These comparison tables illustrate the differences that can exist when the structural steel models are not included in the analysis. In some cases, the results with the structural steel included are many times higher than the results computed without the structural steel. The steel models add flexibility to the piping system. More flexibility means lower natural frequencies and more modes to be excited by the shock. A comparison of the natural frequencies of the two models is given as follows:
In the previous table, there are only five extra mode shapes for the system.
The restraint moment at node 55 in the Z-direction is much larger without the steel model than it is with the steel model. Even though the piping is tied to the steel, the steel frame does not support much moment in the Z-direction. The steel frame bends slightly about the Z-axis, and the moment is carried through from the pipe. In the piping only model, the rigid anchor at node 55 does not rotate about the Z-axis (or any other axis), and ends up carrying the entire moment load.