Scientia Iranica
Volume:18 Issue: 3, 2011

This paper and a companion paper present a state-of-the-art review of significant research performed in the area of smart structures. The focus of the review is journal articles published since 1997. This paper reviews articles on active and-semi active control of structures using a variety of systems. Active control systems include active tuned mass dampers, distributed actuators, active tendon systems and active coupled building systems. Semi-active control systems include: magnetorheological (MR) fluid dampers, semi-active stiffness dampers, semi-active tuned liquid column dampers, and piezoelectric dampers. A review of hybrid control systems and control strategies is presented in the companion paper.

In a companion paper, the writers reviewed significant work done on active and semi-active vibration control of structures performed in the past decade or so. In this paper, journal articles on the hybrid vibration control of structures and control strategies, published during the same period, are reviewed. They include hybrid mass dampers, semi-active base isolators, actuators with passive dampers, and semi-active Tuned Liquid Column Dampers (TLCD) with passive dampers. A key element in successful implementation of smart structure technology is an effective control algorithm to compute the magnitudes of internal forces to be applied to the structure. Various improved or new control strategies developed for civil structures are reviewed in this paper. They include modified LQR and LQG, neural network-based, fuzzy logic, sliding mode and wavelet-based controllers. Studies on optimum placement of control devices are also summarized.

This paper presents an economical solution to improve the seismic response of simply supported precast concrete girder bridges under transverse excitation. This solution employs an energy dissipation system, which consists of conventional elastomeric bearings that transfer gravity load to the substructure, and energy dissipating shear keys that replace conventional shear keys on bent caps and abutments. The proposed shear key, which is a yielding steel damper, transfers a good portion of the seismic load to the substructure, while dissipating the seismic energy through inelastic deformation. An experimental study on four full scale specimens is conducted to evaluate the behavior of the proposed shear keys under cyclic loading condition. The specimens with good ductility and energy dissipation capacity are identified. These specimens were able to sustain large inelastic deformation without any strength degradation under cyclic loading. The nonlinear time history response of a three span precast concrete girder bridge, with and without the proposed shear key, is also studied. The results of analyses indicate that seismic demands on the substructure are greatly reduced when conventional shear keys are replaced by the proposed shear keys.

A Cellular Automata approach is introduced in this paper for the optimal design of sewer network problems. The solution of sewer network optimization problems requires the determination of pipe diameters and average pipe cover depths, minimizing the total cost of the sewer network subject to operational constraints. One constraint of the problem, namely, the maximum relative depth of flow, is first used to define the pipe diameters in terms of the pipe slope and, hence, the nodal elevation of the network. As a result, the sewer network optimization problem is redefined, in terms of the network nodal elevations, and the resulting problem is solved using a Cellular Automata (CA) approach. The nodes of the network are used as the CA cell, with the corresponding elevations as CA cell states. The updating rule of the proposed CA is then derived by requiring the stationary total penalized objective function of the problem, with respect to the cell state. The proposed method is used to solve two benchmark examples and the results are compared to those obtained by other methods. The results indicate that the proposed method is highly efficient compared to alternative methods, while producing comparable results.

Suction caissons offer certain advantages over other underwater foundation systems by virtue of large bearing capacity, ease of installation, and efficiency. They are typically built with upright walls. The behaviour of upright suction caissons in regard to their applications, installation, load-bearing, etc. has already been investigated by a number of researchers. However, the performance of tapered suction caissons has not been formerly studied. This paper addresses the pull-out capacity of tapered suction caissons under vertical pull-out loads. A numerical approach was used. The finite element model was first calibrated against available test results on upright suction caissons and then used to simulate the pull-out of tapered caissons. It is admitted, however, that further validation of the model against experimental results on tapered suction caissons will increase the acceptability of the results. It was observed that positive wall slopes may noticeably improve the pull-out capacity. A change from local to global failure modes was postulated as the main reason for this improved resistance. With negative wall slopes, however, the pull-out capacity slightly decreased. In addition, effects from the caisson wall slope were investigated for a number of caisson geometries, drainage conditions and soil properties.

In this paper, application of the Endurance Time (ET) method in seismic analysis of concrete gravity dams has been investigated. The ET method is based on time history analysis of structures, subjected to specially designed intensifying acceleration functions. It is expected that by developing its application in analysis of concrete dams, useful information on the seismic behavior of such dams at various excitation intensities can be obtained. Results from linear analysis of Folsom and Koyna dams under real earthquakes and ET acceleration functions have been compared. It is shown that the ET method can predict the response of concrete gravity dams to individual earthquakes with reasonable accuracy in linear analysis. In order to evaluate concrete gravity dams by linear analysis, the target time has been assumed to correspond to the OBE level of earthquake. Endurance criteria can be set as the time when the desired damage index(es), e.g. maximum principal stress, reaches its allowable value. The design can be considered satisfactory if endurance time is more than target time and vice versa. Also, potential application of the ET method in non-linear analysis of concrete gravity dams has been investigated. It is shown that crack profiles during earthquakes could be traced by the ET method.

A study was conducted to simplify and broaden the procedures for estimating scour dimensions at the downstream of cascades. There are many studies about energy dissipation, aeration and oxygen transfer in stepped-channel chutes and cascades. There is little attention paid to the scour downstream of the stepped chutes and their downstream scour hole geometry. Measurements were taken to obtain a scour hole profile in the equilibrium state of the scour, the value of the maximum depth of the scour, the location of the maximum scour depth and the length of the scour hole. It was also found that step geometry, stepped chute angle, downstream water level and sill types of a stilling basin are very important parameters for the geometry of the scour hole.