Is it possible that the people who set the constraints are different from the people who design the bridge, who again are different from the people who approve

Is it possible that the people who set the constraints are different from the people who design the bridge, who again are different from the people who approve the design? Yes, that's how it works in the real world. As a design engineer, all you can do is explain to the stakeholders how the constraints will affect the outcome and suggest alternatives. Ultimately, the engineers will have to work with what they are given, and as long as the outcome is safe and its limitations are communicated, they can't be blamed.