Estudio del comportamiento de pilares de edificacion frente a cargas de impacto

Resumen

Accidental actions cause stresses on the structures that must be taken into account in the designing process. The latest regulations related to the structural design of buildings include some considerations regarding this type of action, which are usually associated with equivalent static loads. However, the indications given in these codes for car crashes are very different for the same kind of impact. Moreover, there isn't any research regarding car crash against building structures. All these considerations justify the need and relevance of the work being done in this thesis. The overall objective of this study is to determine the static load that is equivalent to an impact on some particular conditions. Since the impacts on buildings happen inside a city or in a car park, the speeds to take into account are low, i.e., speeds between 10 and 30 km/h. In order to cover various possibilities, we have used different types of cars in the study of the impact, with different masses, structural and buffer characteristics. In addition, we have placed a box inside of one of the cars to evaluate the influence of mass increase on the vehicle. To perform a primary numerical approach to the problem, we have made a finite element model with ideal boundary conditions for the column, and a realistic car model. Using this model, some simulations have been performed with different static horizontal forces located at the impact point and also some dynamic simulations in which we have used different initial velocities in the car. The equivalent static load was obtained through a comparison between static and dynamic simulations, using the maximum displacement on the pillar as the benchmark. The obtained equivalent static loads through this first approach were much higher than those prescribed by the regulations analysed. To verify the results obtained so far, a group of full scale car crash tests have been completed in which the impact of a car against a concrete column was performed. The column was post-tensioned to simulate the compression due to an upper structure. During these tests, the pillar was monitored with accelerometers, linear displacement sensors and strain gauges. Furthermore we have developed a procedure to measure the displacement of the column from the images obtained with a high-speed camera. The results of these tests have shown that the linear displacement sensors were unable to properly register the movement of the column, whereas it has done by using the high-speed camera. Once the experimental phase was implemented, a new finite element model was performed under the same conditions in which the previous tests were completed. On this model a last generation concrete model was used. This material model was specifically designed to represent the behaviour of concrete under impulsive loads. In addition, this model was adjusted to ensure that its behaviour during impact is the same as was measured during the tests. The procedure followed to obtain the equivalent static loads with this new model was the same as that used in previous simulations, but in this case the benchmark was the maximum curvature of the column, instead of the maximum displacement. In addition, other interesting results have been obtained, e.g. the variation of force in the contact or damage to the concrete due to the impact. The results show that aside from impact velocity, the characteristic that most influences the results is the structural design of the vehicle and not the mass, as one might think. In general, more modern vehicles cause minor consequences in the structure during an impact than older vehicles. This is due to the incorporation of damping materials and technologies that enable a lot of energy to dissipate during the impact, thereby preventing transmission to the structure. In terms of the obtained equivalent static loads and their comparison with the analysed codes, it was determined that the recommendations given by Parts 1.7 and 2.7 respectively of Eurocode 1 are not safe enough, as they are lower than those obtained in this work for any car and speed. However, the instructions on Annex C and A of the Parts 1.7 and 2.7 respectively of Eurocode 1 are probed to be safe in almost all cases. Only for a car with separated chassis and velocity higher than 20 km/h, these values are lower than those obtained. Moreover, the analysis of concrete damage shows that only at speeds above 20 km/h and for a vehicle without the latest shock absorber systems, there is a seriously damaged section on the column. However, as a security measure, it is advisable to consider the possibility that any kind of car can move around, so that the static load to consider is in the region of 500 kN for a speed of 30 km/h. However, it is hoped that these old models of cars stop circulating and the design of new lines of shock absorber systems are still better than existing ones, so that consideration of such actions on buildings will tend to disappear in the foreseeable future.