucial influence on structural response, there is a clear need to develop robust and computationally efficient numerical models. This experimental work provides the essential validation basis for future modeling efforts. The primary goal of this future research is to develop numerical strategies that can reliably capture the observed effects on stiffness, strength, and energy dissipation, while remaining easily implementable for practicing engineers. Achieving this balance between high-fidelity models (for validation) and the development of simplified, yet reliable, design tools (for ease of application) is necessary to fully integrate these findings into a modern performance-based seismic design methodology.

In summary, the interaction between non-structural drywall infill and reinforced concrete frames is a complex, nonlinear phenomenon. While infills enhance initial stiffness and peak strength, they fail at relatively low drifts, requiring a critical re-examination of design assumptions for reinforced concrete buildings with drywall partitions.

While this study provides valuable full-scale experimental data regarding the seismic interaction between drywall infills and reinforced concrete frames, several limitations inherent to the experimental design must be acknowledged:

• First, the experimental campaign focused on a specific drywall configuration. This setup (double 15 mm panels on 70 mm studs) was selected to strictly adhere to the installation specifications of the Spanish standard UNE 102043 [21], ensuring representativeness of current construction practice. Consequently, the reported stiffness and strength contributions are specific to this standard assembly; variations in board thickness, stud gauge, or screw spacing—different from those prescribed by the norm—could alter the interaction.
• Second, the specimen geometry consisted of a single-storey, single-bay frame. While effective for isolating fundamental interaction mechanics at the component level, this setup does not capture global system effects present in multi-storey buildings. However, this sub-assembly approach provides the essential experimental data required to calibrate macro-models which can subsequently be extrapolated to assess complex structures.
• Third, tests were conducted under pinned-base boundary conditions. In real buildings, foundation flexibility (Soil–Structure Interaction) can modify the dynamic response; however, the pinned base was chosen to strictly quantify the stiffness of the superstructure components without external variables.
• Finally, the instrumentation setup was primarily designed to capture the in-plane global response (stiffness, strength, and drift). While the failure mechanism of the drywall involved out-of-plane buckling and tearing—inherently 3D phenomena—these were monitored visually and correlated with the in-plane force drop.

Future research should address these points through advanced numerical modelling calibrated with these experimental results to assess multi-storey behaviour and the influence of foundation flexibility. Furthermore, experimental campaigns on full-scale multi-storey buildings, the use of 3D instrumentation and dynamic testing in shake table are recommended to validate the global interaction effects and local deformation patterns in a realistic structural assembly.