Fig. 11 shows a simple bridge structure made from steel members. The metal has been selected for its specific properties that make it suitable for this appl...

(a) The structural technique used in the bridge framework is triangulation [1]. The diagonal members divide the rectangular panels of the frame into triangles, creating a rigid structure known as a truss.

(b) This technique makes the structure rigid because a triangle is the only polygon that cannot be distorted under load without changing the length of its sides [1]. A rectangular or square frame can collapse into a parallelogram when a sideways force is applied, because the angles at the joints can change. In a triangle, the three sides lock the angles in place, making the shape inherently rigid. The diagonal members distribute forces along their length as tension or compression, so the entire frame resists deformation.

(c) The steel plates used at the joints labelled A to reinforce the connections are gusset plates [1]. These are flat plates, typically rectangular or triangular, that are bolted, riveted, or welded across the joint where two or more structural members meet. They spread the concentrated forces at the joint over a larger area of each member, preventing localised failure and stiffening the connection.

(d) The top horizontal member is in compression when the load is applied to the bridge deck [1]. When a load pushes down on the bridge, the top chord of the truss is squeezed together (shortened) as the structure tries to sag in the middle. The bottom chord, by contrast, is in tension (being pulled apart).