Suspension Bridges and the Effects of Wind
By Joan Whetzel
Suspension bridges span rivers and shipping canals, and by their very nature, must be flexible and as light weight as possible. However, they are susceptible to collapse if all the required elements are not taken into account during the design and construction process. Bridge engineers must design around the weight of the cars driving across the bridge on an hourly, daily, yearly basis. They must also consider the weight of the materials used to construct the bridge, the effect of the river (motion of the water on the bridge supports), and even the weather - most especially, the wind. On November 7, 1940, the suspension bridge known as the Tacoma Narrows Bridge (nicknamed Galloping Gertie) collapsed due to just such an engineering failure, which took 70 years to fully comprehend.
The main components of suspension bridge construction includes steel cables, two pillars centered along the bridge span, concrete anchorages on opposite shorelines, and a deck. The two main cables (pilot cables) extend from a concrete anchor on one end of the bridge, through a pillar saddle in the center of the bridge, to a concrete anchor on the far side of the bridge. A set of evenly spaced vertical cables connect the bridge span to the main cables, suspending it above the river. Two pillars (towers), made of concrete, are situated along both sides of the deck, about halfway between the concrete anchors. The anchors are made from molded concrete (constructed to resist the tension from the cables) and steel eye-bars for attaching the main cables. The deck (a.k.a. the span length, roadbed) is steel reinforced concrete, that is joined to the main cables, by way of the vertical cables. The construction of the span begins at the center point, where it connects to the pillars.
The engineers that designed the Tacoma Narrows Bridge, at the time of its construction, were of the opinion that bridges built thin, and constructed of light-weight materials, would not only be durable, but safe and dependable. However, only 4 months after opening to traffic, the bridge began a bucking and rolling motion that, to drivers, felt much like a roller coaster ride (hence the nickname Galloping Gertie). The twisting and roiling action became so intense that it heaved the main cables over 100 feet into the air. It caused the main cables to pull out of their tower saddles, then crash back down onto the bridge spans. Finally, the bundle of wires making up the main cable on the Northern side of the bridge, frayed and ripped apart, causing the remaining wires to stretch beyond their limits. With no support system holding it up, the concrete and steel bridge span crumbled and fell into the river.
Taking the Wind into Account
After decades of studying the bridge designs, the construction techniques of the time, and weather conditions it was determined that the engineers had not taken the effect of wind, especially the force from wind traveling down a river channel, into account when designing the bridges. Wind speed and intensity for the channel to be spanned should be determined before any bridge designs are drawn up. The wind load calculation must be a major part of the bridge design process, so that wind vibrations can be visualized on scale models - before the investment in materials and man-hours - and adjustments to the design made. In Galloping Gertie's case, the ration between the depth and the width of the road bead were too great, causing it to be far too elastic, flexible, or bendable. Steel girders, like those used on the Tacoma Narrows Bridge, are flexible. The high winds traveling along the river channel placed a great amount of stress on the steel, bending it out of shape. The higher the span is above the river, the greater the wind load that is placed on the span.
What's happening is that, the wind sets up an "harmonic vibration pattern" (turbulence) that sets the ball in motion. The wind blowing across the span pushes downward, and the wind blowing below the span pushes upward. The strong wind load along with the weight of all those cars, cause the cables and the span to begin flexing. The upward and downward motion and the flexing builds up momentum and torsion until the span begins its undulating, roller-coaster-like dance, ending in a catastrophic failure of the suspension bridge.
Causes of the Tacoma Narrows Bridge Collapse
The Tacoma Narrows engineers worked under strict government supervision, due to the cost of the project. However, that did not prevent the failure of this bridge. Ultimately, it was decided that the Tacoma Narrows Bridge collapse came down to 3 causes.
- The span was design and construction was too light weight, too thin, and too flexible or elastic.
- The design and construction of the span's steel girders and concrete created a "lift and drag" motion, much like that required around airplane wings, allowing the airplanes to fly.
- The engineers didn't understand the wind load concept, or the aerodynamics of river channels well enough to do a wind load test during their design process.
What Came Out of the Tacoma Narrows Bridge Failure
The Tacoma Narrows Bridge failure produced a wealth of engineering knowledge that has lead to stronger, sturdier - and safe - suspension bridges. The knowledge from this failure has lead to success in:
- reducing wind drag.
- counteracting the wind load with larger diameter cables, greater vertical cable lengths, and larger bridge spans (thicker and wider).
- wind load testing at varying wind speeds to assess the bridge design for sideways (lateral) and twisting (torsional) motions caused by the high winds.
- reinforcement of the decks for reduced flexibility, as well as lateral and torsional buckling from the wind.
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