Abstract: The structural system reliability is an important factor of the structural overall performance. However, in civil engineering the concept of structural system reliability still stays at the stage of theoretical analysis, and there are hardly any practical applications due to the difficulties in calculation. With the high redundancy of civil engineering structures, the number of failure modes is numerous and the correlation between them is very complex. In the present paper, the property of hazard loads is studied by introducing the factor-load Roughness Index. Correlation between structural failure modes and the approximate approach of structural system reliability under hazard loads are studied. The following conclusions are reached: the structural failure modes under hazard loads are fully correlated and thus the structural system reliability under hazard loads can be determined by the weakest failure mode.