Ioannis G. Raftoyiannis

Use of non-linear localization for isolating structures from earthquake-induced motions

The dynamic response due to earthquake-induced excitations of multi-storey buildings simulated by a cantilever (with attached concentrated masses) supported on a flexible foundation, is reconsidered when stiffness non-linearities are included. To this end, a suitable non-linear spring-mass device is placed between the ground and the mass of the foundation, which under certain conditions can absorb a significant amount of seismic energy over a large frequency range, thus drastically reducing the seismic response of the foundation. This is achieved by the stiffness non-linearity that gives rise to a localization phenomenon, according to which motions generated by external disturbances remain passively localized close to the point of seismic excitation instead of ‘spreading’ to the entire structure. The implications of these findings to the design of earthquake-resistant structures are discusssed. Copyright

Dynamic buckling of 2-DOF systems with mode interaction under step loading

This paper deals with a qualitative dynamic buckling and global stability analysis of autonomous non-dissipative systems which under the same loading applied statically exhibit a mode coupling. The energy, topological and geometrical criteria valid for dynamic buckling of autonomous systems, which under static loading experience simple critical points, are extended to the aforementioned type of statical systems. Simple and readily applied formulas leading to practically very reliable dynamic buckling loads of a 2-DOF model for various types of initial geometric imperfections are comprehensively presented.

DYNAMIC STABILITY OF CABLE-STAYED BRIDGE PYLONS

This paper deals with the stability of the pylons of a cable-stayed bridge under the action of time-dependent loads, due to the vibration of the bridge deck. The stability of such problems of cable-stayed bridges is solved by a technique developed in the Laboratory of Metal Structures and Steel Bridges, of National Technical University of Athens (NTUA), as well as Bolotins technique for the solution of nonlinear problems of dynamic stability. Three cases are studied: pylons with damping, pylons under forced vibration, and pylons subjected to an arbitrary external dynamic load. Useful relations are established by the aforementioned solution method, examples for a variety of pylons are presented, and interesting results regarding the stability of each case are given in diagrams.

A new approach for loads moving on infinite beams resting on elastic foundation

The present work deals with the problem of the dynamic behavior of infinite beams resting on a Winkler-type elastic foundation under moving loads. The beam is subjected to a moving point load traveling with constant speed. Determining the effective length of the beam for a non-moving (static) load, and using the so-called quasi-stationary state of the beam, a solution is proposed with the aid of modal analysis and useful conclusions are obtained. Moving loads have a significant influence on the dynamic behavior of elastic or inelastic solid elements of an entire structure or parts of the structure, while they may produce strong vibrations on such systems, especially when high speeds are reached. The expressions obtained in this work allow the use of the same formulae for any values of speed or damping parameters. The analytical results obtained by the relations presented herein are compared to the ones given by Frýba in the relative chapter of his classic work. Finally, the possible speeds of moving loads that can be reached in the case of such types of beams are estimated and the application field of the gathered relations is determined.

CURVED-IN-PLANE CABLE-STAYED BRIDGES: A MATHEMATICAL MODEL

A mathematical model suitable for static and dynamic analyses of curved-in-plane cable-stayed bridges is proposed. By expressing the tensile forces of the cables in relation to the deck and pylon deformations, the problem is reduced to the solution of a beam curved-in-plane that is subjected to the usual permanent and external loads and to the tensile forces of the cables, the latter being functions of the deformation of the beam. The theoretical formulation presented is based on a continuum approach, which is suitable for the three-dimensional (3D) analysis of long span cable-supported bridges. Numerical examples will be analyzed to illustrate the applicability of the proposed approach.

Critical Lateral-Torsional Buckling Moments of Steel Web-Tapered I-beams

This paper deals with the stability of steel web-tapered I-beams subjected to bending loads. Tapered beams can carry a maximum bending moment at a single location while in the rest of the member the moment carrying capacity is considerably lower. This results in appreciable savings in materials as well as in construction. Numerous researchers have focused on the investigation of the elastic behavior of tapered I-beams and many theoretical findings have been incorpo- rated into the current specifications. According to Eurocode 3, the elastic critical moment is used for determining the de- sign strength against lateral-torsional buckling (LTB) of I-beams with uniform cross-section and a number of coefficients is employed accounting for the boundary conditions, the cross-sectional geometry and the type of transverse loading, while no detailed information is given regarding non-uniform members. In this work a simple numerical approach is pre- sented for determining the critical lateral-torsional buckling loads of web-tapered I-beams. Modification factors of the elastic critical moment with reference to the mean cross-section are given for various taper ratios. The results presented in graphical form are compared with those of previous investigations. The approach presented herein can be very easily ap- plied for the design of tapered beams against lateral-torsional buckling.

ROCKING AND SLIDING OF ANCIENT TEMPLE COLUMNS UNDER EARTHQUAKE EXCITATIONS

This paper deals with the rocking and sliding behavior of a monolithic stone block or a system of stone blocks under earthquakes, a problem commonly observed for ancient temples in Greece and Southern Italy. The analysis for the above monolithic or multi-drum columns is conducted by a simple process based on generally accepted simplifications. The effects of column geometry, earthquake characteristics and restitution ratio due to impact are also studied herein. Furthermore, an analytical approach for the solution of the complete nonlinear equations of motion (including the one for the vertical earthquake excitation) for the subject considered is proposed. Finally, characteristic representative examples are presented with useful conclusions drawn. It was found that the stability criteria based on static conditions are reliable, while the corresponding dynamic criteria may lead to erroneous results.

Stability of Steel Columns with Non-Uniform Cross-Sections

In this work, non-uniform steel members with or without initial geometrical or loading imperfections, that are loaded by axial forces applied concentrically or eccentrically and by concentrated moments applied at the ends or at intermediate points, are studied. More specifically, steel members with varying cross-sections, tapered or stepped or members consisting by two different tapered parts are considered. The formulation presented in this work is based on solving the governing equation of the problem through a numerical method where the eigenshapes of the member are employed. A failure plasticity criterion is introduced for members especially the short ones that will never reach the elastic critical buckling load. Although only the simply supported beam-column case is studied herein, it is obvious that the method can be extended to multi-span beams and frames, by employing the corresponding eigenshapes. Useful diagrams are presented for both the critical buckling loads and the equilibrium paths showing the influence of the main characteristics of the beam-column. non-uniform steel members with or without imperfections (of any form), loaded by axial forces (concentrically or eccentrically applied) and by concentrated moments applied at its ends or intermediate points are studied. The steel members with cross-section that may vary along the length, can be tapered or stepped or they can be members consisting by two unequal tapered parts. The imperfections considered may have any form. The formulation presented in this paper is based on solving the governing equation of the problem through the Galerkin method using the eigenshapes of the member. A plasticity failure criterion is introduced for stub or short members that will never reach the elastic critical buckling load. Although in this paper only the simply supported single-span beam-column is studied, it is obvious that the formulation presented may be extended to any type of frame members or frames using the corresponding eigenshapes. The results are presented in the form of diagrams either for the critical buckling loads or for the equilibrium paths, showing the influence of the main members characteristics, as for example the cross-sectional variation law, the interme- diate loads and bending moments or the existing imperfec- tions on the above mentioned buckling loads and equilibrium paths. Although these diagrams are derived for a simply sup- ported beam-column they can be readily employed for the design of steel frames with such members through the use of the equivalent buckling length factor concept.

Eigenfrequencies and Critical Speeds on a Beam due to Travelling Waves

A dynamic load, suddenly applied at a point of a beam, produces a local disturbance that propagates or diffuses to the rest of the beam. This propagation takes place with a speed depending on the material and geometrical characteris- tics of the beam. It has been demonstrated that an impulsive disturbance involving shear and moment will result in two wave types, one that propagates with the shear wave velocity and a second that propagates with a moment-wave velocity. It is observed that tampering with the cross-section of the beam may result to equal shear wave and moment-wave veloci- ties and the two types of disturbances will travel together along with additionally interfering shear waves from beams ends reflections. In this paper, the effect of the traveling waves on the dynamic characteristics of a beam is studied. A complete beam model is presented, which motion is governed by the Timoshenko equation. Two main cases are exam- ined, namely a simply supported beam, and a beam resting on a Winkler-type elastic foundation. Analytical results are presented in graphical form showing the influence of the traveling waves on the eigenfrequencies and critical speeds of such a beam and useful conclusions are drawn.

Mathematical modeling for seismic response of steel bridges with internal damping systems

The present work deals with internal damping systems aiming to reduce the torsional and torsional-flexural modal amplitudes in steel bridges with open or closed cross sections. The effect of internal damping systems on the vibration modes and modal amplitudes is studied using simple models of bridges subjected to earthquake actions and useful results are gathered that can be used for design purposes.