Did you know that bridges are intentionally designed to move? Bridges join gaps we’d otherwise have a hard time crossing, so as disturbing as this sounds, it’s true. Engineers build bridges to sway and move side to side, so they don’t break but expand and contract.
Bridges expand and contract thanks to thermal expansion and contraction. The former increases a material’s volume, and the latter shrinks it. Both happen due to temperature changes. Bridges have expansion joints built, so movement from thermal differentials doesn’t damage them.
Thermal expansion and contraction aren’t the most accessible subjects in the world, and neither is engineering. Don’t worry, though; you’ll get a quick and easily understandable explanation for both below.
Why Can Bridges Move?
Bridges can move because everything on and around them can move also. In the event of an earthquake, certain buildings can rock back and forth so that they don’t collapse. Bridge swaying works on the same principle – protection.
A bridge may move in the wind and bounce with traffic, so they don’t snap or break against the movement. Another great comparison is young trees. When trees are saplings, they’re relatively elastic, and this rubber-like nature helps them withstand strong winds.
However, there are instances where a moving bridge isn’t something you’d want. You may have heard of it, but the first Tacoma Narrows Bridge is a particularly infamous example of a frighteningly wobbling bridge.
Generally, there are 4 reasons bridges fail: resonance, aeroelastic flutter, and thermal expansion and contraction (thermal movement).
Let’s go over resonance and flutter first.
The Tacoma Narrows Bridge Went Down in 1940
The Tacoma Narrows Bridge’s first iteration eventually went down on November 7, 1940, but a lot led to the spectacular failure. It took a little over a year to build the bridge (19 months), but by the time the project was finally done and opened on July 1, 1940, the builders realized that it swayed far too much in the wind.
Unfortunately, Puget Sound, where construction teams built the Tacoma Narrows, had many windy days. The bridge was too narrow and loosely structured to keep stable in the wind, so much so that people began calling it “Galloping Gertie.”
There were various attempts to keep the original Tacoma Narrows from shaking, but it wasn’t enough – come November 7, the bridge went down. You can see footage of the bridge falling apart here
Why the Tacoma Narrows Collapsed
One reason people suspected for years why the bridge collapsed was resonance, which is one of the negative reasons bridges move. It’s what happens when the frequency of a bridge and the wind synchronize. When resonant frequencies build up too much energy, the objects holding said energy collapse.
But the real reason why the Tacoma Narrows Bridge catastrophically failed has to do with aeroelastic flutter. Flutter is yet another “negative bridge movement,” and it happens when “rubber-like” structures move too much in a “fluid flow,” which in the Tacoma Narrrow’s case was windy weather.
Windy weather began the bridge’s twisting, but Galloping Gertie kept moving under its own power after a while. But the more the Tacoma Narrows twisted around, the more energy it built, which enabled even more twisting. Once the flutter started, there was no stopping it.
Why Do Bridges Expand and Contract?
Now let’s go over what thermal expansion and contraction or thermal movement is.
Thermal movement is when matter stretches out or shrinks in reaction to temperature changes. To specify, when objects are heated, they get larger. Thermal contraction is when matter diminishes and becomes smaller.
Bridges expand and contract because of thermal expansion. Expansion happens when molecules inside of objects become spread further apart. Thermal contraction is when molecules become closer together – or the thing is pressing into itself.
If you’d like to know more information on thermal expansion and contraction, then you can watch this video
What Thermal Movement Has To Do With Bridges
You’re probably wondering what thermal movement has to do with why and how bridges move.
To put it simply, when things such as buildings, railroads, and bridges expand or contract, they move, and this movement can have disastrous consequences.
Like the above Tacoma Narrows Bridge example, weather plays an enormous part in engineering projects. When the weather becomes too hot or too cold, it can cause bridges to distort, shrink or swell, which results in breaks and cracks.
The contraction and expansion may not seem like a huge deal at first, but eventually, repeated cycles will become problematic.
If it helps, think of how inflatable balls, like basketballs and soccer balls, expand in the summer and shrink in the winter. Unlike your soccer ball, though, it’s imperative to consider how the weather will affect construction projects, much less a bridge.
What Are Expansion Joints and What Is Their Function?
An expansion (or movement) joint is used by civil engineers to mitigate thermal movement. It’s a construction component builders use to hold structures together in the event of contraction or expansion caused by temperature changes.
Additionally, expansion joints can help reduce resonant frequency build-up and reduce sway and potential collapse. A movement joint can be made of many materials and come in various sizes; copper is the most common material used because it’s a sturdy and malleable metal.
Movement joints are integral parts of building, sidewalk, railroad, and bridge construction. Their primary function is to mitigate any potential give in a bridge. Builders also use these joints to ensure that traffic can continuously and safely move over bridges.
Expansion joints keep bridges together in the event of extreme weather and make it easier to replace bearings. Bridge bearings transfer load (weight) away from the bridge superstructure and enable regulated movement. So it’s crucial construction crews can reach them quickly if they’re damaged.
There are three general types of expansion joints: small, medium, and large.
Small movement joints allow for up to 45 mm (1.77 inches) of sway, and medium movement joints permit bridges to move from 45 to 130 mm (1.77 to 5.11 inches). Large movement joints, meanwhile, let bridges shift past 130 mm (5.11 inches).
There are also different types of joints for different scenarios. For example, finger plate joints are suitable for medium and long-length bridges. Stip seal joints, on the other hand, have the most negligible impact on bridge traffic. Poured sealant joints (great for speedy repairs) and asphaltic plug joints (useful for smooth roads) are also options.
Bridges can move for two reasons: engineers build them to move, or sometimes natural forces (or poor construction) cause movement. The harmful types of shifting that can affect bridges are resonant frequencies, aeroelastic flutter, and thermal expansion and contraction.
Thermal movement is what happens when bridges expand or contract, depending on hot or cold weather respectively.
Expansion joints are important construction components because they stop thermal activity and other kinds of harmful shifting from damaging and destroying bridges. They also help traffic move along without interruption and make it easier to replace bridge bearings that carry weight and assist with reducing thermal movement.
- The New York Times: Buildings Can Be Designed to Withstand Earthquakes. Why Doesn’t the U.S. Build More of Them?
- Exploratorium: Faultline: Seismic Science at the Epicenter
- Construction News: Bridge Expansion Joints and Their Benefits
- Wikipedia: Tacoma Narrows Bridge (1940)
- APS Physics: November 7, 1940: Collapse of the Tacoma Narrows Bridge
- Science Direct: Expansion Joints
- Wikipedia: Expansion joint
- Forbes: Science Busts The Biggest Myth Ever About Why Bridges Collapse
- Bridge Masters Incorporated: Aeroelastic Flutter & the Collapse of the Tacoma Narrows Bridge
- Wikipedia: Aeroelasticity
- Practical Engineering: Why do Bridges Move?
- Popular Mechanics: How and Why Bridges Are Made to Move
- Popular Science: We’d rather that bridges never wobbled – but here’s why they do