Engineers choose from many bridge designs because each one handles force in a different way. Building three bridges and placing weights on each reveals how well each structure spreads a heavy load across its members. The amount of flex tells you which design resists force the best, showing that shape determines how load distributes across a structure.

Force distribution depends on the relationship between an arch's width and its height. As the arch gets wider while staying the same height, the span increases and the structure holds less weight before it collapses. The wider arches fail under less weight, showing that increasing a dome's diameter without raising its height weakens the structure because the shape can no longer spread the load across itself as effectively.

Arches distribute force along their curve, spreading a heavy load so no single spot bears all the weight. The arch's height changes how well this spreading works. A 650 mm strip bent into an arch between pillars set 500 mm apart holds 16.5 kg, more than shorter or longer strips. The taller curve gives force a longer path to travel, distributing the load more evenly across the shape.