Jorge Daniel Riera

On the stress analysis of structures subjected to aircraft impact forces

Abstract Total reaction versus time relationships expected in case of an accidental impact of large commercial aircrafts against a rigid surface are presented. Maximum response curves for an elastic, undamped, one-degree of freedom system are evaluated and examples of dynamic analyses of representative reinforced or prestressed concrete structures in nuclear power plants are discussed in some detail. The above information and results serve as a basis for proposed simplified approaches.

A Critical Reappraisal of Nuclear Power Plant Safety against Accidental Aircraft Impact

Abstract The overall problem of nuclear power plant safety against an accidental aircraft impact is discussed in relation with its structural analysis and design. Associated risks, such as fire, which is a potential source of damage for buildings and other structures, are not considered. The paper is divided in two parts. In part I different approaches used for determining the reaction-time curve are discussed. The influence on the results of target motions is examined next. It is shown that for the evaluation of structural response an aircraft-structure interaction analysis is usually an unnecessary refinement, “mean” reaction-time and impact area-time curves being sufficient to define the excitation. Preliminary results for oblique impact are also given. Since the conditional probability of a normal impact is very small, the consideration of oblique impact may become acceptable in future design criteria. In part II, available solutions for the resulting structural dynamic problem are reviewed. The feasibility of resorting to a static analysis is also discussed. Present practices to evaluate floor response spectra are reviewed next. The short-comings of the “deterministic” approach are pointed out. It is proposed to define the excitation as a mean plus a fluctuating force. The latter is treated as a nonstationary random process and the problem solved by numerical integration in the time domain. Although such solutions get prohibitively expensive when the number of degrees of freedom becomes large, results obtained for simple models may help to clarify which are the important variables of the problem.

Discrete elements model for evaluating impact and impulsive response of reinforced concrete plates and shells subjected to impulsive loading

The application of a discrete element representation of solids to the analysis of reinforced concrete plates and shells is discussed. Yielding of steel as well as fracture of concrete are duly accounted for by means of constitutive criteria that quantifies coupling between both effects. Comparison with experimental results show excellent correlation.

Generation of New Cracks Accompanied by the Dynamic Shear Rupture Propagation of the 2000 Tottori (Japan) Earthquake

The 2000 Tottori (Japan) earthquake caused fracture zones of surface rupture at some places away from the trace of the main causative fault. In order to explain this observation, the 3D dynamic rupture process of the 2000 Tottori (Japan) earthquake was simulated. The attractive feature of the problem under consideration is the possibility of introducing internal new cracks that propagate under tensile stress as a consequence of the dynamic process of shear slip propagation. The discrete element method was used to solve this problem because it can introduce internal tensile cracks. For the shear rupture propagation the simple slip-weakening model was used as a friction law on the fault. The new tensile cracks occur, following the classical Griffith theory, when the critical value of tensile fracture surface energy has been reached. The first step to solve the problem was the estimation of the dynamic parameters, such as stress drop, strength excess, and critical slip, which were recov- ered from the results of waveform inversion. In the second step, a shear dynamic rupture process was simulated assuming that shear slip occurs only on the pre- existing fault, and the tensile stress concentration resulting from shear slip causes the new cracks that propagate away from the pre-existing fault. The results, which are consistent with the observations, show a free-surface rupture caused by the new cracks and the development of a flowerlike structure springing from the borders of the pre-existing fault and main asperity.

A modified Vlasov theory for dynamic analysis of thin-walled and variable open section beams

The equations of motion of thin-walled beams with open cross-section considering the effects of shear flexibility, rotatory inertia in the stress resultants and variable cross-sectional properties are presented. These equations are based on Vlasovs theory of thin-walled beams, which is modified to include the effects indicated above. The formulation is also applicable to solid beams, constituting therefore a general theory of coupled flexure and torsion of straight beams. The differential equations are shown to be particularly suitable for analysis in the frequency domain using a state variables approach. The resulting equations are used in the determination of the natural frequencies and the seismic response of a building provided with a thin-walled reinforced concrete core to resist lateral loads. Finally, an example of a variable section channel beam is presented, which provides additional evidence on the generality of the formulation.

Damage detection under ambient vibration by harmony search algorithm

Highlights? A new approach for vibration-based (SHM) procedures is presented. ? Stochastic system identification with the evolutionary harmony search algorithm. ? Numerical investigation considering noise levels and a series of damage scenarios. ? Experimental study of three cantilever beams under different damage conditions. ? The methodology has shown potential for use in the damage assessment of structures. Damage in structural systems induced by vibrations, alternating load cycles, temperature changes, corrosion, etc., constitute a serious technical problem. Smart methods of control and structural health monitoring (SHM) for large structures are, therefore, highly needed. In certain structural applications, moreover, a lack of access to the damaged area imposes an additional constraint on damage identification procedures. One method that may fulfill those requirements is dynamic nondestructive testing, which consists of monitoring changes in the structures natural frequencies, vibration modes and damping.In this paper, a new approach for vibration-based (SHM) procedures is presented, in an ambient vibration context; this method combines a time domain modal identification technique (SSI) with the evolutionary harmony search algorithm. A series of numerical examples with different damage scenarios and noise levels have been carried out under impact and ambient vibration. Thereafter, an experimental study of three cantilever beams with several different damage scenarios is conducted and the proposed methodology has shown potential for use in the damage diagnosis assessment of the remaining structural life.

Wind excitation of neighbouring tall buildings

Abstract There is a growing concern about the importance of interaction effects between neighbouring tall buildings subjected to wind loading. Available results indicate that both the peak dynamic response as well as the mean wind loads may increase substantially due to building interaction. An extensive wind-tunnel study of the interaction between two square prisms with a height-to-base length equal to six was undertaken at the LAC, UFRGS. Fifteen relative prism positions were studied, for wind orientations at every 15°, under smooth-uniform as well as turbulent-shear wind. Mean force and torsional moment coefficients for uniform flow were reported in previous publications. Herein, complete results for turbulent wind are given. In addition, the most important interaction effects observed under both flow conditions are discussed in detail.

Penetration, scabbing and perforation of concrete structures hit by solid missiles

Abstract The basic features of the penetration, scabbing and perforation processes in concrete structures hit by solid missiles are discussed. The influence of the missile slenderness and crushing strength is then evaluated. The tensile strength of concrete is introduced, as a single strength parameter of the target material, in the simplified equations proposed for prediction purposes. The equations also account for size effects and for the influence of steel reinforcement.

Estimation of the time-dependent frequency content of earthquake accelerations

Abstract In order to define seismic random processes, attention is devoted to the evolutionary spectra method. There is shown that the estimation of the evolutionary spectra (ESP) carried out by the multifilter technique with an approximate consideration of the transient behaviour of the filter element may in fact lead to an improved estimator.

Fault dynamic rupture simulation of the hypocenter area of the thrust fault of the 1999 Chi-Chi (Taiwan) Earthquake

A 2D Discrete Element Model was employed to simulate the rupture propagation and near-source ground motion in the epicentral area of the 1999 Chi-Chi (Taiwan) earthquake. The observations show that the hanging wall side is characterized by larger particle motions than the footwall side. The simulation results reproduce the main features of the recorded ground motion and show that the particle velocity on the hanging wall side and on the footwall side are symmetric in the deeper fault region, however, as the crack approaches the free surface, the hanging wall side is characterized by larger particle motions than the footwall side. These results suggest that the difference in the particle motion on the hanging wall and on the footwall is due mainly to the asymmetric geometry of hanging wall and footwall. Thus, the model used leads to good approximations in the vicinity of the epicenter.