"Duty" is a term of art, it usually means an obligation imposed by law.
Public policy plays a big role in determining who has a duty to do what. In
a building construction setting , there is a major publc policy to ensure
the safety of the public. By imposing such a duty on the structural engneer
"in charge", it seems it is not an unreasonable burden on the engineering
community considering the overwhelming benefit to the public safety in
Civil/Structural engineer, Attorney at Law