State Road 414 Bridge

Soon after the bridge was opened to traffic, motorists began to report that its distinctive arches were visibly moving in light winds. Subsequent study by RWDI indicated that the arches moved – consistently and conspicuously – in winds of just eight miles per hour. Our wind tunnel analysis also identified the reason: eight-mile winds produced vortex shedding that matched the arches’ natural frequency. The issue was further compounded by the structure’s very low inherent damping, which was measured to be only 0.03% of critical – meaning that without some intervention, it would take a long time for vibration in the structure to subside once it had begun.

The movement presented no immediate safety threat since the arches were not important to the bridge’s structural stability, but it could be distracting to motorists both on the bridge and on the highway below. Over time, it would also cause fatigue damage to accumulate in the joints that connected the arches to the bridge deck. Once we’d helped the bridge’s operators to understand why the arches were moving so noticeably, they engaged us to find a way to mitigate the wind-induced motion.

We worked closely with our clients to understand the unique design constraints of the bridge. One consideration was aesthetic: the arches had been added to create a visual effect, so any solution to the wind-induced motion challenge should avoid detracting from their aesthetic impact. Florida’s hurricane-prone climate was also a consideration: our team needed to devise a damping solution that would, like the rest of the bridge, be resilient to extreme wind.

We developed a set of tuned mass dampers (TMDs) that would sit inconspicuously between the double arches. Each TMD contained a leaf spring: a simple device invented in the 18^th century, composed of a fixed centre and two flexible ends, like the wings of a bird. One early leaf spring application was on horse-drawn carriages: the central piece (the bird’s body) would be affixed to the axle and the “wings” would hold the chassis, absorbing shocks from bumpy roads.

In our TMD design, the central piece is affixed to the bridge arches, while each of the flexible wings holds two 150-pound weights. The position of the weights was adjusted to ensure that the movement of each damper matched the arches’ natural frequency of 1.34 Hz. When the wind begins to act on the arches, the weights suspended from the wings offer passive resistance, dissipating the wind’s energy and preventing the arches from visibly moving.

Our solution includes four TMDs in total, two installed on each double arch – at the 1/3 and 2/3 marks respectively. We ensured that all the devices were robustly designed and securely installed, with redundancy to ensure that they would perform safely and effectively in Florida’s sometimes volatile wind climate.

Our TMDs were installed in 2012 and there have been no further reports of arch movement since.