Equivalent Force Control Method for Real-time Testing of Nonlinear Structures

The equivalent force control (EFC) method replaces numerical iteration with a feedback control strategy to solve the nonlinear equations of motion using an implicit integration method for real-time substructure tests (RSTs). The method, however, requires the conversion of the equivalent forces to structural displacements using a conversion matrix. It is demonstrated in this article that with the use of a proportional-integral (PI) controller for the EFC, one has the convenience of choosing the initial stiffness matrix of a structure to construct the conversion matrix regardless of the properties and degree of nonlinearity of the system. The stability condition of the EFC using a PI controller has been derived with the Routh stability criterion. Methods for designing and tuning a PI controller for RST using EFC have been presented and excellent system performance has been obtained from numerical simulations and actual tests. The simulation results showed that the EFC method using a PI controller and the initial stiffness matrix to construct the conversion matrix can deliver excellent performance even for structural systems that develop a severe strain-softening behavior. Its superiority over iteration method proposed by Jung et al. [2007] Jung, R. Y., Shing, P. B., Stauffer, E. and Thoen, B. 2007. Performance of a real-time pseudo dynamic test system considering nonlinear structural response. Earthquake Engineering and Structural Dynamics, 36: 1785–1809.  [Google Scholar] was demonstrated through numerical simulation. This provides an efficient means to test nonlinear multiple-degrees-of-freedom structures.     

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.     

Transverse Seismic Behavior Studies of a Medium Span Cable-Stayed Bridge Model with Two Concrete Towers

Due to lack of investigation on nonlinear seismic behavior of cable-stayed bridges under strong earthquake excitation, the concrete towers, as the main gravity-carrying component, are usually required to remain nearly elastic. However, in order to achieve this high seismic performance objective, the reinforcement ratio of the tower legs and the tower struts need to be greatly increased in addition to its static loading requirement. To study the potential plastic region and possible failure mode of the cable-stayed bridge, a 1/20-scale full bridge model from a typical medium span concrete cable-stayed bridge was designed, constructed and tested on 4 linear shake tables using a site specific artificial wave in the transverse direction. Test results showed that the damage characteristics of the bridge model were as follows: (1) the severe damage was observed at the upper strut, with several steel bars fractured at both ends; (2) the repairable damage was observed at tower legs at the bottom and the middle part, with concrete cover spalling and exposure of steel bars; (3) the minimal damage was observed at the lower strut and the both sides of the side bents, with only slightly concrete spalling; and (4) no damage was observed at the auxiliary bents, the superstructure and the cables. Numerical results and test results were further compared and showed good agreement in low amplitudes of excitations. The test also proved that the bridge system was stable in flexural failure of upper struts, and had the negligible residual displacement subjected to high amplitudes of excitations.     

Seismic Resistant Effects of Composite Reinforcement on Rockfill Dams Based on Shaking Table Tests

In this article, the seismic resistant effect of a new reinforcement measure on rockfill dams, the Composite Reinforcement, is investigated based on 1-g large-scale shaking table tests. A 1 m high rockfill dam, which is scaled from the Yele rockfill dam in China, is tested by inputting a series of motions designed from a realistic Wenchuan earthquake record. The model dam is divided into two sections, with one section representing a normal rockfill dam (RD) while the other section employing the Composite Reinforcement (composite reinforced rockfill dam (CRRD)). The seismic-resistant effect of the Composite Reinforcement is investigated by comparing the recorded dynamic response of the two dams, in terms of the peak acceleration amplification factor distribution along the dam core, the crest settlement, and the visualized failure pattern. Results show that the application of the Composite Reinforcement can effectively mitigate the seismically induced deformation of the rockfill dam by reducing the peak crest acceleration and residual crest settlement by 18% and 55%, respectively, when subjected to a 0.5 g input motion. The investigation of the captured failure pattern on the two dams further indicates the effective reinforcing effect of the Composite Reinforcement on preventing the sliding failures of rockfill materials in the top 20% height of the dam body.     

Constituting Corporate Europe: A Study of Elite Social Organization

Abstract: This article explores the emerging shape and form of the European corporate community since 1996. We examine the cohesion of corporate Europe through the network of interlocking corporate directorates and memberships in the European Round Table of Industrialists. We focus on the unequal structure of representation; the interplay of national and transnational aspects of the network; the role of finance capitalists as a signpost of a regime of internationalized finance capital; and the embeddedness of corporate Europe in the global corporate network. Although the transnational European network gained in strength while national networks eroded, expansion of the European network did not negate a structure of representation favoring the northwest. Bankers became less dominant, yet industrialists with financial connections formed the core of the European corporate community, signaling a departure from national corporate communities centered upon banks. At the threshold of the current economic crisis, corporate Europe comprised the most integrated segment of the global corporate elite.     

Experimental Study of Large Area Suspended Ceilings

Damage of nonstructural components during past seismic events was shown to be not only a critical threat to life safety in extreme cases but also led to substantial reduction of functionality of buildings and other facilities. Because of the complex construction of nonstructural and architectural components, current standards provide only limited guidance for the seismic design. Suspended ceiling systems are among the less understood important nonstructural/architectural components in buildings for which design standards provide limited guidance. To understand the dynamic behavior of suspended ceiling systems, a series of full scale shake table tests of 20 ft × 53 ft and 20 ft × 20 ft ceiling systems were conducted at the Structural Engineering and Earthquake Simulation laboratory (SEESL) at University at Buffalo (UB). For the full scale dynamic testing, a new test frame providing a continuous suspended ceiling area of 1,060 ft² was constructed on the tandem shake tables and was equipped with an open-loop shake table compensation procedure. The combined designs of the physical frame and of the shake table motion controllers allowed simulating the required floor/roof motion according to ICC-ES AC156 standard at the roof of the test structure. Various test configurations were selected in order to determine the influence of different system conditions and the effects or efficiency of various protective systems required by the current standard ASTM E580 for seismic design. Based on the test data and the failure mechanisms observed, damage states are defined, and fragility curves are developed. The results of the fragility analysis show that a ceiling system becomes more vulnerable as (a) it is subjected to multi-directional input motions, (b) heavier tiles are installed, (c) the size of a ceiling area increases, and (d) lateral restraints are not installed. In addition, simplified numerical models that can capture the special behavior of ceiling systems are developed and presented in a companion paper. This paper presents the experimental study of large area suspended ceiling systems involving test setup and configurations, test motions generated by a unique control system, and basic lessons gained from the experiments.     

Seismic Behavior of RC Wide Beam-Column Connections Under Dynamic Loading

In this article, the seismic behavior of RC wide beam-column connections designed primarily for gravity loads is evaluated experimentally. A 2/3 scale model of one exterior and one interior connection is constructed and subjected to seismic simulations on a shaking table until collapse. The results of the tests indicate that: (a) the drift at yield is from 3 to 6 times higher than, for example, the 0.5% admitted by Eurocode 8 to satisfy the “damage limitation requirement;” (b) the beam-column joints do not fail; (c) the torsion in the spandrel beams governs the overall load-displacement relationship of the exterior connections and limits the ultimate strength. Based on the test results, a nonlinear model for predicting the hysteretic behavior and the failure of the connections is suggested. The model can be implemented in a computer code for evaluating the vulnerability of this type of structure through nonlinear dynamic response analyses.     

Seismic Evaluation and Retrofit of Asymmetric Multi-Story Wood-Frame Building

The presented research focuses on large-scale seismic testing under multi-directional ground motion of a three-story high, wood-frame residential building representing late 1960's California construction. Earthquake lateral resistance is provided by plywood shear walls around the perimeter of the building with an open front in the first story for tuck-under parking. Accordingly, the as-built structural configuration is asymmetric in plan and discontinuous in elevation with tendency to twist about a vertical axis and to form a weak story mechanism. The test results confirm this tendency. They also reveal the sensitivity of the response to multi-direction ground motion. Asymmetric damage patterns are induced by the multi-component motions in the walls oriented perpendicular to the open front for the as-built test structure, with or without finish materials. However, the observed damage remained noncritical as far as structural integrity is concerned even for ground accelerations exceeding 120% of that recorded during Northridge earthquake. This is viewed as a consequence of the better construction of the test building compared to actual construction. Investigated retrofit includes adding a welded moment resisting steel frame around of the garage opening and strengthening the diaphragm to header beam connections. The study indicates that the retrofit significantly reduced the maximum story drift in the open front. Moreover, the finish material and the retrofit greatly reduce the maximum rotation of the building about the vertical axis.     

Effect of SMA Braces in a Steel Frame Building

Pull-back and shaking table test results on a simple model of a three-storey structure that includes shape memory alloys (SMA) copper-based dampers are presented and discussed. The model corresponds to a rigid-framed steel structure and the dampers to austenite CuAlBe wires inserted as bracing at each story. The inclusion of the dissipators in the structure increases the percentage of critical damping from 0.59% for the bare case to 5.95% for the braced system. At the same time, the structural stiffness increases making the first fundamental frequency change from 2.5–3.7 Hz (0.4–0.27s). The net effect of these two factors is a 30–60% reduction of peak relative displacements compared to the ones obtained without dissipation devices when the structure is subjected to earthquake records. Depending on records frequency contents, a reduction of the peak accelerations to near 58% also can be obtained. Additionally, a crude nonlinear analytical model has been studied that can predict the earthquake responses reasonably well.     

Experimental and Numerical Seismic Response of a 65 kW Wind Turbine

A full-scale shake table test is conducted to assess the seismic response characteristics of a 23 m high wind turbine. Details of the experimental setup and the recorded dynamic response are presented. Based on the test results, two calibrated beam-column finite element models are developed and their characteristics compared. The first model consists of a vertical column of elements with a lumped mass at the top that accounts for the nacelle and the rotor. Additional beam-column elements are included in the second model to explicitly represent the geometric configuration of the nacelle and the rotor. For the tested turbine, the experimental and numerical results show that the beam-column models provide useful insights. Using this approach, the effect of first-mode viscous damping on seismic response is studied, with observed experimental values in the range of 0.5–1.0% and widely varying literature counterparts of 0.5–5.0%. Depending on the employed base seismic excitation, damping may have a significant influence, reinforcing the importance of more accurate assessments of this parameter in future studies. The experimental and modeling results also support earlier observations related to the significance of higher modes, particularly for the current generation of taller turbines. Finally, based on the outcomes of this study, a number of additional experimental research directions are discussed.