On August 4th, 2020, a tremendous explosion occurred in Beirut, Lebanon’s capital, specifically in its port. The resulting damage was humongous, impacting the city along a radius of about 600m.
A series of grains silos adjacent to the center of the explosion, at 85m proximity, was importantly damaged.
The cause was the explosion of Ammonium Nitrate that had been stored for over 6 years in the port, of “said” quantity equal to 2750t, whereby different opinions have emerged regarding the amount of the explosive materials and the magnitude of the explosion.
This article is based on a research report by Yehya Temsah, Ali Jahami, Bassam Al Timani, and Charles Aouad, defining the magnitude of the explosion, using a structural engineering approach through non-linear finite element modeling of the silos.
The magnitude of the explosion is defined as the numerical model magnitude that generated the silos damages. The study is based on silos detailed drawings, data collected from site visits, and the use of the Conventional Weapons Effects Blast Loading (CONWEP) method.
In addition, a damage assessment for the “standing” silos has been conducted and final recommendations are included.
This article shows that the magnitude of the explosion is significantly smaller than the equivalent TNT of the original declared quantity of stored Ammonium Nitrate.
Beirut grain silos (Figure 1) were built in the port of Beirut in the late 1960s and were inaugurated in 1970. The silos structure consists of 42 cylinders, with an internal diameter of 8.5 meters (each cylinder), walls thicknesses of 17cm, and a height of 48 meters.
Over the years, expansion and rehabilitation works had been executed. In the late 1990s, 6 new silos were added to the main 42 silos. In 2002, due to concrete carbonization, the silos had undergone restoration works.
The silos construction process was performed in three stages as illustrated below:
The research team, in cooperation with the Directorate of Engineering in the Lebanese Army, conducted field visits to assess the damages to the silos.
In addition, the Lebanese Air Force provided the research team with aerial high-resolution photos of the silos, as shown below.
In order to specify the detonation center point of the explosion, the aerial photos captured by the Lebanese Army Air Force are used to preliminary define the detonation center point, showing ward number 12, which contained the explosive materials and the silos.
Since the explosive material in ward number 12 was Ammonium Nitrate (AN), a scaling factor was used to transform the AN mass to equivalent TNT mass, to enable using it in the numerical study as input.
Referring to the data provided by the Lebanese army (based on their investigations), a scaling factor of 0.39 is recommended to transfer the explosive mass from AN to TNT. This implies that the total original mass of AN of 2750t, is equivalent to 1100t of TNT.
The analytical program is divided into 8 cases, results for all study cases are summarized in the table below.
It is noted that for equivalent TNT mass of 1100t, 500t, 400t, and 300t respectively, all silos will be totally destroyed, reducing the mass of explosion to 50t results in the destruction of just one row of silos.
Two parameters are considered to calibrate the model, the degree of damage as shown below:
In this phase, the blast centroid is shifted by 10m toward the zone where the last two silos were totally destroyed, to provide results matching the damage level in the actual case.
In order to refine the determined magnitude of the explosion, five additional cases are studied, considering the small variations of both detonation centroid location and detonation mass.
The analysis results led to 209t TNT, as a final magnitude of the explosion corresponding to 535t of AN. This implies that the percentage of AN mass exploded was 19% of the 2750t, the original declared stored quantity.
The total work dissipated by the silos is 2.58×108 J. This value is used to determine the energy absorbed by the silos, which is around 0.003% of the total released energy by 200t of TNT equivalent mass.
This refutes the claims that the silos protected Beirut city from total destruction, yet they helped in diffracting the wave away from the buildings lying behind.