Why is deflection important in structural design?

Deflection is the displacement of a structural element under load and is critical for serviceability design. Excessive deflection can cause cracking in partitions, damage to finishes, psychological discomfort to occupants, and ponding of water on roofs. Design codes limit deflection to span/250 or span/360 depending on conditions. While a structure may be strong enough, it must also be stiff enough to limit deflections.