Seismic vulnerability assessment methodology for slender masonry structures

ABSTRACT

Slender masonry structures such as towers, minarets, chimneys, and Pagoda temples can be characterized by their distinguished architectural characteristics, age of construction, and original function, but their comparable geometric and structural ratios yield to the definition of an autonomous structural type. These structures constitute a part of the architectural and cultural heritage. Their protection against earthquakes is of great importance. This concern arises from the strong damage or complete loss suffered by these structures during past earthquakes. Seismic vulnerability assessment is an issue of most importance at present time and is a concept widely used in works related to the protection of buildings. However, there is few research works carried out on developing the seismic vulnerability assessment tools for such structures.

This article presents a new method for assessing the seismic vulnerability of slender masonry structures based on vulnerability index evaluation method. The calculated vulnerability index can then be used to estimate structural damage after a specified intensity of a seismic event. Here, 12 parameters are defined to evaluate the vulnerability index for slender masonry structures. Implementation of this methodology is carried out in different types of slender masonry structures to develop vulnerability curves for these structure types.     

Seismic Load Capacity of Historical Masonry Mosques by Rigid Body Kinetics

ABSTRACT

Kinetic analysis methods based on linear and nonlinear rigid body dynamics are used to evaluate earthquake safety of masonry structures. In this study, the formulas used to calculate the in-plane and out-of-plane load capacities of masonry load-bearing walls were evaluated and a procedure based on rigid body mechanism was proposed to calculate the out-of-plane load capacities of the walls of Ottoman period masonry mosques. New aspects of the method with respect to existing formulations is the inclusion of dynamic axial load and definition of the collapse limit spectral acceleration on the overturning wall. The calculated capacities of the mosque and individual walls were compared with the results of nonlinear pushover analysis and time history analyses performed under 1.0 and 0.5 scaled forms of nine different 3-component ground motion records. It was displayed that the seismic load capacity estimated by the proposed method is very close to the values calculated by pushover and time history analyses. The method was developed on Lala Pasha Mosque, and the reliability and applicability of the proposed methodology is verified on a different historical masonry mosque in comparison to finite element analyses results.     

Empirical Formulation for Estimating the Fundamental Frequency of Slender Masonry Structures

The fundamental frequency of a structure enables better assessment of its seismic demand for an efficient design and planning of its maintenance and retrofit strategy. The frequency is independent of the type of external loads, however, depends on structural stiffness, mass, damping and boundary conditions. In the case of slender masonry structures such as towers, minarets chimneys, and pagoda temples, it is influenced by mass and stiffness distribution, connection to adjacent structures, material properties, aspect ratio and slenderness ratio. In this present article, the data collected from various literature reviews on the slender masonry structures regarding dynamic, geometrical, and mechanical characteristics have been correlated to identify the major parameters influencing the fundamental frequency of such structures. The database has been used for developing an empirical formulation for predicting the fundamental frequency of such structures. The comparison between the experimental fundamental frequencies and the estimated fundamental frequencies are carried out in order to define reliability and accuracy of these empirical formulae.     

Dynamic Analysis and Seismic Response of Planar Circular Arches with Variable Cross-Section

The aim of this paper is to investigate the dynamic response of planar circular arches with variable cross-section subjected to seismic ground motions. Arches have a wide range of application (e.g. bridges, roofs) thanks to their capacity to span large areas by resolving vertical actions into compressive stresses and confining tensile stresses. The full understanding of their dynamic response is a challenging technical and computational problem, especially when seismic loading is considered. For example, the assumption of axial inextensibility simplifies the differential equations but overestimates the vibration frequencies, especially those of shallow arches since axial forces are of paramount importance (as opposed to beams). In lieu of the above, our formulation incorporates the effect of axial extension, and the arches are modeled using a new generic curved beam model that includes both axial (tangential) and transverse (normal) to the arch centerline deformations, and is able to account for variable mass and stiffness properties, as well as elastic support or restraint. The resulting dynamic governing equations of the circular arch are formulated in terms of the displacements, and solved using an efficient integral equation method. Three circular arches with variable rectangular cross-section are analyzed in order to investigate their dynamic properties and seismic performance. Using both time history and modal analysis useful conclusions are drawn with regard to the contribution of each mode on the calculation of different response quantities.     

Experimental Study on Postponing the Deboning of FRP Sheets in Masonry Walls

ABSTRACT

A large number of buildings all around the world are constructed of unreinforced masonry. These structures do not act well during earthquakes because of their vulnerable behavior. In last two decades, fiber-reinforced polymers (FRPs) has been used widely in seismic rehabilitation and strengthening unreinforced concrete and masonry structures. One important issue in using FRP composites for strengthening masonry walls is the inopportune debonding of composites from the wall surface; thus, in this article new methods are proposed to further delay the mentioned debonding issue. For this purpose, 13 masonry panels with 100x870x870 mm dimension are strengthened by using carbon and glass FRPs (CFRPs and GFRPs). A variety of strengthening methods such as surface preparation, boring, grooving, nailing, and plaster are used to mount FRP composites to the walls. For each specimen subjected to diagonal compression test, the loading level along with tensile and compressive diagonal displacements are evaluated. In order to assess the effect of FRP composites, four unreinforced masonry walls are tested as well. The results show 110% increase in ductility index of reinforced specimens compared to the unreinforced ones.     

Discrete Element Modeling of Groin Vault Displacement Capacity

Displacements experienced by many historic masonry structures concentrate at masonry joints and can be large before collapse is a concern, making modeling of stability using discrete element modeling (DEM) particularly suitable. In this study, masonry groin vault and arch models with several geometries were subjected to horizontal and vertical support displacements using DEM. Support movements were applied in a quasi-static manner to simulate the support settlement process. Displacements at collapse and at the point when the first block fell from the vault were recorded. Block separation and mechanisms were also noted during the simulations. A two-dimensional (2D) analytical model using thrust line analysis was developed to help evaluate the DEM results. In general, the displacements at first block fall were relatively large but significantly less than those at collapse. The groin vaults and arches exhibited significantly higher capacity to sustain vertical support displacement compared to horizontal displacements. For many geometries, the DEM collapse displacements of the groin vaults compared reasonably well to similar arches, indicating that the displacement capacity of groin vaults can be reasonably estimated using 2D simplifications. However, for certain geometries, three-dimensional effects were found to significantly affect displacement capacity.     

Nonlinear Structural Performance of a Historical Brick Masonry Inverted Dome

ABSTRACT

Traditional domes are obtained by double curvature shells, which can be rotationally formed by any curved geometrical plane figure rotating about a central vertical axis. They are self-supported and stabilized by the force of gravity acting on their weight to hold them in compression. However, the behavior of inverted domes is different since the dome is downward and masonry inverted domes and their structural behaviors in the literature received limited attention. This article presents a nonlinear finite element analysis of historical brick masonry inverted domes under static and seismic loads. The brick masonry inverted dome in the tomb of scholar Ahmed-El Cezeri, town of Cizre, Turkey, constructed in 1508 is selected as an application. First, a detailed literature review on the masonry domes is given and the selected inverted dome is described briefly. 3D solid and continuum finite element models of the inverted masonry dome are obtained from the surveys. An isotropic Concrete Damage Plasticity (CDP) material model adjusted to masonry structures with the same tensile strength assumed along the parallel and meridian directions of the inverted dome is considered. The nonlinear static analyses and a parametric study by changing the mechanical properties of the brick unit of the inverted masonry dome are performed under gravity loads. The acceleration records of vertical and horizontal components of May 1, 2003 Bingöl earthquake (Mw = 6.4), Turkey, occurred near the region, are chosen for the nonlinear seismic analyses. Nonlinear step by step seismic analyses of the inverted dome are implemented under the vertical and horizontal components of the earthquake, separately. Static modal and seismic responses of the inverted masonry dome are evaluated using mode shapes, minimum and maximum principal strains and stresses, and damage propagations.     

Displacement-Based Seismic Design Approach for Prestressed Precast Concrete Shear Walls and its Application

ABSTRACT

Prestressed precast concrete shear wall (PPCW) is a new kind of shear wall utilizing a combination of unbonded post-tensioning steel and mild steel for flexural resistance across horizontal joints. A simple procedure of direct displacement-based design approach for PPCW based on concept of inelastic design spectra is proposed. Section design is then carried out according to base overturning moment. A detailed design example demonstrating a step-by-step application of the design procedure is also provided. Nonlinear time-history analysis verified that this approach is applicable to control the target displacement to the performance acceptable limit.     

Seismic Analysis of Masonry Gravity Dams Using the Discrete Element Method: Implementation and Application

Much research in recent years has focused on the seismic analysis of concrete and earthfill dams, and few works have addressed the case of masonry dams. The structural behavior of masonry dams is controlled essentially by its discontinuous nature, which may induce significant nonlinear response during an intense earthquake. In this article, a numerical tool based on the Discrete Element Method is presented, aimed at the static, dynamic, and hydromechanical analysis of masonry gravity dams. The use of discontinuous models is mandatory for the study of failure mechanisms involving the masonry discontinuities, the dam-rock interface or the rock mass joints. The Discrete Element Method is able to assemble continuous and discontinuous meshes simultaneously in the same model, providing a versatile tool to consider various assumptions and levels of analysis, ranging from simplified to detailed structural representations. A comprehensive study of the seismic behavior of Lagoa Comprida Dam, located in Portugal, is presented. Both continuous and discontinuous models were developed to assess the main failure mechanisms, including overstress, partial and global sliding, and overturning.     

Seismic Behavior of L-Shaped RC Squat Walls under Various Lateral Loading Directions

Considerable progress has been made on the research of non-rectangular reinforced concrete (RC) squat walls over the past decades. However, the experimental data of L-shaped RC squat walls remain limited, especially for their seismic behaviors under non-principal bending actions. This paper presents an experimental and numerical investigation on L-shaped RC squat structural walls with an emphasis on how varying the directions of lateral cyclic loading influences the seismic responses of these walls. Four L-shaped specimens are tested under lateral cyclic displacements and low levels of axial compression The variables are axial loads and lateral loading directions. The performance of specimens is discussed in terms of cracking patterns, failure mechanisms, hysteretic responses, deformation components and strain profiles. Furthermore, three-dimensional finite element models are developed to supplement the experimental results. The direction of lateral loading is found to have a significant effect on the peak shear strength of L-shaped RC squat walls.     

OUT-OF-PLANE SEISMIC RESISTANCE OF MASONRY WALLS

Seismic vulnerability of unreinforced masonry buildings is studied by means of simplified out-of-plane collapse mechanisms that take into account connections with transversal walls. According to experimental evidence, the analysis assumes that failure is reached with a rigid body motion of a part of the facade that falls down. Two classes of mechanism are examined: the overturning of the facade due either to a vertical crack at the connection or a diagonal crack on the transversal wall, both defined resorting to a simple model of masonry fabric, viewed as a regular assembly of rigid blocks and elastic plastic joints with friction but no cohesion. The use of simplified mechanisms give rise to an explicit evaluation of the seismic resistance to changes in the geometry and in the masonry fabrics, that could be used by practising engineers. This formulation is developed for both static horizontal actions and ground velocity peak, in the belief that the latter probably gives a better approximation of seismic action, while also providing, by comparison with the results of static forces, an estimate of the behaviour factor for unreinforced masonry. Eventually, the analytical forecasts are compared with numerical results obtained by means of the distinct element method.     

Dynamic One-Sided Out-Of-Plane Behavior of Unreinforced-Masonry Wall Restrained by Elasto-Plastic Tie-Rods

Past earthquakes have shown the high vulnerability of existing masonry buildings, particularly to out-of-plane local collapse mechanisms. Such mechanisms can be prevented if façades are restrained by tie rods improving the connections to perpendiculars walls. Whereas in the past only static models have been proposed, herein the nonlinear equation of motion of a monolithic wall restrained by a tie rod is presented. The façade, resting on a foundation and adjacent to transverse walls, rotates only around one base pivot and has one degree of freedom. Its thickness is explicitly accounted for and the tie rod is modeled as a linear elastic—perfectly plastic spring, with limited displacement capacity. The model is used to investigate the response to variations of wall geometry (height/thickness ratio, thickness), tie rod features (vertical position, length, prestress level), and material characteristics (elastic modulus, ultimate elongation, yield strength) typical of historical iron. The most relevant parameter is the steel strength, whereas other characteristics play minor roles allowing to recommend reduced values for pre-tensioning forces. The force-based procedure customary in Italy for tie design is reasonably safe and involves protection also against collapse, although probably not enough as desirable.     

Seismic vulnerability assessment and fragility analysis of stone masonry monastic temples in Sikkim Himalayas

Buddhist monasteries in Sikkim Himalayas constitute important religious and architectural heritage. These random rubble (RR) stone masonry structures located in high seismically active regions of the Himalayas have suffered varied degrees of damages in the past earthquakes. The study presents seismic vulnerability assessment of four archetypal monastic temples using finite element (FE) analyses. Linear and nonlinear analyses of these structures were conducted in Abaqus FE environment. These analyses identified the damage prone areas of the structures and provided load-deformation behavior under lateral loads. Fragility analyses indicate a high probability of collapse for the specified design level earthquake of the region. The study shows that performance of the structure can be enhanced by improving the strength and stiffness of the stone masonry walls.     

Review of Out-of-Plane Seismic Assessment Techniques Applied To Existing Masonry Buildings

ABSTRACT

Observations after strong earthquakes show that out-of-plane failure of unreinforced masonry elements probably constitutes the most serious life-safety hazard for this type of construction. Existing unreinforced masonry buildings tend to be more vulnerable than new buildings, not only because they have been designed to little or no seismic loading requirements, but also because connections among load-bearing walls and with horizontal structures are not always adequate. Consequently, several types of mechanisms can be activated due to separation from the rest of the construction. Even when connections are effective, out-of-plane failure can be induced by excessive vertical and/or horizontal slenderness of walls (length/thickness ratio). The awareness of such vulnerability has encouraged research in the field, which is summarized in this article. An outline of past research on force-based and displacement-based assessment is given and their translation into international codes is summarized. Strong and weak points of codified assessment procedures are presented through a comparison with parametric nonlinear dynamic analyses of three recurring out-of-plane mechanisms. The assessment strategies are marked by substantial scatter, which can be reduced through an energy-based assessment.     

Self-Centering Behavior of Unbonded,Post-Tensioned Precast Concrete Shear Walls

Self-centering ability of unbonded post-tensioned precast concrete shear walls has been attributed to the presence of post-tensioning force. However, the experimental results presented in this paper indicate that the post-tensioning force may completely die out during cyclic loading while the walls are able to retain their superior self-centering characteristic. Moreover, the analytical study presented in this article indicates that with proper configuration of end-anchorages for post-tensioned tendons, self-centering of post-tensioned walls can be achieved even when the post-tensioning force vanishes. This study also investigates the effects of tendon layout, tendon end-anchorage configuration, and external vertical load on the self-centering ability of unbonded precast concrete shear walls subjected to earthquake loading.     

Numerical Study on the Peak Strength of Masonry Spandrels with Arches

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.     

Stability of Slender Masonry Columns with Circular Cross-Section under Their Own Weight and Eccentric Vertical Load

This work concerns the stability of unreinforced masonry slender circular cross-sectional columns subjected to their own weight and eccentric vertical load. Cantilever columns are examined, considering that the material has infinitely linear elastic behavior in compression and has no tensile strength. For the analysis, an existing numerical model and solution procedure developed for the stability analysis of masonry elements with rectangular cross-section are utilized and adapted to the circular columns. For the instability of the columns, an appropriate criterion that relates the top lateral deflection to the intensity of the applied eccentric vertical load is employed. By considering a reference column, critical buckling load is obtained, behavior of the column interpreted and efficiency of the numerical model emphasized. Performing a nonlinear buckling analysis using a general purpose software on this reference column, obtained results are compared with those of the adapted procedure of the present study. Implementing parametric analyses on reference column, effects of the column slenderness, eccentricity of vertical load, elastic modulus, and self-weight on the buckling load are investigated. Presented calculation procedure provides a useful tool in order to calculate the critical loads or to check the stability of masonry circular columns.     

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.     

Pisa goes critical

Abstract

The Leaning Tower of Pisa is not just a quaint curiosity but a magnificent architectural treasure. Over the years its inclination has been increasing inexorably to the point where it is about to fall over. Moreover the stresses in the masonry induced by the lean have brought it close to structural failure. Stabilisation of the tower represents the ultimate civil engineering challenge. This paper describes the history of the tower and the mechanics of its behaviour, understanding of which has proved vital in the development of stabilisation measures. Early stabilisation was first achieved by placing temporary lead weights on the foundation masonry on the opposite side from the lean. Long term stabilisation commenced in February 2000 using a controversial method of soil extraction to reduce the inclination by about 10% – not enough to be visible but enough to substantially increase the safety. A long, tense journey lies ahead of the tower.     

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.