Elide Nastri

Seismic Design of MRF-EBF Dual Systems with Vertical Links: EC8 vs Plastic Design

Despite the fact that Eccentrically Braced Frames with Vertical Links (VL-EBFs), also referred to as inverted Y-scheme, are codified in Eurocode 8, the issues related to their seismic response and design have not been widely investigated, so that design criteria commonly applied for Eccentrically Braced Frames with Horizontal Links (HL-EBFs) are commonly applied. However, the Theory of Plastic Mechanism Control (TPMC) has been recently extended to the case of VL-EBFs. The aims of this article, on one hand, are to provide a further validation of the recently proposed design procedure, based on TPMC, and, on the other hand, are to compare the seismic performance of dual systems composed by a moment-resisting part and VL-EBF part designed by means of TPMC with those occurring when Eurocode 8 design criteria are applied. The validation of the proposed design procedure is carried out by means of Incremental Dynamic Analyses (IDA). The main purpose of such analyses is the check of the fulfilment of the design goal of TPMC, i.e., the development of a pattern of yielding consistent with the collapse mechanism of global type. Such mechanism is universally recognized as the one leading to the highest energy dissipation capacity. In case of MRF-EBF dual systems, it is characterized by the yielding of all the links and all the beams at their ends. Conversely, all the columns and the diagonal braces remain in elastic range. Obviously, exception is made for the base sections of first story columns. In particular, two case studies are analyzed which are characterized by a different number of stories. Each building structure is designed according to both TPMC and Eurocode 8 provisions. The seismic response obtained is investigated by both push-over and IDA analyses. The attention is focused on the pattern of yielding obtained, the maximum interstory drift demand, the link plastic rotation demand and sharing of the seismic base shear between the moment-resisting part and the bracing part of the structural system. The results obtained point out improvement of the seismic response, compared to Eurocode 8 provisions, achieved by means of TPMC.

Theory of plastic mechanism control: State-of-the-art

In this paper, the state-of-the-art regarding the “Theory of Plastic Mechanism Control” (TPMC) is presented. TPMC is aimed at the design of structures assuring a collapse mechanism of global type. The theory has been developed in the nineties with reference to moment-resisting steel frames (MRFs) and progressively extended to all the main structural typologies commonly adopted as seismic-resistant structural systems. In particular, the outcome of the theory is the sum of the plastic moments of the columns required, at each storey, to prevent undesired failure modes, i.e. partial mechanisms and soft-storey mechanisms. The theory is used to provide the design conditions to be satisfied, in the form of a set of inequalities where the unknowns are constituted by the column plastic moments. Even though the set of inequalities was originally solved by means of an algorithm requiring an iterative procedure, now, thanks to new advances, a “closed form solution” has been developed. This result is very important, because the practical application of TPMC can now be carried out even with very simple hand calculations. In order to show the simplicity of the new procedure, numerical applications are herein presented in detail with reference to Moment Resisting Frames (MRFs) and dual systems both composed by Moment Resisting Frames and Eccentrically Braces Frames (MRF-EBFs) with inverted Y scheme and composed by Moment Resisting Frames and Concentrically Braced Frames (MRF-CBFs) with X-braced scheme and V-braced scheme. Finally, the pattern of yielding obtained is validated by means of both push-over analyses and incremental dynamic analyses. A comparison in terms of structural weight of the designed structures is also presented and the corresponding seismic performances are discussed.

Validation of a Design Procedure for Failure Mode Control of EB-Frames: Push-Over and IDA Analyses

The paper is devoted to the investigation of the seismic response of eccentrically braced frames characterised by links having different length. In addition, the analysed structures have been designed according to a methodology, already proposed by the authors, aiming to guarantee a collapse mechanism of global type. Therefore, the results of the nonlinear analyses herein presented provide the validation of the proposed design procedure, by testifying that all the designed structures exhibit a global failure mode where all the links are yielded while all the columns remain in elastic range with the exception of the base section of first storey columns, leading to high energy dissipation capacity and global ductility. Furthermore, two different distributions of the link lengths are examined. The first one is characterised by short links with uniform lengths along the height of the structure. The second one is characterised by the use of link elements having different length at the different storeys which are selected to assure the same value of the non-dimensional link length. The seismic response of EB-Frames with such distributions of the link length is investigated by means of both push-over analyses and dynamic non-linear analyses. The comparison of the performances is mainly carried out in terms of plastic hinges distribution, local ductility demand and frame lateral stiffness.

Evaluation of Rotation Capacity of RHS Aluminium Alloy Beams by FEM Simulation: Temper T4

The aim of this work is the numerical assessment of the ultimate behaviour of aluminium alloy beams subjected to non-uniform bending. An extensive numerical analysis has been performed by means of FE code ABAQUS with reference to RHS sections considering different values of the main geometrical and mechanical parameters. In particular, regarding the geometrical parameters the flange slenderness, the flange-to-web slenderness ratio and the moment gradient parameter have been considered. In particular, their influence on the ultimate behaviour of such beams has been investigated by adopting the material constitutive law proposed by Eurocode 9 based on the Ramberg-Osgood model. The investigations concern these parameters considered separately as well as their interaction. The results are herein reported with reference to temper T6 and show the importance of the investigated parameters on the buckling strength and the rotational capacity of aluminium alloy beams. Temper T6 gives rise to a quite low hardening compared to temper T4, which is analysed in a companion paper.

Proposal for an empirical evaluation of rotation capacity of RHS aluminium alloy beams based on fem simulations

The aim of this work is the development of an empirical relationship for evaluating the rotation capacity of RHS aluminium alloy beams, for temper T4 and T6. The proposed relationships are based on the numerical results coming from an extensive parametric analysis performed by means of FE code ABAQUS for different materials, which gain insight into the influence of all the geometrical and mechanical parameters affecting the ultimate behaviour. In particular, the influence of the materials strain hardening, flange slenderness, web stiffness, shape factor and moment gradient the on the plastic behaviour of such beams has been investigated. Successively, by means of monovariate and multivariate non linear regression analyses, empirical relationships are provided in order to predict the rotation capacity of RHS aluminium alloy beams starting from their geometrical and mechanical properties. This paper is focused on this issue.

The Use of TPMC for Designing MRFs Equipped with FREEDAM Connections: A Case Study

Within European Research Project “FREEDAM” supported by RFCS a design procedure for MRFs equipped with friction dampers has been developed. In particular, beam-to-column joints are equipped with friction dampers located at the bottom flange level namely “FREEDAM” dampers. Therefore, all the connections are conceived in order to prevent structural damage. A prototype structure has been designed according to a design procedure assuring a collapse mechanism of global type whose name is Theory of Plastic Mechanism Control, and its seismic performances have been investigated by means of IDA analyses in a companion paper.

The Use of TPMC for Designing MRFs Equipped with FREEDAM Connections: Performance Evaluation

The work herein presented is devoted to the validation of TPMC design procedure applied to steel MRFs equipped with FREEDAM dampers located at beam-to-column joints. The seismic performances evaluations of the designed structure have been carried out by means of both Push-over analysis and Incremental Dynamic Analysis. In particular, the Push-over analysis aims to confirm the real development of a collapse mechanism of global type, while, through IDA analysis, Maximum Interstorey Drift and Top Residual Displacement performed by the designed structures have been pointed out. For this reason, a MRF whose design procedure by TPMC is detailed in a companion paper has been subjected to both push-over and IDA analysis.

Comparative Response of Earthquake Resistant CBF Buildings Designed According to Canadian and European Code Provisions

In this study, both Canadian and European code provisions for steel concentrically braced frames (CBF) are discussed and issues addressing ductility classes for brace cross-sections, q factor value and brace configurations as covered in Eurocode 8 are presented. From comparison with the Canadian provisions it is concluded that beams and columns of CBFs designed according to Eurocode 8 could be under-design when braces perform in the inelastic range. A prototype 8-storey CBF building with multi-storey X-braces is designed and analysed in agreement with both code provisions. The nonlinear seismic responses are presented in terms of interstorey drift, residual interstorey drift and floor acceleration. It was concluded that both buildings are able to yield similar base shear, show similar floor acceleration while the European building undergoes larger residual interstorey drift.

Comparison Between Different Design Strategies For Freedam Frames: Push-Overs and Ida Analyses

Modern seismic code design rules are known to be based on capacity design principles. They try to assure the damage to occur in the ductile parts of the structure, such as beam ends while the other have to remain in elastic range. Therefore, in the aftermath of design earthquakes, plastic deformations at member or connection level will imply high repair costs. In the last decades, innovative structural solutions based on the so-called supplementary energy dissipation strategy allow increasing the dissipative capacity of structures through equipping it with special damping devices. In the case of substitution of dissipative zones with dissipative devices the strategy takes the name of substitutive strategy. This is the case of Moment Resisting Frames investigated in this paper, where traditional dissipa-tive zones, are equipped with innovative low damage frictional devices. However, the current version of codes does not provide any rules to design of MRFs equipped with this type of friction joints.

Thematic Issue on Advances in Modeling, Analysis and Design of Steel Connections

Modeling, analysis and design of steel connections between structural members are of primary importance in structural steel design, because the connections’ behavior significantly affects the response of steel structures under monotonic loading conditions, both in elastic and in plastic range, and exceptional impact loading conditions. In addition, also the seismic response of steel structures is strongly affected by the ultimate behavior of structural connections under cyclic loading conditions [1 3]. In particular, seismic design of steel structures is commonly carried out to assure the dissipation of the seismic input energy in the so-called “Dissipative Zones” which has to be properly detailed in order to assure wide and stable hysteresis loops. Once the yielding of non-dissipative structural members is avoided, connections play a role of paramount importance. In fact, they can be designed either as Full Strength (FS) or Partial Strength (PS). In the first case, the seismic input energy is dissipated by means of plastic cyclic excursions in structural members. In the second case, dissipation requires the plastic engagement of ductile connection components.