Reason for kolkata bridge collapse

Not exact but the only reason which caused collapse was overconfidence, mostly senior and experienced engineers check the design thrice or more than that before execution(in first check though they know the design is correct but they do confirm by check another few times) but youngsters get overwhelmed at the first itself that the design is accurate and applicable. As stated by IIT-Kharagpur the design was faulty and as well as the execution both can be termed as HUMAN ERRORS so in the field of construction everything must be checked even though we know everything is fine but a simple crack or a simple mistake can take lives of many.