Nuno Mendes

Analysis of Masonry Structures Without Box Behavior

Assessment of the seismic performance of structures is still a challenge. Historic masonry structures exhibit peculiar properties (low tensile strength and lack of box behavior) that make the task of the analyst even more difficult. It seems that traditional design and assessment methods, similar to the ones currently used for reinforced concrete structures, are not applicable. This article provides a review of the seismic analysis of masonry structures without box behavior. Different methods of structural analysis are discussed and a comparison is made between pushover methods and non-linear dynamic analysis with time integration. Three cases studies (San Torcato church, Qutb Minar and “Gaioleiro” buildings) were used and the results show that traditional, adaptive or modal pushover analyses are not totally in agreement with non-linear dynamic analysis or experimental observations.

Seismic Assessment of Masonry ''Gaioleiro'' Buildings in Lisbon, Portugal

This article presents a numerical study for the seismic assessment of a Portuguese building masonry typology - “Gaioleiro.” Numerical analysis was performed using a finite element model calibrated with experimental results obtained in 1:3 reduced scale tests carried out in the LNEC 3D shaking table. Nonlinear dynamic analysis with time integration and pushover analysis are carried out. Using nonlinear dynamic analysis it was verified that the buildings of “Gaioleiro” type with appropriate floor-wall connections are in the limit of their loading capacity. The pushover analyses performed in this study were incapable of simulating correctly the damage of the structure under seismic action.

Experimental investigation on the seismic performance of masonry buildings using shaking table testing

Masonry buildings worldwide exhibited severe damage and collapse in recent strong earthquake events. It is known that their brittle behavior, which is mainly due to the combination of low tensile strength, large mass and insufficient connection between structural elements, is the main limitation for their structural implementation in residential buildings. A new construction system for masonry buildings using concrete blocks units and trussed reinforcement is presented here and its seismic behavior is validated through shaking table tests. Dynamic tests of two geometrically identical two-story reduced scale (1:2) models have been carried out, considering artificial accelerograms compatible with the elastic response spectrum defined by the Eurocode 8. The first model was reinforced with the new proposed system while the second model was built with unreinforced masonry. The experimental analysis encompasses local and global parameters such as cracking patterns, failure mechanisms, and in-plane and out-of-plane behavior in terms of displacements and lateral drifts from where the global dynamic behavior of the two buildings is analyzed comparatively. Finally, behavior factors for the design recommendations in case of unreinforced masonry are also evaluated.

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.

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.

Methods and Challenges for the Seismic Assessment of Historic Masonry Structures

ABSTRACT Despite the high vulnerability of historic structures to earthquakes, the approaches for evaluating seismic demand and capacity still appear inadequate and there is little consensus on the most appropriate assessment methods to use. To develop an improved knowledge on the seismic behavior of masonry structures and the reliability of analysis tools, two real-scale specimens were tested on a shake table, and several experts were invited to foresee failure mechanism and seismic capacity within a blind prediction test. Once unveiled, experimental results were simulated using multi-block dynamics, finite elements, or discrete elements. This article gathers the lessons learned and identifies issues requiring further attention. A combination of engineering judgment and numerical models may help to identify the collapse mechanism, which is as essential as it is challenging for the seismic assessment. To this purpose, discrete modeling approaches may lead to more reliable results than continuous ones. Even when the correct mechanism is identified, estimating the seismic capacity remains difficult, due to the complexity and randomness of the seismic response, and to the sensitivity of numerical tools to input variables. Simplified approaches based on rigid body dynamics, despite the considerable experience and engineering judgment required, provide as good results as do advanced simulations.

Simulation of shake table tests on out-of-plane masonry buildings. Part (IV): macro and micro FEM based approaches

ABSTRACT This article presents a study on the out-of-plane response of two masonry structures without box behavior tested in a shaking table. Two numerical approaches were defined for the evaluation, namely macro-modeling and simplified micro-modeling. As a first step of this study, static nonlinear analyses were performed for the macro models in order to assess the out-of-plane response of masonry structures due to incremental loading. For these analyses, mesh size and material model dependency was discussed. Subsequently, dynamic nonlinear analyses with time integration were carried out, aiming at evaluating the collapse mechanism and at comparing it to the experimental response. Finally, nonlinear static and dynamic analyses were also performed for the simplified micro models. It was observed that these numerical techniques correctly simulate the in-plane response. The collapse mechanism of the stone masonry model is in good agreement with the experimental response. However, there are some inconsistencies regarding the out-of-plane behavior of the brick masonry model, which required further validation.

Seismic vulnerability assessment of ancient masonry building : an experimental method

This paper presents a review of the methods for seismic vulnerability assessment, together with an experimental method based on shaking table testing. This method is applied to a Portuguese masonry building typology with stone walls and timber floors, subjected to increasing earthquake damage. Traditional-like materials and techniques are used in the building. The vulnerability curves are presented and the damage indicator is correlated with the crack patterns and EMS 98.

Masonry macro-block analysis

The structural analysis involves the definition of the model and selection of the analysis type. The model should represent the stiffness, the mass, and the loads of the structure. The structures can be represented using simplified models, such as the lumped mass models, and advanced models resorting the finite element method (FEM) and discrete element method (DEM). Depending on the characteristics of the structure, different types of analysis can be used such as limit analysis, linear and nonlinear static analysis, and linear and nonlinear dynamic analysis. Unreinforced masonry structures present low tensile strength, and the linear analyses seem to not be adequate for assessing their structural behavior. On the other hand, the static and dynamic nonlinear analyses are complex, since they involve large time computational requirements and advanced knowledge of the practitioner. The nonlinear analysis requires advanced knowledge on the material properties, analysis tools, and interpretation of results. The limit analysis with macro-blocks can be assumed as a more practical method in the estimation of maximum load capacity of structure. Furthermore, the limit analysis requires a reduced number of parameters, which is an advantage for the assessment of ancient and historical masonry structures, due to the difficulty in obtaining reliable data. The observation of the damage in masonry buildings caused by earthquakes in the past has been shown that the masonry structures can be discretized into several macro-blocks and interfaces. The macro-blocks are portions of a structure with similar material properties and structural behavior, to which the mechanical properties of the material can be assigned or, by simplification, they can be assumed to be infinitely rigid. The interfaces in a macro-modelling represent, in general, the cracks associated to the failure mechanisms. However, in a micro-modelling strategy applied to masonry, in which the units and the joints are individually considered, the interfaces simulate the behavior of the joints. Furthermore, different criteria for the strength parameters of macro-blocks and interfaces can be considered. The limit analysis (Fig. 1) with macro-blocks is a simplified and powerful structural analysis tool to evaluate the ultimate capacity of masonry structures by static models, involving the equilibrium of the macro-blocks through the limit analysis basic concepts.

Empirical seismic vulnerability analysis for masonry buildings based on school buildings survey in Iran

School facilities in Iran, in particular masonry schools, have shown poor performance during past earthquakes and can be identified as one of the parts of the country’s infrastructure that is most vulnerable to earthquakes. Hence, in this paper a method to perform index-based damage assessment for brick masonry schools located in the province of Yazd, the central region of Iran, using a comprehensive database of school buildings, is proposed. The database was obtained from the field survey forms applied for each observed school to collect the features of and damage to the structure. The results of a vulnerability index method developed in Iran are employed as input data to obtain empirical fragility curves for the school inventory. The Macroseismic model and GNDT II level method are two empirical methods combined in this procedure. Finally, the procedure is verified using damage survey data obtained after recent earthquakes (1990 Manjil–Rudbar earthquake and 2003 Bam earthquake) that occurred in Iran.