ABSTRACT
5.1 Background
Reliability theory arose out of the engineer's practice of trial and error. Before the theories of statics and dynamics were developed, structures were designed and constructed very much on the basis of experience, being informed by the performance of earlier structures. After Newton's work on forces and dynamics, a consistent framework for designing structures evolved that was based on the principles of mechanics. Structures were still not perfect because of several factors. Firstly, the behaviour and performance of materials such as timber, stone and brick depend on their chemical and physical microstructure, which can vary between individual beams or blocks. No two timber beams are identical because the growth rates and internal structure of the fibres differ. Similarly, no two stone blocks are identical at the microscale because the arrangement of the crystalline structure is not repeated exactly from one block to the next. Construction materials have associated with them some uncertainty in their performance due to unavoidable small-scale differences in their composition. Secondly, the diligence with which the construction of a structure follows the design is important. Errors and carelessness in the process of construction can lead to underperformance of the structure. Thirdly, the loads that the structure is intended to withstand may be underestimated. Finally, the prevailing environmental conditions may alter to an extent that diminishes the integrity of the structure. For example, tunnelling beneath buildings can weaken the load-bearing capacity of the earth, leading to subsidence or collapse. Similarly, changes in river flow due to the construction of weirs and dams may lead to higher water levels than previously experienced and designed for along some reaches, thereby leading to localised flooding.
