The operation of a fixed trunnion bascule bridge is modeled after a balanced seesaw. On the playground, an ideal balanced seesaw has both the weights and the distances (arms) from the pivot point equal (as shown below). However, for a bridge, having the lengths of the counterweight side and the leaf side equal, is impractical.
The concept of torque can be used to modify the design of the top seesaw to make it more suitable for bridge applications. Torque is a rotational force. A good example is the loosening of a nut by a wrench. The force applied at the end of the wrench results in rotational force (torque) applied to the nut. If you have had to loosen a particularly stubborn nut, you have no doubt noticed that a longer wrench is more effective than a shorter one. This is because the amount of torque is calculated by multiplying the applied force by its distance from the pivot point. Equivalent torques can be produced by either changing the amount of force applied or by modifying distance from the force to the pivot point.
In the playground seesaw shown above, with everything being equal, the torques are also equal. To maintain a balanced seesaw with different arm lengths, the weights will have to change accordingly. In the example in the seesaw below, reducing the counterweight arm by one-fourth, requires that the weight of the counterweight be increased by a factor of four. This represents the bridge leaf of the fixed trunnion bascule bridge.