Optimum Design of Passive Control Devices for Reducing the Seismic Response of Twin-Tower-Connected Structures

Based on the 3-single-degree-of-freedom (SDOF) model of twin-tower structures linked by the sky-bridge and passive control devices, the frequency functions and the vibration energy expressions of the structures are derived by using the stationary white noise as the seismic excitation. The analytical formulas for determining the connecting optimum parameters of viscoelastic damper (VED) represented by the Kelvin model and the viscous fluid damper (VFD) represented by Maxwell model are proposed using the principle of minimizing the average vibration energy of either the single tower or the twin tower. Three pairs of representative numerical examples of twin-tower-connected structures are used to verify the correctness of the theoretical approach. The optimum parametric analysis demonstrates that the control performance is not sensitive to damper damping ratio of VED and relaxation time of VFD. The effectiveness of the proposed control strategies based on the 3-SDOF models is also proved to be applicable to multi-degree-of-freedom systems. The theoretical analysis and numerical results indicate that the seismic response and vibration energy of the twin-tower-connected structures are mitigated greatly under the two types of dampers. The presented control strategies of VED and VFD can help engineers in application of coupled structures.     

Seismic Demand on Shear Panel Dampers Installed in Steel-Framed Bridge Pier Structures

This study is aimed at investigating the demand on shear panel dampers (SPDs) installed in steel structures under strong earthquake motions to serve as guidance for the recommended capacity of SPDs in seismic design. For this purpose, an extensive dynamic analysis is carried out on steel bridge pier structures with SPD devices. To describe the restoring force characteristics of SPDs, the analysis uses a newly developed combined hardening model based on experimental data. The seismic demands made on SPD devices are examined and then summarized to give recommended values for determining the necessary deformation capacity of SPDs.     

Displacement-Based Design of an Energy Dissipating System for Seismic Upgrading of Existing Masonry Structures

A displacement-based method for the design of an energy dissipating system is proposed in this article. The device, which is composed of added concrete walls equipped with hysteretic Added Damping and Stiffness (ADAS) dampers, is aimed at upgrading the seismic behavior of existing masonry structures. The design method is based upon a simplified model of the overall structure-dissipating system. The proposed displacement-based design procedure was tested by means of inelastic response-time history analyses considering different masonry structures. The results of the analyses were compared with the seismic behavior expected from the design.     

Physical and Numerical Approaches for the Optimal Insertion of Seismic Viscous Dampers in Shear-Type Structures

This article 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 numerical approach (based upon the use of genetic algorithms) and a physically based approach (based upon the properties of classically damped systems). 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 obtained using both approaches 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 optimizing 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.     

SEISMIC RESPONSE CONTROL OF CABLE-STAYED BRIDGE USING DIFFERENT CONTROL STRATEGIES

The goal of this research is to study the effect of cable vibration through a number of control cases of a cable-stayed bridge. In order to consider the complicated dynamic behaviour of the full-scale bridge, a three-dimensional numerical model of the MATLAB-based analysis tool has been developed by the complete simulation of the Gi-Lu bridge. The dynamic characteristics of cables in the cable-stayed bridge are verified between the field experiment and the result from numerical simulation using geometrically nonlinear beam elements in MATLAB program. Three types of control devices are selected to reduce the response of the bridge deck which includes: actuators, viscous-elastic dampers with large capacity, and base isolations. Moreover, two types of control devices, MR dampers and viscous dampers, are installed either between the deck and cables and/or between two neighbouring cables for controlling the cable vibration. A modified bi-viscous model combined with convergent rules is used to describe the behaviour of MR dampers. Finally, through evaluation criteria the control effectiveness on the cable-stayed bridge using different control strategies is examined.     

Passive Control of Steel Storage Racks for Parmigiano Reggiano Cheese under Seismic Accelerations

Recent earthquakes in Italy resulted in the collapse of steel storage racks for aging Parmigiano Reggiano cheese, damaging a huge number of cheese wheels. In order to improve the seismic performance of surviving racks with low-budget solutions, when replacement cannot be considered, we investigate the possibility of using viscous dampers that connect the racks to a surrounding structure. Time-history analyses with natural and spectrum-compatible artificial accelerograms allow one to determine the optimal damping factor that reduces both the stress in the rack and the actions transmitted to the constraining structure. Results confirm the noteworthy benefits of the proposed retrofitting system.     

Tuned Mass Damper Positioning Effects on the Seismic Response of a Soil-MDOF-Structure System

Tuned mass dampers (TMDs) are effective structural vibration control devices. However, very little research is available on the experimental investigation of TMDs and their performance in systems undergoing dynamic soil-structure interaction. Geotechnical centrifuge tests are conducted to investigate story positioning effects of single and multiple TMDs in a soil-MDOF-structure system. The criteria for optimal story positioning will be established, and it is shown that story positioning influences TMD performance more than the number of TMDs used. Non-optimal story positioning was found to have the potential of reducing damping efficiency, amplifying peak structural response, and inducing lengthier high-intensity motion.     

Shaking Table Tests of a Structure Equipped with Superelastic Dampers

The energy dissipation capacity of the NiTi alloy was evaluated as part of a series of shake table tests. A superelastic damper was developed to take advantage of the hysteretic energy dissipation associated with this type of shape memory alloy. Each device was tested at different intensity levels. A vertical steel cantilever with 600 kg mass on top was subjected to a series of ground motions with different spectral characteristics. The dampers were placed as part of a tie system, restraining the horizontal movement of the top mass. The devices showed stable hysteretic behavior allowing for energy dissipation.     

Seismic Overstrength of Shear Walls in Parking Structures with Flexible Diaphragms

The objective in current design practice for parking structures is that energy is dissipated through the formation of plastic hinges at the base of shear walls while floor diaphragms remain elastic and are vertically supported by a combination of shear walls and gravity resisting columns. Unfortunately, this objective is not always achieved due to inaccuracies in current methods for calculating demands on shear walls and in calculating the capacity of shear walls (IBC 2003 International Building Code. International Conference of Building Officials. Whittier, CA.  [Google Scholar], ACI code). When demands are overestimated and capacity underestimated, then diaphragm can fail prior to flexural yield of shear walls as was observed in several parking structures in the 1994 Northridge earthquake.

Eigenvalue and inelastic dynamic response analyses were performed in order to investigate the effects of diaphragm flexibility on wall responses and of wall overstrength on diaphragm responses. The elongated periods of parking structures due to diaphragm flexibility were found to significantly decrease seismic force demand on shear walls relative to what is calculated using codes of practice in which diaphragms are assumed to be rigid. This leads to the over design of shear walls, which further compounds the problem by preventing the flexural yielding of these walls and thereby driving inelastic response to diaphragms. Various degrees of diaphragm flexibility, shear wall layout, seismic zone, and the number of stories were considered in these analyses.

Inelastic static pushover analyses were preformed to investigate the design and capacity evaluation of shear walls. The results illustrate that the shear capacity of walls may be close to twice that calculated by codes of practice. The largest overstrengths were observed in shear walls with low height-to-length ratios in which a significant portion of the lateral load was taken by direct strut action to the foundation and without placing demands on the longitudinal tension reinforcement in the shear walls. The article concludes that methods in codes of practice for calculating shear wall demands and capacities need to be improved if good seismic performance of parking structures is to be achieved.     

Estimation of the Displacement of Viscously Damped Pinching Hysteretic Structures Subjected to Near-fault Ground Motions

To fulfill a displacement-based design or response prediction for nonlinear structures, the concept of equivalent linearization is usually applied, and the key issue is to derive the equivalent parameters considering the characteristics of hysteretic model, ductility level, and input ground motions. Pinching hysteretic structures subjected to dynamic loading exhibit hysteresis with degraded stiffness and strength and thus reduced energy dissipation. In case of excitation of near-fault earthquake ground motions, the energy dissipation is further limited due to the short duration of vibration. In order to improve the energy dissipation capability, viscous-type dampers have been advantageously incorporated into these types of structures. Against the viscously damped pinching hysteretic structure under the excitation of near-fault ground motions, this study aims to develop a seismic response estimation method using an equivalent linearization technique. The energy dissipation of various hysteretic cycles, including stationary hysteretic cycle, amplitude expansion cycle, and amplitude reduction cycle, is investigated, and empirical formulas for the equivalent damping ratio is proposed. A damping modification factor that accounts for the near-fault effect is introduced and expanded to ensure its applicability to structures with damping ratios less than 5%. An approach for estimating the maximum displacement of a viscously damped pinching hysteretic structure, in which the pinching hysteretic effect of a structure and the near-fault effect of ground motions are considered, is developed. A time history analysis of an extensive range of structural parameters is performed. The results confirm that the proposed approach can be applied to estimate the maximum displacement of a viscously damped pinching hysteretic structure that is subjected to near-fault ground motions.     

HYSTERESIS AND PARAMETER ESTIMATION OF MDOF SYSTEMS BY A CONTINUOUS-TIME LEAST SQUARES METHOD

Civil structures could undergo hysteresis cycles due to cracking or yielding, when subjected to severe earthquake motions. Seismic instrumentation, structural monitoring and system identification techniques have been used in the past years to assess civil structures under lateral loads. The present research makes use of a continuous-time Least Squares Method, modified herein to consider the nonlinear behaviour of the structure. The proposed on-line algorithm simultaneously estimates the hysteretic component and structural parameters such as damping and stiffness of a shear building structure. Simulations are carried out using the El Centro and the Mexico City seismic records. Fair convergence speed is obtained for the identification of the parameters and the estimation of the hysteretic component.     

Seismic Fragility Evaluation of RC Frame Structures Retrofitted with Controlled Concrete Rocking Column and Damping Technique

Acceleration response of simple yielding structure is proportional to its own weight, but it is limited by yield strength. Thus, using rocking columns that reduces global yield strength, a limited acceleration is achieved. However, the displacement becomes large due to lower strength and higher inelasticity, but it can be controlled by adding damping. Performing fragility analyses, the seismic response of R/C frame structures with rocking columns and viscous dampers is investigated. Near field MCEER ground motions are considered. The analyses show that the story accelerations are reduced by using rocking columns, while the story displacements are controlled by using viscous dampers.     

DIAPHRAGM ACTION OF SANDWICH PANELS IN PIN-JOINTED STEEL STRUCTURES: A SEISMIC STUDY

The paper focuses on the seismic response of steel pin-jointed frames braced by lightweight cladding panels. In particular, with the aim to investigate the performance of such a structural scheme when acting as a dissipative system, a wide numerical study has been developed. It is based upon available shear tests on screwed sandwich panels, whose experimental cyclic responses are properly incorporated into a scope-oriented, computer program. The goal is firstly to check the possibility of using cladding panels as shear diaphragms in seismic areas and then to assess an appropriate design behaviour factor, accounting for their actual hysteretic response. Key findings from the nonlinear dynamic analyses are: (1) a portal frame steel building in a low-medium seismicity zone may be braced by common cladding panels, completely avoiding the use of other bracing systems; (2) this structural solution, if compared with a conventional one, appears to be more efficient and cost-effective, giving rise to a weight saving which, in the case under examination, reaches a value of about 20%; (3) on the basis of the numerical study a design behaviour factor q _d =2 seems to be realistic for such a structural typology.     

The Effect of Stiffened Floor and Roof Diaphragms on the Experimental Seismic Response of a Full-Scale Unreinforced Stone Masonry Building

An extensive experimental program was carried out at EUCENTRE, within a research project on the evaluation and reduction of the seismic vulnerability of stone masonry structures. The main part of the experimental program has been devoted to the shaking table tests on three full-scale, two-story, single-room prototype buildings made of undressed double-leaf stone masonry. The first building tested was representative of existing unreinforced stone masonry structures with flexible wooden diaphragms, without any specific anti-seismic design nor detailing. In the second and third buildings, strengthening interventions were simulated on structures theoretically identical to the first one, improving wall-to-floor and wall-to-roof connections and increasing diaphragm stiffness. In particular, in the third specimen, steel and r.c. ring beams were used to improve the diaphragm connection to the walls and collaborating r.c. slab and multi-layer plywood panels were used to stiffen floor and roof diaphragms, respectively. This article describes the strengthening interventions applied to the third building prototype and presents the experimental results obtained during the shaking table tests. The results obtained permitted the calibration of a macroelement model representative of the nonlinear behavior of the structure.     

A Five-Step Procedure for the Dimensioning of Viscous Dampers to Be Inserted in Building 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.     

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.     

An Enhanced Finite Element Macro-Model for the Realistic Simulation of Localized Cracks in Masonry Structures: A Large-Scale Application

ABSTRACT

Finite element macro-modeling approaches are widely used for the analysis of large-scale masonry structures. Despite their efficiency, they still face two important challenges: the realistic representation of damage and a reasonable independency of the numerical results to the used discretization. In this work, the classical smeared crack approach is enhanced with a crack-tracking algorithm, originating from the analysis of localized cracking in quasi-brittle materials. The proposed algorithm is for the first time applied to a large-scale wall exhibiting multiple shear and flexural cracking. Discussion covers structural aspects, as the response of the structure under different assumptions regarding the floor rigidity, but also numerical issues, commonly overlooked in the simulation of large structures, such the mesh-dependency of the numerical results.     

Numerical Investigation of Residual Strength and Energy Dissipation Capacities of Corroded Bridge Members Under Earthquake Loading

Recent damage examples of aged steel bridge infrastructures around the world are so alarming. They intensified the importance of careful evaluation of existing structures for the feasibility of current usage and to ensure public safety. Corrosion and fatigue cracking may be the two most important types of damages in aging structures. Furthermore, recent earthquakes demonstrated potential seismic vulnerability of some types of steel bridges. Corrosion and its effects can trigger the damages caused by earthquakes, and it will be vital to understand the behavior of existing steel bridges which are corroding for decades in future severe seismic events as well. This article comprises the results of nonlinear FEM analysis of many actual corroded plates with different corrosion conditions and proposes a simple and reliable methodology to estimate remaining seismic strength and energy dissipation capacities by measuring only the minimum thickness of a corroded surface, which can be used to make rational decisions about the maintenance management plan of steel infrastructures.     

Distributed Tuned Mass Dampers for Multi-Mode Control of Benchmark Building under Seismic Excitations

The effectiveness of the multi-mode control of seismically excited building installed with distributed multiple tuned mass dampers (d-MTMDs) is investigated by comparing dynamic response with the other controllers, such as passive friction dampers, semi-active dampers, single tuned mass damper (STMD), and multiple tuned mass dampers (MTMDs-all.top), both installed at the top of the building, and arbitrarily distributed MTMDs (ad-MTMDs). It is concluded that the d-MTMDs exhibit improved performance as compared with the STMD, MTMDs-all.top, and ad-MTMDs. The d-MTMDs are also convenient to install owing to the reduced space requirements, being placed at various floors.     

SEISMIC DESIGN AND ANALYSIS OF A KNEE BRACED FRAME BUILDING

A five-storey steel frame incorporating dissipative knee elements is designed using the Eurocode 8 pushover analysis method. The non-linear analysis makes use of a novel knee element model capable of accurately simulating the bending and shear behaviour observed in full-scale tests. The performance of the structure is assessed using non-linear time-history analysis. This shows that the knee elements can be designed to yield under small earthquakes or early in a strong one (maximising their energy dissipation) while still being able to withstand a large event without collapse. Knee elements thus have the potential to give excellent seismic performance in steel framed structures. The time history analysis results are compared to those obtained with the three different pushover analysis methods (Eurocode 8, FEMA 356 and ATC 40). The FEMA 356 method, which includes a more accurate representation of the structure's significant post-yield stiffness, gave the closest agreement with the time history analyses, while the Eurocode 8 method gave rather conservative results and the ATC 40 method appears non-conservative for this type of structure.