Seismic Resilient Steel Frames Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damagefree behaviour of the system. However, to date, little attention has been paid to their column bases, which represent fundamental components in order to achieve resilience. In fact, column bases designed by current conventional approaches lead to significant seismic damage and residual drifts leading to difficult-to-repair structures. This work assesses the seismic performance of steel MRFs equipped with an innovative damage-free, self-centering, rocking column base joints, developed in accordance with the aims of the European project FREEDAM. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behaviour of the connection. Non-linear dynamic analyses have been carried out to investigate the effectiveness of the column base in protecting the first storey columns from yielding and reducing the residual storey drifts. The results show that the damage-free behaviour of the column bases is a key requirement when self-centering of MRFs is a design objective

Design and analysis of a seismic resilient steel moment resisting frame equipped with damage-free self-centering column bases

Many recent research studies focused on the development of innovative seismic resilient structures by chasing the objectives of minimising both seismic damage and repair time, hence allowing the definition of structures able to go back to the undamaged, fully functional condition, in a short time. In this context, the present study investigates an innovative type of self-centring damage-free steel column base (CB) connection and its beneficial effects when used within steel moment-resisting frames (MRFs). The proposed connection consists of a rocking column equipped with a combination of friction devices, providing energy dissipation capacity, and post-tensioned bars with disk springs, introducing restoring forces in the joint. Contrary to conventional steel CBs, the proposed connection exhibits moment–rotation behaviours that can be described by simple analytical equations, allowing the definition of an easy-to-apply design procedure. Numerical models of the connection, developed in OpenSees, are validated against experimental results and successively implemented within a four-storey case study steel MRF. Incremental Dynamic Analyses are performed to derive the samples of the demand for the engineering demand parameters of interest while accounting for the record-to-record variability. Fragility Curves show the effectiveness of the proposed solution in reducing the residual storey drifts and in protecting the first-storey columns from damage, hence providing significant advantages in terms of repairability, and hence resilience of the structure with a negligible increase on the overall cost. The results show that the damage-free behaviour of the CBs is a key requirement when self-centring of MRFs is a design objective

Numerical simulations of a two-storey steel moment resisting frame with free from damage beam-to-column connections and self-centering column bases

In the last two decades, increasing efforts have been devoted to the definition of innovative seismic design philosophies, with the aim of reducing direct and indirect costs deriving from the occurrence of destructive seismic events. Among others, beam-to-column connections equipped with friction devices have emerged as an effective solution able to dissipate the seismic input energy, while also ensuring the damage-free behaviour within steel Moment Resisting Frames (MRFs). Additionally, recent extensive numerical studies have demonstrated the benefits deriving from the replacement of conventional full-strength column bases (CBs) with innovative damage-free self-centring CBs, in reducing the residual storey drifts and in preventing the first-storey columns' yielding of low-rise MRFs. This work introduces the preliminary study of an experimental campaign which has been planned on a one-bay two-storey large-scale MRF, equipped with the proposed damage-free self-centring CBs. The present study has the objective to foresee the response that will be observed during the experimental test by advanced numerical simulations in OpenSees

Seismic Response of a Steel Resilient Frame Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damage-free behaviour of the system. How-ever, to date, little attention has been paid to their column bases, which represent fundamental com-ponents in order to achieve resilience. In fact, column bases designed by current conventional ap-proaches lead to significant seismic damage and residual drifts leading to difficult-to-repair structures. The present paper evaluates the seismic performance of steel MRFs equipped with an innovative dam-age-free, self-centring, rocking column base joints. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behav-iour of the connection. Non-linear dynamic analyses have been carried out on a set of ground motions records to investigate the effectiveness of the column base in protecting the first storey columns from yielding and in reducing the residual storey drifts. Incremental Dynamic Analyses are used to investigate the influence of the record-to-record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters of the frame and of the column base connection. The results show that the damage-free behaviour of the column bases is a key requirement when self-cen-tering of MRFs is a design objective

Experimental investigation and modelling of T-stubs undergoing large displacements

This paper investigates the development of second (2nd) order effects, arising from geometric and material non-linearities of T-stubs bolted to a rigid support, through a combination of experimental, numerical and analytical approaches. Experimental data is presented for a broad range of T-stub geometries, designed to ensure that significant 2nd order effects always develop, that will complement the existing library of limited test results. Finite element models, incorporating combined tensile (ductile) and shear damage initiation, evolution and failure in both the flange and bolt, are also developed to elucidate how key geometric/material parameters influence the resistance and ductility of T-stubs undergoing large displacement. It will be shown that the restraining effect from the bolt is integral to the activation of catenary action in the flange and the development of a second hardening branch in the tensile response, leading to identification of two new modes of failure that are not currently considered in classical theory or by EC3 (Part 1.8). A mechanical model is formulated to identify the key geometric and material parameters controlling the initiation, and development, of the second hardening branch. Finally, a criterion is proposed to estimate the critical displacement from when 2nd order effects become active

Experimental Response of a Large-Scale Steel Structure Equipped with Innovative Column Bases

In the last few decades, increasing research efforts have been devoted to the definition of innovative seismic design philosophies aiming at reducing seismic-induced direct and indirect losses. For steel Moment Resisting Frames (MRFs), the use of Friction Devices (FDs) in beam-to-column connections has emerged as an effective solution to dissipate the seismic input energy while also ensuring their damage-free behaviour. Additionally, more recent research studies have revealed the benefits of replacing traditional full-strength Column Bases (CBs) with innovative CBs for both damage and residual drift reductions of steel MRFs. In this direction, an experimental campaign has been performed on a two-storey large-scale steel structure equipped with innovative Self-Centring CBs (SC-CBs). The present paper illustrates the preparatory work required for the specimen's design, the experimental program and the preliminary results. The tests' outcomes demonstrated the effectiveness of the SC-CB connections in minimising the residual drifts of the structure and in protecting the first-storey columns from damage

Performance-based assessment of seismic-resilient steel moment resisting frames equipped with innovative column base connections

Low-damage and self-centring column base connections have been proposed in the last two decades as innovative solutions able to provide the seismic resilience in Moment Resisting Frames (MRFs). Although many works have demonstrated the benefits deriving from the adoption of these systems, only a few research studies investigated the significant parameters influencing their self-centring capability. This paper investigates the influence of the frame layout (i.e., sto-reys and bays number) on the seismic performance of perimeter MRFs equipped with damage-free self-centring column bases previously studied by the authors. Nine case-study perimeter steel MRFs are designed and modelled in OpenSees. Incremental Dynamic Analyses are per-formed monitoring both global and storey-level Engineering Demand Parameters, including peak and residual interstorey drifts. Fragility curves are successively used to evaluate the self-centring capability of the structures. The present study provides insights on the use of the adopted con-nections for the residual drift reduction of MRFs and defines the boundaries of the investigated parameters for their application. Results highlight that the self-centring behaviour is particularly sensitive to the number of storeys and tends to reduce with the increasing height of MRFs equipped with the proposed connections

Seismic Resilient Steel Frames Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damage-free behaviour of the system. However, to date, little attention has been paid to their column bases, which represent fundamental components in order to achieve resilience. In fact, column bases designed by current conventional approaches lead to significant seismic damage and residual drifts leading to difficult-torepair structures. This work assesses the seismic performance of steel MRFs equipped with an innovative damagefree, self-centering, rocking column base joints, developed in accordance with the aims of the European project FREEDAM. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behaviour of the connection. Non-linear dynamic analyses have been carried out to investigate the effectiveness of the column base in protecting the first storey columns from yielding and reducing the residual storey drifts. The results show that the damage-free behaviour of the column bases is a key requirement when self-centering of MRFs is a design objective

Finite element analysis of bolted T-stubs undergoing large displacement: a preliminary study

To properly assess the robustness of steel Moment Resisting Frames (MRFs), the non-linear response of structural members and connections would need to be quantified. Under the influence of extreme load cases, structural joints are subjected to both material and geometric nonlinearities, known commonly as second-order effects. These effects cannot be disregarded if catenary actions develop in the connecting beam member. The rotational capacity of bolted joints is directly dependent on the deformation capacity of its components in bending which are typically represented by the equivalent T-stub. A T-stub is composed of a single T-section bolted to a support whose stiffness may be equivalent or greater than that of the T-element. To accurately characterise the response of a T-stub undergoing large displacement, the non-linear behaviour of its flange will need to be thoroughly investigated. In the flange, second order effects are caused by the development of axial (or membrane) forces which can be significant for those T-stubs connected to a rigid support. Hitherto, little information exists on the influence of second-order effects on the response of bolted T-stubs and, consequently, there are no existing guidelines on how to include these effects in design. In this paper, we present the results of a parametric investigation, using finite element (FE) analysis, to assess the influence of second-order effects in T-stubs bolted to a rigid support. Both material and geometrical non-linearities were considered since they are known to have a critical impact upon the performance of T-stubs. A benchmark FE model is first generated and validated against experimental data; it is then used to carry out a parametric investigation, by alternately considering and neglecting geometric non-linearity, to identify the geometric configurations that experience significant second order effects. A method to assess the contributions of membrane forces to the overall deformation response of a T-stub is also propose

numerical assessment of the influence of different joint hysteretic models over the seismic behaviour of moment resisting steel frames

The main aim of this work is to understand how the prediction of the seismic performance of moment-resisting (MR) steel frames depends on the modelling of their dissipative zones when the structure geometry (number of stories and bays) and seismic excitation source vary. In particular, a parametric analysis involving 4 frames was carried out, and, for each one, the full-strength beam-to-column connections were modelled according to 4 numerical approaches with different degrees of sophistication (Smooth Hysteretic Model, Bouc-Wen, Hysteretic and simple Elastic-Plastic models). Subsequently, Incremental Dynamic Analyses (IDA) were performed by considering two different earthquakes (Spitak and Kobe). The preliminary results collected so far pointed out that the influence of the joint modelling on the overall frame response is negligible up to interstorey drift ratio values equal to those conservatively assumed by the codes to define conventional collapse (0.03 rad). Conversely, if more realistic ultimate interstorey drift values are considered for the q-factor evaluation, the influence of joint modelling can be significant, and thus may require accurate modelling of its cyclic behavior