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This paper presents the first main results of quasi-static cyclic tests performed on a half-scaled two-storey one-bay steel pendular frame equipped with Crescent Shaped Braces (CSBs). The CSB is a yielding steel bracing element characterized by a boomerang-like shape, designed to provide the structure with an optimal combination of lateral stiffness, strength, ductility capacity and hysteretic dissipation, thus capable of meeting multiple seismic performance objectives within the context of Performance Based Seismic Design. In previous research studies, the behaviour of single CSBs under cyclic loads was analytically, numerically, and experimentally investigated through tests conducted on 1:6 scaled specimens. These results verified the potential capabilities of the device. As a further step of the experimental validation, a set of tests have been designed to evaluate the performances of CSBs when inserted into realistic frame structures. A two-storey prototype structure has been designed to meet selected seismic performance objectives, which could not be achieved with traditional diagonal braces. From that, a single-bay two-storey frame, representative of one braced frame of the whole structure, has been designed, detailed, and manufactured (half-scaled) for the experimental tests. For a full assessment of the behaviour of the braced frame, two configurations have been designed and tested. This paper presents the results of the test performed on the first configuration with one CSB device placed at the first storey only. The attention has been mainly focused on the global force-displacement response and the contribution of the connection plates, energy dissipation capacities and local strains around the knee-point region of the CSB. The results show that the CSB behaves following the theoretical predictions, while the connection plates provide a non-negligible contribution in the whole response of the system that should be properly accounted for in the design phase.

A novel experimental study of the dynamic and seismic response of a 2 m × 2 m in plan - 0.7 m in height groin vault model, involving 266 tests conducted on the shaking table of EQUALS laboratory, University of Bristol, UK, is reported. The experimental rig consists of blocks formed by a 3D-printed plastic skin to provide stiffness and strength, filled with mortar. Dry joints between the voussoirs are formed for ease of testing and vault reconstruction. No investigations of this kind and size have been attempted in the past. Two support boundary conditions involving four lateral confinement modes, leading to various vault configurations, were tested. White-noise, sinusoidal and earthquake motions were imposed in one horizontal direction, with progressively increasing amplitude and different frequencies, up to collapse. The model exhibited a strong non-linear behaviour, characterised by decreasing fundamental frequency and increasing damping with increasing table acceleration. Failure mechanisms and collapse accelerations were found to mainly depend on base restraint conditions. The SEBESMOVA3D project (SEeismic BEhaviour of Scaled MOdels of groin VAults made by 3D printers, https://sera-ta.eucentre.it/index.php/sera-ta-project-22/) was funded by European Union’s Horizon 2020 research and innovation programme SERA, under grant agreement No 730900.

The technical note investigates the tensile force-displacement response of the hysteretic steel yielding brace known as Crescent Shaped Brace and characterized by a boomerang-like geometrical shape. The force-displacement curve is governed by three key performance points which correspond to the transition points separating the initial elastic behaviour, the flexural plastic behaviour, the geometrical hardening behaviour and the final axial plastic behaviour. In particular, the influence of the main geometrical parameter of the device, the so-called “lever arm”, on the strongly non-linear force-displacement behavior is analyzed by means of a simplified kinematic model. Based on this, analytical estimations of the key performance points are derived and compared with numerically simulated force-displacement curves.

This work investigates the behaviour of unreinforced and reinforced cross vaults subjected to static shear deformation at the springings through an experimental campaign. An earlier companion paper discussed the results of a pseudo-static test performed on a 1:4 scaled model, built with timber blocks and lime mortar, applying shear displacements to two abutments until failure. This paper illustrates the effects of the same action applied to the same model after extrados reinforcement with Fibre Reinforced Polymer strips. Crack patterns and displacement evolution are presented and compared with those of the unreinforced model to evaluate the effectiveness of this seismic strengthening technique.

The dynamic response of complex structures can be estimated by means of Generalized Single Degree of Freedom (G-SDOF) systems. The original concept of the G-SDOF system is revisited using an alternative viewpoint based on the equilibrium of the three resultant dynamic forces associated with the stiffness, mass and damping components, and applied to frame structures equipped with viscous dampers. In particular, it is shown that a generic structure with supplemental dampers can be reduced into two different G-SDOF idealizations, based on the global translational or rotational equilibrium. The mechanical analogies of the two G-SDOF systems provide physical insight into the dynamic behavior of structures with viscous dampers, since their energy dissipation capacities can be graphically assessed in terms of the resultant dynamic forces and corresponding lever arms. The approach is applied to shear-type structures with specific dampers configurations resulting in proportionally damped systems. Novel analytical estimations of their first circular frequencies and modal damping ratios are obtained, providing upper- and lower-bounds of the exact values. Limitations in the use of the approach for structures with different dampers configurations resulting in highly non-proportionally damped systems are also discussed.

In the present work a direct procedure for the preliminary seismic design of building structures with added dampers is described which represents the simplification of the so-called “five-step procedure” originally developed in 2010 by some of the authors. The procedure is applicable to yielding frame structures with a generic along-the-height distribution of inter-storey viscous dampers. It is aimed at guiding the structural engineer through the sizing of both viscous dampers and structural elements making use of an equivalent static analysis approach. First, the peak structural response under earthquake excitation is reduced by imposing an overall reduction factor accounting for both the ductility demand and the viscous damping provided by the added dampers. Second, linear damping coefficients are calculated in order to reduce the structural response according to the selected target damping ratio. Then, analytical formulas allow the estimation of peak velocities and forces in the dissipative devices, and an energy criterion is used to identify the non-linear mechanical characteristics of the actual manufactured viscous dampers. Finally, the internal actions in the structural elements are estimated through the envelope of two equivalent static analyses (ESA). At this initial stage of the research, the procedure appears suitable for the preliminary design phase, while correction factors for the higher modes contributions need to be applied to improve its accuracy, especially for high-rise buildings. A numerical verification of the final behaviour of the system by means of non-linear time-history analyses is recommended. An applicative example is finally developed to highlight the soundness of the procedure.

This work investigates the dynamic behaviour of a multi-storey frame building, assumed as the structure to be controlled, connected with an adjacent support structure by means of horizontal fluid viscous dampers. The dampers connection system has two main effects: (i) energy dissipation provided by the viscous dissipative forces, (ii) energy transfer between modes leading to modes coupling effects between the two buildings without a significant dissipation of energy. The relative contribution of the two effects is highly dependent on the main properties of the coupled system. To investigate this complex behaviour, first the minimal coupled dynamic system composed by two Single-Degree-Of-Freedom systems connected by a viscous damper is analysed. The theory of complex damping is used to determine complex frequencies and damping ratios, while analytical expressions of steady state response under harmonic excitation are determined to investigate the influence of the system parameters on the maximum dynamic amplifications. The seismic response is also investigated through a wide parametric study with the aim of evaluating the trends of the damping reduction factors with respect to the main dynamic parameters of the coupled system. Minimum values of the damping reduction factors and corresponding optimal damping coefficients are determined. Then, the analyses are extended to uniform multi-storey structures for which minimum damping reduction factors are provided for a wide range of the key system parameters. Comparisons between one-storey and uniform multi-storey systems are provided. The results can be useful for a preliminary design of the dampers connection system and for the evaluation of its effectiveness based on the fundamental dynamic properties of the connected buildings.

Crescent Shaped Braces, CSBs, are steel hysteretic devices which, thanks to their geometrical configuration, are characterized by an enhanced seismic behavior that makes them a promising alternative to conventional diagonal steel braces and other seismic devices such as Buckling Resisting Braces and Scorpions. The present work reports the results of the first experimental campaign devoted to assess the cyclic experimental behavior of CSBs. The main goals of the experimental campaign are: (i) to verify/compare the effectiveness of design formulas for the seismic design of CSB introduced by the authors in a previous research work and the predictions of a simplified non-linear model in terms of force-displacement envelop response; (ii) to assess the experimental non-linear cyclic behavior in terms of ductility capacity, energy dissipation capacity, failures.

It is well known that peak inter-storey drifts and peak inter-storey velocities are fundamental response quantities to assess the seismic response of a frame structure equipped with added viscous dampers. In the present work, analytical estimations, grounded on the first mode response, of the two response quantities are introduced. Then, a large parametric study is carried out to assess the effectiveness of the proposed predictions. A special attention is devoted to the peak inter-storey velocities and to their sensitivity on the higher modes contribution.

The construction sector could provide solutions for the safe utilization of industrial by-products as construction materials, if proper characterization and control of the materials properties is undertaken. Under this consideration, fines produced from marble cutting and fines produced from concrete truck washing were investigated as fine material for use in cement mortars. Both these by-products are produced in large amounts in the form of sludge. Marble Sludge Fines (MSF) and Cement Sludge Fines (CSF) were characterized in terms of fineness, density, chemical analysis and suitability for use with cement. Mortars with variable rate (10%, 20% and 30%) of cement substitution with MSF or CSF were tested and compared to a reference cement mortar in respect to their fresh and hardened properties. Packing ability and viscosity were measured in fresh mortars, while strength development, water absorption and porosity were measured in hardened mortars. The results confirm the suitability of both as filler material; although MSF performed better regarding fresh mortar properties and CSF showed better results regarding strength development.

Summary The primary objective of the “performance-based seismic design” is to provide stipulated seismic performances for building structures. However, a certain degree of design freedom is needed for matching a specific seismic response. This design freedom is not obtainable by the conventional lateral resisting systems because their stiffness and strength are coupled. Here, we put emphasis on the role of the unconventional lateral resisting systems in adding more flexibility to the design. In this paper, we seek to explore the seismic design of moment-resisting frame structures equipped with an innovative hysteretic device, known as “crescent-shaped brace.” One conspicuous feature of this device is its distinctive geometrical configuration, which is responsible for the enhanced nonlinear force-displacement behavior exhibited by the device. A new performance-based approach for the seismic design of the crescent-shaped brace is proposed. The performance of the device is evaluated, and its application in multistory shear-type structures is investigated. Two case studies were established to illustrate the design methodology. The first is a new two-story RC structure, and the second is an existing three-story RC structure. Nonlinear time history and pushover analyses are performed to evaluate the behavior of the controlled structures. The analyses show that for each of the two case studies, the acceleration–displacement capacity spectrum conforms to the performance objectives curve. This finding confirms the validity of the proposed design approach and the effectiveness of the new hysteretic device in resisting lateral forces.

This paper gives a new insight into the linear dynamic behavior of one-storey eccentric systems, with particular attention devoted to provide a comprehensive physically based formulation of the maximum corner displacement magnification, which involves three contributions (translational response, torsional response and their combination). It is shown that the largest magnifications, which mainly occur for the class of torsionally-flexible systems, are due to the translational contribution which is caused by the shift of the fundamental period of the eccentric system with respect to that of the equivalent not-eccentric system. A simplified method for the estimation of the maximum corner displacement under seismic excitation, based on the physical properties of the eccentric system, is finally proposed.

Over the years, the belief that the first mode of vibration governs the seismic response of shear-type frame structures has been widely accepted and proved to be effective for preliminary structural design. Indeed, most of the actual seismic design procedures are based on drift profiles which are typically an approximation of the shape of the fundamental mode of vibration. In this paper, an analytical study on the dynamic properties of multi-storey shear-type frames is carried out with the purpose of precisely identifying the contribution of the modes of vibration to the seismic response of such structures, both in terms of maximum inter-storey displacement profiles (which govern the beams and columns maximum actions) and maximum inter-storey velocity profiles (which govern the viscous dampers maximum forces, of fundamental importance for building structures equipped with additional viscous dampers). A new parameter, referred to as Seismic Modal Contribution Factor, which represents the contribution of the generic mode to the seismic response of the structure, is introduced. With respect to the well-known Modal Contribution Factor, grounded on the concept of modal static response, the Seismic Modal Contribution Factor explicitly takes into account also the dynamic nature of the response due to earthquake excitation. The Seismic Modal Contribution Factor could be a meaningful parameter to be implemented in a professional structural design software and used in conjunction with the common modal participating mass ratios to identify the number of modes to be included in the analyses.

The non-linear seismic response of in-plan asymmetric systems has been extensively studied since the 1980s. Nevertheless, most of the research effort has been devoted to the study of specific asymmetric buildings and, even though relevant progresses have been achieved in the understanding of the complex translational-to-torsional coupled response of such systems, a complete understanding of the seismic behaviour of in-plan asymmetric buildings is still missing. In this paper a systematic study of the seismic response of linear and non-linear one-storey asymmetric structures is presented. A mixed analytical–numerical approach, the so-called “alpha” method, is used to investigate the non-linear seismic response of a wide range of in-plan asymmetric structures with the aim of providing general trends of behavior.

The development of performance-based design methodologies and displacement-based approaches requires a special attention to the correct definition of the seismic input to be used for the analyses and in particular of reliable displacement response spectra. In this paper, the characterization of the schematized response spectra has been deeply investigated with the purpose of obtaining a more accurate displacement design spectrum and a simple but efficient criterion for the selection of recorded ground motions. First, a correlation analysis between peak ground motions parameters and other intensity measures has been performed. The independent parameters which are both necessary and sufficient for an engineering characterization of the seismic input at a specific site have been identified as the three peak parameters ( PGA , PGV and PGD ). Second, the amplification factors have been modeled as random processes in order to identify their fundamental properties in terms of mean values and variability. Based on the obtained results, an updated formulation for the design displacement spectrum is proposed and compared with that suggested by the Eurocode 8. Finally, a criterion for the selection of recorded ground motions for time-history analyses is presented.

This paper introduces a special lateral-resisting element, herein referred to as Crescent Shaped Brace (CSB). CSBs are characterised by a geometrical configuration which is ‘‘ad hoc’’ defined in order to provide the structure with prescribed multiple seismic performances, within the performance based seismic design framework. In detail, the present work gives analytical, numerical and limited experimental results in order to provide a Comprehensive understanding of the mechanical behaviour of these devices, with special attention devoted to the seismic design purposes.

Most of the actual formulas available in both scientific literature and seismic codes for the damping reduction factors to be used for the seismic design of highly damped structures are derived through statistical analyses of the time-history response of SDOF systems subjected to earthquake accelerograms. In this study, analytical formulations of the damping reduction factor based on power spectral density functions are derived and compared with the main formulas available in the scientific literature. The surface ground motion is modelled with the Kanai–Tajimi power spectral density, i.e. as an “ideal white noise” at the bedrock filtered through the soil deposit. It is found that the ratio between the ground predominant period and the structure fundamental period significantly influences the reduction due to high damping and that, in some cases (e.g. stiff low-rise buildings), the actual code formulations may become unconservative. Finally, simple code-like formulas for the estimation of the damping reduction factor accounting for the ratio between the ground predominant period and the structure fundamental period are proposed.

In the last decades, the use of added viscous dampers for the mitigation of the effects due to the seismic action upon the structural elements has been worldwide spread. In this respect, several design methods aimed at sizing the viscous dampers to be inserted in building structures have been proposed. Among others, some of the authors proposed a five-step procedure which guides the practical design from the choice of a target reduction in the seismic response of the structural system (with respect to the response of a structure without any additional damping device), to the identification of the corresponding damping ratio and the mechanical characteristics (i.e. the damping coefficient values for chosen damping exponent, the oil stiffnes, the maximum damper forces) of the commercially available viscous dampers. The procedure requires the development of numerical simulations for the evaluation of the peak inter-storey velocity profiles, necessary for the evaluation of the damper forces. In the present paper a comprehensive study on the interstorey velocity profiles developed in shear-type building structures under seismic excitation is conducted with the purpose of deriving analytical formulae for their estimation. The analytical estimations of the peak inter-storey velocities are then used to simplify the original five-step procedure leading to a direct (i.e. fully analytical) procedure. The direct procedure is suitable for the preliminary design of the added viscous dampers, in particular for practitioners not dealing everyday with the design of added viscous dampers. In the last decades, the use of added viscous dampers for the mitigation of the effects due to the seismic action upon the structural elements has been worldwide spread. In this respect, several design methods aimed at sizing the viscous dampers to be inserted in building structures have been proposed. Among others, some of the authors proposed a five-step procedure which guides the practical design from the choice of a target reduction in the seismic response of the structural system (with respect to the response of a structure without any additional damping device), to the identification of the corresponding damping ratio and the mechanical characteristics (i.e. the damping coefficient values for chosen damping exponent, the oil stiffnes, the maximum damper forces) of the commercially available viscous dampers. The procedure requires the development of numerical simulations for the evaluation of the peak inter-storey velocity profiles, necessary for the evaluation of the damper forces. In the present paper a comprehensive study on the inter-storey velocity profiles developed in shear-type building structures under seismic excitation is conducted with the purpose of deriving analytical formulae for their estimation. The analytical estimations of the peak inter-storey velocities are then used to simplify the original five-step procedure leading to a direct (i.e. fully analytical) procedure. The direct procedure is suitable for the preliminary design of the added viscous dampers, in particular for practitioners not dealing everyday with the design of added viscous dampers.

Most of the actual seismic design procedures for structures equipped with viscous dampers requires the development of non-linear time-history analyses, which, even though of quite common use among the researchers, still represent a challenge for professional engineers. The paper describes a simple Equivalent Static Analysis procedure for the seismic design of building structures equipped with added viscous damper. The procedure, which is based on the fundamentals of the dynamics of multi-storey frame structures with added viscous dampers, is aimed at estimating the maximum forces in the viscous dampers as well as the peak internal actions in the structural members. The simplified procedure is applied for the design of a 3-storey reference steel frame analysed by other researchers.

The Crescent Shaped Brace (CSB) is a new steel dissipative device recently proposed by some of the authors within the context of Performance Based Seismic Design (PBSD). Contrarily to the conventional concentric stiff diagonal braces, the CSB allows the practical designer to choose the lateral stiffness independently from the yield strength of the device, due to its peculiar ad-hoc shape. In previous studies, analytical formulations and first experimental tests were described. In the present paper, the main results of further experimental tests conducted on scaled CSB specimens of full circular cross-section under both monotonic pseudo-static and cyclic loads are presented. The results of the experimental tests are compared with analytical formulations.

The seismic design of a two-storey precast reinforced-concrete building structure equipped with viscous dampers is presented in this paper with twofold purpose. The first goal is to verify the applicability of a practical procedure for the identification of the mechanical characteristics of the viscous dampers which allow to achieve target performance levels, originally proposed by the authors for moment-resisting building frames, also with reference to “pendular” structures. The second goal is to investigate the effectiveness of the use of viscous dampers (as compared with traditional lateral-resisting stiff braces) for the seismic design of precast not moment-resisting concrete structures.

Viscous dampers have widely proved their effectiveness in mitigating the effects of the seismic action upon building structures. In view of the large impact that use of such dissipative devices is already having and would most likely have soon in earthquake engineering applications, this article presents a practical procedure for the seismic design of building structures equipped with viscous dampers, which aims at providing practical tools for an easy identification of the mechanical characteristics of the manufactured viscous dampers which allow to achieve target levels of performances. Selected numerical applications are developed with reference to simple, but yet relevant, cases.

In the last recent years, structural monitoring has acquired an increasing importance in the diagnosis and control of buildings, especially for historical buildings whose preservation is essential to the safeguard of cultural heritage. The aim of this study is to introduce a standardized approach for the interpretation of the large amount of data acquired from a monitoring system of historic buildings. This approach is based on the definition of specific reference quantities (extrapolated from the recorded time series) able to characterize the main features of the structural response and the preliminary identification of the order of magnitudes of these quantities. These reference quantities may be collected in a database and may become fundamental for comparing the response of similar buildings. This type of analysis has been applied to the data obtained from the Static Structural Health Monitoring (SSHM) system of two significant Italian monuments: the Cathedral of Modena and the Two Towers of Bologna (Asinelli and the Garisenda Towers), North Italy. This paper provides, also, the preliminary results of the experimental data as obtained from the Dynamic Structural Health Monitoring (DSHM) of the Asinelli Tower conducted by the Italian National Institute of Geophysics and Volcanology (INGV).

In recent years, the seismic behaviour of structural systems composed of squat concrete walls has been the object of a renewed interest. This paper presents the results obtained in a large experimental campaign carried out as a joint effort between the University of Bologna and the Eucentre labs in Pavia. This effort aims at the assessment of the seismic performances of cellular structures composed of cast in situ sandwich squat concrete walls. In order to shed light upon the seismic behaviour (stiffness, strength, ductility, energy-dissipation) of such structures, a number of tests were performed upon two dimensional (3.0 m by 3.0 m) cast in situ sandwich squat concrete walls (with and without openings). In the experimental tests a number of horizontal in-plane loading cycles were imposed to the specimens, while the vertical load was kept constant. The results obtained have shown that the tested elements are characterized by: (i) high values of the maximum horizontal load applied to the structural systems (higher than the applied vertical load); (ii) residual bearing capacity with respect to the vertical loads, also when large lateral deformations are imposed; and (iii) high ductility.

The effectiveness of viscous dampers in mitigating the seismic excitation impacts upon building structures has been widely proved. Recently, with reference to the specific case of equal mass, equal stiffness, shear-type structures, the authors developed a direct practical procedure which gives the mechanical characteristics of the manufactured viscous dampers capable of providing the frame structure with a prescribed value of the first damping ratio. In this paper, a comprehensive rational framework is presented, which allows to formally extend the validity of the proposed procedure to the more realistic case of a generic moment-resisting frame structure. Also the influence of various lateral stiffness distributions is investigated.

The seismic behavior of flat-bottom silos containing grain-like material still presents strong uncertainties and current design codes tend to provide too conservative formulations for the estimation of the seismic actions. Over the years, some researchers focused on the dynamic behavior of such silos mainly through numerical investigations. Analytical formulations for the evaluation of the pressures exerted by the ensiled grain on the silo wall under seismic excitation were developed by Younan and Veletsos (J Struct Eng ASCE 124(1):62–70 1998) and, more recently, by Silvestri et al. (Bull Earthq Eng 10(5):1535–1560 2012). Experimental shaking-table tests were performed on silo specimens (Silvestri et al. EESD 2015, submitted), which showed good agreement with the Silvestri’s analytical formulations, even if some theoretical limits of validity were not satisfied. This has encouraged a complete revision and refinements of the theoretical framework, which is the object of this paper. In detail, the static and the dynamic actions exchanged between different grain portions and between the grain and the silo wall are idealised in a more physically consistent way. The analytical developments are carried out by means of simple free-body dynamic equilibrium equations. The refinements yield to a significant extension of the theoretical limits of validity and to a new set of analytical formulas for the wall pressures and for the wall shear and bending moment. A comparison of the analytical formulas with (i) the consolidated Janssen (Zeitschrift des vereines deutcher Ingenieure 39:1045–1049 1895) and Koenen (Centralblatt der Bauverwaltung 16:446–449 1896) theory for static design of silos and (ii) with the Eurocode 8 provisions for seismic design of silos and with the experimental results is also performed in order to (i) check the updated theoretical model in static conditions and (ii) verify the reliability of the different formulations in accelerated conditions, respectively. The refined theory confirms that the portion of ensiled material that interacts with the silo wall is significantly smaller than the effective mass suggested by Eurocode 8.

In this paper the authors investigate the dynamic performances of multi-degree-of-freedom shear-type structures equipped with manufactured viscous dampers. The system of added viscous dampers that leads to a mass proportional damping (MPD) matrix is identified as the one providing the best overall performance in terms of minimum top-storey response variance to a Gaussian band limited white noise input within the class of classically damped systems. A performance index developed by the authors and based upon modal damping ratios confirms that, within the class of Rayleigh damping systems, the MPD limiting case provides the largest damping effect. For a 6-d.o.f. system, a comparison with results available in literature also shows that the MPD system provides better performance than optimised (as suggested by other research works) non-classical damping schemes. A physical explanation for the optimum damping performance offered by MPD systems is also provided herein, together with clear indications about how ...

In the last few decades, the use of cast in situ reinforced concrete sandwich panels for the construction of low- to mid-rise buildings has become more and more widespread due to several interesting properties of this construction technique, such as fast construction and high thermal and acoustic performances. Nonetheless the level of knowledge of the structural behavior of systems made of squat reinforced concrete sandwich panels is still not so consolidated, especially with reference to the seismic response, due to the lack of experimental studies. In recent years, while various experimental tests have been conducted on single panels aimed at assessing their seismic capacity, only few tests have been carried out on more complex structural systems. In this paper, the experimental results of a series of shaking-table tests performed on a full-scale 3-storey building are presented in detail. The main goal is to give to the scientific community the possibility of develop independent interpretation of these experimental results. An in-depth interpretation of the discrepancies between the analytical predictions and the experimental results is beyond the objective of this paper and is still under development. Nonetheless, preliminary interpretations indicate that both the stiffness and the strength of the building under dynamic excitation appear quite superior with respect to those expected from the results of previous pseudo-static cyclic tests conducted on simple specimens. The authors acknowledge the financial support received from the European Community’s Seventh Framework Program [FP7/2007-2013] under grant agreement nº 227887 for the SERIES Project.

This research work focuses on the analysis of the hysteretic seismic behaviour of inelastic SDOF systems equipped with viscous dampers. In detail, it is aimed at obtaining a practical tool useful for the seismic design of building structures with added dampers, within the framework of the traditional seismic design based on ductility. The objective is to evaluate the appropriate force reduction factor for highly damped (i.e. damping ratio greater than 5 %) SDOF systems able to guarantee a prescribed level of structural safety.

In previous research works, the authors have identified a key system parameter which controls the maximum rotational response under free and forced vibrations of one-storey linear-elastic systems representative of asymmetric seismic isolated building structures. This parameter (called “ALPHA”) has also led to the identification of a simplified procedure (called “ALPHA method”) for the estimation of the maximum rotational response of such systems. Main goal of this paper is to verify the properties of the ALPHA parameter and the predictive capabilities of the ALPHA method, when applied to one-storey systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of 11600 numerical simulations developed with reference to different representative structures subjected to historically recorded ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and the ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response, (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation, and (c) indicate that the ALPHA method is only weakly sensitive upon the period of vibration of the structure. The paper also introduces a simple code-like provision for conservative estimations of the maximum rotation developed under seismic input by asymmetric structures based upon the confidence interval concepts.

This paper presents the results of an exhaustive parametric analysis which compares the performances offered by various systems (which lead to both classical and non classical damping matrices) of added viscous dampers in shear type structures. The aim of the research work here presented is the identification of the system of added viscous dampers which maximizes the dissipative properties under an equal “total size” constraint. The choice of the systems of added viscous dampers considered in the comparison is carried out both using a physically based approach (based upon the properties of classically damped systems) and a numerical approach (based upon the use of genetic algorithms). The comparison is carried out through the numerical evaluation of the dynamic response of representative shear-type structures to both stochastic and recorded earthquake inputs. The results indicate that a damping system based upon the mass proportional damping component of the Rayleigh viscous damping matrix (referred to as MPD system) is capable of optimising simultaneously a number of different performance indexes, providing the best “overall” damping performances. The MPD system is characterised by viscous dampers (a) which connect each floor to a fixed point and (b) which are sized proportionally to the corresponding floor mass.

This paper presents an illustrative example of the advantages offered by inserting added viscous dampers into shear-type structures in accordance with a special scheme based upon the mass proportional damping (MPD) component of the Rayleigh viscous damping matrix. In previous works developed by the authors, it has been widely shown that, within the class of Rayleigh damped systems and under the "equal total cost" constraint, the MPD system provides best overall performance both in terms of minimising top-storey mean square response to a white noise stochastic input and maximising the weighted average of modal damping ratios. A numerical verification of the advantages offered by the application of MPD systems to a realistic structure is presented herein with reference to a 4-storey reinforced-concrete frame. The dynamic response of the frame subjected to both stochastic inputs and several recorded earthquake ground motions is here analysed in detail. The results confirm the good dissipative properties of MPD systems and indicate that this is achieved at the expense of relatively small damping forces.

Different design strategies, within the Performance Based Design framework, have been proposed through the years in order to design a structure of high seismic performances, including seismic isolation, viscous dampers and hysteretic dampers. A new lateral-resisting device, referred to as Crescent Shaped Brace (CSB), has been recently proposed to be used for the story isolation of multi-story frame structures. The main property of the CSB is that its lateral stiffness is uncoupled from its yield strength so that the practical designer may choose them independently, unlike conventional diagonal braces whose lateral stiffness is directly proportional to the yield strength. Past studies were devoted to the development of analytical formula for the design of such devices and to the study (through numerical simulations) of their non-linear response. In the present paper, the main results of experimental tests conducted on scaled CSB specimens (monotonic tests, cyclic tests, pseudo-static tests) are presented in order to assess the seismic behavior of such devices. The results of the experimental tests are compatible with the analytical predictions.