Effect of URM infills on seismic vulnerability of Indian code designed RC frame buildings

Unreinforced Masonry(URM) is the most common partitioning material in framed buildings in India and many other countries.Although it is well-known that under lateral loading the behavior and modes of failure of the frame buildings change significantly due to infill-frame interaction,the general design practice is to treat infills as nonstructural elements and their stiffness,strength and interaction with the frame is often ignored,primarily because of difficulties in simulation and lack of modeling guidelines in design codes.The Indian Standard,like many other national codes,does not provide explicit insight into the anticipated performance and associated vulnerability of infilled frames.This paper presents an analytical study on the seismic performance and fragility analysis of Indian code-designed RC frame buildings with and without URM infills.Infills are modeled as diagonal struts as per ASCE 41 guidelines and various modes of failure are considered.HAZUS methodology along with nonlinear static analysis is used to compare the seismic vulnerability of bare and infilled frames.The comparative study suggests that URM infills result in a significant increase in the seismic vulnerability of RC frames and their effect needs to be properly incorporated in design codes.     

Procedures for calibration of linear models for damage limitation in design of masonry‐infilled RC frames

Recent earthquakes have confirmed the role played by infills in the seismic response of reinforced concrete buildings. The control and limitation of damage to such nonstructural elements is a key issue in performance‐based earthquake engineering. The present work is focused on modeling and analysis of damage to infill panels, and, in particular, it is aimed towards linear analysis procedures for assessing the damage limitation limit state of infilled reinforced concrete frames. First, code provisions on infill modeling and acceptance criteria at the damage limitation limit state are reviewed. Literature contributions on damage to unreinforced masonry infill panels and corresponding displacement capacity are reported and discussed. Two procedures are then proposed aiming at a twofold goal: (i) the determination of ‘equivalent’ interstory drift ratio limits for a bare frame model and (ii) the estimation of the stiffness of equivalent struts representing infill walls in a linear model. These two quantities are determined such that a linear model ensures a reliable estimation of seismic capacity at the damage limitation limit state, providing the same intensity level as that obtained from nonlinear analyses carried out on structural models with infills. Finally, the proposed procedures are applied to four‐story and eight‐story case study‐infilled frames, designed for seismic loads according to current technical codes. The results of these application examples are presented and discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Fragility functions and floor spectra of RC masonry infilled frames: influence of mechanical properties of masonry infills

A wide number of experimental studies conducted in latest years pointed out the high influence of the mechanical properties of masonry units and mortar bed joints on lateral strength and stiffness of masonry panels. This feature significantly modifies the global response of infilled frames under seismic actions as well as the local interaction phenomena. Despite a wide investigation on the influence of the infills on global behaviour of reinforced concrete (RC) frames has already been provided, different features characterizing the seismic performances of buildings suggest the need of accurately evaluating local interaction phenomena as well as the influence of the panel on specific and relevant aspects, as the accelerations transferred to non-structural components. This study provides a parametrical analysis of the influence of shear strength and elastic modulus of masonry infills on the seismic behaviour of RC frames originally designed for gravity loads. Regular buildings with different height were analysed using the Incremental Dynamic Analysis in order to provide fragility curves, investigate on the collapse mechanisms and define the floor spectra depending on the properties of the infills. Results obtained pointed out the high influence of the considered parameters on the fragility of existing RC frames, often characterized by inadequate transversal reinforcement of columns, which may lead to brittle failure due to the interaction with the infills. Floor response spectra are also significantly affected by the influence of masonry infills both in terms of shape and maximum spectral accelerations. Lastly, on the basis of the observed failure mechanisms, a parameter defining the ductility of the frames depending on the properties of the infills was also provided (Capacity Design Factor). The correlation between the mechanical properties of the infills and this parameter suggests its reliability in the simplified vulnerability analysis of existing buildings as well as for the design of new buildings.     

Effect of masonry infills on seismic performance of a 3‐storey R/C frame with non‐seismic detailing

The objective of this study is to investigate the effect of masonry infills on the seismic performance of low‐rise reinforced concrete (RC) frames with non‐seismic detailing. For this purpose, a 2‐bay 3‐storey masonry‐infilled RC frame was selected and a 1 : 5 scale model was constructed according to the Korean practice of non‐seismic detailing and the similitude law. Then, a series of earthquake simulation tests and a pushover test were performed on this model. When the results of these tests are compared with those in the case of the bare frame, it can be recognized that the masonry infills contribute to the large increase in the stiffness and strength of the global structure whereas they also accompany the increase of earthquake inertia forces. The failure mode of the masonry‐infilled frame was that of shear failure due to the bed‐joint sliding of the masonry infills while that of the bare frame appeared to be the soft‐storey plastic mechanism at the first storey. However, it is judged that the masonry infills can be beneficial to the seismic performance of the structure since the amount of the increase in strength appears to be greater than that in the induced earthquake inertia forces while the deformation capacity of the global structure remains almost the same regardless of the presence of the masonry infills. Copyright © 2001 John Wiley & Sons, Ltd.     

Seismic response of masonry-infilled steel frames via multi-scale finite-element analyses

Effects of masonry infills on the seismic vulnerability of steel frames is studied through multi-scale numerical modelling. First, a micro-modelling approach is utilized to define a homogenized masonry material, calibrated on experimental tests, which is used for modelling the nonlinear response of a one-story, single span, masonry-infilled portal under horizontal loads. Based on results of the micro-model, the constitutive behavior of a diagonal strut macro-element equivalent to the infill panel is calibrated. Then, the diagonal strut is used to model infill panels in the macro-scale analysis of a multi-span multi-story infilled moment-resisting (MR) steel frame. The seismic vulnerability of the MR frame is evaluated through a nonlinear static procedure. Numerical analyses highlight that infills may radically modify the seismic response and the failure mechanism of the frame, hence the importance of the infill correct modelling.     

Strength and stiffness reduction factors for infilled frames with openings

Framed structures are usually infilled with masonry walls. They may cause a significant increase in both stiffness and strength, reducing the deformation demand and increasing the energy dissipation capacity of the system. On the other hand, irregular arrangements of the masonry panels may lead to the concentration of damage in some regions, with negative effects; for example soft story mechanisms and shear failures in short columns. Therefore, the presence of infill walls should not be neglected, especially in regions of moderate and high seismicity. To this aim, simple models are available for solid infills walls, such as the diagonal no-tension strut model, while infilled frames with openings have not been adequately investigated. In this study, the effect of openings on the strength and stiffness of infilled frames is investigated by means of about 150 experimental and numerical tests. The main parameters involved are identified and a simple model to take into account the openings in the infills is developed and compared with other models proposed by different researchers. The model, which is based on the use of strength and stiffness reduction factors, takes into account the opening dimensions and presence of reinforcing elements around the opening. An example of an application of the proposed reduction factors is also presented.     

In‐plane and out‐of‐plane behavior of confined masonry walls for various toothing and openings details and prediction of their strength and stiffness

Eight half‐scale brick masonry walls were tested to study two important aspects of confined masonry (CM) walls related to its seismic behavior under in‐plane and out‐of‐plane loads. Four solid wall specimens tested to investigate the role of type of interface between the masonry and tie‐columns, such as toothing varying from none to every course. The other four specimens with openings were tested to study the effectiveness of various strengthening options around opening to mitigate their negative influence. In the set of four walls, one wall was infilled frame while the other three were CM walls of different configurations. The experimental results were further used to determine the accuracy of various existing models in predicting the in‐plane response quantities of CM walls. Confined masonry walls maintained structural integrity even when severely damaged and performed much better than infill frames. No significant effect of toothing details was noticed although toothing at every brick course was preferred for better post‐peak response. For perforated walls, provision of vertical elements along with continuous horizontal bands around openings was more effective in improving the overall response. Several empirical and semi‐empirical equations are available to estimate the lateral strength and stiffness of CM walls, but those including the contribution of longitudinal reinforcement in tie‐columns provided better predictions. The available equations along with reduction factors proposed for infills could not provide good estimates of strength and stiffness for perforated CM walls. However, recently proposed relations correlating strength/stiffness with the degree of confinement provided reasonable predictions for all wall specimens. Copyright © 2016 John Wiley & Sons, Ltd.     

Seismic fragility of lightly reinforced concrete frames with masonry infills

Seismic fragility of lightly reinforced concrete frames with masonry infills is assessed through numerical simulations considering uncertainty in ground motion and building materials. To achieve this aim, a numerical model of the components is developed, a rational approach to proportion and locate individual struts in the equivalent three‐strut model is proposed, and an explicit nonlinear column shear response model accounting for the infill–column interaction and soft‐story mechanism is employed. The proposed numerical model is used to (1) generate probabilistic seismic demand models accounting for a wide range of ground motion intensities with different frequency content and (2) determine limit state models obtained from nonlinear pushover analysis and incremental dynamic analysis. Using the demand and limit state model, fragility curves for the masonry‐infilled frames are developed to investigate the impact of various infill properties on the frame vulnerability. It is observed that the beneficial effect of the masonry infill diminishes at more severe limit states because of the interaction with the boundary frame. In some cases, this effect almost vanishes or switches to an adverse effect beyond a threshold of ground motion intensities. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerically based proposals for the stiffness and strength of masonry infills with openings in reinforced concrete frames

Aimed at investigating the effect of openings on the in‐plane behaviour of masonry infills in reinforced concrete frames, a parametric study is presented based on model calibration via experimental tests. Two types of openings are investigated: central window openings and different combinations of door and window openings based on the typologies of southern European countries. First, a finite element model of the structure is made using the DIANA software program. Then, after calibration with experimental results, a parametric analysis is carried out to investigate the effect of the presence and location of the different types of openings on the in‐plane behaviour of the infilled frame. Finally, different equations for predicting the initial stiffness and lateral strength of infilled frames with any types of openings were obtained. An α factor related to the geometry of the piers between openings is proposed to take into account the location of the openings in the developed equations. Subsequently, the masonry infill panel is replaced by a diagonal strut. An empirical equation is also proposed for the width of an equivalent strut to replace a masonry infill panel with openings in such a way that they possess the same initial stiffness. Copyright © 2015 John Wiley & Sons, Ltd.     

Nonlinear pushover analysis of infilled concrete frames

Six reinforced concrete frames with or without masonry infills were constructed and tested under horizontal cyclic loads. All six frames had identical details in which the transverse reinforcement in columns was provided by rectangular hoops that did not meet current ACI specifications for ductile frames. For comparison purposes, the columns in three of these frames were jacketed by carbon-fiber-reinforced-polymer （CFRP） sheets to avoid possible shear failure. A nonlinear pushover analysis, in which the force-deformation relationships of individual elements were developed based on ACI 318, FEMA 356, and Chen＇s model, was carried out for these frames and compared to test results. Both the failure mechanisms and impact of infills on the behaviors of these frames were examined in the study. Conclusions from the present analysis provide structural engineers with valuable information for evaluation and design of infilled concrete frame building structures.     

Empirical‐based out‐of‐plane URM infill wall model accounting for the interaction with in‐plane demand

The role of masonry infills in the seismic behavior of reinforced concrete buildings has been widely studied in terms of their strength and stiffness contribution in the in‐plane (IP) direction, while fewer studies have been carried out on their response and modeling in the out‐of‐plane (OOP) direction. In this paper, the state of the art in code and literature provisions regarding infills' OOP capacity and seismic demand is presented, together with a review of the experimental tests that have been carried out to investigate infills' OOP behavior and the effects of IP‐OOP interaction. This review aims to collect an experimental database that is used to evaluate the effectiveness of literature and code provisions and to propose a semiempirical approach both for predicting infills' OOP strength, stiffness, and displacement capacity and for modeling the effects of IP displacement demand on OOP behavior and vice versa. Then, the state of the art on modeling of infills' OOP behavior and IP‐OOP interaction is presented together with a new macro model based on the proposed formulations and conceived to represent the IP and OOP behavior by taking into account the mutual interaction effects. Finally, the proposed model is used for an example application on two case‐study buildings, showing the effects of taking into account or neglecting the IP‐OOP interaction phenomena.     

Implementation and verification of a masonry panel model for nonlinear dynamic analysis of infilled RC frames

The effect of infill panels on the response of RC frames subjected to seismic action is widely recognised and has been subject of numerous experimental investigations, while several attempts to model it analytically have been reported. In this work, the implementation, within a fibre-based Finite Elements program, of a double-strut nonlinear cyclic model for unreinforced masonry panels is carried out. The adequacy of the model in predicting the cyclic/seismic response of multi-storey infilled reinforced concrete frames is then verified through comparisons against experimental results.     

Pseudo‐dynamic seismic response of reinforced concrete frames infilled with non‐structural brick masonry

This paper presents pseudo‐dynamic test results on the in‐plane seismic behaviour of infilled frames. Thirteen single‐storey, single‐bay, half‐size‐scale, reinforced concrete‐frame specimens, most of which infilled with non‐structural masonry made of perforated bricks and cement mortar are tested. The infills are in contact with frames, without any connector; openings are not covered. The frames are different in their strength and details, reinforcement grade, and aspect ratio. Seismic input is the 1976 Tolmezzo (Friuli, Italy) ground acceleration, to which specimens are subjected two times: virgin and damaged by the previous test. The global seismic response of initially virgin infilled specimens considerably differs from that of bare specimens. This follows a dramatic change of properties: compared to a bare frame, the initial stiffness increases by one order of magnitude, and the peak strength more than doubles. The peak drift lessens; however, the displacement ductility demand does not. The energy demand is greater. Nevertheless, the influence of infill decreases as damage proceeds. Displacement time histories of damaged specimens are quite similar. At the local level, infill causes asymmetry and concentration of the frame deformation. Copyright © 2005 John Wiley & Sons, Ltd.     

Masonry infill walls in reinforced concrete frames as a source of structural damping

This paper presents the results of an experimental study on the determination of damping characteristics of bare, masonry infilled, and carbon fiber reinforced polymer retrofitted infilled reinforced concrete (RC) frames. It is well known that the masonry infills are used as partitioning walls having significant effect on the damping characteristics of structures as well as contribution to the lateral stiffness and strength. The main portion of the input energy imparted to the structure during earthquakes is dissipated through hysteretic and damping energies. The equivalent damping definition is used to reflect various damping mechanisms globally. In this study, the equivalent damping ratio of carbon fiber reinforced polymer retrofitted infilled RC systems is quantified through a series of 1/3‐scaled, one‐bay, one‐story frames. Quasi‐static tests are carried out on eight specimens with two different loading patterns: one‐cycled and three‐cycled displacement histories and the pseudo‐dynamic tests performed on eight specimens for selected acceleration record scaled at three different PGA levels with two inertia mass conditions. The results of the experimental studies are evaluated in two phases: (i) equivalent damping is determined for experimentally obtained cycles from quasi‐static and pseudo‐dynamic tests; and (ii) an iterative procedure is developed on the basis of the energy balance formulation to determine the equivalent damping ratio. On the basis of the results of these evaluations, equivalent damping of levels of 5%, 12%, and 14% can be used for bare, infilled, and retrofitted infilled RC frames, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

Prediction of inter-storey drifts for regular RC structures with masonry infills based on bare frame modelling

The traditional construction of masonry infills adjacent to RC structural elements is still widely adopted in European countries, including seismically active regions. Given the repeated field observations from damaging earthquakes, pointing to unacceptably high levels of masonry infill damage, the present study is motivated by the need to improve further the European seismic design approach for new RC structures with masonry infills, in order to exclude the poor seismic behaviour probably caused by deficiencies in the verification procedure. Since the in-plane damage to non-structural panels is commonly controlled through the limitation of inter-storey drifts, the possibility to introduce more effective verification criteria, accounting for structural properties, infill layouts and masonry properties is explored. Therefore, starting from the assumption that analyses and verifications in the design of buildings are commonly accomplished neglecting the presence of infills, results of extensive nonlinear numerical analyses for different building configurations are examined. As a result, a simplified procedure for the prediction of expected inter-storey drifts for infilled structures, based on the corresponding demands of bare configurations, in function of a simple parameter accounting for structural properties and the presence of infills, is introduced. Possible implications of the proposed approach aimed at the improvement of the current design provisions are discussed.     

Displacement-based seismic design of hysteretic damped braces for retrofitting in-elevation irregular r.c. framed structures

A displacement-based design procedure is proposed for proportioning hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level of a reinforced concrete (r.c.) in-elevation irregular framed building which has to be retrofitted. To check the effectiveness and reliability of the design procedure, a numerical investigation is carried out with reference to a six-storey r.c. framed building, which, originally designed according to an old Italian seismic code (1996) for a medium-risk zone, has to be retrofitted by inserting of HYDBs to attain performance levels imposed by the current Italian code (NTC08) in a high-risk zone. To simulate a vertical irregularity, a change of use of the first two floors, from residential to office, is also supposed; moreover, masonry infill walls, regularly distributed along the perimeter, are substituted with glass windows on these floors. Nonlinear dynamic analyses of unbraced (UF), infilled (IF) and damped braced infilled (DBIF) frames are carried out considering sets of artificially generated and real ground motions, whose response spectra match those adopted by NTC08 for different performance levels. To this end, r.c. frame members are idealized by a two-component model, assuming a bilinear moment–curvature law whose ultimate bending moment depends on the axial load, while the response of an HYDB is idealized by a bilinear law, to prevent buckling. Finally, masonry infills are represented as equivalent diagonal struts, reacting only in compression, with an elastic–brittle linear law.     

R/C frame–infill interaction model and its application to Indonesian buildings

This paper proposes a new analytical model for masonry‐infilled R/C frames to evaluate the seismic performance considering R/C frame–infill interactions. The proposed analytical model replaces masonry infill with a diagonal compression strut, which represents distributed compression transferred between frame and infill interfaces. The equivalent strut width is presented as a function of the frame–infill contact length, which can be evaluated by static equilibriums related to compression balance and lateral displacement compatibility at the frame–infill interfaces. The proposed analytical model was verified through comparisons with experimental results obtained for several brick masonry‐infilled R/C frames representing a typical R/C building with nonstructural masonry infill in Indonesia. As a result, good agreements were observed between the experimental and analytical values of the lateral strength and ductility of the infilled frames. The seismic performances of two earthquake‐damaged R/C buildings with different damage conditions were evaluated considering infill effects by applying the proposed analytical model. Consequently, the nonstructural brick masonry infill significantly affected the seismic resistances of the buildings, which seemed to lead to differing levels of damage for each building. These results indicate that the proposed analytical model can be an effective tool for more precisely screening earthquake‐vulnerable existing R/C buildings in Indonesia. Copyright © 2016 The Authors. Earthquake Engineering & Structural Dynamics Published by John Wiley & Sons Ltd.     

Analysis of the in‐plane response of earthen masonry infill panels partitioned by sliding joints

The vulnerability of infilled frames represents a critical issue in many regions with high seismicity around the world where infills are typically made of heavy masonry as they are used for thermal control of the buildings because of their thermal inertia. In this context, the use of earthen masonry infills can give a superior performance because of their ability to regulate thermal‐hygrometric performance of the building and sustainability of its life‐cycle. This paper presents a numerical study on the seismic behaviour of infill walls made of earthen masonry and partitioned with horizontal wooden planks that allow the relative sliding of the partitions. The combination of the deformability of earthen masonry and the sliding mechanism occurring along the wooden planks gives a high ductility capacity to the in‐plane response of the infill and, at the same time, significantly reduces its stiffness and strength, as compared with traditional solid infills made of fired clay units. As a result, the detrimental interaction with the frame and the damage in the infill when subjected to in‐plane loading can be minimized. The numerical model is validated with results from an experimental study and is used to perform a parametric analysis to examine the influence of variations in the geometry and mechanical properties of the infill walls, as well as the configuration of the sliding joints. Based on the findings of this study, design guidelines for practical applications are provided, together with simple formulation for evaluating their performance. Copyright © 2016 John Wiley & Sons, Ltd.     

A capacity curve model for confined clay brick infills

Experimental studies have proven that clay brick infills, confined with carbon-fiber-reinforced polymers (CFRP) in reinforced concrete (RC) frames, have some advantages in terms of stiffness, strength, energy dissipation capability and damage intensity. Owing to these advantages, existing infill walls in RC frames may be retrofitted with CFRP strips, especially in low-rise buildings in earthquake-prone areas. There is a gap in the literature concerning their behavior model, for use in structural analysis. A piecewise linear capacity curve model called “DUVAR” is proposed here, which estimates the envelope of force-vs.-displacement hysteresis, depending on the data compiled from the literature and the completed experimental studies. A nonlinear shear spring element is utilized in the model to represent the bare and retrofitted infills. The ultimate shear strength and the corresponding displacement, the ratio of cracking stiffness to initial stiffness, the ratio of ultimate strength to cracking strength, and the ductility ratio are the five key parameters of the model. The model is validated against the experimental results of two sovereign studies. Finally, the model is employed in the performance evaluation of an existing three-story RC building to exemplify its straightforward application.     

Local effects on RC frames induced by AAC masonry infills through FEM simulation of in-plane tests

Unreinforced masonry infills are widely used in many parts of the world and it is common practice for seismic design to use simplified methods that usually do not take into account the interaction between the infill and the structure. Starting from the 1950s, many researchers have investigated the lateral response of masonry infills focusing on several different topics. The scientific interest on masonry infills is continuously raising due to the unsatisfactory seismic response of the infilled frame structures observed during post-event inspections and to the difficulty to contrive a widely scientifically and practical recognized solution. Although some modern codes consider the presence of infills with some specifications to prevent damage in the masonry panels and global and local effects on the structure, an effective evaluation of these detrimental effects has not been achieved yet. Within this paper, a FEM simulation of in-plane pseudo-static cyclic tests on a RC frame specimen infilled with unreinforced Autoclaved Aerated Concrete (AAC) masonry infill has been performed in order to study accurately the influence and the interaction of the infill with the RC structure. The experimental results performed by Calvi and Bolognini (J Earthq Eng 5:153–185, 1999), and Penna and Calvi (Campagna sperimentale su telai in c.a. con tamponamenti in Gasbeton (AAC) con diverse soluzioni di rinforzo” (in Italian), 2006) on one-bay one-storey full scale specimens are taken as reference. Non-linear static analyses using a “meso-modelling” approach have been carried out. The masonry used in the model has been calibrated according to tests of mechanical characterization and to in-plane cyclic tests on load-bearing AAC masonry conducted by Costa et al. (J Earthq Eng 15:1–31, 2011). The analyses performed have allowed to investigate the local effects on the frame and, in particular, the changes in the moment and shear demands on the RC elements due to the presence of the AAC infill in comparison with the ones in the bare structure, and to estimate the thrust and the contact length activated by the infill on the frame.