Fabio Freddi | University College London (original) (raw)

Papers by Fabio Freddi

Lecture notes in civil engineering, 2024

Lecture notes in civil engineering, 2024

Proceedings of the 8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2015)

ce/papers

In the last few decades, increasing research efforts have been devoted to the definition of innov... more In the last few decades, increasing research efforts have been devoted to the definition of innovative seismic resilient design strategies with the aim of reducing direct and indirect losses. Among others, the use of Friction Devices (FDs) for Beam‐to‐Column joints (BCJs) has emerged as an effective solution to ensure the damage‐free behaviour of steel Moment Resisting Frames (MRFs). Additionally, more recent research studies have revealed the benefits of replacing traditional full‐strength Column Bases (CBs) with innovative CBs promoting the residual drift reduction of steel MRFs. Additionally, several research studies developed and investigated structures that could be easily repaired after an extreme event, promoting a functional recovery of the structures and able to reinstate the original seismic performance in a short time. However, experimental research on suitable structural repair techniques is still marginal and the lack of confidence in the performance of repaired structu...

Engineering failure analysis, Jun 1, 2024

Passive control systems, such as buckling restrained braces (BRBs), have emerged as an efficient ... more Passive control systems, such as buckling restrained braces (BRBs), have emerged as an efficient tool for the seismic response control of new and existing structures by providing strength and stiffness to buildings, in addition to high and stable energy dissipation capacity. Systems equipped with BRBs have been widely investigated in literature, however, only deterministic description of the BRBs' properties is usually considered. These properties are provided by the manufacturer and are successively validated by qualification control tests. The acceptance criteria specified by codal standards allows for some variation in the response of a single BRB by introducing a tolerance limit. Therefore, the 'real' properties of these devices could differ from the design values. This difference can affect the seismic response and potentially lead to an undesired seismic performance at the global level. This paper provides some preliminary insights on the influence of the BRBs' uncertainty on the seismic response of a retrofitted RC frame. For the case-study, a benchmark two-dimensional RC frame is considered. A single retrofit condition is analyzed and the BRBs' uncertainty is defined according to the tolerance limits of devices' quality control tests. Cloud analysis and probabilistic seismic demand models are used to develop fragility functions for four different damage states. Fragility curves are defined for the bare and retrofitted frame while considering both the design and the 'real' values of the BRBs properties. The preliminary results show that the BRBs' uncertainty could lead to an increase of the vulnerability up to 26.80% for the considered case-study.

ce/papers, Sep 1, 2017

Column bases are critical components of a seismic-resistant steel building as they transfer gravi... more Column bases are critical components of a seismic-resistant steel building as they transfer gravity and lateral forces to the foundations. This paper focuses on the finite element (FE) modelling of a rocking damage-free steel column base, which uses post-tensioned high strength steel bars to achieve self-centering behavior and friction devices to provide energy dissipation capacity. Contrary to conventional steel column bases, the monotonic and cyclic moment-rotation behavior of the column base can be easily described using simple analytical equations. An advanced threedimensional FE model for the column base is developed in ABAQUS. The techniques used to overcome convergence issues in numerical simulations as well as the constitutive laws for the nonlinear behavior of materials and friction interfaces are described in detail. The FE model provides results that are in very good agreement with the results from the analytical momentrotation equations. The FE model results are also used to validate a simplified numerical model in OpenSees. Moreover, the FE model provides results that help to assess the level of stress concentration in critical areas of the column base and to evaluate a step-by-step design procedure that ensures damage-free behavior, self-centering capability, and adequate energy dissipation capacity.

Journal of Structural Engineering-asce, Mar 1, 2023

Several recent studies proposed and investigated the use of unbonded post-tensioned (PT) bars in ... more Several recent studies proposed and investigated the use of unbonded post-tensioned (PT) bars in bridge substructure systems to improve their self-centering behavior. However, the lateral loading resistance and self-centering capacities of reinforced concrete (RC) piers with PT bars (i.e., the post-tensioned RC bridge piers-PRC piers) could be easily compromised by early crushing of the base compression toe during a seismic event. Hence, in order to enhance the pier base integrity, the present study proposes and experimentally investigates three simple strategies for enhancing the seismic damage-resistance of PRC piers based on the use of: 1) a steel tube to encase the PRC pier's end segment, 2) ultra-high performance concrete (UHPC) at the PRC pier's end segment, and 3) engineered cementitious composite (ECC) mortar bed underneath the pier bottom. The performance of these three PRC piers was assessed by comparing them to a conventional PRC pier under cyclic loading and considering both the damage evolution and the cyclic forcedisplacement response. The comparison shows that the proposed enhanced PRC solutions allow improving the seismic performance of the system sustaining large lateral drifts with good self-centering behavior. Moreover, based on the test results, finite element models accounting for PT force loss and the rocking characteristics were validated to reproduce the piers' cyclic response, highlighting the importance of considering PT force loss during numerical simulations.

Procedia structural integrity, 2023

The paper illustrates a probabilistic methodology for assessing the vulnerability of existing rei... more The paper illustrates a probabilistic methodology for assessing the vulnerability of existing reinforced concrete (RC) buildings with limited ductility capacity retrofitted by means of dissipative braces. The aim is to highlight the most important parameters controlling the capacity of these coupled systems and specific aspects concerning the response uncertainties. The proposed methodology is based on the use of local engineering demand parameters for monitoring the seismic response and on the development of component and system fragility curves before and after the retrofit. In the first part of the paper, the methodology is illustrated by highlighting its advantages with respect to the existing approaches. Then, its capability and effectiveness are tested by considering a benchmark two-dimensional RC frame designed for gravity-loads only. The frame is retrofitted by introducing elasto-plastic dissipative braces designed for different levels of base shear capacity. The obtained results show the effectiveness of the methodology in describing the changes in the response and in the failure modalities before and after the retrofit, for different retrofit levels. Moreover, the retrofit effectiveness is evaluated by introducing proper synthetic parameters describing the fragility curves and by stressing the importance of employing local engineering demand parameters (EDPs) rather than global EDPs in the seismic risk evaluation of coupled systems consisting in low-ductility RC frames and dissipative braces.

Buckling-restrained braces (BRBs) are energy dissipation devices which have proven to be very eff... more Buckling-restrained braces (BRBs) are energy dissipation devices which have proven to be very effective in improving the performance of existing and new building frames. However, their post-elastic stiffness may lead to excessive residual deformations in the systems to be protected, and this may cause irreparable damage and jeopardize the capability of withstanding multiple shocks. Previous studies demonstrated that buckling-restrained braced frames (BRBFs) can be used in conjunction with special moment-resisting frames to form a dual system, able to minimize the residual drifts and optimize the performances of the BRBFs. The objective of this present paper is to provide recommendations regarding the proportioning in terms of forces, stiffness and ductility of the two systems. For this purpose, an extensive parametric analysis under the single degree of freedom approximation for the dual system is carried out to shed light on the parameters that control the seismic performance and residual capacity of frames equipped with BRBs. A non-dimensional formulation of the problem allows investigating wide ranges of configurations, including the case of BRBFs and the case of BRBFs forming a dual system with moment-resisting frames. The results of this study provide useful information for the preliminary sizing and the optimal choice of the design parameters of structural systems equipped with BRBs.

This paper presents the numerical assess of the robustness of a seismic-resistant steel building ... more This paper presents the numerical assess of the robustness of a seismic-resistant steel building with self-centering moment resisting frames against progressive collapse. The numerical analyses were carried out using a 3D model developed in ABAQUS. The 3D model considers the effect of the composite slab, where composite beams and their shear connectors were modeled with a combination of shell, beam and nonlinear connector elements. All the beam-column and beam-to-beam connections were modeled using nonlinear connector elements with appropriate failure criteria, calibrated against previous experimental results. The self-centering moment resisting frame where a sudden column loss was simulated was modelled using 3D solid elements to accurately capture its local and global nonlinear behavior. Quasi-static nonlinear analyses were carried out to identify all possible failure modes and to investigate the effect of the floor slab on the overall progressive collapse resistance. Nonlinear dynamic analyses were also carried out to predict the true dynamic response and evaluate the acceptance criteria of current building design guidelines.

The optimum combination of steel and concrete elements to constitute hybrid steelconcrete seismic... more The optimum combination of steel and concrete elements to constitute hybrid steelconcrete seismic-resistant structural systems should exploit the stiffness of concrete and the ductility and dissipative capacity of steel. If the seismic damage is limited to some, easy to replace, steel components only and the residual deformations are limited, the structural system can be quickly repaired and go back to the full functionality even in the aftermath of major earthquakes. This design strategy allows obtaining structural resilient systems. In this context, the present work aims at reviewing the recent outcomes of a European research project where two hybrid structural systems were numerically and experimentally investigated. A proposal for design recommendations consistent with the framework of the Eurocodes is also presented. The first hybrid system considered is a steel frame with reinforced concrete infill walls designed as a truss structure where seismic damage is concentrated in the vertical steel components with reduced sections undergoing yielding in tension. All other steel elements as well as the reinforced concrete infill walls are designed to work within their elastic range. The second hybrid system considered is a reinforced concrete wall coupled to two steel columns by means of steel links. Both columns and the wall are designed to work in their elastic range while the yielding of the coupling links allows dissipating the seismic energy. Design criteria aiming at activating all links along the building height and the effect of the coupling ratio are discussed.

ce/papers

Significant progress has been made in recent years for the definition of seismic‐resilient struct... more Significant progress has been made in recent years for the definition of seismic‐resilient structures, chasing the urgent need for structures able to return to undamaged, fully functional conditions in a short time after an extreme event. Seismic‐resilient steel structures have been widely investigated, mainly considering solutions based on moment‐resisting and concentrically braced frames. However, while the first category may be characterised by low stiffness, the second is often characterised by low ductility. Eccentrically Braced Frames (EBFs) allow overcoming these drawbacks and well‐balance stiffness, strength and ductility. This paper presents the numerical and experimental study of a damage‐free self‐centring link for EBFs. The proposed device uses post‐tensioned bars to control the self‐centring behaviour and friction devices to provide dissipation capacity. Experimental cyclic tests have been carried out to evaluate the response of the device. A 3D ABAQUS model has been de...

Lecture notes in civil engineering, 2024

Lecture notes in civil engineering, 2024

Proceedings of the 8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2015)

ce/papers

In the last few decades, increasing research efforts have been devoted to the definition of innov... more In the last few decades, increasing research efforts have been devoted to the definition of innovative seismic resilient design strategies with the aim of reducing direct and indirect losses. Among others, the use of Friction Devices (FDs) for Beam‐to‐Column joints (BCJs) has emerged as an effective solution to ensure the damage‐free behaviour of steel Moment Resisting Frames (MRFs). Additionally, more recent research studies have revealed the benefits of replacing traditional full‐strength Column Bases (CBs) with innovative CBs promoting the residual drift reduction of steel MRFs. Additionally, several research studies developed and investigated structures that could be easily repaired after an extreme event, promoting a functional recovery of the structures and able to reinstate the original seismic performance in a short time. However, experimental research on suitable structural repair techniques is still marginal and the lack of confidence in the performance of repaired structu...

Engineering failure analysis, Jun 1, 2024

Passive control systems, such as buckling restrained braces (BRBs), have emerged as an efficient ... more Passive control systems, such as buckling restrained braces (BRBs), have emerged as an efficient tool for the seismic response control of new and existing structures by providing strength and stiffness to buildings, in addition to high and stable energy dissipation capacity. Systems equipped with BRBs have been widely investigated in literature, however, only deterministic description of the BRBs' properties is usually considered. These properties are provided by the manufacturer and are successively validated by qualification control tests. The acceptance criteria specified by codal standards allows for some variation in the response of a single BRB by introducing a tolerance limit. Therefore, the 'real' properties of these devices could differ from the design values. This difference can affect the seismic response and potentially lead to an undesired seismic performance at the global level. This paper provides some preliminary insights on the influence of the BRBs' uncertainty on the seismic response of a retrofitted RC frame. For the case-study, a benchmark two-dimensional RC frame is considered. A single retrofit condition is analyzed and the BRBs' uncertainty is defined according to the tolerance limits of devices' quality control tests. Cloud analysis and probabilistic seismic demand models are used to develop fragility functions for four different damage states. Fragility curves are defined for the bare and retrofitted frame while considering both the design and the 'real' values of the BRBs properties. The preliminary results show that the BRBs' uncertainty could lead to an increase of the vulnerability up to 26.80% for the considered case-study.

ce/papers, Sep 1, 2017

Column bases are critical components of a seismic-resistant steel building as they transfer gravi... more Column bases are critical components of a seismic-resistant steel building as they transfer gravity and lateral forces to the foundations. This paper focuses on the finite element (FE) modelling of a rocking damage-free steel column base, which uses post-tensioned high strength steel bars to achieve self-centering behavior and friction devices to provide energy dissipation capacity. Contrary to conventional steel column bases, the monotonic and cyclic moment-rotation behavior of the column base can be easily described using simple analytical equations. An advanced threedimensional FE model for the column base is developed in ABAQUS. The techniques used to overcome convergence issues in numerical simulations as well as the constitutive laws for the nonlinear behavior of materials and friction interfaces are described in detail. The FE model provides results that are in very good agreement with the results from the analytical momentrotation equations. The FE model results are also used to validate a simplified numerical model in OpenSees. Moreover, the FE model provides results that help to assess the level of stress concentration in critical areas of the column base and to evaluate a step-by-step design procedure that ensures damage-free behavior, self-centering capability, and adequate energy dissipation capacity.

Journal of Structural Engineering-asce, Mar 1, 2023

Several recent studies proposed and investigated the use of unbonded post-tensioned (PT) bars in ... more Several recent studies proposed and investigated the use of unbonded post-tensioned (PT) bars in bridge substructure systems to improve their self-centering behavior. However, the lateral loading resistance and self-centering capacities of reinforced concrete (RC) piers with PT bars (i.e., the post-tensioned RC bridge piers-PRC piers) could be easily compromised by early crushing of the base compression toe during a seismic event. Hence, in order to enhance the pier base integrity, the present study proposes and experimentally investigates three simple strategies for enhancing the seismic damage-resistance of PRC piers based on the use of: 1) a steel tube to encase the PRC pier's end segment, 2) ultra-high performance concrete (UHPC) at the PRC pier's end segment, and 3) engineered cementitious composite (ECC) mortar bed underneath the pier bottom. The performance of these three PRC piers was assessed by comparing them to a conventional PRC pier under cyclic loading and considering both the damage evolution and the cyclic forcedisplacement response. The comparison shows that the proposed enhanced PRC solutions allow improving the seismic performance of the system sustaining large lateral drifts with good self-centering behavior. Moreover, based on the test results, finite element models accounting for PT force loss and the rocking characteristics were validated to reproduce the piers' cyclic response, highlighting the importance of considering PT force loss during numerical simulations.

Procedia structural integrity, 2023

The paper illustrates a probabilistic methodology for assessing the vulnerability of existing rei... more The paper illustrates a probabilistic methodology for assessing the vulnerability of existing reinforced concrete (RC) buildings with limited ductility capacity retrofitted by means of dissipative braces. The aim is to highlight the most important parameters controlling the capacity of these coupled systems and specific aspects concerning the response uncertainties. The proposed methodology is based on the use of local engineering demand parameters for monitoring the seismic response and on the development of component and system fragility curves before and after the retrofit. In the first part of the paper, the methodology is illustrated by highlighting its advantages with respect to the existing approaches. Then, its capability and effectiveness are tested by considering a benchmark two-dimensional RC frame designed for gravity-loads only. The frame is retrofitted by introducing elasto-plastic dissipative braces designed for different levels of base shear capacity. The obtained results show the effectiveness of the methodology in describing the changes in the response and in the failure modalities before and after the retrofit, for different retrofit levels. Moreover, the retrofit effectiveness is evaluated by introducing proper synthetic parameters describing the fragility curves and by stressing the importance of employing local engineering demand parameters (EDPs) rather than global EDPs in the seismic risk evaluation of coupled systems consisting in low-ductility RC frames and dissipative braces.

Buckling-restrained braces (BRBs) are energy dissipation devices which have proven to be very eff... more Buckling-restrained braces (BRBs) are energy dissipation devices which have proven to be very effective in improving the performance of existing and new building frames. However, their post-elastic stiffness may lead to excessive residual deformations in the systems to be protected, and this may cause irreparable damage and jeopardize the capability of withstanding multiple shocks. Previous studies demonstrated that buckling-restrained braced frames (BRBFs) can be used in conjunction with special moment-resisting frames to form a dual system, able to minimize the residual drifts and optimize the performances of the BRBFs. The objective of this present paper is to provide recommendations regarding the proportioning in terms of forces, stiffness and ductility of the two systems. For this purpose, an extensive parametric analysis under the single degree of freedom approximation for the dual system is carried out to shed light on the parameters that control the seismic performance and residual capacity of frames equipped with BRBs. A non-dimensional formulation of the problem allows investigating wide ranges of configurations, including the case of BRBFs and the case of BRBFs forming a dual system with moment-resisting frames. The results of this study provide useful information for the preliminary sizing and the optimal choice of the design parameters of structural systems equipped with BRBs.

This paper presents the numerical assess of the robustness of a seismic-resistant steel building ... more This paper presents the numerical assess of the robustness of a seismic-resistant steel building with self-centering moment resisting frames against progressive collapse. The numerical analyses were carried out using a 3D model developed in ABAQUS. The 3D model considers the effect of the composite slab, where composite beams and their shear connectors were modeled with a combination of shell, beam and nonlinear connector elements. All the beam-column and beam-to-beam connections were modeled using nonlinear connector elements with appropriate failure criteria, calibrated against previous experimental results. The self-centering moment resisting frame where a sudden column loss was simulated was modelled using 3D solid elements to accurately capture its local and global nonlinear behavior. Quasi-static nonlinear analyses were carried out to identify all possible failure modes and to investigate the effect of the floor slab on the overall progressive collapse resistance. Nonlinear dynamic analyses were also carried out to predict the true dynamic response and evaluate the acceptance criteria of current building design guidelines.

The optimum combination of steel and concrete elements to constitute hybrid steelconcrete seismic... more The optimum combination of steel and concrete elements to constitute hybrid steelconcrete seismic-resistant structural systems should exploit the stiffness of concrete and the ductility and dissipative capacity of steel. If the seismic damage is limited to some, easy to replace, steel components only and the residual deformations are limited, the structural system can be quickly repaired and go back to the full functionality even in the aftermath of major earthquakes. This design strategy allows obtaining structural resilient systems. In this context, the present work aims at reviewing the recent outcomes of a European research project where two hybrid structural systems were numerically and experimentally investigated. A proposal for design recommendations consistent with the framework of the Eurocodes is also presented. The first hybrid system considered is a steel frame with reinforced concrete infill walls designed as a truss structure where seismic damage is concentrated in the vertical steel components with reduced sections undergoing yielding in tension. All other steel elements as well as the reinforced concrete infill walls are designed to work within their elastic range. The second hybrid system considered is a reinforced concrete wall coupled to two steel columns by means of steel links. Both columns and the wall are designed to work in their elastic range while the yielding of the coupling links allows dissipating the seismic energy. Design criteria aiming at activating all links along the building height and the effect of the coupling ratio are discussed.

ce/papers

Significant progress has been made in recent years for the definition of seismic‐resilient struct... more Significant progress has been made in recent years for the definition of seismic‐resilient structures, chasing the urgent need for structures able to return to undamaged, fully functional conditions in a short time after an extreme event. Seismic‐resilient steel structures have been widely investigated, mainly considering solutions based on moment‐resisting and concentrically braced frames. However, while the first category may be characterised by low stiffness, the second is often characterised by low ductility. Eccentrically Braced Frames (EBFs) allow overcoming these drawbacks and well‐balance stiffness, strength and ductility. This paper presents the numerical and experimental study of a damage‐free self‐centring link for EBFs. The proposed device uses post‐tensioned bars to control the self‐centring behaviour and friction devices to provide dissipation capacity. Experimental cyclic tests have been carried out to evaluate the response of the device. A 3D ABAQUS model has been de...