When a bridge is built as a straight line. The force exerted by a vehicle or body on the bridge is equal to the weight ( mg ) of that object.
But
When the bridge is constructed in a convex shape, then the objects (vehicles) moving on the bridge go in a circular path. Hence, the equation for balancing forces will be:
where
V = velocity of the vehicle.
r = radius of curvature of the bridge
N = normal force exerted by a bridge on the vehicle
Hence the force exerted by vehicles (called the load) is less than the weight ( mg ) of that object.
Thus the stress on the bridge reduces. Hence the stability of the bridge is better.