The resource of shear resistance provided by the dowel mechanism of rebar in reinforced concrete ... more The resource of shear resistance provided by the dowel mechanism of rebar in reinforced concrete (RC) structures can significantly affected by the simultaneous presence of axial loading. This occurs for example of plastic hinges of seismic resistant structures. In fact, at load reversals in cycles of large deformation demand, rebars are subjected to combined axial and shear loading, particularly in those section where the shear transfer via aggregate interlocking is jeopardized by the opening of the crack throughout the entire section depth. Thus, a reliable assessment of the shear capacity of dowels under combined shear and axial load is required to check the element shear resistance. The paper describes the results of a specific experimental campaign on rebar dowels subjected to shear loading in presence of different levels of axial load. Both smooth and ribbed rebar dowels were investigated. A marked reduction of the dowel shear strength and stiffness in presence of increasing axial loading was experimentally observed, only partially compensated by the kinking effect. The latter was found to characterize the entire resource of dowel capacity when the axial load was close to the rebar yielding strength. The paper proposes an analytical model, adapted from others available in the literature, to predict the dowel shear-displacement response accounting for the applied axial load. The model helps the understanding of the dowel response, is suitable for hand calculation and can easily assist the dowel design. The experimental response was quite well captured for smooth dowels, while the prediction was less accurate for ribbed ones. Future refinements may address the local damage induce by the rebar pullout, typical of ribbed rebars, which is neglected in present form of the model.
The continued interest in technological innovation in construction has greatly broadened the hori... more The continued interest in technological innovation in construction has greatly broadened the horizons of material science, developing a specific sector closely related to the recycling of waste products. This paper examines the thermal, mechanical and structural behaviour of an insulating light weight fibre reinforced concrete (ILWFRC), which is made by replacing natural sand and gravel with artificial aggregates resulting from the process of glass recycling. ILWFRC offers low density (approximately 650 kg/m 3), excellent thermal characteristics (thermal conductivity 0.1 W/mK), a compressive strength similar to brick masonry (3.5 MPa) with low cement content (265 kg/m 3) and stable postcracking behaviour. The mechanical and physical properties of ILWFRC were employed for the construction of a full-scale infill wall (having dimensions of 2.9 9 2.6 9 0.2 m), which was experimentally studied under in-plane and out-of-plane actions. Inplane response showed a maximum lateral load of 359 kN at 1.5% drift, with a residual capacity of more than 75% at 4% drift. The subsequent out-of-plane test was performed up to failure with a maximum lateral load of 67 kN, corresponding to about 7 times the infill self-weight. Keywords Artificial aggregates Á Circular economy Á Fibre reinforced concrete Á Infill wall Á Performance levels Á Insulating concrete Á Recycling Á Quasi-static cyclic response 1 Research significance In order to address the challenges of Directive 2002/ 91/EC on the energy performance of buildings [1] and of Directive 2008/98/EC on waste [2], several new construction materials have been developed over the years. Among these, and the focus of this article, is an innovative material, called insulating lightweight fibre reinforced concrete (ILWFRC). This new cement-based material is characterised by a reduced unit weight and excellent thermal and mechanical characteristics, due to the presence of 70% (by volume) lightweight recycled glass aggregates (patented by [3]) and synthetic fibres. These characteristics make it suitable for use in large structural and non-structural elements which are subject to moderate compressive stresses, such as load-bearing walls, partitions and infill walls. In this paper, an ILWFRC infill prototype is investigated. The characteristics of
ABSTRACT A new technique based on the use of high-performance fibre-reinforced concrete (HPFRC) i... more ABSTRACT A new technique based on the use of high-performance fibre-reinforced concrete (HPFRC) in seismic structural walls with unbonded tendons is investigated herein. The aim of using this kind of structural wall, which develops a rocking behaviour under horizontal loading, is to limit or avoid damage during seismic events. The technique consists of strengthening the base of reinforced concrete (RC) walls provided with unbonded tendons by replacing the regular concrete at the wall toes with HPFRC, which can enhance the wall properties and be used to strengthen or repair existing RC walls. A full-scale test was performed on a rocking wall, before and after strengthening at the wall toes. Local deformations at the toes were measured in detail to quantify the extent of concrete damage in relation to wall drift and to assist future theoretical modelling. The results show that the strengthening technique can considerably limit damage at the ultimate state.
In this research, the feasibility of strengthening self-centering walls by high-performance concr... more In this research, the feasibility of strengthening self-centering walls by high-performance concrete was investigated through an experimentally validated finite element model. The effects of the wall's axial stress and tendons' prestressing ratios on the wall's damage, and the effectiveness of potential strengthening approaches were studied through 40 independent scenarios, and 360 different cases. Using the estimated damage from numerical results at the compression zone near the bottom corners, the maximum compressive strain of the concrete at the walls' toe regions was estimated. Using the calibrated
In reinforced concrete (RC) multi-storey buildings, the important role of the seismic interaction... more In reinforced concrete (RC) multi-storey buildings, the important role of the seismic interaction of structural frames with masonry infills has been revealed by several earthquakes and investigated by many authors. Recently, several innovative infill solutions have been proposed to mitigate such interaction, which could result in widespread damage in both the masonry and the RC structure and sometimes jeopardize the building stability and the occupants' safety. One solution consists in the partitioning of the masonry infill into several sub-panels, relatively sliding along specific joints. This paper investigates the seismic assessment of this technological solution in the framework of performance based earthquake engineering. A two-dimensional five-storey RC seismic-resistant frame is selected as case study and the performance is assessed by comparing the responses of the same structure infilled with different solutions, made of sliding joints or traditional masonry, or in the bare configuration. Incremental Dynamic Analyses (IDA) is used for the probabilistic determination of fragility curves of the structures. Results show the seismic fragility and reliability of the different investigated structures, especially addressing the probabilities of occurrence of damage at different limit states and quantifying the associated expected annual loss.
In the present paper a numerical study is reported, aimed at investigating the role of openings i... more In the present paper a numerical study is reported, aimed at investigating the role of openings in the in-plane response of infilled RC frames when masonry infills with sliding subpanels are adopted. Such innovative infills are meant to reduce post-earthquake damage and infill-frame interaction, with respect to traditional solid masonry infills. Their study was carried out mainly on the fully infilled configuration, and limited information is available about its response in presence of an opening. In the paper focus is made on the interaction of the infill with both the surrounding frame and with the specific post placed at the opening side, which is essential to confine the infill and protect the opening fixtures. The adopted numerical model is calibrated on the experimental results of a test performed on a real scale infill wall with horizontal sliding joints and a full-height opening on a side. A parametric study is carried out to investigate the influence of different design parameters. In detail, the focus is placed on the stiffness of the post and on the geometry of the infill (in terms of position of the opening, infill length and sliding joint configurations). The obtained results show the beneficial effect of the post deformability in reducing the infillframe interaction, with a significant reduction of the shear action exerted on the frame columns and on the post itself. Moreover, the obtained results allowed to define some preliminary recommendations for the design of the infill and of the post at the opening side.
The most of the reinforced concrete frame structures of the European building stock have been bui... more The most of the reinforced concrete frame structures of the European building stock have been built without attention to the seismic action or according to obsolete code. Before the 1970's, in all the Mediterranean area, plain rebars were employed for the longitudinal reinforcement of structural members. Due to their smooth surface, they are characterized by poor bond capacity; this results in a significant slip of the loaded bar from the surrounding concrete, which strongly increases the structural deformation capacity compared to modern ribbed rebars. Whereas the cyclic response of non-conforming reinforced concrete members and structural sub-assemblies is deeply investigated in literature, a scarce knowledge about anchored plain bar cyclic behaviour is noticed. In the recent years, detailed non-linear modelling of gravity load designed structures is widely used for the seismic vulnerability assessment; a deep investigation on full scale anchorage detailing is then fundamental for the correct modelling of such structures.. In the present paper an experimental investigation on different anchorage solution of plain rebar embedded in concrete is presented; straight, hook-ended and 45° bent rebar were analysed, monitoring their axial stress versus slip behaviour. A non-linear stress-slip behaviour was observed for all the anchorage solutions since the lower loading level. Stiffness degradation due to cyclic loading was observed; on the other hand, strength decay was visible only for straight anchorage length. Hook-end device and bar bent resulted effective in providing a good anchorage performance, on the other hand their response showed a progressive plasticization.
The seismic vulnerability of masonry infilled frames represents a critical issue in reinforced co... more The seismic vulnerability of masonry infilled frames represents a critical issue in reinforced concrete and steel buildings, widely highlighted by ruinous collapses during earthquakes and studied by many authors. In these structures, the interaction between the flexible frame and the rigid masonry infill modifies the dynamic response of the frame, inducing possible undesired collapse mechanism, and, on the other hand, can cause widespread damage in the infills. To avoid these detrimental effects, a technological solution for the design of the infills has been studied and tested in the last years at the University of Brescia, consisting in partitioning the masonry with wooden planks, working as sliding joints. Tests have shown the potential of the solution for providing a superior performance than traditional masonry infills, thanks to the reduction of the detrimental effects of the infill frame interaction. The benefit comes from a significant reduction of the infill strength and stiffness, a limitation of its damage under in-plane loading and a ductile behaviour, with energy dissipation capacity that can be easily predicted. In this paper a numerical study that extends test results is presented. A parametric analysis of the response of the infills has been performed as a function of some geometrical and mechanical properties: stiffness and strength of the materials, dimension of the infill and configuration of the sliding joints. The results offer information necessary for the application of the construction technique, which is proposed to improve the infilled frame seismic response.
Masonry-infilled RC frames can sustain significant damage in the event of strong ground shak-ing.... more Masonry-infilled RC frames can sustain significant damage in the event of strong ground shak-ing. The introduction of sliding joints in masonry infills can reduce the infill-frame interaction, ensure the in-fill out-of-plane stability, and minimize the infill damage under both in- and out-of-plane loading as shown by previous experimental and analytical studies. This paper summarizes previous studies and provides recom-mendations for the design and detailing of such structures. The proposed simplified design procedure can quantify the shear action on the frame columns due to the infill interaction which is compared to the case of traditional continuous infills
A refined design procedure for the seismic retrofit of warehouses or, more generally, single-stor... more A refined design procedure for the seismic retrofit of warehouses or, more generally, single-storey RC frames bounded by “drift-sensitive” masonry infills and glazed curtain walls, is proposed in this paper by means of hysteretic braces. The calculation method is based on displacement-based design (DBD) procedures in which both the as-built frame and the dissipative braces are modelled through simple linear equivalent SDOF systems arranged in parallel. In this regard, with respect to the provisions of the Italian Building Code, two refinements are introduced: (1) the definition of two performance targets tailored to the protection of glazed curtain walls (among most expensive non-structural components) and to ensure an acceptable level of damage level for masonry infills; and (2) the adoption of a more accurate formulation for the estimation of the equivalent viscous damping developed both by the main frame and the dissipative braces. The refined design method is applied to a case-s...
The resource of shear resistance provided by the dowel mechanism of rebar in reinforced concrete ... more The resource of shear resistance provided by the dowel mechanism of rebar in reinforced concrete (RC) structures can significantly affected by the simultaneous presence of axial loading. This occurs for example of plastic hinges of seismic resistant structures. In fact, at load reversals in cycles of large deformation demand, rebars are subjected to combined axial and shear loading, particularly in those section where the shear transfer via aggregate interlocking is jeopardized by the opening of the crack throughout the entire section depth. Thus, a reliable assessment of the shear capacity of dowels under combined shear and axial load is required to check the element shear resistance. The paper describes the results of a specific experimental campaign on rebar dowels subjected to shear loading in presence of different levels of axial load. Both smooth and ribbed rebar dowels were investigated. A marked reduction of the dowel shear strength and stiffness in presence of increasing axial loading was experimentally observed, only partially compensated by the kinking effect. The latter was found to characterize the entire resource of dowel capacity when the axial load was close to the rebar yielding strength. The paper proposes an analytical model, adapted from others available in the literature, to predict the dowel shear-displacement response accounting for the applied axial load. The model helps the understanding of the dowel response, is suitable for hand calculation and can easily assist the dowel design. The experimental response was quite well captured for smooth dowels, while the prediction was less accurate for ribbed ones. Future refinements may address the local damage induce by the rebar pullout, typical of ribbed rebars, which is neglected in present form of the model.
The continued interest in technological innovation in construction has greatly broadened the hori... more The continued interest in technological innovation in construction has greatly broadened the horizons of material science, developing a specific sector closely related to the recycling of waste products. This paper examines the thermal, mechanical and structural behaviour of an insulating light weight fibre reinforced concrete (ILWFRC), which is made by replacing natural sand and gravel with artificial aggregates resulting from the process of glass recycling. ILWFRC offers low density (approximately 650 kg/m 3), excellent thermal characteristics (thermal conductivity 0.1 W/mK), a compressive strength similar to brick masonry (3.5 MPa) with low cement content (265 kg/m 3) and stable postcracking behaviour. The mechanical and physical properties of ILWFRC were employed for the construction of a full-scale infill wall (having dimensions of 2.9 9 2.6 9 0.2 m), which was experimentally studied under in-plane and out-of-plane actions. Inplane response showed a maximum lateral load of 359 kN at 1.5% drift, with a residual capacity of more than 75% at 4% drift. The subsequent out-of-plane test was performed up to failure with a maximum lateral load of 67 kN, corresponding to about 7 times the infill self-weight. Keywords Artificial aggregates Á Circular economy Á Fibre reinforced concrete Á Infill wall Á Performance levels Á Insulating concrete Á Recycling Á Quasi-static cyclic response 1 Research significance In order to address the challenges of Directive 2002/ 91/EC on the energy performance of buildings [1] and of Directive 2008/98/EC on waste [2], several new construction materials have been developed over the years. Among these, and the focus of this article, is an innovative material, called insulating lightweight fibre reinforced concrete (ILWFRC). This new cement-based material is characterised by a reduced unit weight and excellent thermal and mechanical characteristics, due to the presence of 70% (by volume) lightweight recycled glass aggregates (patented by [3]) and synthetic fibres. These characteristics make it suitable for use in large structural and non-structural elements which are subject to moderate compressive stresses, such as load-bearing walls, partitions and infill walls. In this paper, an ILWFRC infill prototype is investigated. The characteristics of
ABSTRACT A new technique based on the use of high-performance fibre-reinforced concrete (HPFRC) i... more ABSTRACT A new technique based on the use of high-performance fibre-reinforced concrete (HPFRC) in seismic structural walls with unbonded tendons is investigated herein. The aim of using this kind of structural wall, which develops a rocking behaviour under horizontal loading, is to limit or avoid damage during seismic events. The technique consists of strengthening the base of reinforced concrete (RC) walls provided with unbonded tendons by replacing the regular concrete at the wall toes with HPFRC, which can enhance the wall properties and be used to strengthen or repair existing RC walls. A full-scale test was performed on a rocking wall, before and after strengthening at the wall toes. Local deformations at the toes were measured in detail to quantify the extent of concrete damage in relation to wall drift and to assist future theoretical modelling. The results show that the strengthening technique can considerably limit damage at the ultimate state.
In this research, the feasibility of strengthening self-centering walls by high-performance concr... more In this research, the feasibility of strengthening self-centering walls by high-performance concrete was investigated through an experimentally validated finite element model. The effects of the wall's axial stress and tendons' prestressing ratios on the wall's damage, and the effectiveness of potential strengthening approaches were studied through 40 independent scenarios, and 360 different cases. Using the estimated damage from numerical results at the compression zone near the bottom corners, the maximum compressive strain of the concrete at the walls' toe regions was estimated. Using the calibrated
In reinforced concrete (RC) multi-storey buildings, the important role of the seismic interaction... more In reinforced concrete (RC) multi-storey buildings, the important role of the seismic interaction of structural frames with masonry infills has been revealed by several earthquakes and investigated by many authors. Recently, several innovative infill solutions have been proposed to mitigate such interaction, which could result in widespread damage in both the masonry and the RC structure and sometimes jeopardize the building stability and the occupants' safety. One solution consists in the partitioning of the masonry infill into several sub-panels, relatively sliding along specific joints. This paper investigates the seismic assessment of this technological solution in the framework of performance based earthquake engineering. A two-dimensional five-storey RC seismic-resistant frame is selected as case study and the performance is assessed by comparing the responses of the same structure infilled with different solutions, made of sliding joints or traditional masonry, or in the bare configuration. Incremental Dynamic Analyses (IDA) is used for the probabilistic determination of fragility curves of the structures. Results show the seismic fragility and reliability of the different investigated structures, especially addressing the probabilities of occurrence of damage at different limit states and quantifying the associated expected annual loss.
In the present paper a numerical study is reported, aimed at investigating the role of openings i... more In the present paper a numerical study is reported, aimed at investigating the role of openings in the in-plane response of infilled RC frames when masonry infills with sliding subpanels are adopted. Such innovative infills are meant to reduce post-earthquake damage and infill-frame interaction, with respect to traditional solid masonry infills. Their study was carried out mainly on the fully infilled configuration, and limited information is available about its response in presence of an opening. In the paper focus is made on the interaction of the infill with both the surrounding frame and with the specific post placed at the opening side, which is essential to confine the infill and protect the opening fixtures. The adopted numerical model is calibrated on the experimental results of a test performed on a real scale infill wall with horizontal sliding joints and a full-height opening on a side. A parametric study is carried out to investigate the influence of different design parameters. In detail, the focus is placed on the stiffness of the post and on the geometry of the infill (in terms of position of the opening, infill length and sliding joint configurations). The obtained results show the beneficial effect of the post deformability in reducing the infillframe interaction, with a significant reduction of the shear action exerted on the frame columns and on the post itself. Moreover, the obtained results allowed to define some preliminary recommendations for the design of the infill and of the post at the opening side.
The most of the reinforced concrete frame structures of the European building stock have been bui... more The most of the reinforced concrete frame structures of the European building stock have been built without attention to the seismic action or according to obsolete code. Before the 1970's, in all the Mediterranean area, plain rebars were employed for the longitudinal reinforcement of structural members. Due to their smooth surface, they are characterized by poor bond capacity; this results in a significant slip of the loaded bar from the surrounding concrete, which strongly increases the structural deformation capacity compared to modern ribbed rebars. Whereas the cyclic response of non-conforming reinforced concrete members and structural sub-assemblies is deeply investigated in literature, a scarce knowledge about anchored plain bar cyclic behaviour is noticed. In the recent years, detailed non-linear modelling of gravity load designed structures is widely used for the seismic vulnerability assessment; a deep investigation on full scale anchorage detailing is then fundamental for the correct modelling of such structures.. In the present paper an experimental investigation on different anchorage solution of plain rebar embedded in concrete is presented; straight, hook-ended and 45° bent rebar were analysed, monitoring their axial stress versus slip behaviour. A non-linear stress-slip behaviour was observed for all the anchorage solutions since the lower loading level. Stiffness degradation due to cyclic loading was observed; on the other hand, strength decay was visible only for straight anchorage length. Hook-end device and bar bent resulted effective in providing a good anchorage performance, on the other hand their response showed a progressive plasticization.
The seismic vulnerability of masonry infilled frames represents a critical issue in reinforced co... more The seismic vulnerability of masonry infilled frames represents a critical issue in reinforced concrete and steel buildings, widely highlighted by ruinous collapses during earthquakes and studied by many authors. In these structures, the interaction between the flexible frame and the rigid masonry infill modifies the dynamic response of the frame, inducing possible undesired collapse mechanism, and, on the other hand, can cause widespread damage in the infills. To avoid these detrimental effects, a technological solution for the design of the infills has been studied and tested in the last years at the University of Brescia, consisting in partitioning the masonry with wooden planks, working as sliding joints. Tests have shown the potential of the solution for providing a superior performance than traditional masonry infills, thanks to the reduction of the detrimental effects of the infill frame interaction. The benefit comes from a significant reduction of the infill strength and stiffness, a limitation of its damage under in-plane loading and a ductile behaviour, with energy dissipation capacity that can be easily predicted. In this paper a numerical study that extends test results is presented. A parametric analysis of the response of the infills has been performed as a function of some geometrical and mechanical properties: stiffness and strength of the materials, dimension of the infill and configuration of the sliding joints. The results offer information necessary for the application of the construction technique, which is proposed to improve the infilled frame seismic response.
Masonry-infilled RC frames can sustain significant damage in the event of strong ground shak-ing.... more Masonry-infilled RC frames can sustain significant damage in the event of strong ground shak-ing. The introduction of sliding joints in masonry infills can reduce the infill-frame interaction, ensure the in-fill out-of-plane stability, and minimize the infill damage under both in- and out-of-plane loading as shown by previous experimental and analytical studies. This paper summarizes previous studies and provides recom-mendations for the design and detailing of such structures. The proposed simplified design procedure can quantify the shear action on the frame columns due to the infill interaction which is compared to the case of traditional continuous infills
A refined design procedure for the seismic retrofit of warehouses or, more generally, single-stor... more A refined design procedure for the seismic retrofit of warehouses or, more generally, single-storey RC frames bounded by “drift-sensitive” masonry infills and glazed curtain walls, is proposed in this paper by means of hysteretic braces. The calculation method is based on displacement-based design (DBD) procedures in which both the as-built frame and the dissipative braces are modelled through simple linear equivalent SDOF systems arranged in parallel. In this regard, with respect to the provisions of the Italian Building Code, two refinements are introduced: (1) the definition of two performance targets tailored to the protection of glazed curtain walls (among most expensive non-structural components) and to ensure an acceptable level of damage level for masonry infills; and (2) the adoption of a more accurate formulation for the estimation of the equivalent viscous damping developed both by the main frame and the dissipative braces. The refined design method is applied to a case-s...
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