Tube reliability from the tube manufacturer's viewpoint

As part of the investigation of electron tube reliability, it is well to examine other products for which there is no reliability problem so that we may determine the nature of the various factors involved. One such product is a steel beam. We never talk about reliable steel beams and there must be a good reason for this. Is it not that the requirements on steel beams are well known, and that engineers use them well within their capabilities by making use of well-defined safety factors?