Zero! As a designer I do the calculations and drawings for bridges and then a ‘Contractor’ (person or company) do the actual construction work. Sometimes I do go on site and help inspect or supervise the work to make sure they are built correctly.
I have designed many other bridges at ‘Structures Options’ stage. This is when we are investigating different types of bridge for a project and assessing which is best from a cost, safety, environmental, construction, aesthetic, function point of view.