Analysis of building collapse under blast loads

Abstract

The analysis of the structural failure of a reinforced concrete building caused by a blast load is presented in this paper. All the process from the detonation of the explosive charge to the complete demolition, including the propagation of the blast wave and its interaction with the structure is reproduced. The analysis was carried out with a hydrocode.

The problem analysed corresponds to an actual building that has suffered a terrorist attack. The paper includes comparisons with photographs of the real damage produced by the explosive charge that validates all the simulation procedure.

Introduction

Due to different accidental or intentional events, related to important structures all over the world, explosive loads have received considerable attention in recent years. The design and construction of public buildings to provide life safety in the face of explosions is receiving renewed attention from structural engineers [1], [2], [3]. Such concern arose initially in response to air attacks during World War II [4], [5], [6], it continued through the Cold War [7] and more recently this concern has grown with the increase of terrorism worldwide [1], [2], [3]. For many urban settings, the proximity to unregulated traffic brings the terrorist threat to or within the perimeter of the building. For these structures, blast protection has the modest goal of containing damage in the immediate vicinity of the explosion and the prevention of progressive collapse. In this sense, computer programs simulations could be very valuable in testing a wide range of building types and structural details over a broad range of hypothetical events [3].

With the rapid development of computer hardware over the last decades, it has become possible to make detailed numerical simulations of explosive events in personal computers, significantly increasing the availability of these methods. On the other hand, new developments in integrated computer hydrocodes [8], such as AUTODYN software [9] used in this paper, complete the tools necessary to carry out the numerical analysis successfully. Nevertheless, two important features must be taken into account when performing computer blast resistance assessment of buildings. The first one is related to the need of experimental validation of both the models and the analysis procedures used. The second problem is related to the computational cost that makes almost impracticable, a realistic blast analysis of an actual reinforced concrete building with all its details.

Much research has been carried out in last years concerning the behaviour of structural elements and materials under blast loads. The experimental results about the behaviour of steel [10], [11], concrete [12], [13], [14] and fibre reinforced [15] panels subjected to explosions can be found in the bibliography. Nevertheless, most of the results related to full-scale structures are those concerning structures that have actually suffered explosions [16]. One of the first tests was a full-scale blast test on a four-story building at the White Sands Missile Range in New Mexico as part of a research and development contract from the Defense Threat Reduction Agency (DTRA) to investigate measures to retrofit US Embassies and other critical structures worldwide against blast loads. At White Sands, DTRA Field Command has constructed a full-scale prototype of a concrete flat-structure called CST-1 to test windows, walls and structural elements under realistic threat conditions (i.e., the blast effects of large vehicle bombs [17]). Although there are still many uncertainties, material behaviour under blast loads has been widely studied experimentally [18], [19], [20], [21] and many sophisticated numerical models have been proposed, especially for steel and concrete [22], [23], [24], [25], [26], [27], [28], [29], [30]. These models have been included in different computer programs [23], [29], [31], which can be used for the analysis of the blast behaviour of structural elements and small structures and validated with available experimental results. Nowadays, the analysis of a complete reinforced concrete structure taking into account all the reinforcement details is almost impracticable due to the elevated computational costs. Many simplifying assumptions should be done in order to perform the analysis [32], [33] and most of these assumptions are related to constituents’ materials that cannot be yet considered as individual materials but have to be interpreted as homogenised materials with average properties.

This paper is related to the effect of blast loads in reinforced concrete buildings and presents the results of the numerical simulation of the structural collapse of an actual building, the AMIA (Israel’s mutual society of Argentina) building, that has suffered a terrorist attack that produced the demolition of part of it in 1994.

The location and magnitude of the explosive load were previously obtained from the blast analysis and comparison with actual damage of the complete block of buildings where the target was located [34] and are supposed to be known in this paper. It is assumed that the damage was caused by an explosive load equivalent to 400 kg of TNT placed in the entrance hall of the building.

In order to reproduce the structural collapse, the complete building was modelled, including the reinforced concrete structure and the masonry walls. Appropriate numerical models were used for the different materials that are in the structure. The mechanical properties of the materials were obtained from tests on parts of the actual structure. The constitutive model used for concrete was proved and calibrated with experimental results of a concrete plate subjected to blast loads [13]. A homogenised model was used for reinforced concrete.

In order to reproduce the complete phenomenon, the volume of air in which the structure was immersed was also modelled. The analysis began with the modelling of the detonation and propagation of the pressure wave inside the explosive and in the air in contact with the explosive. As this analysis must be performed with much detail, it was done in a previous stage in which a spherical explosive was modelled [34]. Then the results of this first analysis were mapped into the 3D model [9]. Starting from this point, the propagation of the blast wave in air and its interaction with the building was simulated. The complete collapse process was reproduced and compared with the rest of the actual building validating the analysis procedure and the assumptions made.