Above: High winds through the strait can cause loud sounds for communities living on either side of the Golden Gate Bridge

August, 2023- Extruder designs custom product to prevent excess noise during high winds through the Golden Gate Bridge.

One of the most iconic structures in North America, the Golden Gate Bridge, opened in May 1937. It straddles the strait between San Francisco Bay and the Pacific Ocean. This monumental beauty stretching 1.7 miles, 90 feet wide, 500 feet tall at the towers and hovering 220 feet above the water needs constant maintenance.

When the Golden Gate Bridge, Highway and Transportation District realized it had a sound problem during high winds, it turned to Richardson Metals Inc. to produce a solution.

Richardson Metals Inc. is a precision aluminum extruder specializing in close-tolerance extrusions, rapid prototype extrusions and small- to medium-quantity production runs. The Colorado Springsbased company also provides cut-to-length, CNC machining, drilling, tapping, anodizing and powder coating services.

      When the bridge authority had a noise problem, it found an extruder that could make a clip, with a rubber insert, that would dampen the sound.

The district first conducted extensive analysis and testing, then developed its idea to install aluminum U-shaped clips with a rubber lining to eliminate many of the wind-induced sounds altogether and make most remaining sounds inaudible.

“Tones may still be audible at the bridge during rare instances of very specific and severe wind conditions,” the agency states. “However, the proposed solution was shown to reduce those sounds by 75 percent” compared with bridge railing that had no such solution installed.

12,000 PARTS

Designed by district engineers and leading bridge aerodynamics and acoustics experts, thin, Ushaped clips are being attached to both edges of all 12,000 vertical slats on the west railing of the bridge. That railing was retrofitted in 2021.

    Extrusion die created by Richardson Metals to produce the clips used to dampen loud noises during high winds that strike the Golden Gate Bridge.

The clips, made from 1/8-inch-thick grade 6061 aluminum, cover the edge of every slat from top to bottom, and the thin rubber sleeve underneath dampens vibrations that contribute to the sounds. The rubber insert also prevents the aluminum from touching the steel railings, which prevents corrosion.

The clips are painted in the Golden Gate Bridge’s famous international orange, making them invisible to most bridge users.

The U clips work to reduce or eliminate sounds by lightly disrupting the flow of air as it passes over each slat, minimizing “vortex shedding,” or the creation of small vibrations in the air, according to district engineers.

Sounds are created when these vibrations match the natural structural vibrations of the slats or of the air between the slats, which are then propagated in sound waves, similar to a cone speaker. By disrupting airflow, the clips and rubber sleeves reduce the vibrations in both the air and slats, thereby eliminating or reducing the sounds to mostly inaudible levels.

The district estimated the cost to fabricate the clips and rubber inserts for the 12,000 affected slats at $450,000. Installation will be completed this year. Engineers tested the clip design in a wind tunnel under 110 different combinations of wind speed and direction, where it was shown to eliminate or make inaudible the sounds in all but two very specific and severe wind scenarios.

The wind retrofit “is necessary to ensure the structural integrity of the bridge is not jeopardized during high winds and will allow the bridge to withstand extreme sustained winds up to 100 miles per hour.” This “is especially important in the context of increasing severe weather events due to climate change.” John Tassone, vice president of Richardson Metals, notes that such projects take “forever to get approvals” but the company has made three shipments of the custom clips so far.

MACHINE INSTALLATION

Over the last couple years, Richardson Metals purchased two new 4-axis CNC machines. “We can hold machine tolerances on extruded material. But sometimes the customers want better than that, so we will cut pieces to length to produce to a final dimension. We won’t only make 10-foot bars but we will create a lot of pieces and parts and create preassembly kits. We supply finished parts, too,” according to Tassone. Before the company had that inside capability, its customers would buy 10- to 12-foot-long extrusions and send them to a machine shop to have machining and cut to length performed. “We now do all of that; we are a one-stop shop,” Tassone says.

Richardson Metals Inc., 800/635-1994, http://richardsonmetals.com/