Antonio Di Cesare

Jet-Pacs Project: Dynamic Experimental Tests and Numerical Results Obtained for a Steel Frame Equipped with Hysteretic Damped Chevron Braces

The experimental and numerical results obtained by Research Units of the University of Basilicata and University of Calabria for a steel frame, bare or equipped with metallic yielding hysteretic dampers (HYDs), are compared. The shaking table tests were performed at the Structural Laboratory of the University of Basilicata within a wide research program, named JETPACS (“Joint Experimental Testing on Passive and semiActive Control Systems”), which involved many Research Units working for the Research Line 7 of the ReLUIS (Italian Network of University Laboratories of Earthquake Engineering) 2005–2008 project. The project was entirely founded by the Italian Department of Civil Protection. The test structure is a 1/1.5 scaled two-story, single-bay, three-dimensional steel frame. Four HYDs, two for each story, are inserted at the top of chevron braces installed within the bays of two parallel plane frames along the test direction. The HYDs, constituted of a low-carbon U-shaped steel plate, were designed with the performance objective of limiting the inter-story drifts so that the frame yielding is prevented. Two design solutions are considered, assuming the same stiffness of the chevron braces with HYDs, but different values of both ductility demand and yield strength of the HYDs. Seven recorded accelerograms matching on average the response spectrum of Eurocode 8 for a high-risk seismic region and a medium subsoil class are considered as seismic input. The experimental results are compared with the numerical ones obtained considering an elastic-linear law for the chevron braces (in tension and compression), providing that the buckling be prevented, and the Bouc-Wen model to simulate the response of HYDs.

Post-Tensioned Glulam Beam-Column Joints with Advanced Damping Systems: Testing and Numerical Analysis

This article describes tests investigating a feasible source of passive damping for post-tensioned glue-laminated (glulam) timber structures. This innovative structural system adapts precast concrete PRESSS technology [Priestley et al., 1999] to engineered wood products combining the use of post-tensioned tendons with large timber members. Current testing is aimed at further improvement of the system through additional energy dissipation. Testing has favorably compared glue-laminated timber (not previously implemented in this way) with laminated veneer lumber (LVL) used in New Zealand. After initial benchmark testing with post-tensioning only, a simple, minimally invasive and replaceable type of hysteretic damper was added.

Experimental and numerical behaviour of hysteretic and visco-recentring energy dissipating bracing systems

An extensive program of shaking table tests under the name Project JetPacs (Joint experimental testing on Passive and semi active control systems) has been developed with the goal of assessing the effectiveness of seven different passive and semi-active energy dissipating bracing (EDB) systems in controlling the seismic vibrations of framed buildings. The experimental program, carried out considering a 3D 1/1.5 scaled steel frame, was entirely funded by the Italian Department of Civil Protection as part of the RELUIS 2005–2008 project. The following article focuses on the experimental tests carried out considering only two EDB systems, based on hysteretic dampers (HD) and visco-recentring devices (SMA + VD) respectively. Specially shaped low carbon steel plates were used to provide hysteresis in the HD based devices, while the innovative SMA + VD visco-recentring system was made up of a combination of viscous dampers (VD) and shape memory alloy (SMA) wires. In this paper a displacement focused design procedure based on non linear static analysis has been proposed in order to evaluate the mechanical characteristics of both types of energy dissipating device. The aim of this design procedure is to limit inter-storey drifts after frame yielding. In order to assess the robustness of the design procedure and to evaluate the effects of the viscous and recentring components, two different sets of HD and SMA + VD devices characterized by slight alterations in the mechanical properties have been tested and compared. Finally, the experimental seismic response of the structure equipped with and without the HD and SMA + VD elements is reported and compared with numerical results obtained using non linear time history analysis.

Assessment of the performance of hysteretic energy dissipation bracing systems

The advantages of passive supplemental dampers for performance enhancement of new and existing structures have been demonstrated extensively in the past. The big amount of experimental tests carried out all over the world on framed structures upgraded by energy dissipating bracing (EDB) systems based on hysteretic dampers (HDs), have shown their effectiveness in reducing seismic effects on buildings. The mechanical characteristics of the HDs in some cases may be different from those arising by the design procedure due to industrial tolerance or because of some damage suffered during previous earthquakes. In order to assess the robustness of this technique, in terms of capacity of seismic vibrations control even for significant changes in the mechanical characteristics of the EDB system respect to the design ones, in this paper experimental tests and parametric nonlinear time history analysis have been carried out changing the characteristics of the HD stiffness and strength. The experimental results refer to the shaking table tests performed at the Structural Laboratory of the University of Basilicata within a wide research program, named Joint Experimental Testing on Passive and semiActive Control Systems. The program has been completely funded by the Italian Department of Civil Protection within the activity of the Research Line 7 of the ReLUIS (Italian Network of University Laboratories of Earthquake Engineering) 2005–2008 project. A displacement-focused design procedure has been considered to evaluate the mechanical characteristics of the dissipating system, with the aim of limiting inter-storey drifts after frame yielding. From the experimental point of view, two design solutions have been tested for chevron braces equipped with HD, assuming the same stiffness but different values of both ductility demand and yield strength of the HDs. Moreover, parametric studies have been performed through numerical simulations. This paper provides an overview of the experimental set up and briefly summarizes the experimental outcomes and the comparison with the results of numerical nonlinear time history analysis. Moreover, the results of the parametric analysis for the assessment of the performances of the dissipating system in controlling structural response are presented.

Seismic Retrofit of Reinforced Concrete Frame Buildings with Hysteretic Bracing Systems: Design Procedure and Behaviour Factor

This paper presents a design procedure to evaluate the mechanical characteristics of hysteretic Energy Dissipation Bracing (EDB) systems for seismic retrofitting of existing reinforced concrete framed buildings. The proposed procedure, aiming at controlling the maximum interstorey drifts, imposes a maximum top displacement as function of the seismic demand and, if needed, regularizes the stiffness and strength of the building along its elevation. In order to explain the application of the proposed procedure and its capacity to involve most of the devices in the energy dissipation with similar level of ductility demand, a simple benchmark structure has been studied and nonlinear dynamic analyses have been performed. A further goal of this work is to propose a simplified approach for designing dissipating systems based on linear analysis with the application of a suitable behaviour factor, in order to achieve a widespread adoption of the passive control techniques. At this goal, the increasing of the structural performances due to the addition of an EDB system designed with the above-mentioned procedure has been estimated considering one thousand case studies designed with different combinations of the main design parameters. An analytical formulation of the behaviour factor for braced buildings has been proposed.

RINTC PROJECT: NONLINEAR ANALYSES OF ITALIAN CODE-CONFORMING BASE-ISOLATED BUILDINGS FOR RISK OF COLLAPSE ASSESSMENT

This paper reports on the results of an ongoing Research Project aimed at computing the risk of collapse of new buildings conforming to the Italian Seismic Design Code. Companion papers describe the overall Research Project, funded by the Italian Civil Protection Department (DPC), its different areas of application (reinforced concrete, masonry, steel buildings, etc), the overall seismic risk calculation procedure and the ground motion selection process followed to identify the recorded ground motions used for the multi-stripe analyses for twenty different ground motion intensities. This paper describes the nonlinear analyses on a number of reinforced concrete (RC) buildings equipped with different isolation systems. Base isolation is one of the most widespread techniques currently used for seismic protection of buildings and their equipment. The present study presents the results for a six storey RC building isolated with different base seismic isolation systems based on: (i) rubber bearings; (ii) rubber bearings and flat sliding bearings; (iii) friction pendulum systems. The isolation systems have been designed according to current Italian seismic code considering different suitable combinations of stiffness and damping or friction. The failure conditions of the proposed isolation systems and peak responses of building have been evaluated through nonlinear dynamic analysis performed under bidirectional ground motions, considering twenty couple of natural earthquakes for ten different peak ground accelerations. The results point out that all isolation systems work effectively in limiting the building damage for seismic intensities much higher than the design earthquake. On the other hand, they have a little margin to collapse beyond the design seismic intensity.

Electromagnetic Sensing Techniques for Non-Destructive Diagnosis of Civil Engineering Structures

Health Assessment Methods (HAM) and Structural Health Monitoring (SHM) aim to improve the standard of knowledge regarding the safety and maintenance of structures and infrastructure acquiring information about geometrical, mechanical and dynamical characteristics of structures. In earthquake-prone areas, this activity has the double aim of assessing the buildings structural integrity and extracting information regarding their response during a seismic event in order to define appropriate activities for risk mitigation. A number of factors afflict buildings and infrastructure safety in seismic areas:  Outdated codes of practice: a significant number of highly urbanized areas are present globally, where a high percentage of structures have been designed and erected considering only gravity loading.  The age of the structures and the real in-situ performance of construction material significantly affect their overall behaviour.  Structural deficiencies such as poor material qualities and/or degradation of structural materials (rust, spalling etc.), inadequate construction detailing, low levels of ductility, brittle collapse mechanisms. The seismic assessment of structures is performed in terms of the estimation of the earthquake intensity that would lead to a certain damage condition and/or collapse. The assessment of the seismic vulnerability of existing buildings is generally based on the knowledge of building characteristics and through a complex analysis of the possible collapse mechanisms in order to identify the most probable failure for the given structure (as example: Ansari, 2005; Douglas, 2007; Moustafa et al., 2010). The methodological approach for the evaluation of a structure resistance is represented in Figure 1 where structural knowledge obtained through a series of test assessments is needed in order to define vulnerability and thus design suitable retrofit strategies.