Tacoma Narrows Bridge Collapse

posted September 29, 2015 at 11:26 am in Design

TacomaNarrowsBridgeCollapse_220
The collapse of the Tahoma Narrows Bridge, on 7th November 1940, is a classic textbook lesson on bridge design  The bridge was designed with a slim, elegant design together with cost savings from shallow supports. Bridge design in the 1930s had forgotten key features that 19th century designers had learnt.

Early suspension bridge failures resulted from light spans with very flexible decks that were vulnerable to wind (aerodynamic) forces. In the late 19th century engineers moved toward very stiff and heavy suspension bridges.

Only a handful of skilled engineers had the opportunity to build a large, highly expensive bridge building project. There were no peers to remind them of mistakes of the past. This was the root of the problem. According to bridge historian David P. Billington, at that time among suspension bridge engineers, “there seemed to be almost no recognition that wind created vertical movement at all.”

Bridge Construction

The bridge, situated in the US state of Washington, was opened to traffic on July 1, 1940, and dramatically collapsed into Puget Sound on November 7 of the same year. At the time of its construction (and its destruction), the bridge was the third longest suspension bridge in the world in terms of main span length, behind the Golden Gate Bridge and the George Washington Bridge.

Construction on the bridge began in September 1938. From the time the deck was built, it began to move vertically in windy conditions, which led to construction workers giving the bridge the nickname Galloping Gertie. The motion was observed even when the bridge opened to the public. Several measures aimed at stopping the motion were ineffective, and the bridge’s main span finally collapsed under 40-mile-per-hour (64 km/h) wind conditions the morning of November 7, 1940.

The wind-induced collapse occurred on November 7, 1940, at 11:00 a.m. (Pacific Time), because of a physical phenomenon known as aeroelastic flutter.

Day of the Collapse

Leonard Coatsworth, a Tacoma News Tribune editor, was the last person to drive on the bridge:

“Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car…I jammed on the brakes and got out, only to be thrown onto my face against the curb…Around me I could hear concrete cracking…The car itself began to slide from side to side of the roadway.

On hands and knees most of the time, I crawled 500 yards (460 m) or more to the towers…My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb…Toward the last, I risked rising to my feet and running a few yards at a time…Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.”

No human life was lost in the collapse of the bridge. Tubby, a black male cocker spaniel, was the only fatality of the Tacoma Narrows Bridge disaster; he was lost along with Coatsworth’s car. Professor Farquharson and a news photographer attempted to rescue Tubby during a lull, but the dog was too terrified to leave the car and bit one of the rescuers. Tubby died when the bridge fell, and neither his body nor the car were ever recovered. Coatsworth had been driving Tubby back to his daughter, who owned the dog. Coatsworth received US$450.00 (US$7,600 with inflation) for his car and US$364.40 (US$6,100 with inflation[ in reimbursement for the contents of his car, including Tubby.

Cause of the collapse

The original Tacoma Narrows Bridge was solidly built, with girders of carbon steel anchored in huge blocks of concrete. Preceding designs typically had open lattice beam trusses underneath the roadbed. This bridge was the first of its type to employ plate girders (pairs of deep I-beams) to support the roadbed. With the earlier designs any wind would simply pass through the truss, but in the new design the wind would be diverted above and below the structure. Shortly after construction finished at the end of June (opened to traffic on July 1, 1940), it was discovered that the bridge would sway and buckle dangerously in relatively mild windy conditions that are common for the area, and worse during severe winds. This vibration was transverse, one-half of the central span rising while the other lowered. Drivers would see cars approaching from the other direction rise and fall, riding the violent energy wave through the bridge. However, at that time the mass of the bridge was considered to be sufficient to keep it structurally sound.

The failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 miles per hour (64 km/h). This is a so-called torsional vibration mode (which is different from the transversal or longitudinal vibration mode), whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line of the road remaining still. Specifically, it was the “second” torsional mode, in which the midpoint of the bridge remained motionless while the two halves of the bridge twisted in opposite directions. Two men proved this point by walking along the center line, unaffected by the flapping of the roadway rising and falling to each side. This vibration was caused by aeroelastic fluttering.

 

Post a comment