Alexandre A. Costa

Seismic Performance of Autoclaved Aerated Concrete (AAC) Masonry: From Experimental Testing of the In-Plane Capacity of Walls to Building Response Simulation

The need to assess the seismic performance of autoclaved aerated concrete (AAC) masonry arose in different countries in the last years. The use of AAC for load-bearing walls is quite common in low seismicity areas in Central and Northern Europe, where its thermal insulation properties, together with lightness and workability, are particularly appreciated. Increasing attention to energy-efficient buildings is now supporting the adoption of a material with such characteristics also in higher seismicity regions. Hence, in order to correctly study the seismic performance of this constructive system, the in-plane response of unreinforced AAC masonry panels has been assessed through an experimental test campaign aiming at obtaining a reliable description of the lateral cyclic behavior. The experimental results are summarized in the article and the derived essential seismic design parameters are presented. The test results allowed the calibration of a macro-element model representative of the nonlinear response of single piers, simulating their cyclic experimental behavior. Three-dimensional models of unreinforced AAC masonry buildings were then obtained using the TREMURI program. Their seismic performance assessment has been carried out through both a nonlinear static (pushover) procedure and nonlinear dynamic time history analyses. Nevertheless, the obtained results allow for some preliminary considerations on the global response of this type of construction and its potential for application in moderate and high seismicity countries.

Analysis of the Out-Of-Plane Seismic Behavior of Unreinforced Masonry: A Literature Review

Although the issue of the out-of-plane response of unreinforced masonry structures under earthquake excitation is well known with consensus among the research community, this issue is simultaneously one of the more complex and most neglected areas on the seismic assessment of existing buildings. Nonetheless, its characterization should be found on the solid knowledge of the phenomenon and on the complete understanding of methodologies currently used to describe it. Based on this assumption, this article presents a general framework on the issue of the out-of-plane performance of unreinforced masonry structures, beginning with a brief introduction to the topic, followed by a compact state of art in which the principal methodologies proposed to assess the out-of-plane behavior of unreinforced masonry structures are presented. Different analytical approaches are presented, namely force and displacement-based, complemented with the presentation of existing numerical tools for the purpose presented above. Moreover, the most relevant experimental campaigns carried out in order to reproduce the phenomenon are reviewed and briefly discussed.

Experimental testing, numerical modelling and seismic strengthening of traditional stone masonry: comprehensive study of a real Azorian pier

Stone masonry is one of the oldest building techniques used worldwide and it is known to exhibit poor behaviour under seismic excitations. In this context, this work aims at assessing the in-plane behaviour of an existing double-leaf stone masonry pier by experimental testing. Additionally, a detailed 3D finite element numerical analysis based on micro-modelling of the original pier is presented (fully describing the geometry and division of each individual elements, namely infill, blocks and joints) aiming at simulating the experimental test results. This numerical strategy can be seen as an alternative way of analysing this type of constructions, particularly useful for laboratory studies, and suitable for the calibration of simplified numerical models. As part of a wider research activity, this work is further complemented with the presentation of an effective retrofit/strengthening technique (reinforced connected plaster) to achieve a significant improvement of its in-plane cyclic response which is experimentally verified in the results presented herein.

Experimental Assessment of the Out-of-Plane Performance of Masonry Buildings Through Shaking Table Tests

ABSTRACT This article presents the results of the LNEC-3D shaking table tests on two mock-ups, Brick House and Stone House, carried out in the scope of the workshop “Methods and challenges on the out-of-plane assessment of existing masonry buildings.” The mock-ups have a U shape with one facade wall and two orthogonal sidewalls. The facade has a central opening and a gable on top, whereas the two sidewalls, acting as abutments, are either blind or have a window. A unidirectional seismic action, in the perpendicular direction to main wall, was applied. Out-of-plane behavior of the facade was found, even if the response was clearly influenced by the presence of the window in one of the sidewalls, which led to significant torsion of the structure. The detailed description of the two tests and the conclusions are presented. The response of the mock-ups was evaluated based on the displacements, damage, and collapse mechanisms developed as function of an increasing intensity earthquake testing protocol, in which a pre-processed strong ground motion component of the Christchurch (New Zealand) earthquake (February 21, 2011) was used.

Out-of-plane behaviour of existing stone masonry buildings: experimental evaluation

Masonry structures can be considered as the simplest type of structures concerning its assemblage but, at the same time, it is one of the most complex construction materials in terms of mechanical properties and correct behaviour assessment. In this context, the work herein presented aims at describing an experimental testing campaign recently carried out in order to characterize the out-of-plane behaviour of traditional masonry constructions. Taking advantage of the existence of a traditional two-storey masonry building abandoned after the 1998 Azores earthquake, several in-situ tests were defined and performed with the application of quasi-static cyclic loads at the building top level in the out-of-plane direction. In addition, the efficiency of retrofitting and/or strengthening techniques applied during the 1998 Azores reconstruction process was also experimentally evaluated. Finally, an overall discussion of these techniques is presented, resorting also to previous tests’ results carried out by the same authors, aiming at inferring and suggesting quantifications of strengthening techniques’ contributions for future interventions on existing buildings. For this purpose, simple analytical mechanical approaches were adopted in order to provide numerical estimates of strength that were found in good agreement with the experimental results.

Methods and Approaches for Blind Test Predictions of Out-of-Plane Behavior of Masonry Walls: A Numerical Comparative Study

ABSTRACT Earthquakes cause severe damage to masonry structures due to inertial forces acting in the normal direction to the plane of the walls. The out-of-plane behavior of masonry walls is complex and depends on several parameters, such as material and geometric properties of walls, connections between structural elements, the characteristics of the input motions, among others. Different analytical methods and advanced numerical modeling are usually used for evaluating the out-of-plane behavior of masonry structures. Furthermore, different types of structural analysis can be adopted for this complex behavior, such as limit analysis, pushover, or nonlinear dynamic analysis. Aiming to evaluate the capabilities of different approaches to similar problems, blind predictions were made using different approaches. For this purpose, two idealized structures were tested on a shaking table and several experts on masonry structures were invited to present blind predictions on the response of the structures, aiming at evaluating the available tools for the out-of-plane assessment of masonry structures. This article presents the results of the blind test predictions and the comparison with the experimental results, namely in terms of formed collapsed mechanisms and control outputs (PGA or maximum displacements), taking into account the selected tools to perform the analysis.

In situ Out-of-Plane Cyclic Testing of Original and Strengthened Traditional Stone Masonry Walls Using Airbags

This article presents a field experimental campaign carried out on an existing one-story stone masonry building, aiming at characterizing the out-of-plane behavior of its walls. A bidirectional test set-up based on a self-equilibrated airbag system was developed and used on three similar masonry walls under distinct conditions: original, retrofitted, and strengthened. The results obtained and the main advantages and shortcomings found in the test set-up are discussed. Finally, some of the most popular retrofitting and/or strengthening techniques applied during the 1998 Azores reconstruction process are critically analyzed, aiming at suggesting quantifications for future interventions on existing unreinforced masonry buildings.

Strengthening of Stone and Brick Masonry Buildings

Today, the scientific community has recognised that the structural safety aspects of existing masonry buildings cannot be treated according to standard procedures that are fit for new constructions. Hence, new approaches for assessing the actual structural performance of existing masonry buildings and developing more appropriate methods and criteria for their repair and strengthening are in progress. The basic idea is that the usual design approaches naturally imply a certain level of “over-design”, and this can lead to unacceptable solutions, under the point of view of costs and conservation, when dealing with existing structures. For these reasons, attention must be paid to the appropriate selection and design of materials and technologies for intervention, taking into account the possibilities offered by traditional solutions and their possible combinations with innovative ones. As existing buildings are usually designed for vertical actions, the “seismic conditions” have the most awkward implications. Indeed, the earthquake actions refer to the very extreme structural resources, i.e. those connected with resistant mechanisms that are normally neglected, and are very difficult to be implemented into structural models. In this chapter, after a general introduction on the characteristics and peculiarities of existing buildings made of stone and clay brick masonry, where we will take into account mainly ordinary buildings, and after some general considerations on the more suitable approaches and criteria for the design of interventions, the most relevant techniques used to strengthen this kind of buildings will be presented.

Methods and Challenges on the Out-Of-Plane Assessment of Existing Masonry Buildings

Historic masonry buildings were built for many centuries taking into account mostly vertical static loads according to the experience of the builder, usually, without much seismic concern. The seismic behavior of ancient masonry buildings is particularly difficult to characterize and depends on several factors, namely the materials properties, geometry of the structure, connections between structural and non-structural elements, stiffness of the horizontal diaphragms, and building condition. However, the different masonry types present common features that lead to high seismic vulnerability of these buildings, such as high specific mass, low tensile strength, low to moderate shear strength, and low to moderate ductility. The out-of-plane behavior of historic masonry structures remains, possibly, the most challenging response in case of seismic action. As demonstrated recurrently all over the world by earthquakes, in case of lack of an integral behavior of the building, out-of-plane failure dominates. This is also favored by the fact that many historic buildings possess large spaces inside, with insufficient connections to transverse structural elements. Even if adequate connections between elements can be ensured, e.g., by tying walls, enforcing connections between walls and enforcing wall to floor connections, it is necessary to avoid disintegration of the walls themselves when subjected to out-of-plane actions, especially in case of rubble masonry. The out-of-plane response of these structures or their local mechanisms, often assumed as macroblocks with almost rigid behavior that become independent from the global structure is complex. The dynamics of these local mechanisms close to collapse are very sensitive to the seismic input (e.g., frequency contents, duration or directivity of the signal) but also to the structure itself (e.g., boundary conditions). The methods for structural analysis available for this purpose, both for research and engineering applications, are rather different in terms of formulation, input, conceptual complexity, and computational efficiency. For these reasons, the results obtained can also vary greatly. Taking this into account, during the 9th International Masonry Conference held in Guimarães, Portugal, in 2014, a blind test exercise on the out-ofplane failure of historic masonry structures, involving prediction, testing, and postdiction, was carried out. This involved about 25 international experts in the field and clearly demonstrates that further developments are needed in the field. This special issue addresses the works carried out, including a review article on seismic assessment techniques, a conceptual discussion article on research needs and modeling issues, a thorough description of the experimental tests, a comparison between the predictions of the different participants, postdictions by the six teams, and a final concluding article. The editors sincerely thank all experts and their teams for the participation on this blind test exercise.

Numerical Modelling Approaches for Existing Masonry and RC Structures

Assessment of existing buildings making use of numerical simulation methods, even under the hypothesis of full knowledge of current conditions and materials, it is not an easy and straightforward task due to the limitations and complexities of such analysis tools. In this chapter, a discussion of different approaches for the simulation of structural response is introduced and applied to two of the most common building typologies: masonry structures and reinforced concrete frames. Following a brief introduction of the problematic, an overview of different modelling possibilities for masonry structures is presented. Afterwards, choices made during numerical modelling are discussed, based mainly on the finite element method. Moreover, the problematic of different modelling techniques is addressed, where some paths and best practices are suggested. The last section is devoted to the response simulation of reinforced concrete structures. Efficient frame elements and sectional models, which allow capturing an extended range of elastic and inelastic response, are analysed first. Strut-and-tie modelling is then recalled as a powerful analysis tool, and its application to the assessment of old buildings is studied.