Module of simulation, analysis, evaluation and graphical representation of possible effects on buildings and people under blast in the urban space
1. Software application for defining the threat: Allows the user to define the threat by entering / selecting / calculating the following parameters:
a) Type of bomb vehicle: Different categories of bomb vehicles are predefined. There is the possibility of defining a new one. The amounts of explosive were introduced according to ATF – Department of the Treasury Bureau of Alcohol, Tobacco and Firearms.
Figure 1 Different categories of bomb vehicles and blast effects
b) Type of charge: two types of charges will be defined, namely shell ammunition and bare explosive. There is a possibility of defining a new type of charge;
c) Define the type of explosive: different types of bare explosive and explosive mixtures are predefined. There is a possibility of defining a new type of explosive;
d) Definition of metallic casing: Depending on the type of bomb vehicle, the mass of the metallic casing is calculated.
Figure 2 Possibilities for defining the thereat and bomb vehicle
2. Software application for environmental blast characterization: performs the determination of parameters in the shock wave front and the reflected pressure and impulse depending on the threat set by the application 1 and the stand-off distances. The mathematical models used are based on the Rankine-Hugoniot and Kingery Bulmash equations as well as complex algorithms to determine the visibility of the structures and building elements by the direct shock wave. In engineering calculations, the blast loadings on a building are simulated using pressure-time variation. This pressure time history is characterized by the peak of overpressure, the positive phase duration and the shock wave parameter. To define peak overpressure and impulse it has to determine first the parameters of shock wave. Parameters needed to fully define the shock wave are: peak positive overpressure, ; impulse for positive phase Ip and positive phase duration, tp; arrival time,ts and wave form parameter, b. For this software application the effect of negative pressure phase of the blast wave was negligible.
The most commonly used relation to describe pressure–time variation is modified Friedlander equation:
(1) where: P0 is the atmospheric pressure.
The equation used to determine the overpressure is the Kingery and Bulmash equation:
(2) where A, B, C, D and E are constants.
Also, for a blast wave the positive impulse represent the area under the positive phase of pressure-time curve and can be expressed using the following equation:
The graphical representation of the overpressure on different buildings for the same bomb vehicle can be shown in the following figure.
Figure 3 Graphical representation of incident and reflected pressure on neighbouring buildings.
3. Software application for the estimation of the level of buildings damage under blast: estimates the levels of destruction of buildings based on the destruction thresholds according to national and international standards. Obstacles are not taken into account. The levels of damage are calculated in according with AASTP-1 equation:
where K is a coefficient that depends on the level of the damage and WTNT is the TNT equivalent of the explosive charge, in kg.
The graphical representation of the levels of damages for the detonation of a bomb vehicle (cargo van) is presented in the following figure.
Figure 4 Graphical representation of building damages under blast wave
4. Software application for structure characterization: performs the database with the characteristics of the analyzed structures. Depending on the data contained in the national regulations, the user has the possibility to select the characteristics of the buildings for which the behaviour under blast loadings will be determined. There are predefined some features as followings: the type of building; year of construction; destination; class of importance; the geometry of the building; type of building materials used, etc. The application provides the user with the ability to modify / define new data / features. Output data will be used by other applications to estimate the buildings level of damage and to assess the potential for the occurrence and propagation of the progressive collapse.
Figure 5 Options for building characterisation
5. Software application for the assessment of the level of destruction of buildings and the occurrence of progressive collapse: performs a precise estimation of the effect of the blast on buildings and evaluates the potential for the occurrence and propagation of the progressive collapse. The application uses models with a single degree of freedom and P-I diagram to estimate the strength of the building elements under blast. Estimation is done on three levels of damage: 30%, 60% and 100%. The level of 100% damage corresponds to the moment of element collapse.
Figure 6 Example of P-I diagram for reinforced concrete column evaluation under shock wave.
6. Software application for the estimation of the effects of the shock wave against personnel
The effects of detonation of an explosive charge on personnel may be manifested by:
- the action of blast overpressure in time;
- the action of the fragments resulting from the destruction of structures or from the acceleration of objects;
- the impact of the human body driven by the explosion with different obstacles or with the surface of the ground.
Human injuries caused by explosive devices can be divided into main four categories, as follows:
Primary lesions - due to exposure to excessive pressure, which is most likely to affect the organs in the chest and ears;
- Secondary injuries - due to the impact caused by fragments of the container in which the device is located or elements adjacent to the device, such as fragments from the buildings;
- Tertiary injuries - due to exposure to overpressure that throws the human body, causing cranial fractures and injuries to the entire body;
- Quaternary injuries - includes other methods of injury due to an explosion, for example burns, poisonings due to inhalation of explosive residues and psychological traumas.
At the level of the head the area most sensitive to injury is the ear. In general, it is considered that the rupture of tympanic membranes takes place in the dynamic pressure range of 0.35 - 0.5 bar. A more precise approach to the effect of the shock wave on the head and ears should take into account not only the maximum value of the overpressure in front of the incident shock wave but also the duration of the positive or impulse phase. According to the AASTP-1 and taking into account characteristics of shock wave, the survival levels of 1%, 50 % and 99% for head and ears injuries for a 450 kg TNT equivalent bomb vehicle can be shown in figure 7.
Figure 7 Survival ranges for a 450 kg of TNT equivalent bomb vehicle for head (left) and ears (right) injuries.
In the thoracic area the areas most susceptible to injury due to overpressure, are the zones of passage between environments with different densities. At an overpressure of more than 2.5 bar, potential fatal damage to the lungs and other organs that by their nature have spaces in which a fluid (fluid or gas), such as the stomach, intestines or bladder, can occur. In simple terms, it is enough to describe the lungs as crushed or exploded. To determine the survival ranges for lung injuries under blast action there were used pressure – positive phase duration diagrams, figure 8, 9 and 10.
Figure 8 Survival curves (left) and ranges(right) for the effects of 450 TNT equvalent on lungs of an adult of 70 kg.
Figure 9 Survival curves (left) and ranges(right) for the effects of 450 TNT equvalent on lungs of an adult of 70 kg in the stading position.
Using the same procedure it can be determined the ranges of survival for different amount of explosive charge and for different types of injuries.