Probability Distribution of Variables
Most civil engineering problems deal with quantitative measures using the familiar deterministic formulations of engineering problems. However, there is nothing pre-determined about the real-life case, for example, when you assume you have a reinforced concrete column in drawings mentioning that its section is 500 mm x 500 mm. This means it will be around this exact number when you measure the column. It is possible that there could be some deviation, which is allowable in the code.
The theory of random variables has enables the analyst to furnish useful substitutes for less precise qualitative characteristics. Variables, whose specific values cannot be predicted with certainty before an experiment, can be presented by the probabilistic models and distributions.