The reference “NOTES ON BRIDGE FAILURES, Chapter V Suspension Bridge Accidents Prior to 1940” lists accident records of 111 bridges. Using these records, specifically those with clear construction years (60 collapsed bridges and 20 bridges involved in accidents) to classify the cause of the failure. Based on the relationship between this classification and the first specification (The world’s first comprehensive bridge specifications, e.g., “General Specification for Iron and Steel Railroad Bridges and Viaducts” of the United States of America, 1890.), there are differences in failure causes before and after the first specification. Before the first specification, quantifiable uncertainties caused the most failures. However, after the first specification, unknown uncertainty, which cannot be quantified, became the main cause of failure. Early suspension bridges, which existed before the first specification, were characterized by poor deck stability, weaknesses in the major cable members, and failure of the components that make up the cable members due to corrosion, each of which could lead to the failure of the entire structure. These factors are crucial in modern bridge failures, and we defined them as “lack of out-of-plane stability,’“‘lack of structural stability,’‘and “defective performance of materials and joints,’‘respectively. Further, we provide examples of these failure forms and discuss their relationship with unknown uncertainties.