Why does a suspension bridge need a stiff or heavy deck?

A suspension bridge requires a heavy or stiff deck primarily to ensure stability and prevent excessive movement under various loading conditions. Because the deck is suspended in the air, it is susceptible to dangerous vertical or torsional (twisting) oscillations caused by dynamic wind loads.123

If a deck is too thin or lacks proper support, these vibrations can lead to structural failure, as famously demonstrated by the collapse of the Tacoma Narrows Bridge in 1940. To mitigate these risks, engineers incorporate stiffness—often through the use of web trusses or aerodynamically stable box girders—or sufficient weight to dampen movement.31

While increasing weight can help stabilize a structure, it is important to note that for earthquake resistance, engineers generally prefer lighter dead weights, as horizontal forces from ground acceleration are proportional to the structure's mass. Consequently, modern design often relies on deflection theory, which calculates how the deck and cables work together to manage loads, allowing for optimized stiffness as span lengths increase.31