Examples of categorization of seismic load for civil engineering and mechanical engineering

Both civil and mechanical engineers come across loads of the force-controlled type (qualified as “primary”) or of the displacement-controlled type (qualified as “secondary”). For decades, different authors have been debating whether a seismic load should be classified as primary or secondary. The purpose of this paper is to bring some clarity to the subject by considering oscillators that are characterized by their ductile capacity, μ u, and the corresponding “effective stiffness factor”, τu. It is concluded that a seismic input cannot be a priori regarded as primary or secondary. Both τu, and the slope, p, of the input spectrum in the vicinity of the (dominant) natural frequency of the structure or component should be considered concurrently when making a decision about stress categorization. It appears that τu, is depending on μ u, and differs from concrete structures to piping components and piping systems. Conclusions are drawn accordingly on seismic load categorization.