Evaluation of blast loads on buildings in urban environment

Transcription

1 Evaluation of blast loads on buildings in urban environment A. M. Remennikov School of Civil, Mining and Environmental Engineering, University of Wollongong, Australia Abstract This paper is concerned with an accurate prediction of the effects of adjacent structures on the blast loads on a building in an urban layout. Blast loadings on structures have typically been evaluated using empirical relationships. These relationships assume that there are no obstacles between the charge and the target. In real situations, the actual blast loads can either be reduced due to shadowing by other buildings or can be enhanced due to the presence of other buildings in the vicinity. Results of the numerical simulations presented in this study for a building in an urban environment have demonstrated significant increase in peak pressures and impulses due to the effect of partial confinement. An approach to determining the enhancement factors is described. Keywords: blast loads, urban environment, explosion, numerical simulation. 1 Introduction Protecting civilian buildings from the threat of terrorist activities is one of the most critical challenges for structural engineers today. Events of the past few years have greatly heightened the awareness of structural designers of the threat of terrorist attacks using explosive devices. Extensive research into blast effects analysis and methods of protective design of buildings has been initiated in many countries to develop methods of protecting critical infrastructure and the built environment. The private sector is also increasingly considering measures to protect so-called icon buildings against the threat of external terrorist bomb attack. There are a number of means available to help prevent a successful terrorist attack on a building. One of the most effective measures consists of the gathering

2 74 Structures Under Shock and Impact VIII of intelligence that can be used to stop an attack before it takes place. Another measure, which can be used to protect many new and existing buildings, is the blast resistant design and retrofit of structures. This area of research is currently receiving a great deal of attention by the engineering community. Although it is recognised that no civilian buildings can be designed to withstand any conceivable terrorist threat, it is possible to improve the performance of structural systems by better understanding the factors that contribute to a structure s blast resistance. One of such factors is the ability of the structural designer to accurately predict the blast loadings on structural components using analytical or numerical tools that take into account the complexity of the building, the presence of nearby structures and the surrounding environment. The existing engineering-level techniques for calculating the blast effects on buildings are based on the assumption that the building experiences a load estimated assuming that it is isolated in an open space. Historical evidence suggest that the actual blast loads can either be reduced due to shadowing by other buildings or can be enhanced due to the presence of other buildings in the vicinity. The aim of this paper is to demonstrate importance of considering effects of congestion between buildings and to present numerical techniques available to predict the loads on buildings in an urban environment. 2 Methods for predicting blast load on a single building The following methods are available for prediction of blast effects on building structures: Empirical (or analytical) methods Semi-empirical methods Numerical (or first-principle) methods. Empirical methods are essentially correlations with experimental data. Most of these approaches are limited by the extent of the underlying experimental database. The accuracy of all empirical equations diminishes as the explosive event becomes increasingly near field. Semi-empirical methods are based on simplified models of physical phenomena. They attempt to model the underlying important physical processes in a simplified way. These methods rely on extensive data and case study. Their predictive accuracy is generally better than that provided by the empirical methods. Numerical (or first-principle) methods are based on mathematical equations that describe the basic laws of physics governing a problem. These principles include conservation of mass, momentum, and energy. In addition, the physical behaviour of materials is described by constitutive relationships. These models are commonly termed computational fluid dynamics (CFD) models.

3 3 Effects of neighbouring buildings on blast load on a structure Blast loads in simple geometries can be predicted using empirical or semiempirical methods. These can be used to calculate blast wave parameters for hemispherical or spherical explosive charges detonated near the surface or in a free air to predict blast effects on isolated structures and structural components. Events of the recent years have demonstrated that the most common source of unplanned explosions were terrorist devices in urban environment. In complex urban geometries, the blast wave behaviour can only be predicted from first principles using such numerical tools as AUTODYN [1], CTH [2], Air3D [3], and some others. Such tools solve the governing fluid dynamics equations and can be used to simulate three-dimensional blast wave propagation including multiple reflections, rarefaction and diffraction. In addition, Computational Fluid Dynamics (CFD) techniques can capture such key effects as blast focussing due to the level of confinement, shielding by other buildings and component failure (e.g. a window failure). 3.1 Bomb explosion in urban layout Structures Under Shock and Impact VIII 75 Experimental tests and numerical simulations of the effect of confinement provided by tall buildings which border straight city street have been performed by Rose and Smith [4]. In this paper, an urban layout consisting of a 100-metrelong segment of a straight city street with buildings of variable height and a T - junction at the far end was selected for numerical studies of the effects of the level of confinement of a blast wave in a street. The numerical simulations were performed using the three-dimensional blast simulation program Air3D [3]. Air3D uses an explicit, finite volume formulation to solve one-, two- and three-dimensional forms of the Euler equations. The computational grid uses cubic cells in a regular Cartesian mesh. Typical simulation of blast wave rigid building interaction with Air3D includes three stages with automatic remapping between each stage: (1) onedimensional analysis for the spherically symmetrical region between the centre of the explosive charge and the ground, if the high explosive (HE) explosive source is detonated above the ground level; (2) two-dimensional blast wave propagation for the radially symmetrical region from the time when the blast wave reaches the ground level to when it reaches the nearest surface of the target building; and (3) three-dimensional analysis to capture such effects as multiple reflection, diffraction, blast focusing and shielding. The street layout adopted for the simulation is shown in Figure 1. A 1000-kg TNT hemispherical explosive charge was placed in the middle of the street at the ground level at a standoff distance of 5 m from nearest building bordering the street. A standoff distance to the target building at the T -junction was 100 m. This allowed investigating the blast wave propagation along the street for the 1/3 scaled street distances up to R/ W = 10 m/kg 1/3, where W is the charge mass. The height of buildings bordering the street was varied from 10 m to 40 m with a

4 76 Structures Under Shock and Impact VIII 1/3 10-m increment that represented the scaled building heights h/ W = 1.0, 2.0, 3.0, and 4.0 m/kg 1/3. The height of the target building at the opposite end from the explosion was kept constant of 15 m, which is indicative of medium-size shopping mall building. Figure 1: Street geometry for blast effects simulation. The street numerical model consisted of about 4,000, m cubic cells. The 250 msec simulation required about 4 hours to complete each analysis on a Pentium-IV 2.8GHz machine. The simulation took advantage of symmetry through the centre of the street. In addition to the street layout simulation, the free-field blast parameters for a surface burst charge were derived analytically for the pressure measuring points along the centre of the street and at the surface of the target building thereby ignoring the presence of the surrounding buildings. 3.2 Simulation results In this section, results of numerical simulations of a blast wave propagating along a straight street will be presented. The effect of scaled building height will be evaluated. As the blast wave reaches the target building at the far end of the street, the pressure time history at the base of the building will be computed to evaluate the effects of partial confinement of the blast wave in a street Blast wave propagation along the street Figure 2 shows the effect of scaled building height on positive phase pressure and impulse as a blast wave propagates along the street. The graphs compare the peak values of positive pressure and impulse measured at the centre of the street

5 Structures Under Shock and Impact VIII 77 at the ground level for four scaled building heights with the corresponding values for a free-field surface burst of the same hemispherical charge at the same scaled distance. The computer program ConWep [5] was used to make free-field hemispherical predictions for comparison with the Air3D predictions. It is seen from Figure 2 that the channelling effect along the street is clearly evidenced by the higher pressures and impulses calculated for a street environment compared with those from a free-field surface burst. Considering Figure 2(a), the peak overpressure is significantly enhanced due to multiple reflections from the nearby buildings. It also shows that the pressure-distance relationships for the selected scaled distances are nearly coincident at scaled 1/3 distances below Z = 5.0 m/kg. At more extended distances from the source, 1/3 1/3 the line corresponding to the scaled building height of h/ W = 1.0 m/kg deviates from the remaining curves. This implies that buildings with the scaled 1/3 1/3 height greater than h/ W = 1.0 m/kg provide an equivalent level of confinement with respect to the peak overpressure measured along the street. This fact can be used for practical purposes to develop a single pressure enhancement factor-scaled distance relationship to account for an urban environment without considering the height of the surrounding buildings. The effect of partial confinement of a blast wave in a street on positive phase impulse for the selected scaled building heights are presented in Figure 2(b). The street positive phase impulses are appreciably higher than the ones for a surfaceburst charge in a free-field environment. The curves on the graph become 1/3 1/3 essentially coincident for the scaled building heights h/ W 3.0 m/kg. Therefore, this scaled building height can be considered as a limiting level above which the street positive impulses at ground level do not vary appreciably. This 1/3 finding is in accord with the scaled building height of 3.2 m/kg found by Rose and Smith [4] to be effective maximum height considered for practical applications. One can also notice a significant reduction in positive phase 1/3 impulse beyond the scaled distance of Z = 8.0 m/kg. The corresponding impulse measuring point was located in the middle of a T -junction where the blast wave originating from the street reduced its strength due to diffraction over the vertical corners of the buildings. Numerical simulations, using Air3D code, were used to derive design factors to account for the influence of an urban environment on the blast wave properties as function of distance. Based on the results depicted in Figure 2, the ratios of the pressure and impulse were calculated at each scaled distance. The pressure and impulse enhancement factors for the assumed street layout are shown in Figure 3. Channelling of the blast is shown to increase peak pressure by about 400 percent and peak impulse by about 500 percent at extended distances from the source compared with analytical results for the blast wave expanding hemispherically over a flat surface.

6 78 Structures Under Shock and Impact VIII 2,000 Peak overpressure, kpa 1, Free-field Scaled height = 1.0 Scaled height = 2.0 Scaled height = 3.0 Scaled height = 4.0 (a) Scaled distance along street, m/kg^(1/3) Positive impulse, kpa-msec 10,000 5,000 1, Free-field Scaled height = 1.0 Scaled height = 2.0 Scaled height = 3.0 Scaled height = 4.0 (b) Scaled distance along street, m/kg^(1/3) Figure 2: Comparison of the street pressures and impulses enhanced by blast focussing with the analytical predictions for a free-field surface burst at the same scaled distances.

7 Structures Under Shock and Impact VIII 79 Pressure enhancement factor Impulse enhancement factor Street Scaled distance along street, m/kg^(1/3) Street Scaled distance along street, m/kg^(1/3) Figure 3: Positive phase side-on pressure and impulse enhancement factors representing the effect of a partial confinement for the assumed street configuration Blast loads on target building The results of numerical simulations presented in Section have demonstrated that blast waves reflect off the ground and adjacent structures, reinforcing the intensity of the blast s effects. As the blast wave propagates along the street and is about to reach the target building, the positive phase side-on overpressure and impulse are already enhanced by more than 300 percent compared with the free-field blast wave parameters.

8 80 Structures Under Shock and Impact VIII 120 Reflected pressure, kpa Empirical model Computed pressure history Time, msec Figure 4: Reflected pressure time history at base of target building. The reflected overpressure time history at the base of the target building is shown in Figure 4. The free field reflected overpressures predicted by ConWep are also shown in this figure. The free field curve was developed for a hemispherical surface burst of a 1000-kg explosive charge at a standoff distance of 100 m without considering the neighbouring buildings. The channelling effect is shown to enhance peak reflected overpressure by 300 percent compared with the simplified empirical result. Peak reflected pressure and impulse enhancement factors for the front wall of the target building are shown in Figure 5. Reflected pressure predictions on the front wall were compared with Kingery-Bulmash [6] reflected pressures as computed by ConWep. The enhancement factors were calculated as ratios of numerical and empirical values of peak reflected pressure and impulse at each target point. The target points were along a vertical line at the centre of the front wall. It is seen in Figure 5 that peak pressures are enhanced by a factor of 3 and peak impulses by a factor of 2. The enhancement factor has relatively uniform distribution along the height of the target building. Near the top of the building, the strength of the blast wave is reduced due to diffraction over the roof, and the enhancement factor is reduced for both pressure and impulse.

9 Structures Under Shock and Impact VIII 81 Figure 5: Height above ground, m Reflected pressure Reflected impulse Enhancement factor Reflected pressure and impulse enhancement factors on front wall of target building. 4 Summary At present, available analytical and numerical techniques are based on the assumption that the structure experiences a blast load assuming it is isolated in an open flat terrain. Historical evidence suggests that the majority of actual blast incidents occurred in congested urban environments. Consequently, the actual blast loads experienced by buildings may be enhanced or reduced by the presence of other buildings. To develop an accurate method of predicting loads on a structure, it would be necessary to take this congestion effect into account. The essential information about an urban environment should be provided in the form of street width, height of buildings and street layout in the vicinity of a target building. It is evident that evaluation of all possible street configurations of nearby structures will not be practical. Therefore, a concise set of arrangements of explosive charge and multiple structures should be established within a framework of developing a satisfactory engineering-level model. In this paper, a tentative attempt has been made to characterise the blast environment by considering a simple urban configuration with a relatively long straight street segment and a T -junction at the far end. Numerical simulations, using the Air3D code, have been used to determine the blast effects on a

10 82 Structures Under Shock and Impact VIII building in a typical urban terrain. Each simulation provided the variation with distance of peak overpressure and impulse. When compared with the corresponding variations for a surface burst of a hemispherical charge in a freefield environment, these variations allow calculating the pressure and impulse enhancement factors at each scaled distance from the charge. The resulting enhancement factors can be used to effectively modify the blast parameters obtained from simplified analytical techniques. The use of both analytical techniques and sophisticated CFD numerical simulations can provide an effective approach to determining blast loads in an urban environment. Further work is required to extend blast load numerical simulations to the order of 10 7 cells or more using high performance computing facilities and massively parallel processors. Acknowledgments The author wishes to acknowledge the contribution of Dr. Tim Rose of RMCS, Cranfield University, U.K., for providing the Air3D code and continuous support. This work was undertaken as part of the project funded by the University of Wollongong research grants scheme. References [1] Century Dynamics, AUTODYN-2D & 3D, User s manual, [2] McGlaun, J.M., Thomson, S.L. & Elrick, M.G., CTH: A three-dimensional shock wave physics code. International Journal of Impact Engineering, 10, pp , [3] Rose, T.A., Air3D User s Guide, RMCS, Cranfield University, UK, [4] Rose, T.A. & Smith, P.D., Influence of the principal geometrical parameters of straight city streets on positive and negative phase blast wave impulses. International Journal of Impact Engineering, 27, pp , [5] Hyde, D.W., ConWep Conventional Weapons Effects. Department of the Army, Waterways Experiment Station, US Army Corps of Engineers, Vicksburg, [6] Kingery, C.N. & Bulmash, G., Airblast parameters from spherical air burst and hemispherical surface burst. US Army Armament Research and development Center, Technical Report ARBRL-TR-02555, 1998.