In this simulation we used 5% of critical damping in all modes. This value was chosen from experience, based on the fact that the bolted connections between the trusses and the cross bracing might absorb significant energy as a result of local frictional effects. In cases such as this, where accurate data are not available, it is important to investigate the effect of the choices that you make.

Figure 9–13 compares the history of the reaction force at one of the top connections when 1%, 5%, and 10% of critical damping are used.

As expected, the oscillations at lower damping levels do not diminish as quickly as those at higher damping levels, and the peak force is higher in the models with lower damping. With damping even as low as 1%, the peak pull-out force is 85 kN, which is still less than the strength of the connection (100 kN). Therefore, the cargo crane should retain its integrity under this drop load.