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The design of reinforced concrete (RC) jacketing for retrofitting substandard columns is a common practice when approaching the structural rehabilitation design of existing structures. In this framework, international technical codes suggest to assume the hypothesis of monolithic section and finally affecting the deduced performances by some empirical factors, known in the literature as monolithicity factors. The assessment of these coefficients is a difficult task, and only some rough assumptions can be found in the modern codes. This paper presents the development of a software package able to perform the static non-linear analysis of RC jacketed columns and evaluating the actual value of the monolithicity factor. The code is developed in an integrated environment, taking advantage of finite element (FE) framework OpenSees and adopting Matlab for developing an user-friendly Graphical User Interface (GUI). The fibre-section approach is automatically implemented for modelling the flexural behaviour of the column and the effective bond between the old and new column in considered through a distribution of non-linear springs. Finally, the parametric analysis performed by the proposed tool allows to present the monolithicity factors as a function of the key parameters.

Fabric Reinforced Cementitious Matrix (FRCM) materials are increasingly common for strengthening existing masonry structures. Their popularity is due to their many advantages with respect to resin-based composites, especially when applied to stone supports. The constitutive behaviour of FRCM materials is defined by the combination of their tensile response and the bond behaviour with the masonry support, both depending on complex stress transfer mechanisms between matrix and fabric, especially in the post-cracking stage. This paper presents a numerical study which aims to predict the mechanical behaviour of FRCM systems through simple 2D models of truss elements and non-linear springs to simulate the fabric-to-matrix and composite-to-substrate interaction. The comparisons between results of numerical approach and experimental responses showing that the proposed methodology is an effective and easy tool to predict the mechanical behaviour of FRCM composites.

Structural Health Monitoring (SHM) represents a strategic solution for the preservation of cultural heritage buildings. Existing masonry structures often suffer reductions in mechanical performances due to physiological aging of material constituents, external actions, and effect of catastrophic natural events. In many cases, the prompt prediction of damage in masonry elements is difficult and it can cause sudden collapses, compromising the safety of people. The proposed experimental study examines the effectiveness of two low-cost and innovative stress sensors, i.e. piezoelectric and capacitive stress sensors, for SHM of masonry structures. To this scope, the sensors were embedded in the mortar joints of two small-scale clay brick and calcarenite masonry wall specimens consisting of three panels. Experimental tests were carried out by applying a constant vertical compressive load at the top of each specimen and simulating the damage with a progressive reduction of the cross-section of one of the panels. During the tests, the vertical stress distributions (and their variations), were monitored by the sensors. Experimental outcomes from sensor reading were then compared to that numerically provided by a refined finite element simulation of the test. Results will show that vertical stress variations in masonry structures can be effectively accounted by the adopted sensors and potentially interpreted for the early prediction of structural damage.

The use of the Fabric Reinforced Cementitious Mortars (FRCMs) is nowadays a promising solution for the strengthening of both reinforced concrete and masonry structural elements. The application consists of a bond-dependent face-to-face plastering of an open grid or mesh by means of an inorganic-based matrix, i.e. a cement-based mortar. The main advantage of such a strengthening technique is the good compatibility with different types of substrates since the most suitable matrix can be selected focusing on the most similar breathability and stiffness. On the other side, the strengthening efficiency could be over-estimated if the potential bond failure is neglected. The FRCM-bond behaviour depends on many parameters, e.g., mechanical properties of substrate and reinforcement, environmental conditions, etc. Due to the wide range of variability of these parameters, nowadays, formulations for a reliable prediction of the maximum strain which the FRCM is able to develop for avoiding bond failure are not available. At this scope, the present paper collects a large database of experimental results of bond tests available in literature in order to investigate the main parameters influencing the performance of this strengthening technique, to identify the aspects that have to be further studied, and to calibrate an empirically-based analytical model proposed by the authors to predict the maximum design strain in the FRCM reinforcement. The parameters assumed in the model are the following: type of substrate (i.e. masonry or concrete), type of fabric (i.e. carbon, glass, steel, basalt), type of matrix (i.e. cement, lime, alkali-activated based) and failure mode (detachment, slippage, rupture, etc.). The database was clustered depending on the failure mode in order to calibrate targeted and more accurate equations.

The use of composite beams made with traditional concrete and bio-based materials (such as timber and bamboo) is a valuable solution to reduce the environmental impact of the building sector. Timber-Concrete Composite (TCC) beams have been used for decades in structural applications such as new buildings, refurbishment of old timber structures, and bridges. Recently, different researchers suggested composite beams based on engineered bamboo, commonly named Bamboo-Concrete Composite (BCC) beams. This study presents a systematic comparison of structural performances and connection behavior of TCC and BCC beams under short-term static load. TCCs beams are compared to BCC ones using similar shear connectors. The most important aspects of the two composite systems are compared: mechanical behavior of connectors and structural behaviors of full-scale composite beams (e.g., failure modes, connection stiffness, connection shear strength, ultimate load-carrying capacity, maximum deflection and composite efficiency). This comprehensive review indicates that BCC beams have similar or even better structural performances compared with TCC.

This paper presents an analytical procedure for the design of the longitudinal reinforcement in reinforced concrete rectangular sections subjected to axial load and biaxial bending moment. The integration of the equilibrium equations is performed in analytical form for the case of uniaxial moment, including the effect of the reinforcement placed on the sides of the section and adopting code-based constitutive laws for steel and concrete. The strength domain of the section is calculated in closed form for the case of only main steel and in simplified form when including the reinforcement disposed on the side. Additionally, a formulation is proposed for the interaction domain of the two components of the bending moment for constant axial load, in which the uniaxial moment capacities are calculated including all the rebars within the section. The proposed method is adopted for setting a design procedure of the steel bars of rectangular sections subjected to biaxial bending. Comparisons with numerical results and code provisions show the reliability of the approach, which reduces computational costs and give useful indications for practitioners and designers.

Modelling the tensile behaviour of Fabric Reinforced Cementitious Matrix (FRCM) is not a straightforward task due to the inner complexity of the mechanics of this kind of composite materials. In fact, after that the matrix is cracked, the compatibility between the fibre and the surrounding mortar is lost and the system behaves as two separate elements connected by a brittle interface. For this reason, several research studies proposed computational approaches for evaluating the tensile behaviour of FRCM composites, usually referring to brick-based 3D Finite Element Models (FEM) or to complex numerical procedures. This paper shows the formulation of a simplified coupled truss element for modelling the tensile behaviour of FRCM composites. The proposed element includes the interface slip between fibre and matrix and the brittle failure of the fabric and allows to describe the response of the system between the cracks in closed form. The proposed element is also adopted for assessing the tensile constitutive response of FRCMs through a simplified assembly procedure, proving to be reliable and computationally efficient.

Textile reinforced mortar (TRM) materials are currently receiving great attention for the strengthening of reinforced concrete and masonry structural elements. Mortars with different strength classes have been coupled with different kind of synthetic fabrics, such as carbon, glass, steel, PBO. Recently natural fibres and, in particular, basalt fibres are receiving increasing interest since they proved to be a promising alternative to the most common synthetic fabrics, thanks to the reduced production costs and the consequent benefits in terms of environmental impact. This paper presents an experimental investigation comprising forty-one monotonic tensile tests on textile strips made with a primed basalt fibre bidirectional grid. The tests were performed considering the influence of different setup variables, such as the clamping system, the size of the specimens, the measurement device and the testing elongation rate. Part of the tests was carried out by using a video-extensometer for the measurement of the strains, taking advantage from the digital image correlation (DIC) technique. The goal of the work was to find the most suitable configuration for the mechanical characterization of basalt grids, in order to fully exploit the tensile strength. The results showed to be influenced by the different test setup parameters considered. In particular, the outcomes were affected by the slippage of the grid at the gripping zone and by the testing rate.

This paper presents a critical review of the most established analytical models for the prediction of the compressive strength of FRP and FRCM-confined masonry columns. In particular, two types of fibres are analysed, i.e. glass and basalt. A wide dataset available in the literature is used for the application of the analytical models and for the development of parametric analyses useful for the critical comparison of FRP vs. FRCM confinement technique and glass vs. basalt fibres to be adopted as reinforcement of masonry substrate. The effects of stiffness and strength of the reinforcement, the number of reinforcing layers, the compressive strength of masonry and the cross-section shape are investigated.

The conservation and the preservation of existing masonry buildings, most of them recognized as cultural heritage, require retrofitting techniques that should reduce the invasiveness and assure reversibility and compatibility with the substrate. In this perspective, the strengthening system should be able to improve the bearing capacity of the structural member and, at the same time, to assure mechanical and material compatibility. The use of Fabric Reinforced Cementitious Matrix (FRCM) composites is now recognized to be suitable for these purposes. Indeed, the inorganic matrix has comparable properties with respect to the existing historical mortars while the fabric has relevant tensile strength. At the same time, these systems assure satisfactory level of reversibility (or at least removability). In this scenario, the present research aims to investigate the FRCM-confinement of masonry columns focusing on the influence of specific parameters, still poorly investigated, in order to deeply understand their effect on the mechanical response. In particular, the experimental variables are: the type of masonry substrate (Tuff and clay brick with lime-based mortar), the type of FRCM system (glass dry mesh + lime-based mortar and steel mesh + lime-based mortar) and the number of plies (1, 2 and 3). In addition, a detailed experimental characterization of the utilized materials has been carried out, including bond tests between the reinforcement and the substrate. The results evidenced that the FRCM is an effective solution for masonry columns confinement once a proper design is performed, taking into account all involved parameters. The different strengthening systems exhibited different failure modes. Generally, a single ply of external reinforcement produced a negligible increase of bearing capacity. Both strengthening systems applied with multi-ply strengthening schemes produced a significant increase in terms of strength and ultimate axial deformation. This benefit was observed for both Tuff and clay masonry columns. Two available design-oriented formulas, reported in the Italian CNR (National Research Council) and ACI (American Concrete Institute) guidelines have been utilized, in order to further investigate their accuracy, mostly in the case of multi-layered reinforcement. The performed comparisons highlight that the two design relationships provide similar and accurate results when referred to the GFRCM (Glass-FRCM) system in 1- and 2-layer configurations, while the predictions appear conservative when 3 layers of GFRCM are utilized, for both masonry types. Considering the SRG (Steel Reinforced Grout) system, the results predicted by the two models are more scattered, mostly when the number of layers increases. In addition, the formulation proposed by CNR appears more accurate in the case of Tuff masonry while the ACI predictions are closer to the experimental results in the case of clay brick masonry.

The increasing development of computational power in modern digital devices has spread the use of structural optimization in design applications of different fields of engineering problems. This approach exposes engineers to challenging design procedures aimed to optimize all variables to satisfy the imposed requirements, finding the “optimal” solution. Despite this wide use, the application of optimization algorithms for the design of seismic retrofitting strategies becomes tougher, due to the difficulties in finding a mathematical expression which includes and combines all the key variables (i.e., cost, safety, sustainability, design) and the nonlinearity of the analysis. In this paper, the Genetic Algorithm search method (GA) has been used to optimize the retrofitting, using Buckling-Restrained Braces (BRBs), of a three-storeys RC structure designed to resist only gravity loads. The optimization aims to ensure the safety level required, minimizing intervention costs and use of materials, particularly finding the best topology solution for BRB positioning. To this purpose, an objective function has been applied to the model to minimize retrofit costs intervention using GA. The procedure has been implemented in Python Programming Language. In particular non-linear static analysis has been run using OpenSeesPy framework to evaluate the seismic required safety level at each simulation, while GA has been carried out with a custom design code to provide the best topology position of BRBs to minimize costs and consequently the use of materials. The method brought out the real possibility to ensure the required seismic safety level while reducing intervention cost and use of materials with an optimized and structured BRB positioning.

Concrete and steel jacketing are common strengthening techniques for reinforced concrete (RC) columns subjected to axial loads. In the most of cases the external jacket is made leaving gaps between the strengthening layer and the slabs, avoiding to carry directly a portion of the load. In this way, the external jacket induces a confinement pressure on the inner column, and its beneficial effect is due to the increase of strength and ductility. However, shear stresses develop on the interface between the column and consequently, the external jacket can be considered indirectly loaded. It sustains a portion of the axial load, which depends by the value of the axial shortening, relative slippage between core and jacket and therefore by geometrical and mechanical features of the system. The evaluation of this effect is quite complex, since it depends from several parameters. This paper presents an analytical model able to predict the distribution of relative slips and shear stresses at interface in axially loaded RC columns reinforced with indirectly loaded jackets. Closed form expressions of maximum slip and stress distribution are obtained for linear elastic materials and formulation is extended to RC members via a step-by-step procedure allowing also to obtain the overall load-shortening curve. Results show the dependence of the compressive response from the interface’s stiffness. The analytical solution is verified with results derived by non-linear finite element analyses, showing good agreement for different key parameters. Finally, comparisons with experimental data available in the literature are performed, proving the reliability of the model.

In recent years, the scientific community has focused its interest on innovative inorganic matrix composite materials, namely TRM (Textile Reinforced Mortar). This class of materials satisfies the need of retrofitting existing masonry buildings, by keeping the compatibility with the substrate. Different recent studies were addressed to improve the knowledge on their mechanical behaviour and some theoretical models were proposed for predicting the tensile response of TRM strips. However, this task is complex due to the heterogeneity of the constituent materials and the stress transfer mechanism developed between matrix and fabric through the interface in the cracked stage. This paper presents a state-of-the-art review on the existing constitutive models for the tensile behavior of TRM composites. Literature experimental results of tensile tests on TRM coupons are presented and compared with the most relevant analytical models proposed until now. Finally, a new experimental study is presented and its results are used to further verify the reliability of the literature expressions.

The use of reinforced concrete (RC) jacketing is a common technique for retrofitting existing RC columns with poor structural performances. Literature studies highlighted that the relative slip occurring at the interface between the new and the old concrete can affect substantially the flexibility and the strength of the retrofitted member. For this reason, several interface treatments are usually applied, aiming to induce a monolithic behaviour to the column. However, despite the commonness of RC jacketing, a limited number of studies investigated numerical modelling strategies for including these phenomena, especially with reference to the development of numerical models for design-oriented applications. This paper presents the results of a finite element modelling (FEM) approach for evaluating the flexural behaviour of members reinforced with RC jackets. The proposed method takes advantage of fiber-section beam elements connected through a non-linear interface. Monotonic static non-linear analyses are performed and results highlights the differences with the usual assumption of monolithic behaviour. The comparisons with experimental results confirm the reliability of the approach, which allows to obtain accurate results with a substantial reduction of the computational cost with respect to brick-based FEM.

Composites made with inorganic matrix, namely fabric reinforced cementitious mortar (FRCM) composites are becoming widespread as strengthening materials for existing masonry structures. These composites are made of a dry grid of fibres embedded in an inorganic matrix. FRCMs can be considered a valid alternative to traditional organic composites such as fibre reinforced polymers (FRPs) because of their better compatibility with the masonry support. This work presents an experimental study for the tensile characterization of a basalt fabric reinforced cementitious mortar (BFRCM) composite. Tensile tests were carried out on coupons reinforced with one, two or three layers of grid to investigate the influence of the reinforcement on the load–strain and stress–strain response of the composite. The basalt grid and cementitious mortar matrix were also tested in order to compare the mechanical properties of the constituent materials to the response of the composite. The digital image correlation (DIC) technique was used to check in-plane and out-of-plane bending effect, to measure the crack opening and to analyse the crack pattern evolution and failure modes.

Fabric reinforced cementitious mortar (FRCM) materials have started to be employed during the last years with the aim of overcoming the drawbacks related to the use of fibre reinforced polymer (FRP) composites, proving to be potentially suitable for strengthening masonry structures. Moreover, the will to develop materials able to guarantee a certain degree of sustainability without renouncing to adequate mechanical properties has drawn the attention to the use of basalt fibres, which appear to be a valid alternative to carbon or glass fibres. This work presents an experimental investigation on a basalt FRCM (BFRCM) system to confine circular masonry columns, aimed at evaluating the effectiveness of this system in comparison with data obtained by basalt FRP (BFRP) jacketing. A total of eighteen clay brick masonry cylinders were prepared by using two different assembling schemes and subjected to uniaxial compression. Six cylinders were tested as control specimens, while the rest were reinforced by using either one or two layers of basalt textile. Traditional measuring instruments were integrated with the digital image correlation (DIC) technique. The experimental results are presented in terms of stress–strain curves, and strength and strain enhancements of confined cylinders compared to control specimens. The failure modes are also discussed. All outcomes are compared to those obtained by the authors in a similar study performed on BFRP-confined cylinders realized with the same manufacturing in order to have an effective comparison.

Many investigations have shown that fibre reinforced polymer (FRP) composites can be effectively used to induce a passive confinement action on masonry columns and improve their axial capacity and ductility. This paper presents the results of an experimental study on the compressive behaviour of clay brick masonry cylinders externally wrapped with basalt fiber reinforced polymer (BFRP) grids. The circular section shape was chosen to assess the pure confinement effect on the masonry material. Fourteen clay brick masonry cylinders, cored from two different masonry assembly types, were confined with one or two layers of BFRP grids. The two assemblies were used to investigate the effect of vertical joints on the response of the cylinders. The cylinders were tested under uniaxial compression load. The test results showed a strength increase between 30% and 38% for cylinders wrapped with one layer and between 69% and 71% for those wrapped with two layers of BFRP grids.

Fibre-reinforced cementitious matrix (FRCM) composites have been effectively used during the last ten years for the strengthening of existing concrete and masonry structures. These composite materials are made of medium- and high-strength fibre meshes embedded in inorganic matrices. Synthetic fibres are the ones that are currently the most used, however, natural fibres, such as basalt fibres, have recently been receiving growing attention. This work presents an extensive experimental study on the mechanical characterisation of a primed basalt fibre bidirectional grid. Fifty monotonic tensile tests on basalt grid strips were performed by varying different parameters, such as the dimension of the specimens, the clamping system, the measurement system and the test rate. Some of the tests were carried out using a video-extensometer to measure each specimen&rsquo, s strain. The aim of the study was to find the most suitable setup for the tensile characterisation of basalt textiles, in particular, to prevent slippage of the samples at the gripping area and fully exploit the tensile capacity of the grid.

Several recent works studied the efficiency of inorganic matrix composites, namely Textile Reinforced Mortar (TRM) or Fabric Reinforced Cementitious Matrix, for enhancing the strength and shortening capacity of masonry columns subjected to axial load. Literature studies were addressed to study the great number of variables involved in the problem, such as the nature and the grade of mortar, the strength of the fabric, the number of reinforcing layers, the type, the arrangement and the strength of the masonry, and helped to draw the first technical guidelines for practitioners and designers. All the experimental works highlighted that the actual performance of TRM confinement in masonry members was limited by the premature failure of the external layer in correspondence of the corners of sharp-edged sections, due to the stress concentration, limiting the confinement pressure achievable from the external wrap. This paper presents the result of an experimental investigation which aims to study the effect of a corner strengthening detail on the compressive behaviour of calcarenite masonry columns wrapped with glass TRM. The study was performed by varying the number of textile layers and the grade of the mortar adopted for the composite. The results show that the introduction of a fabric strip in correspondence of the corners can limit the stress concentration at the edges, allowing to exploit the confinement pressure of the TRM and achieving greater strength increases.

Optimizing seismic upgrading interventions in reinforced concrete (RC) structures is a difficult task, due to the inner non-linearity of the analyses usually performed. Additionally, it is well known that the displacement demand to the structure depends from the mass and stiffness of the system, and consequently its definition cannot be made a-priori. This paper presents the application of a soft-computing method -i.e. Genetic Algorithm (GA)- for the shaping optimization of code-compliant seismic upgrading interventions on plane RC frames through Buckling-Restrained Braces (BRB). The metaheuristic procedure allows to minimize the cost while ensuring the required safety level, without the need of evaluating all the possible solutions. The main GA operators (selection, crossover and mutation) contribute to explore the research space and evolve the suitable results toward the optimal one. The procedure is developed in an entirely open-source environment, taking advantage of Python for implementing the GA and OpenSeesPy for the structural analysis. The results of the procedure are compared with the outcomes of Monte Carlo simulations, showing the stability and the efficacy of the proposed algorithm.

Flat-jack testing method is one of the most commonly used techniques for the structural assessment of existing masonry structures. Single and double flat jacks are usually adopted to evaluate the acting normal stress, or the compressive behaviour of masonry material. Test procedures are codified by international standards (e.g., A.S.T.M D4729-87; C1196-04; C1197-04, R.I.L.E.M TC 177–MDT D.4; R.I.L.E.M. TC 177–MDT D.5), which provide the preliminary calibration of an experimental coefficient (km) and of the effective area (Aeff), which determination influences significantly the reliability of the test. This article presents the result of an experimental study on the calibration of flat jacks for masonry testing. The problem is investigated by several tests carried out on two types of common flat jacks, which differ for geometry and producer. Two calibration methods are adopted in order to relate the pressure values of the flat jack with those of the hydraulic press and load cycles are performed in two different pressure ranges. Finally, a theoretical interpretation of results is made, which gives good predictions of calibration parameters. Results of this investigation highlight the influence of constructive features and service pressure of the jack adopted, for obtaining reliable results from the tests on masonry structures.

In the last decade, Fibre Reinforced Polymer (FRP) wrapping technique has become a common method to retrofit masonry piers or columns with poor structural performances. The passive confinement effect induced by the external wrap allows increasing the compressive strength and ductility of the member. Several studies highlighted as the efficacy of this technique is affected by several key parameters, including the shape of the transverse cross section, stress intensification at the strength corner of sharp sections, amount and mechanical properties of adopted composite. Despite this technique has been widely studied from both theoretical and experimental point of view, most of studies focused on short columns and little information is available on the influence of second order effects on its structural efficacy. This paper presents a simplified method able to assess the effect of FRP confinement on slender columns. A preliminary evaluation of the constitutive law in compression of FRP confined masonry is made and the best-fitting model is adopted to model masonry in compression. Sectional analysis is performed by including the tensile strength of masonry and considerations are made on the increase of ultimate moment and curvature. Finally, the effect of column slenderness is considered using a simple numerical procedure, making it possible to calculate the allowable slenderness ratios as a function of the maximum drift, taking into account both strength and stability.

In the last decade, several studies have investigated the application of Fibre Reinforced Polymer (FRP) wraps to masonry piers in order to provide a confinement effect and enhance compressive strength and ductility. Although this technique has proved to be quite effective, various drawbacks arise when organic resins are used to bind fibres. To solve these problems, different techniques have been developed to be used as alternative methods for enhancing the structural performances of weak masonry columns. Among these, the use of Basalt Fibre Reinforced Cementitious Matrix (BFRCM) wrapping, or the application of steel wires at mortar joints. This paper presents the results of an experimental study on the compressive behaviour of clay brick masonry columns reinforced with FRP wraps, Basalt Fibre Reinforced Cementitious Matrix (BFRCM) or with steel wires. Uniaxial compressive tests were performed on twelve retrofitted columns and four control specimens up to failure. Two masonry grades were considered varying the mix used for the mortar. The results are presented and discussed in terms of axial stress-strain curves, failure modes and crack patterns of tested specimens. Comparisons with unreinforced columns show the capabilities of these techniques in increasing the structural efficiency with reduced aesthetical impact.

Fibre Reinforced Cementitious Matrix (FRCM) composites are becoming largely adopted for retrofitting masonry structures. These materials offer several advantages in comparison to Fibre Reinforced Polymer (FRP) composites, such as good resistance to fire and high temperatures, vapour permeability, possibility to be applied on wet surfaces, higher compatibility with the masonry substrate. However, the tensile behavior of FRCM materials is more complex compared to FRP composites, due to the limited tensile strength of the cement-based matrix. For this reason, FRCM materials require appropriate tensile characterization and, in this context, the use of non-conventional measurement systems, such as the Digital Image Correlation (DIC), can offer numerous advantages.This work presents an experimental study on the application of the DIC technique for the tensile characterization of Basalt Fibre Reinforced Cementitious Matrix (BFRCM) strips. Tensile tests were carried out on three series of specimens reinforced with one, two or three layers of basalt gridin order to investigate the effect of the reinforcement ratio on the tensile response of the composite strips. The test setup and the calibration of the DIC analyses are discussed. It is shown as the DIC allows obtaining detailed information on the tensile response, including the evaluation of the fullstrain field on the surface of the BFRCM strips and the location of cracks. Results are discussed also in terms of stress-strain curves and failure modes.

The use of steel jacketing technique is a common practice for retrofitting existing reinforced concrete (RC) columns, as it allows increasing load-carrying capacity and ductility of the member. When the external jacket has no-end connections – i.e. the jacket is indirectly loaded- the load sustained by the column is transferred from the inner RC core to the external jacket through shear stresses along the contact surface. The assessment of this mechanism is quite complex, due to the marked non-linear behaviour of constituent materials and to the calibration of a proper shear stress-relative slip constitutive law of the concrete-to-steel interface. In this paper, a step-by-step analytical approach is proposed to assess the load transfer mechanisms in steel jacketed RC columns loaded in compression. The model is able to predict slip and shear stresses along the core-to-jacket contact interface and then the load-carrying capacity of strengthened members. Shear stress-slip constitutive law on the interface is initially assumed linear and a suitable stress-strain constitutive model of confined concrete is considered. Yielding of steel angles is also taken into account into the process by assuming an bilinear law with strain hardening. The progressive damage of concrete is also considered by dividing the examined member in portions and splitting governing equations. Finally, comparisons are performed in order to validate the proposed procedure against experimental results available in the literature, showing good agreement.

This paper aims at investigating the tensile behaviour of basalt fibres on cementitious matrix for the strengthening of masonry structures. The use of Basalt Fibre Reinforced Cementitious Matrix (BFRCM) is favourably considered by the scientific community because it represents a natural composite material with high compatibility with stone and masonry substrate. The study is developed through the generation of Finite Element (FE) models capable of reproducing the tensile behaviour of BFRCM strips with different number of layers of grid. For the scope, the micro-modelling approach is adopted assuming different levels of detail for the simulation of the interface constitutive behaviour. Fibre and substrate are modelled separately and the interface between the two components is simulated by introducing numerical contact properties for reproducing the perfect bond condition, on one hand, and the cohesive tangential slip, on the other hand. The damage of the composite system is investigated using proper damage numerical models at the interface.

In recent decades, fiber reinforced polymer (FRP) wrapping has become a common technique to retrofit reinforced concrete (RC) columns. Numerous research works have sought to verify analytically and experimentally its effectiveness in terms of enhancement of axial load bearing capacity and ductility. These studies highlighted that in the case of sharp-cornered sections, the maximum allowable confinement pressure is limited by premature failure at corners and, consequently, stress in the FRP, as well as the distribution of the confinement pressure, is not uniform. The prediction of this phenomenon is not straightforward, and existing theoretical studies propose complex numerical simulations, whereas technical codes provide simplified or empirical relationships for its assessment. This paper presents an analytical model for the evaluation of the effective distribution of confinement pressure in FRP confined concrete members with rounded corners. The model allows considering the interaction with the concrete core and different brittle failure modes, including FRP rupture and debonding. It leads to determining the distribution of the confinement pressure along the section. Results are compared with those achieved by finite-element (FE) analyses and with numerical and experimental data available in the literature. Good agreement is obtained in all cases, showing the reliability of the proposed model.

This paper presents the results of an experimental research aiming to investigate on the compressive behavior of full size clay brick masonry columns reinforced with Basalt Fiber Reinforced Cementitious Matrix (BFRCM) or with steel wires collaring. Uniaxial compressive tests were performed on eight retrofitted columns and four control specimens. Two masonry strength were considered by varying the mortar grade. Results are presented and discussed in terms of axial stress-strain relationships, failure modes and crack patterns of the specimens tested up to failure. For the limit of investigated variables, comparisons with unreinforced specimens show the ability of these alternative techniques in increasing ductility with limited strength enhancements, even if further investigations should be addressed on detail effectiveness and scale effect.

Reinforced concrete (RC) jacketing is becoming increasingly common among the different retrofit techniques for poor RC members, due to its economical and practical advantages. Experimental investigations in the literature have shown that the actual axial capacity of RC jacketed members can be substantially lower than that analytically evaluated by adapting the most common theoretical models for confined concrete. This fact can be explained by taking into account the presence of tensile stresses developing in the concrete, due to a mutual interaction between the inner core and the external jacket. This phenomenon is relevant especially in members where the concrete properties of the jacket are different with respect to those of the core, causing the premature failure of the external layer. This paper presents a simplified approach able to evaluate these effects. Circular RC jacketed sections are studied and a model is presented to predict the concrete softening effect. The section is modeled by joining circular hollow layers and circumferential and radial stresses are firstly calculated under the assumption of linear elastic behaviour and plane strain state. The model is extended in the non-linear range by adopting a secant constitutive law. Finally, comparisons are made with experimental data available in the literature, showing good agreement.

This paper presents the results of an experimental and analytical study on the compressive behaviour of small clay brick masonry cylinders reinforced with a basalt fibre reinforced polymer (BFRP) composite. Fourteen cylinders, manufactured using two assembling schemes and confined using either one or two layers of BFRP grids, were tested under monotonic compression loading. Traditional strain measuring systems were integrated with digital image correlation (DIC) technique. The BFRP confined masonry cylinders showed a ductile behaviour characterised by a softening branch of the stress–strain curve. The experimental strains, strength, and full stress–strain curves were modelled using analytical models available in the literature. A modified version of the CNR model proposed in this study provided the best strength prediction.

This paper presents the calibration of a simplified analytical model for concrete columns confined by fiber reinforced polymer (FRP) jackets. The model allows evaluating the increase of strength, ductility and dissipated energy without defining the lateral confinement pressure and it can be easily extended for the assessment of FRP confinement in design applications. This model was obtained by a simplified procedure based on the best fit of experimental data available in the literature and the coefficient of determination (R2) was evaluated in order to estimate the accuracy of the regression analysis. A numerical database resulting from finite element (FE) analyses was compiled and reported for integrating the model’s calibration obtained by the few experimental data. The FE models are built based on results of experimental tests available in the literature and several FE simulations are carried out. The experimental results are then integrated with numerical results and new forms of the simplified expressions are obtained by new best fit. The new values of R2 confirm an improvement of the accuracy of the regression analysis. Finally, the performance of the simplified model is compared with existing formulas available in the literature.

Fibre Reinforced Polymer (FRP) confinement represents an effective tool for retrofitting masonry piers or columns enhancing their structural performance. This technique has been widely studied in the literature mainly with reference to short columns, while no extensive information is available on the influence of second order effects on its efficacy in case of slender members. Within this framework, the presented study concerns a simplified method able to assess the effects of FRP confinement on the compressive behaviour of slender masonry columns. A proper constitutive law of FRP confined masonry in compression is adopted for performing a sectional analysis, in which also considerations are made on the increase of ultimate moment and curvature depending on the role of some key parameters. The effect of column slenderness is evaluated using a simple analytical iterative procedure, making it possible to calculate the allowable slenderness ratios as function of the maximum drift, taking into account both strength and stability. The effect of slenderness on eccentrically loaded columns is finally examined through a finite element model used for validating the results of the proposed analytical procedure, allowing also to draw safety domains useful for design/verification purposes. It is shown as the column’s slenderness affects the efficiency of confinement, being the latter negligible for values of normalized length greater than 20.

The use of Fiber Reinforced Cementitious Mortar (FRCM) systems for structural retrofitting of masonry structures has become increasingly popular in the last years, due to the capability of this technique in overcoming some of the drawbacks related to the adoption of resin-based composites. Recent studies investigated on the effect of FRCM wraps on the compressive behaviour of concrete members and demonstrated as the application of mortar-based composites allows increasing the strength and, above all, the ductility of the column. The main difference with FRP confined columns is related to the different post-peak behaviour, characterized by a softening branch. Differently, few studies are actually available in the literature on the efficiency of FRCM confinement in enhancing the compressive behaviour of masonry columns. This paper presents the results of an experimental investigation on the effect of FRCM wraps on the compressive behaviour of calcarenite masonry columns. A preliminary study on the mechanical performances of the strengthening layer is performed by selecting three different mortar grades for the FRCM and testing these under flexure and compression. Moreover, tensile tests on the glass fibre fabric and on FRCM strips are performed to characterize the behaviour of the reinforcing system. Finally, eleven columns are tested under concentric compression, recording the axial strain, and discussion is made on modes of failure and gains of strength and ultimate strain.

In the present paper, a simplified model is used to determine the axial load-carrying capacity of compressed short reinforced concrete columns subjected to corrosion processes. The model considers members with circular and square cross-sections and accounts for—cover spalling, —concrete core confinement induced by transverse steel reinforcement, —buckling of longitudinal reinforcing bars. Strength reduction in concrete cover and core due to cracking induced by rust formation, reduction of steel area in longitudinal bars and transverse stirrups due to general and pitting corrosion and loss of confinement pressure are considered. The load-carrying capacity and load-axial strain curves here generated analytically fit well the existing experimental data.

The use of Fibre Reinforced Polymer (FRP) wraps has become common in practical applications to retrofit existing columns with poor structural features. Wrapping the member with one or more FRP sheets makes it possible to induce confinement action and enhance strength and ductility. This application has been widely studied and adopted in reinforced concrete members for about twenty years, while its suitability to masonry columns and piers has been investigated during the last decade. The results of several studies were summarized in different design expressions for calculating effective confinement pressure, ultimate compressive stress and strain, and the overall trend of the stress-strain curve. Despite this huge amount of research, some results seem actually to be in contrast with the experimental results available in the literature, due to the different key variables arising in this application. This paper presents a comparative analysis of the main existing theoretical models to predict the stress-strain curve of FRP confined masonry in compression. Clay brick masonry is studied and criteria to obtain the strength increase are initially analyzed and compared with experimental results, evaluating the reliability of each expression. In the second part, models to calculate the overall constitutive law in compression are examined and the average error and coefficient of variation are calculated for each model.

In the present study a numerical model is proposed for the response of out-of-plane loaded calcarenite masonry walls strengthened with vertical CFRP strips applied on the substrate by means of epoxy resin. A simplified structural scheme is considered consisting in a beam fixed at one end, subjected to constant axial load and out-of-plane lateral force monotonically increasing. Two different constraint conditions are taken into account: in the first one, the panel is assumed free to rotate at the top end while, in the second one, the rotation is restrained. Three-dimensional finite elements are used for the calcarenite parts and an equivalent constitutive law available in the literature is considered for the compressive behavior of the system ashlar-mortar. Conversely, shell elements are used for modeling the CFRP strips and linear elastic behavior is assumed for the composite while cohesive contact properties are introduced at the FRP-calcarenite interface. The model is validated using both experimental results available in the literature and simplified analytical formulations recently presented by the authors in a previous paper.

Buckling of longitudinal bars in reinforced concrete (RC) members is definitely a critical issue in framed structures subjected to seismic loads. Second order effects can affect the compressive stress–strain law of steel bars, influencing ductility calculations of RC structures. Moreover, literature studies show that buckling can occur over a length wider than stirrups’ pitch (global buckling mode), involving more stirrups and inducing large deflections in the bar. If the critical length is not carefully estimated, stirrups’ failure can occur, causing also the sudden loss of confining effects in concrete. This paper presents the results of different approaches for calculating the critical conditions in longitudinal bars. A discrete mechanical model is proposed, based on the solution of a continuous beam with elastic supports, with deflections restrained in one side to simulate the presence of the concrete core. It allows describing the transition from local buckling (between the stirrups) and global buckling, on the basis of the relative stiffness stirrup-bar. Two other methods corresponding to different computational efforts are also adopted for the sake of comparison. In particular, non-linear finite element analyses are carried out including the effect of strain hardening in the constitutive law of steel and finally, comparisons are made with a simplified closed-form solution proposed in the literature. This last comparison allows to assess the reliability of these expressions and their applications for obtaining parametric considerations useful for design.

The use of fiber-reinforced polymers (FRP) for structural strengthening has become increasingly popular in recent years. Several applications of FRP have been proposed and applied, depending on the target of the technique, kind and/or material of the structural member. In particular, because of their great tensile strength, FRP materials are commonly used to enhance the out-of-plane behaviour of masonry walls, allowing to increase their strength, ductility and improving safety against overturning. For these reasons, FRP laminates are often applied in vulnerable ancient buildings in seismic areas to reinforce façades and walls with poor structural features. However, some issues arise when adopting composites in historical constructions, the most related to the aesthetical impact of laminates and compatibility between FRP and masonry. Consequently, a correct evaluation of the reinforcement percentages for strength and ductility purposes is crucial, as well as the effective increase of structural performances. This paper presents a numerical-analytical approach able to reproduce the flexural behaviour of out-of-plane loaded masonry walls. The model is based on a simplified representation of the member, the latter modeled as a cantilever beam. Mechanical non-linearity is introduced by means of moment–curvature relationships, deduced with proper constitutive laws of masonry and by taking into account the ultimate debonding strain of FRP. Second order effects are considered by adopting an iterative step-by-step procedure. Comparisons are made in terms of moment–curvature and load–displacement curves with experimental data available in the literature and with non-linear finite element analyses, showing both good agreement. Finally, parametric considerations on the reinforcement percentages are made in terms of strength and ductility.

Granite and heavy stone circular columns could often be found in the cultural heritage among ancient churches and historical buildings in all the Mediterranean area. Their good strength properties allow them to carry big load values, while their bright colours and aesthetical characteristics were used by a lot of architects in the past to achieve structural solutions with great visual impact.Though such materials have great compressive strength values, environmental effects, especially long term effects, can damage the structural members, by cracking them. In this way the slenderness of the column increases, and the presence of an imposed ground motion can be very dangerous, since the column will be more vulnerable to rocking motion, which in critical cases could lead to overturning or material crushing due to stress concentration.This paper focuses on the behaviour of cracked granite and heavy stone columns subjected to rocking motion due to pulse type ground shaking. In this field, the effect of the presence of circular collars is analysed and discussed. The overturning spectra are determined, including the presence of the collars, showing their effectiveness in reducing the overturning risk. Comparisons are shown with numerical analyses and a simplified analytical procedure is proposed. © 2013 Elsevier Ltd.

In the present paper the experimental results relative to flexural tests on out-of-plane loaded large scale panels made of laminated structural glass are presented and discussed. Glass panels are parts of the structural facade of the Fontanarossa International Airport of Catania (Italy). Panels are mounted on a rigid testing frame and loaded with an uniformly distributed lateral pressure. Two different kinds of tests were carried-out, cyclic and monotonic, able to reproduce the serviceability and the ultimate conditions of the glass panels when subjected to the coupled effects of dead load and horizontal forces, the latter reproducing the wind action. Results obtained focused the high performances of structural glasses for structural applications (structural facade) characterised by multiple and progressive failure that did not interest the bearing points neither the zone of the shell near the supports. From a theoretical point of view a simple model based on the structural behaviour of equivalent beams is proposed able to take into account of the flexural behaviour of glass panes including catenary action. The comparison between experimental, analytical and numerical results obtained with non-linear finite element analyses shows good agreement. Finally, the proposed model appears an useful tool for preliminary design of glass panels for practicing engineers.

Among the different strengthening techniques to repair RC structures, reinforced concrete (RC) jacketing is one of the most commonly adopted, especially for columns. Its wide application is due to its easy application and relatively reduced cost with respect to other methods (e.g. FRP wrapping, Shape Memory Alloy active confinement). The target of RC jacketing is to increase axial, flexural capacity and ductility of weak existing members by means of two main effects: confinement action provided by the jacket and composite action between external jacket and inner concrete. Different theoretical studies have been carried out to calculate the strength enhancement due to confinement action and most of these are based on the adaptation of classical models for confinement evaluation in RC members. However, experimental investigations on compressive behaviour of RC jacketed sections have shown that the actual axial capacity could be substantially lower than that analytically evaluated with classical confinement models. This fact could be explained by taking into account the presence of tensile stresses developing in the jacket, due to the different dilatation of the inner core and the external jacket. This phenomenon is relevant especially in members where the concrete properties of the jacket are different with respect to those of the core, causing the premature failure of the external layer. This paper presents a simplified approach able to evaluate these effects. In particular RC jacketed circular columns are analysed as case study and the member is studied by considering the different concrete properties of core, jacket and cover. Circumferential and radial stresses are firstly calculated under the assumption of linear elastic behaviour and plane strain state. The model is extended in the non-linear range by adopting a secant constitutive law. Finally, comparisons are made with experimental data available in the literature, showing good agreement and explaining some experimental results.