Influence of Openings,With and Without Confinement,on Cyclic Response of Infilled R-C Frames — An Experimental Study

This article presents the experimental results of a study on reinforced-concrete frames infilled with masonry with openings. The frames were designed according to current European codes. They were built in a scale 1:2.5 and infilled with masonry walls. Mid-size window and door openings were located centrically and eccentrically and were executed with and without tie-columns around them. Presence of masonry infill, although not accounted for in design, improved the system behavior (increase in stiffness, strength and energy dissipation capacity) at drift levels of up to 1%. During the test, openings did not influence the initial stiffness and strength at low drift levels. Their presence became noticeable at higher drift levels, when they lowered the energy dissipation capacity of the system. The infill wall had a multiple failure mechanism that depended on the opening height and position. Tie-columns controlled the failure type, independent of the opening type, prevented out-of-plane failure of the infill, and increased the system's ductility. Negative effects of the infill on the frame were not observed. The infill's contribution could be deemed positive as it enhanced the overall Structural Performance Level. Analytical expressions commonly used for infilled frames underestimate the infill's contribution to strength and stiffness and overestimate the contribution of the bare frame.     

Quasi-Static Testing of RC Infilled Frames and Confined Stone-Concrete Bearing Walls

This article presents an experimental investigation of the seismic performance of gravity load-designed RC infilled frames and confined bearing walls of limestone masonry backed with plain concrete. Five infilled frames and two bearing walls were constructed at one-third scale and tested using reversed cyclic lateral loading and constant axial loads. Effects of openings, axial loading, and infill interface conditions were examined using quasi-static experimentation. The two structural systems exhibited similar lateral resistance and energy dissipation capacities with higher global displacement ductility for the infilled frames. Hysteretic behavior of the infilled frame models exhibited pinching of the hysteretic loops accompanied by extensive degradation of stiffness whereas loops of the bearing walls were free of pinching. Test results confirmed the beneficial effect of axial loading on lateral resistance, energy dissipation, and ductility of the bearing walls. Higher axial loading resulted in a substantial decrease in ductility with no significant effect on lateral resistance of the infilled frames. Openings within the infill panel reduced significantly the lateral resistance of infilled frames. Using dowels at the infill panel interfaces with the base block and bounding columns enhanced the maximum load-carrying capacity of infilled frames without impairing their ductility.     

Pseudo-Dynamic Testing and Analytical Modeling of AAC Infilled RC Frames

Two concrete frames were tested by the PsD procedure. One frame was bare and the other was infilled with AAC blocks in the middle bay. The objective was to determine the effect of AAC infills on the seismic performance of reinforced concrete frames and developing an AAC strut model. Based on the test results, it was found that AAC infill panels did not modify the deformation response of the RC test frame significantly; however, shear in diagonal strut must be considered in boundary column design. A shear design procedure is proposed for the boundary columns in infilled frames. The drift limits of AAC infill panels measured during the tests were 0.005, 0.008, and 0.014 during diagonal cracking, corner crushing, and severe damage states, respectively.     

Influence of Masonry Strength and Openings on Infilled R/C Frames Under Cycling Loading

The influence of masonry infills with openings on the seismic performance of reinforced concrete (R/C) frames that were designed in accordance with modern codes provisions is investigated. Two types of masonry infills were considered that had different compressive strength but almost identical shear strength. Infills were designed so that the lateral cracking load of the solid infill is less than the available column shear resistance. Seven 1/3 – scale, single–story, single–bay frame specimens were tested under cyclic horizontal loading up to a drift level of 40%. The parameters investigated are the opening shape and the infill compressive strength. The assessment of the behavior of the frames is presented in terms of failure modes, strength, stiffness, ductility, energy dissipation capacity, and degradation from cycling. The experimental results indicate that infills with openings can significantly improve the performance of RC frames. Further, as expected, specimens with strong infills exhibited better performance than those with weak infills. For the prediction of the lateral resistance of the studied single-bay, single-story infilled frames with openings, a special plastic analysis method has been employed.     

Simplified Macro-Model for Infill Masonry Panels

In structural analyses, masonry infill walls are commonly considered to be non structural elements. However, the response of reinforced concrete buildings to earthquake loads can be substantially affected by the influence of infill walls. In this article, an improved numerical model for the simulation of the behavior of masonry infill walls subjected to earthquake loads is proposed and analyzed. First, the proposed model is presented. This is an upgrading of the equivalent bi-diagonal compression strut model, commonly used for the nonlinear behavior of infill masonry panels subjected to cyclic loads. Second, the main results of the calibration analyses obtained with two series of experimental tests are presented and discussed: one on a single frame with one story and one bay tested at the LNEC Laboratory; and the second, on a full-scale four story and three-bay frame tested at the ELSA laboratory.     

MODELLING OF RC BEAM-COLUMN JOINTS AND STRUCTURAL WALLS

The deformation of beam-column joints may contribute significantly to drift of reinforced concrete (RC) frames. In addition, failure may occur in the joints due to cumulative concrete crushing from applied beam and column moments, bond slip of embedded bars or shear failure as in the case of existing frames with nonductile detailing. When subjected to earthquake loading, failure in RC structural wall is similar to failure of frame joints as it may occur due to cumulative crushing from high flexural stresses, bond slip failure of lap splice, shear failure or a combination of various mechanisms of failure. It is important to include these behavioural characteristics in a simple model that can be used in the analysis of RC frames and RC walls to predict their response under earthquake loading and determine their failure modes.

Global macro models for the beam-column joint and for RC structural walls are developed. The proposed models represent shear and bond slip deformations as well as flexural deformations in the plastic hinge regions. The models are capable of idealising the potential failure mechanism due to crushing of concrete, bond slip or shear with allowance for the simultaneous progress in each mode. The model predictions are compared with available experimental data and good correlation is observed between analytical results and the test measurements.     

Experimental and Finite Element Analytical Investigation of Seismic Behavior of Full-Scale Masonry Infilled RC Frames

This article investigates the seismic behavior of masonry infilled RC frames with/without openings. Four full-scale, single-story, and single-bay specimens were tested under constant vertical loads and quasi-static cyclic lateral loads. The experimental results showed that the infill wall was more influential in stiffness than in load-resisting capacity. The opening increased the ductility ratio of the structure due to the uniform distribution and slow propagation of cracks. Finally, simplified micro finite element models are established to simulate the tested specimens, which effectively predict the load-displacement response of the structures and the crack damage of masonry infill wall with acceptable accuracy.     

Effect of AAC Infill Walls on Structural System Dynamics of a Concrete Building

The effect of autoclaved aerated concrete (AAC) infill walls on the structural system dynamics of a two-story reinforced concrete building is investigated using its finite element structural model, which is calibrated to simulate the acceleration-frequency response curves from its forced vibration test. The model incorporating the AAC infill walls by equivalent diagonal struts captures the increase in lateral stiffness of the building and the torsional motions induced due to the asymmetrically placed AAC infill walls. A higher strut width coefficient than in ASCE/SEI 41-06 is recommended to model the stiffness of the AAC infill walls in the elastic range.     

EVALUATION OF THE SEISMIC PERFORMANCE OF EXISTING RC BUILDINGS: II. A CASE STUDY FOR REGULAR AND IRREGULAR BUILDINGS

The results of a parametric study are presented, concerned with the evaluation of the structural overstrength, the global ductility and the available behaviour factor of existing reinforced concrete (RC) buildings designed and constructed according to past generations of earthquake resistant design codes in Greece. For the estimation of these parameters, various failure criteria are incorporated in a methodology established to predict the failure mode of such buildings under planar response, as described in detail in a companion publication. A collection of 85 typical building forms is considered. The influence of various parameters is examined, such as the geometry of the structure (number of storeys, bay width etc.), the vertical irregularity, the contribution of the perimeter frame masonry infill walls, the period of construction, the design code and the seismic zone coefficient. The results from inelastic pushover analyses indicate that existing RC buildings exhibit higher overstrength than their contemporary counterparts, but with much reduced ductility capacity. The presence of perimeter infill walls increases considerably their stiffness and lateral resistance, while further reducing their ductility. Fully infilled frames exhibit generally good behaviour, while structures with an open floor exhibit the worst performance by creating a soft storey. Shear failure becomes critical in the buildings with partial height infills. It is also critical for buildings with isolated shear wall cores at the elevator shaft. Out of five different forms of irregularity considered in this study, buildings with column discontinuities in the ground storey exhibit the worst performance. Furthermore, buildings located in the higher seismicity zone are more vulnerable, since the increase of their lateral resistance and ductility capacity is disproportional to the increase in seismic demand.     

Assessment of Effects of Reductions of Lateral Stiffness along Height on Buildings Modeled as Elastic Cantilever Shear Beams

A simplified model useful for assessing economic losses due to moderate seismicity events in urban areas has been developed by studying the behavior of buildings before yielding their structural system, allowing for nonuniform stiffness along their height. In particular, buildings are modeled as cantilever shear beams with uniform mass and parabolic reduction of lateral stiffness. This particular stiffness distribution is relevant, as it could be expected to occur in buildings where earthquake action is a critical structural design criterion. The equation of motion governing the dynamic behavior of the proposed model is solved analytically, finding mode shapes in terms of first and second zero-order Legendre functions. The solution is verified by comparing it with results obtained from fine mesh finite element models. The effect of reducing the lateral stiffness is then studied in the first five modes of vibration. Results include modal periods, mode shapes, modal participation factors, and derivatives of mode shapes. In general, it is found that effects of reduction of lateral stiffness in mode shapes are moderate when the lateral stiffness in the free end is smaller than about seventy percent of the lateral stiffness at the fixed end, but become significant for larger reductions. Effects are particularly important for the derivative of the mode shapes, which could play a significant role in estimating interstory drift demands in buildings. Model usefulness is showcased by analyzing a test case where both acceleration and drift demands are assessed by considering uniform beams and beams with parabolic stiffness variation, finding notable improvements by considering the latter.     

Simplification and Assessment of Modal-Based Story Damage Index for Reinforced Concrete Frames Subjected to Seismic Excitations

Different relations have been represented for the local damage index of structures to date, while the same application is defined for them as can be an indicator of relative sustained damage by the components or stories. Since different force-resisting systems subjected to the ground motions can behave differently, some well-known story damage indices are evaluated for the reinforced concrete frames with regards to their operation during nonlinear time history analysis. Two general concepts of story damage determination are selected for this purpose. SDI is a modal-based story damage index, which is calculated by the modal frequency and mode shapes. The behavior of this local index is evaluated during the seismic excitations. The results were compared with Park-Ang and modal flexibility story damage indices. Based on analytical study on seismic responses of some RC frames subjected to a suit of earthquake records a new story damage index has been developed. It has been derived from a simple global damage equation (softening index) using a normalized ratio of inelastic story shear to its drift. A procedure is recommended to use the proposed equation without any requirement to perform nonlinear dynamic analysis, which can significantly reduce the computational efforts. Distribution of the new represented SDI along the structural height shows a good agreement with damaged state of the RC frames after seismic excitations.     

ESTIMATION OF NEAR-SOURCE GROUND MOTION AND SEISMIC BEHAVIOUR OF RC FRAMED STRUCTURES DAMAGED BY THE 1999 ATHENS EARTHQUAKE

On September 7, 1999 an earthquake with magnitude M _W =5.9 occurred close to the city of Athens in Greece. More than 80 buildings collapsed, about 150 deaths and hundreds of injuries were reported. Soon after the event a damage investigation was carried out by two of the authors in the most heavily struck areas. The most serious damages were observed in the northern suburbs of Athens, where reinforced concrete frames and masonry buildings represent the prevalent construction systems. The hysteretic energy demands imposed on RC buildings should have been rather severe considering the structural systems characteristics and the inadequate construction details. However, over-strengths, redundancy and especially the presence of infill walls, provided a significant increase of the seismic capacity and contributed to the survival of many buildings.

The objective of the present work is to reproduce and analyse the response of typical RC frames subjected to the 1999 Athens earthquake in areas where the observed damage was particularly severe but no recordings of the ground motion were available. After a general overview of the seismotectonic environment, seismological data, observed macro-seismic intensities, structural typologies and observed building behaviour, an attempt is made to identify representative excitations in the meizoseismal area. Specifically, the required accelerograms are obtained by modifying available records so as to reproduce a given global energy content and to be consistent with the observed damage. To study the seismic response of RC models, the obtained accelerograms are used to perform nonlinear dynamic analyses.     

EVALUATION OF DEFORMATION CAPACITY FOR RC T-SHAPED CANTILEVER WALLS

If RC structural walls are properly designed and proportioned, these walls can behave in a ductile manner. To achieve this goal, the designer should provide adequate strength and deformation capacity of structural walls corresponding to each performance level (e.g. immediate occupancy, life safety and collapse prevention). This study investigates the drift and ductility capacities of T-shaped structural walls on the basis of results from experimental tests and sectional analyses. To determine proper deformation capacities for T-shaped structural walls, structural performances of T-shaped walls were evaluated with several parameters such as longitudinal reinforcement ratio, distribution of longi-tudinal reinforcement, lateral confinement ratio, and axial load ratio. Based on these results, the level of deformation capacity specified in current design provisions (ICBO, UBC 1997), which were expressed as both strain-based damage limit and interstorey drift ratio, were evaluated.     

Seismic Vulnerability Assessment of Modular Steel Buildings

Contemporary seismic design is based on dissipating earthquake energy through significant inelastic deformations. This study aims at developing an understanding of the inelastic behavior of braced frames of modular steel buildings (MSBs) and assessing their seismic demands and capacities. Incremental dynamic analysis is performed on typical MSB frames. The analysis accounts for their unique detailing requirements. Maximum inter-story drift and peak global roof drift were adopted as critical response parameters. The study revealed significant global seismic capacity and a satisfactory performance at design intensity levels. High concentration of inelasticity due to limited redistribution of internal forces was observed.     

Displacement-Based Assessment of Non Ductile Exterior Wide Band Beam-Column Connections

Results from testing two half-scale exterior wide band beam-column sub-assemblages under cycles of lateral displacement are presented in this article. The first subassemblage represents the current level of detailing adopted in low to moderate seismic regions, such as Australia, for connections where seismic provisions are not normally a consideration in design. Minor (inexpensive) detailing changes in the reinforcement distribution and anchorage were introduced to the second test specimen. These changes significantly improved the connection performance in terms of increased displacement capacity and a reduction in strength deterioration. Using a displacement-based assessment approach to assess primary moment-resisting band beam frames of up to eight stories, it was found that the current level of detailing is adequate for the drift demands resulting from the expected Australian seismicity for a 500-year return period. However, for the displacement demands corresponding to a 2500-year return period, the frames sited on very soft soils and frames over four stories sited on intermediate soils would require improved detailing such as that used in the second sub-assemblage. A strength hierarchy of strong column-weak beam was assumed in this assessment.     

Experimental Evaluation of Seismic Performance of Squat RC Structural Walls with Limited Ductility Reinforcing Details

This article describes an experimental study carried out on of reinforced concrete (RC) walls of less confining reinforcement than that recommended by ACI 318. A total of eight RC walls with boundary elements comprising of five walls with aspect ratio of 1.125 and three walls with aspect ratio of 1.625 were tested by subjecting them to low levels of axial compression loading and simulated seismic loading, to examine the structural performance of the walls with limited transverse reinforcement. Conclusions are reached concerning the failure mode, drift capacity, strength capacity, components of top deformation, and energy dissipation characteristics of walls on the seismic behavior with limited transverse reinforcement. The influences of axial loading, transverse reinforcement in the wall boundary elements, and the presence of construction joints at the wall base on the seismic behavior of walls are also studied in this paper. Lastly, reasonable strut-and-tie models are developed to help in understanding the force transfer mechanism in the walls tested.     

Seismic Design and Response of Framed Structures with Stiffening Bracing Systems

Stiffening Bracing System (SBS) is proposed as an alternative to conventional braced frames. SBS is intended to reduce the floor accelerations while maintaining uniform inter-story drift along the building height. The system ensures that additional damping devices distributed over the building’s height work efficiently. An iterative design procedure is developed to maintain a desired target performance. The procedure accounts for higher mode effects and supplemental damping. A series of nonlinear response history analyses on braced frames with various heights demonstrated the adequacy of the proposed procedure in achieving target structural performance and seismic demand prediction.     

Pseudo Dynamic Testing of an RC Frame Retrofitted with Chevron Braces

Two-story three-bay reinforced concrete frames with and without chevron brace was tested using pseudo dynamic test method. The chevron braces were implemented to the interior span of the RC frame. Chevron-braced frame was observed to be effective to control inter-story drift demands. Based on the observed damage state and dynamic response of the test frames, performance states were discussed for different scales of Duzce ground motions. The test results were compared with the results of the nonlinear time history analysis. The analysis results were capable of estimating the base shear capacity and displacement demands with a reasonable accuracy.     

Experimental Out-Of-Plane Behavior of Brick Masonry Infilled Frames

ABSTRACT

The vulnerability of masonry infills within reinforced concrete (rc) frames under out-of-plane loading induced by earthquakes has been observed in several past earthquakes through severe damage and often total collapse. Although the infill panels are assumed as non-structural elements, their damage or collapse is not desirable, given the possible consequences in terms of human life losses and repair or reconstruction costs.

Therefore, it is important to gather better insight on the out-of-plane behavior of existing infills so that strengthening guidelines can be derived. In this scope, the main objective of this study is to analyze the out-of-plane experimental behavior of masonry infilled frames that are characteristic of Portuguese buildings and can be seen in other south European countries. In the experimental study carried out, different parameters affecting the out-of-plane response of infilled frames were considered, namely, workmanship, existence of openings and prior in-plane damage. The experimental program was designed to test six half-scale specimens. The out-of-plane loading was applied uniformly to the brick infills by means of an airbag to simulate the effect of earthquakes.     

Seismic Behavior Prediction of Flanged Unreinforced Masonry (FURM) Walls

In most available studies, unreinforced masonry (URM) walls are idealized as rectangular sections, while in reality walls have effective sectional shapes such as C, I, T, and L. In this article, the results of experimental and analytical assessment of flange effects on the behavior of I- and C-shaped URM walls are reported. Four clay brick walls at half scale were tested. Two specimens were designed with I- and C-shaped sections, and for comparison, two additional specimens were designed without flanges. The tests showed that under constant axial load the strength of the I-shaped wall increases, but that of the C-shaped wall decreases, because of out-of-plane distortion effects. Despite the loss of strength, both flanged walls indicated almost similar initial stiffness, deformation capacity, and mode of failure in comparison with walls without flanges. A mixed-mode analytical model is proposed to predict the lateral force displacement curve of flanged URM (FURM) walls. The proposed analytical model is based on section analysis of the walls and shows good agreement with previous experimental results.