Showing papers by "Katrin Beyer published in 2014"

Understanding Poor Seismic Performance of Concrete Walls and Design Implications

Abstract: The 2010–2011 Canterbury earthquakes in New Zealand revealed (1) improved structural response resulting from historical design advancements, (2) poor structural performance due to previously identi...

Scaling unreinforced masonry for reduced-scale seismic testing

Abstract: When testing multi-storey structures, most testing facilities require the testing of a reduced-scale model. A literature review on tests of scaled masonry structural components revealed that scaling of masonry was rather challenging and often significant differences in stiffness, strength and failure mechanisms between the different sized masonry were reported. This paper addresses the scaling of hollow clay brick masonry with fully mortared head and bed joints. We investigate different choices of scaling brick units and mortar joints. Based on the results of an extensive test programme including standard material tests and quasi-static cyclic tests on masonry walls subjected to horizontal and axial loads, we formulate recommendations for the production of a half-scale model of unreinforced masonry structures. The experimental results show a good match between full-scale and half-scale masonry. We discuss the differences in material properties that remained and compare the force-displacement hystereses obtained for the wall tests.

Loading protocols for European regions of low to moderate seismicity

Abstract: Existing loading protocols for quasi-static cyclic testing of structures are based on recordings from regions of high seismicity. For regions of low to moderate seismicity they overestimate imposed cumulative damage demands. Since structural capacities are a function of demand, existing loading protocols applied to specimens representative of structures in low to moderate seismicity regions might underestimate structural strength and deformation capacity. To overcome this problem, this paper deals with the development of cyclic loading protocols for European regions of low to moderate seismicity. Cumulative damage demands imposed by a set of 60 ground motion records are evaluated for a wide variety of SDOF systems that reflect the fundamental properties of a large portion of the existing building stock. The ground motions are representative of the seismic hazard level corresponding to a 2% probability of exceedance in 50 years in a European moderate seismicity region. To meet the calculated cumulative damage demands, loading protocols for different structural types and vibration periods are developed. For comparison, cumulative seismic demands are also calculated for existing protocols and a set of records that was used in a previous study on loading protocols for regions of high seismicity. The median cumulative demands for regions of low to moderate seismicity are significantly less than those of existing protocols and records of high seismicity regions. For regions of low to moderate seismicity the new protocols might therefore result in larger strength and deformation capacities and hence in more cost-effective structural configurations or less expensive retrofit measures.

Numerical Study on the Peak Strength of Masonry Spandrels with Arches

Abstract: This article presents a numerical study on the force-deformation behavior of masonry spandrels supported on arches which are analyzed using simplified micro models. The model is validated against results from quasi-static cyclic tests on masonry spandrels. A large range of spandrels with different arch geometries, material properties, and axial load ratios are studied. The numerical results are compared to peak strength values predicted with an existing mechanical model. Finally, estimates for the initial stiffness and the spandrel rotation associated with the onset of strength degradation are derived.

Capacity Design of Coupled RC Walls

Abstract: Capacity design aims to ensure controlled ductile response of structures when subjected to earthquakes. This article investigates the performance of existing capacity design equations for reinforced concrete coupled walls and then proposes a new simplified capacity design method based on state-of-the-art knowledge. The new method is verified through a case study in which a set of 15 coupled walls are subject to nonlinear time-history analyses. The article includes examination of the maximum shear force in individual walls in relation to the total maximum shear force in the coupled wall system, and subsequently provides recommendations for design.

Modelling shear–flexure interaction in equivalent frame models of slender reinforced concrete walls

Abstract: Keywords: reinforced concrete walls, shear-flexure interaction, beam element models Reference EPFL-CONF-181574 Record created on 2012-10-01, modified on 2016-08-09

Seismic shear distribution among interconnected cantilever walls of different lengths

Abstract: SUMMARYMid-rise to high-rise buildings in seismic areas are often braced by slender reinforced concrete (RC) walls,whichareinterconnected byRC floor diaphragms. Indesign,itistypically assumed that the lateral forces aredistributed in proportion to the wall’s elastic stiffness. Pushover analyses of systems comprising walls ofdifferent lengths have, however, shown that large compatibility forces can develop between them, whichshould be considered in design, but the analyses have also shown that the magnitude of the computed forcesis very sensitive to the modelling assumptions. Using the results of a complex shell element model asbenchmark, different simple hand-calculation methods and inelastic beam element models are assessedand improved to yield reliable estimates of the base shear distribution among the individual wallscomprising the interconnected wall system. Copyright © 2014 John Wiley & Sons, Ltd. Received 5 February 2013; Revised 10 December 2013; Accepted 18 December 2013KEYWORDS:

Towards displacement-based seismic design of modern unreinforced masonry structures

Abstract: Unreinforced masonry (URM) structures are known to be rather vulnerable to seismic loading. Modern URM buildings with reinforced concrete (RC) slabs might, however, have an acceptable seismic performance for regions of low to moderate seismicity. In particular in countries of moderate seismicity it is often difficult to demonstrate the seismic safety of modern URM buildings by means of force-based design methods. Displacement-based design methods are known to lead to more realistic and less conservative results, opening up hence new opportunities for the use of structural masonry. An effective implementation of displacement-based design approaches requires reliable estimates of the structure’s force and displacement capacity. This paper contributes to this endeavour by taking a fresh look at the drift capacity of URM walls with hollow clay bricks and mortar joints of normal thickness. It discusses in particular the influence of the size of the test unit and the applied loading history and loading velocity on the drift capacities of URM walls.

Comparison of force-based and displacement-based design approaches for RC coupled walls in New Zealand

Abstract: Reinforced concrete coupled walls are a common lateral load resisting system used in multi-storey buildings. The effect of the coupling beams can improve seismic performance, but at the same time adds complexity to the design procedure. A case study coupled wall building is designed using Force-Based Design (FBD) and Direct Displacement-Based Design (DDBD) and in the case of the latter a step by step design example is provided. Distributed plasticity fibre-section beam element numerical models of the coupled walls are developed in which coupling beams are represented by diagonal truss elements and experimental results are used to confirm that this approach can provide a good representation of hysteretic behaviour. The accuracy of the two different design methods is then assessed by comparing the design predictions to the results of non-linear time-history analyses. It is shown that the DDBD approach gives an accurate prediction of inter-storey drift response. The FBD approach, in accordance with NZS1170.5 and NZS3101, is shown to include an impractical procedure for the assignment of coupling beam strengths and code equations for the calculation of coupling beam characteristics appear to include errors. Finally, the work highlights differences between the P-delta considerations that are made in FBD and DDBD, and shows that the code results are very sensitive to the way in which P-delta effects are accounted for.

Influence of longitudinal reinforcement layouts on RC wall performance

Abstract: Design recommendations for longitudinal reinforcement layouts of reinforced concrete (RC) walls have been derived from plane section analyses. Such an analysis generally favours wall layouts with boundary elements containing large amounts of vertical reinforcement, providing higher moment resistance and larger ductility capacity than the same reinforcement distributed evenly along the wall length. The main disadvantage of a design method based on a plane section analysis is that it disregards the beneficial influence that the distribution of longitudinal reinforcement has on the member shear performance and reduction of crack widths. In order to better understand the seismic performance of RC walls with concentrated and distributed longitudinal reinforcement layouts, this paper starts with a review of simple mechanical models and code prescriptions on stability of rectangular walls, sliding shear resistance, and required confinement reinforcement. The relevant expressions from this survey are then applied to a case study comprising two walls. In addition, the latter are numerically simulated with an advanced nonlinear membrane model to avoid the limitations of plane section hypothesis. Pushover analyses show that distributed reinforcement layouts can lead to an improved wall behaviour in terms of crack widths and spacing along the wall height, as well as a different failure mechanism due to crushing of the compressed zone instead of a premature sliding shear failure at the base of the wall.

Non-rectangular RC walls: A review of experimental investigations

Abstract: When compared to tests on reinforced concrete (RC) walls with a rectangular or barbelled cross-section, only very few tests on RC walls with open cross-section exist. Most of these walls were subjected to unidirectional or bidirectional loading along one or both of the principal axes of the wall section. It was rare that tests were done using load paths that did not follow the principal axes. This article presents a review of experimental tests of non-rectangular RC walls with open cross-section. The summary comprises tests on individual non-rectangular walls with different cross-sections such as for example L-shaped or U-shaped walls which were tested under quasi-static or dynamic loads. The tests are described with their prototype structure, test objectives, investigated parameters, loading protocols and test conclusions. Emphasis is placed on observations that are specific to non-rectangular walls, which include out-of-plane buckling of the free end of wall segments and significant deviations from the hypothesis that plane sections remain plane. The importance of bidirectional loading for nonrectangular walls is stressed because the critical loading direction for design may be different from loading in the principal directions of the section and because loading along one direction reduces the stiffness of the wall for subsequent loading in the direction orthogonal to the previous one.

Seismic conservation strategies for cultural heritage buildings in Switzerland

Abstract: Cultural heritage buildings were typically built without considering seismic action and are therefore potentially susceptible to earthquake damage. The performance states and protection objectives developed for ordinary buildings are not directly applicable since they do not address the cultural importance of the heritage buildings. This paper proposes a seismic conservation strategy for cultural heritage buildings in Switzerland, which is a country of low to moderate seismicity. Based on the performance states for cultural heritage buildings that were developed within the Pereptuate project, a matrix of performance states has been developed in function of the importance category of the cultural property, the conservation strategy chosen and the return periods for seismic events to be considered. It is proposed that the performance states and return periods for ordinary buildings should define the lowest performance that is acceptable for cultural heritage buildings since these performance states were derived considering minimum protection requirements for people. For cultural heritage buildings such low performance states are acceptable if the importance of the heritage is minor and/or if a conservation strategy is chosen, which is based on the documentation of the status quo with the objective of reconstruction after a seismic event rather than retrofit and improvement of the seismic performance.

Investigating the in-plane mechanical behavior of URM piers via DSFM

Abstract: In this paper we use the novel Disturbed Stress Field Model (DSFM) from Facconi et al. (2013) to model the lateral in-plane behaviour of different unreinforced masonry (URM) walls which we tested at the laboratory of the EPF Lausanne, Switzerland. The model uses the failure criterion developed by Ganz (1985) for URM and is implemented in the software VecTor 2. In the first part of this article, we show that the DSFM gives good estimates for the initial stiffness of the walls but underestimates the displacement and force capacity of our URM walls. This is due to the confinement of the mortar base joint which is confined by the wall foundation. However, we show that this phenomenon can be accounted for by replacing the compression strength of the masonry of the first brick layer by the strength of the brick itself. Using this improved model, we compare the numerical results to the experimental results with regard to displacements and strains. We show that the simplification of the masonry to a continuous material is correct when comparing the global engineering demand parameters (EDPs), e.g., force-displacement behaviour of the walls or crack pattern. However, when comparing local EDPs, e.g., strains and crack widths, significant differences can be obtained. These differences are mainly caused by the torque of the bricks inside the walls (Mann and Muller, 1982). We show that this distortion gets less important with increasing shear span and that for a wall with higher shear span the assumption of a continuous material gives reasonable results even at the local level.

Observations on out-of-plane behaviour of URM walls in buildings with RC slabs

Abstract: In Switzerland many new residential buildings are constructed as unreinforced masonry (URM) structures or as mixed structures where URM walls are coupled with reinforced concrete (RC) walls by RC slabs. At present the boundary conditions of URM walls subjected to out-of-plane accelerations are still not well quantified. In the framework of a large research activity on RC-URM wall structures a shake-table test on a four-storey mixed structure was performed. The test specimen, which was built at half-scale, was designed in such a way to allow the collection of data on the effect of boundary conditions provided by the RC slabs and adjacent RC walls on the out-of-plane seismic behaviour of URM piers. This paper presents observations on the out-of-plane behaviour of the URM walls from the shake-table test and draws conclusions for the boundary conditions to be applied in numerical studies on URM piers in mixed RC-URM wall structures.

Flexural deformations of URM piers: Comparison of analytical models with experiments

Abstract: Despite the fact that displacement-based methods are now frequently applied when assessing the seismic performance of unreinforced masonry (URM) structures, the displacement capacity of in-plane loaded URM piers is still estimated from empirical rather than mechanical relationships. One idea for estimating a pier’s force-displacement curve including its displacement capacity that has been put forward in the past is based on the double integration of the curvature profile. The curvature is derived assuming a bilinear stress-strain relationship in the compression domain and neglecting the tensile strength of the masonry. We tested several identical URM piers under different constant axial load and shear span ratios while applying quasi-static horizontal cyclic in-plane loading. During testing, we tracked with a set of cameras the displacement of four LEDs on each full brick of the masonry piers. Therefore, we were able to determine the local deformations, e.g. average strains in the bricks and deformations of the joints. In this paper we will present experimentally determined local and global deformation quantities of one pier dominated by flexural deformations and compare these to the estimates from the analytical model. Differences between model and experiment will be discussed and first estimates of performance limits describing local deformations will be proposed.

Lateral force resisting mechanisms in slab-column connections: An analytical approach

Abstract: In many countries reinforced concrete (RC) flat slabs supported on columns is one of the most commonly used structural systems for office and industrial buildings. To increase the lateral stiffness and strength of the structure, RC walls are typically added and carry the largest portion of the horizontal loads generated during earthquakes. While the slab-column system is typically not relevant with regard to the lateral stiffness and strength of the structure, each slab-column connection has to have the capacity to follow the seismically induced lateral displacements of the building while maintaining its capacity to transfer vertical loads from the slab to the columns. If this is not the case, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the slab-column connection. This article presents an analytical approach for predicting the moment resistance of all mechanisms that contribute to the strength of the slab-column connection when subjected to earthquake-induced drifts. The approach is based on the Critical Shear Crack Theory (CSCT). The performance of the model is verified comparing the analytical predictions with experiments from the literature. The influence of gravity induced loads on the flexural behaviour of slab-column connections under seismic loading as well as the contribution of the various resistance-providing mechanisms for increasing drifts are discussed.

From plastic hinge to shell models: Recommendations for RC wall models

Abstract: The severe damage and collapse of many reinforced concrete (RC) wall buildings in the recent earthquakes of Chile (2010) and New Zealand (2011) have shown that RC walls did not perform as well as expected based on the design calculations required by the modern codes of both countries. In this context, it seems appropriate to intensify research efforts in more accurate simulations of damage indicators, in particular local engineering demand parameters such as material strains, which are central to the application of performance-based earthquake engineering. Potential modelling improvements will necessarily build on a thorough assessment of the limitations of current state-of-the-practice simulation approaches. This work aims to compare the response variability given by a spectrum of numerical tools commonly used by researchers and specialized practitioners, namely: plastic hinge analyses, distributed plasticity models, and detailed finite element simulations. It is shown that a multi-level assessment—wherein both the global and local levels are jointly investigated from the response analysis outcomes—is fundamental to define the dependability of the results. The latter is controlled by the attainment of material strain limits and the occurrence of numerical problems. Finally, the influence of shear deformations is analysed according to the same methodological framework.

Mechanical model for flexural behaviour of slab-column connections under seismically induced deformations

Abstract: Reinforced concrete (RC) flat slabs supported on columns are one of the most widely used structural systems for office and industrial buildings. In regions of medium to high seismic risk RC walls are typically added as lateral force resisting system and to increase the lateral stiffness and strength. Although slab-column systems are not expected to contribute to the lateral resistance of the structure due to their low stiffness, the slab-column connection have to have the capacity to follow the seismically induced lateral displacements of the building while maintaining the capacity to transfer the vertical loads from the slab to the columns. Otherwise, brittle punching failure of the slab occurs and the deformation capacity of the entire building is limited by the deformation capacity of the connection. The present paper presents a model for predicting the flexural behaviour of slab-column connections without transverse reinforcement when subjected to earthquake-induced deformations, considering both the load and the deformation of the slab. The model is based on the Critical Shear Crack Theory (CSCT) and presents a rational approach for predicting the transferred moment-rotation relationship of slab-column connections as well as the contribution of all resistance-providing mechanisms respecting equilibrium principles in both local and global level. The model proved to be accurate enough when compared with tests (monotonic and cyclic) found in the literature.

A case study in the capacity design of RC coupled walls

Abstract: In the seismic design of structures, capacity design is typically employed to ensure that a desirable ductile response is obtained. In this paper three different capacity design approaches for reinforced concrete coupled walls are investigated. For a simple case study building, the expected capacity design shear forces and bending moments are calculated using the different approaches. The results are then assessed against the corresponding actions found from nonlinear time-history analysis. The performance of each approach is discussed, along with some of the difficulties associated with undertaking the capacity design of coupled walls.

Experimental investigation on seismic behaviour of slab-column connections

Abstract: Code design of slab-column connections to resist seismically induced drifts is currently based on empirical relationships for both moment and deformation capacity. The results of tests on slabcolumn connections are however sensitive to the conception of the test setup. This paper presents a new setup configuration as well as first results of an experimental campaign carried out at EPFL. Within this campaign slabs without shear reinforcement will be subjected to constant vertical loads and increasing seismic moments. Parameters investigated are the vertical loads, the longitudinal reinforcement content of the slabs, the column size and the loading history (monotonic vs. cyclic). Results of the first three out of sixteen tests showed that the drift capacity of relatively thick slabs is smaller when compared to thinner ones tested by other researchers [7]. The influence of the gravity induced shear force on the seismic moment and deformation capacity of the connections is found to be well pronounced. A comparison of a monotonic and cyclic test for high vertical loads showed that the deformation capacity of the cyclically loaded slab is smaller than of the monotonically loaded one.

Quasi-static cyclic tests on U-shaped reinforced concrete walls subjected to diagonal loading

Abstract: This article presents the test setup of two quasi-static cyclic tests on U-shaped walls under horizontal diagonal loading recently performed at EPF Lausanne. The results of the first test are discussed in terms of global behaviour: failure mechanisms and force-displacement hystereses as well as in terms of local behaviour. The emphasis is placed on behaviour specific to U-shaped walls under diagonal loading, such as: 1) proneness to out-of-plane buckling of the free ends of the flanges; and 2) plane sections do not remain plane after deformation. Implications of these phenomena in the design and analysis of U-shaped walls are also discussed.

Seismic response of a 4 storey building with reinforced concrete and unreinforced masonry walls

Abstract: In Switzerland many new residential buildings are designed as mixed structures with unreinforced masonry (URM) walls and reinforced concrete (RC) walls while existing URM structures are often retrofitted by replacing selected URM walls by RC walls. The lateral bracing system of the resulting structure consists therefore of URM walls and some RC walls. All walls are coupled by RC slabs and/or masonry spandrels. Since the seismic performance of such mixed structures– despite being very common in design and retrofit– is still not well understood, a shake-table test on a four-storey RC-URM wall structure was performed at the TREES laboratory of the EUCENTRE in Pavia (Italy). In this paper we present the details of the shake-table test and discuss its results. We compare the observed behaviour to that generally observed for URM structures and conclude on the effect of adding slender RC walls to URM buildings in terms of damage evolution. The force-displacement response of the structure is presented, its response for the two loading directions discussed and different methods for computing the base shear force compared.

Performance-based approach for the retrofit of URM wall structures by RC walls

Abstract: In several countries of moderated seismicity, with the re-evaluation of the seismic hazard most unreinforced masonry (URM) buildings with reinforced concrete slabs failed to satisfy the seismic design check. A possible seismically retrofit solution consists of adding RC walls to the existing structure or replacing selected critical URM walls with RC ones. Experimental and numerical studies have shown that this retrofit technique can be very effective since it modifies the global deformed shape of the structure, leading to an increase in the system’s displacement capacity. The paper proposes a Displacement-Based Design methodology for the retrofit of URM structures by replacing selected URM walls by RC walls. The methodology follows the Direct Displacement-Based Design (DBD) approach by Priestley et al. (2007) and is based in particular on the DBD procedure for frame-wall buildings (Sullivan et al., 2005 and 2006). The design procedure consists of three main phases: (i) A preliminary design check of the URM building by means of the DBD approach. (ii) If the structure does not fulfil the design check and exhibits at the same time a dominant shear behaviour, replacing the critical URM wall or walls with RC ones leads to an improved system’s behaviour. (iii) In the final phase, the DBD design of the mixed RC-URM wall structure, in which both the URM and the RC walls are taken into account, is carried out. The design methodology is then investigated through non-linear dynamic analyses of one case study.

Lessons from the 2011 Christchurch earthquake for improving the seismic performance of concrete walls

Abstract: The 2011, 6.3 magnitude Christchurch earthquake in New Zealand caused considerable structural damage. It is believed that this event has now resulted in demolition of about 65-70% of the building stock in the Central Business District (CBD), significantly crippling economic activities in the city of Christchurch. A major concern raised from this event was adequacy of the current seismic design practice adopted for reinforced concrete walls due to their poor performance in modern buildings. The relatively short-duration earthquake motion implied that the observed wall damage occurred in a brittle manner despite adopting a ductile design philosophy. This paper presents the lessons learned from the observed wall damage in the context of current state of knowledge in the following areas: concentrating longitudinal reinforcement in wall end regions; determining wall thickness to prevent out-of-plane wall buckling; avoiding lap splices in plastic hinge zones; and quantifying minimum vertical reinforcement.

Review and improvement of simple mechanical models for predicting the force-displacement response of URM walls subjected to in-plane loading

Abstract: In performance-based seismic design the global displacement capacity of structures is predicted on the basis of local engineering demand parameters and mechanical models that link local and global deformation quantities. Although unreinforced masonry (URM) is one of the most used construction materials for residential structures all over the world, the displacement capacity of in-plane loaded URM walls is still mainly estimated from empirical drift capacity models rather than mechanical relationships between local and global deformation capacities. In this article, we present an analytical model which links the top displacement of an URM wall to the applied in-plane shear load. In order to verify the model, we compare the predicted results to global and local deformation quantities from own URM wall tests. Comparison of global deformation quantities shows that we are able to predict the initial stiffness of the walls with the simple assumption of a no-tension material and a linear-elastic material in compression. For walls developing a diagonal shear failure or hybrid failure, the predicted force-displacement curve starts diverging from the experimental envelope with the formation of the first diagonal cracks. With comparison of local deformation quantities, e.g. curvature profiles and shear strain profiles, we show that this is due to the formation of a significant diagonal shear crack.