How did the engineers mess up the design of the Tacoma narrow bridge?

The engineers underestimated the effects of wind on the Tacoma Narrows Bridge's design, opting for a slender and lightweight structure without considering aerodynamic stability. The bridge's flexibility and lack of sufficient damping allowed it to oscillate violently in windy conditions, leading to a phenomenon known as aeroelastic flutter. This design oversight culminated in the bridge's dramatic collapse in 1940, just months after its opening. The failure highlighted the importance of considering wind dynamics in the design of suspension bridges.