Investigation of in-plane and out-of-plane seismic behavior of aac infill walls with innovative bed-joint reinforcement configurations

Abstract

The infill walls, which are widely used in reinforced concrete (RC) structures as separators between areas and thermal insulators, in general, are made of brittle materials and are often overlooked in the structural design procedures without performing comprehensive safety assessments. Recent earthquakes (e.g., 2009 L'Aquila, 2011 Van, 2020 Izmir and 2023 Kahramanmaras) indicated that infill walls can be subjected to severe damages which results in significant injuries and life losses. In addition, failure of an infill wall may cause serious damages to other non-structural elements and costly equipment (i.e. in hospitals, laboratories and offices) which may disrupt the functionality of critical facilities that are intensively needed after destructive earthquakes. In addition to this, considering the fact the nonstructural content of buildings covers the majority of the total building cost, the safety assessment and performance enhancement of infill walls have a respectable contribution to reduce the economic burden after seismic events. During seismic events, the infill walls are subjected to both in-plane (IP) and out-of-plane (OOP) actions. Consequently, the prior IP damage in the infills due to earthquake actions is found to reduce the OOP capacity of the infill walls, which causes the expulsion and collapse of the infills. Even though seismic design codes enforce safety checks in the IP and OOP direction of infills separately, the effect of IP damage on the OOP performance of infills is not considered. Enhancing the seismic performance of infill walls is crucial for creating a resilient building environment. There are various methods available to improve the infill performance under seismic actions, including the external application of fiber reinforced polymers and the use of flexible joints between infills and the structural frames. One practical and effective solution for enhancing the OOP performance of infill walls is the utilization of bed-joint reinforcement. This approach was initially explored in the last quarter of the 20th century for infill panels and continues to be the subject of ongoing scientific research projects. The industry is also developing new types of bed-joint reinforcements, such as cord reinforcements and polymer meshes. During construction, bed-joint reinforcement is applied to the infills by placing horizontal reinforcement on top of a completed infill course before continuing with the assembly of the next course. The reinforcement area and vertical spacing can be adjusted based on the wall's design requirements. This thesis specifically focuses on evaluating the performance improvements achieved by utilizing new-generation bed-joint reinforcement systems in infill walls from a structural engineering perspective. The works carried out in this Ph.D. project aim to assess the enhancements in strength, stiffness, and energy dissipation capacity achieved by employing both traditional and new-generation bed-joint reinforcement systems in infill walls. Additionally, the works seek to evaluate the potential delay in damage propagation resulting from the use of bed-joint reinforcement, as the condition of infills plays a crucial role in the serviceability of earthquake-damaged buildings. The project also investigates the impact of pre-existing IP damage on the OOP response of infill walls. To ensure design reliability, the experimental strengths are compared with the strength values calculated according to Eurocode 6, aiming to determine whether the lack of consideration for prior IP damage in the design of infill walls leads to conservative or unconservative design strength values. To achieve these purposes the objectives of the research are as follows: (i) execution of full-scale experimental tests to investigate the effect of bed-joint reinforcement on the IP and OOP performance of infill walls from strength, damage propagation and energy dissipation capacity points of view; (ii) experimentally observing the effects of prior IP damage on the OOP response of the infill walls; (iii) analytical evaluation of the experimental OOP strength values with the capacities obtained from the methodology adopted in Eurocode 6; (iv) development and validation of a numerical modeling approach that can sufficiently represent the specimen responses, which would be used to generalize the experimental findings. Although infill walls can be constructed using a variety of infill units (such as hollow clay bricks, clay bricks, hollow concrete blocks, and Autoclaved Aerated Concrete (AAC), etc.) with various thicknesses, the seismic performance of 150 mm thick AAC infill walls is the main emphasis of this thesis. The major reason for this is that the number of studies evaluating the response of AAC infills during earthquakes is rather limited, and those investigating the response of AAC infills dealt with relatively thicker walls. Additionally, one of the most typical infill layouts utilized in Turkey is AAC infill walls with a thickness of 150 mm. Two types of bed-joint reinforcement configurations are used to investigate their contribution to the OOP performance of infill walls namely truss-type and cord-type. After introducing the research motivations and scope of the thesis (Chapter 1), a literature review study is conducted in Chapter 2. Past experimental research items investigating the IP/OOP interaction and those evaluating the benefits of using bed-joint reinforcement are reviewed. The results of earlier experimental research showed that the OOP strength of infills is decreased as a result of the IP effects of the seismic events, which increases their vulnerability. It is currently difficult to make a thorough and quantitative conclusion from the existing research because of the variability in specimen geometries, infill unit types, and the applied IP drift levels prior to the OOP testing. In addition, in the light of the previous experimental research, the use of bed-joint reinforcement is found to substantially contribute to the OOP performance of infill walls. There is a need to investigate the seismic performance enhancement of bed-joint reinforcement to the relatively thin AAC infill walls under parametric IP/OOP interaction. In Chapter 3, the details of the experimental test setup and instrumentation, which enables the IP and OOP loading of the specimens without reassembling are presented. The reinforcement details of the full-scale RC frame, which is used to surround the infill walls, are presented together with material test results. The mechanical properties of the bed-joint reinforcements are presented, and the reinforcement configurations of the specimens are given. The test matrix covers 8 infill wall specimens. All specimens are tested in the OOP direction, and 5 of them are also imposed to cyclic IP displacement reversals up to 0.005 and 0.010 drift ratios. The loading protocols followed in the experimental test scheme included cyclic IP displacement reversals and cyclic OOP displacements. The OOP displacements are applied only in the pushing direction. Chapter 4 is dedicated to the presentation of the full-scale experimental test results. The specimen responses in the IP (if tested) and OOP directions are evaluated and force-displacement relationships measured during the experimental tests are presented. Also, key damage observations in the IP and OOP tests are presented considering the performance criteria given in design codes and performance evaluation guidelines. In Chapter 5, a comprehensive evaluation of the experimental observations is presented. The IP and OOP responses of the specimens are evaluated whether the presence of bed-joint reinforcement alters and improves the specimen response under representative seismic actions. Based on the experimental responses of the specimens, the effect of various prior IP displacement reversals on the OOP performance of infill walls is evaluated. In addition, analytical calculations for the OOP capacities are performed and the experimental responses are compared with the capacities calculated according to Eurocode 6. The experimental results indicated that the use of flat-truss and cord-type bed-joint reinforcement provides OOP strength enhancement for infill walls ranging from 21.5% to 36.8% compared to the undamaged specimens in the IP direction. In addition, the damages that are formed through the application of 0.005 IP drift ratio, which is the upper limit for service requirements defined in Eurocode 6, resulted in 30.2-36.3% reduction in the OOP strength of infill specimens. When the prior IP drift ratio is increased to 0.010, the OOP strength reduction is ranging between 40.5% and 45.0% compared to that of the undamaged specimens in the IP direction. In Chapter 6, a numerical modeling methodology that has the ability to represent the specimen responses in the IP and OOP directions is developed. Simplified micro modeling approach is adopted in the model development processes. The model is first validated and evaluated through the IP and OOP experimental test results from strength and damage propagation points of view. The observations indicated that the model has a great potential to represent the damage formation of AAC infill walls in the IP and OOP directions. The major weakness of the developed approach is the representation of pinching in the IP direction and the initial stiffness in the OOP direction. Details and potential reasons for these deficiencies are explained in detail. Chapter 7 comprises of the general conclusions obtained from the experimental tests and numerical analyses regarding the IP and OOP response of the AAC infill walls. Additionally, in the light of the observations, recommendations for future studies are presented.