Overall load on a structure

The velocity of impact (assumed normal to the impacted surface) is 215 m s The equivalent diameter of the loading area is 2.60 m. 

The two-step shape of the load curve is due to the presence of two phases initial impact of the body and subsequent impact of the engines (more rigid). 

In practice (with reference to Fig. 17-2), the second impact of the engines is eliminated. The impact area is assumed, as in the first case, equal to 7 m. These assumptions are not accepted by all the experts because they do not take account of the fact that the engines, in the first phase of the impact, may break off the aircraft body and proceed towards the target as autonomous missiles, without the energy absorbing effect of the body itself. 

In order to perform an indicative evaluation of the load which could correspond to other types of aircraft and to other impact speeds, the following simple concepts are suggested  

The preceding relationships agree with the data for the Phantom F-4 within 10 per cent compared with those of a completely different aircraft, the Learjet of roughly 10 t studied in report CEA-IPSN, 1977, for various impact velocities. The influence of velocity too, according to these last data, is well represented by the above discussed formulae. 

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The overall load on a structure is also a function of the size of the structure. The effect of the lateral distribution of load local to an obstruction (front wall of a structure facing the blast) should be carefully analysed. For structures with depths less than about 50 m parallel to the direction of travel of the blast wave, the blast load would have largely passed the structure before the structure had time to respond to the blast wave since the wave is always travelling at or above the speed of sound. For a 50 m deep structure, the peak blast wave front would only engage the structure for about 0.02 s, which is typically well below the global fundamental natural period of the structure but not necessarily the local element response period. 

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