A VBIED attack took place in Istanbul on 15 November 2003. Explosions of two separate VBIEDs 2-3 meters away from two synagogues on very narrow streets in Istanbul resulted in local collapse of the front part of very old one storey buildings and nearby two to three storey residential buildings on the other side of the road. None of the buildings collapsed completely even though parts of concrete columns at a distance of about 3-4 meters were severely damaged as a result of the blast. Light concrete walls at a distance of 5 meters suffered local damage of plastic bending by a few centimeters but were not breached.

The vehicles were small box vans. According to the following table, the threat level should be calculated as 4545 Kg of explosive. However, according to analysis of the crater size in the asphalt and on glass damage, it was estimated that the explosion event was equivalent to 120 – 150 Kg of TNT. Sources from the investigation related that the terrorists packed 600-700 Kg of urea-based home made explosives and initiated the explosion with commercial detonators and boosters they had acquired. In fact, parts of the explosive charge and even a booster were found unexploded at the scene. The 5-fold difference between the quantity of explosive which appeared to have been loaded into the vehicle and the apparent amount used according to the resulting damage can be explained mainly by explosive performance.  Certainly it could be argued that the difference between the quantity of explosive actually used (600-700 Kg) and the quantity expected according to the size of the vehicle (4545 Kg) from the ATF tables can be resolved by arguing in favor of protection levels ‘erring on the side of caution’. However, here it seems that the ATF tables suggest protection levels designed for a 4545 Kg explosive device whereas the actual event involved the equivalent of only 120 Kg TNT, a 40-fold difference.

The synagogue buildings are hundreds of years old, located in the old part of Istanbul where massive perimeter walls or concrete reinforcement retrofitting walls would be unacceptable so the design of protection elements to meet the 4545 Kg level is impossible. In contrast, protection against the real threat of 120 Kg TNT equivalent as was seen in the 2003 attack would be relatively uncomplicated. This encapsulates the dilemma posed by inaccurate assessment of the explosive threat.

View of the street (Istambul) about 8 m wide	View of the crater in th asphalt road (Istanbul)

The second well known VBIED attack that illustrates this problem is the attack against the American embassy in Nairobi, Kenya which took place on Friday 7^th August 1998. A light truck was used, equivalent to a "small boxed van" according to the AFT definitions. In comparison to the attack in Istanbul, in this attack the terrorist's bomb technician seemed to have been professional and was probably using military grade explosive and semi-military design methods (assuming it was a Sudan-based El Qaida group who planned and executed the event). The type of target was a different, as was the attack method and level of professionalism, so it is interesting to determine if the VBIED device was according to the worst case scenario of the ATF/FEMA.

The result of the attack was total collapse of a light structure seven story building that stood about 9 meters from the center of the bomb, very light structural damage to the American embassy building 11 meters away (Figure 5) and glass damage up to 300 plus meters away. From the size of the crater – about 10 meters wide and 2-3 meters deep - it was calculated that the explosive charge was 1000 Kg TNT equivalent. Comparison of glass damage on site to the results of tests that were conducted in Israel four months earlier confirmed this assessment. It was calculated from this comparison that the back façade's windows were exposed to 13-26 kpa (2-4 psi) and 130-260 kpa m sec (20-40 psi m sec) reflected pressure and impulses and it would have been predicted that the damage in glass laminated with ASF would be "break safe" as was the result in the event. The calculated peak reflected pressure of the embassy's concrete wall opposite the blast at 11 meters was 6285 kpa and average reflected impulse on the ground floor opening between columns was 7019 kpa m sec. From comparison with the test results and calculations, we would have expected to see breaching of the ground floor envelope wall and local damage to structural elements. This however did not occur as can be seen from the photograph shown in Figure 5.

	View of the American Embassy Wall, 11 m from VBIED, Kenia	View of the crater in asphalt paved car park

The suggested explanation is the difference in efficacy between improvised terror explosive device compared to the military type explosive devices that are used as references in testing, calculation and most of the modeling software that are used by experts.

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Uzi More, M.I.P. Security Ltd., Israel