A Review of Some Statistical Concepts

Abstract

The forces measured on existing ocean structures have rarely had simple time histories• As a result, the structural responses such as deflection, strain, and acceleration to these “random” type forces do not have simple time histories either. Typical data for either the time behavior of a force or the response to that force at a point on an ocean structure might look like the trace shown in Fig. 9.1, where, for generality, the ordinate is designated as z = z(t). At first glance, such a trace appears to have no definite frequency of fluctuation, although there appears to the an upper bound for z. If z were identified as a force, one might be tempted to simply use the upper bound for z and design a structure according to classical static methods. One might ask, the, what if an approximate forcing frequency could be associated with z over a short time period and what if this frequency were tuned to the natural frequwncy of the structure? Could no the structural deflections then exceed the static design conditions? It is the purpose of this chapter to define som of the statistical tools with an engineer can obtain some answers to these questins. Crandall and Mark (1964) elaborate on these ideas.