Panos Tsopelas

Constitutive Model for Cyclic Response of Structural Steels with Yield Plateau

A rate-independent cyclic plasticity model for structural steels with a yield plateau is proposed. The model couples nonlinear kinematic hardening with a memory surface in the plastic strain space, to account for the progressive cyclic hardening/softening effects and a pseudomemory surface in the deviatoric stress space to correctly describe the plateau response. A simple identification procedure to calibrate the material dependent parameters is outlined and the accuracy of the proposed model is verified against experimental data available in the literature for proportional and nonproportional loading paths.

Load Path Effects in Circular Steel Columns under Bidirectional Lateral Cyclic Loading

AbstractRigorous numerical analyses are conducted to study the effects of bidirectional lateral cyclic loading on the hysteretic response of stocky circular thin-walled steel columns. Four unidirectional and eight bidirectional lateral displacement controlled load paths are considered; different performance indices obtained from the lateral load paths under consideration are compared. The results obtained from these comprehensive numerical analyses indicate that bidirectional lateral cyclic loading has a very limited effect on the ultimate strength of stocky circular thin-walled steel columns, but significantly decreases the ductility when compared with unidirectional lateral loading.

Direct mechanical landmine excitation with scanner laser Doppler vibrometer surface measurements

Remote acoustic or seismic forms of excitation for laser Doppler vibration landmine detection are low false alarm rate detection strategies. A more recent approach now under investigation includes a direct mechanical excitation through a prodder or probe. In this research, we report on simple laboratory measurements of the VS-1.6 landmine undergoing direct mechanical excitation from a modified prodder while measuring the landmines pressure plate vibrational response with a scanning laser Doppler vibrometer. The direct mechanical excitation mechanism, located near the prodding end of a rod, consists of a miniature piezoelectric stack actuator. We additionally compare direct excitation to both acoustic and seismic methods in a large sandbox filled with dry sand. We show that for the landmine buried almost flush, direct contact mechanical excitation compares favorably to both seismic and acoustic excitation responses for the (0,1) mode of the pressure plate. We also observe additional features not previously seen in either seismic or acoustic excitation.

Recent Advances in Seismic Isolation: Methods and Tools

Highlights of the problem of seismic protection of structures and their contents are presented. The seismic isolation technology currently in use is essentially the same technology proposed almost 30 years ago when first applied in building and bridges. The limitations of these isolation systems in the design of structures are briefly presented. The potential of recently proposed seismic isolation hardware to achieve high performance multi objective designs is discussed. Seismic isolators providing uplift restraint, double curved friction pendulum (FP) isolators and Triple FP bearings are presented and their behavior described. A Plasticity-based model for the double curvature FP isolator and a mechanics-based model for the triple FP bearing are introduced. It is critical to provide to engineers reliable tools and software with the ability to perform detailed parametric analyses for the preliminary design phase and detailed realistic analyses for final design. The models of the pendulum type of bearings are implemented in 3DBASIS-ME-MB software which, in addition, can be utilized as independent verification tool of more complex commercial software.

ACTIVE/PASSIVE SEISMIC CONTROL OF STRUCTURES

In civil engineering, structural integrity and safety are of utmost importance as the consequences of failure are devastating. Maintaining the structural integrity becomes particularly important when the structures are subjected to severe earthquakes and strong wind-loading. Various passive and active control means have been considered to avoid catastrophic failure due to seismic or wind excitations. In this paper, a new class of hybrid actuators is presented which consists of a piezoelectric stack actuator combined with a viscoelastic damper to form a passive/active brace system (PAB). The actuator is used for mitigating structural dynamic responses of a three-storey structure subjected to simulated earthquakes loading. The proposed hybrid actuator is selected because it combines the attractive attributes of active and passive systems as well as because it has high stiffness-weight ratio, high frequency bandwidth, and low power consumption. The theoretical and experimental performance characteristics of the three-storey structure with the hybrid PAB actuator are presented. Comparisons are also included when the structure is controlled with conventional viscoeiastic dampers or conventional active control braces that operate without any viscoelastic damping. These comparisons emphasize the effectiveness of the hybrid actuator in damping out simulated seismic vibrations.

The Response of the Seismically Isolated Bolu Viaduct Subjected to Fault Crossing

The effect of fault crossing on the response of the seismically isolated Bolu Viaduct is investigated. The seismic ground motions at the site of the Viaduct are generated following a rigorous methodology, which is based on seismological data available in the literature and finite-fault modelling techniques of the extended source. The generated motions are used to study the effect of fault crossing and fault crossing angle on the nonlinear behaviour of the seismically isolated Viaduct. It is shown that fault crossing is an important factor in the earthquake response of seismically isolated bridges, and that this effect needs to be considered in the design and detailing of the isolation system.

AT mine overpass capability of ground-vehicle mine detection system

An overpass solution for antitank pressure-fused mines was developed and demonstrated for a teleoperated, four-wheeled experimental unmanned ground vehicle hosting a mine detection system using a downlooking ground-penetrating radar. The capability to overpass pressure-activated antitank mines is one way to protect the vehicle. The requirement was to make the vehicle overpass capable by giving it an average footprint pressure of no more than 5 psi using commercially available equipment and without requiring any vehicle modification. An overpass solution was developed and demonstrated using low-pressure, minimal casing rigidity tires that produce a uniformly low ground pressure and enable the vehicle to exert less than the minimum force required to activate the large majority of pressure-fused antitank mines. Overpass requirements are discussed in terms of antitank mine threats, pressure plate size, activation forces, and ground pressure distribution uniformity. A variable-load tire footprint pressure measurement system and laboratory were developed and laboratory evaluation of a number of tire candidates was completed. Laboratory results were demonstrated through field performance demos of the selected low-pressure tire. Results present the successful overpass of various threat representative antitank mines with the pressure plate elevations/exposures at various positions relative to local grade.