The observation of an existing structure supporting a particular maximal load provides a direct constraint on the possible range of values its resistance capacity may take. The implied update of structural reliability allows monitoring and maintenance planning to be done from a risk optimal perspective. Existing proof load-based reliability updating techniques require multiple numerical computations which are often too cumbersome for routine use. By building on the assumptions of the first order reliability method, this study develops and validates a first order reliability updating approach which is computationally efficient. The resulting formulation is shown to be applicable to reliability problems tractably considered using the first order reliability method. This method is illustrated for two example structures: a reinforced concrete beam forming part of a highway bridge to which traffic loading data is applied, and a granular embankment forming a seawall on a shoreline mining operation for which the phreatic surface level is monitored.