When approaching certain bridges, you might come across a road sign noting there are “open joints.” Because these are warning signs, they’ll often be diamond-shaped with a yellow background and black lettering. Sometimes, they’ll also include an illustration of a bicycle or motorcycle. Essentially, these signs are meant to call attention to small gaps in the road ahead that could cause two-wheeled vehicles to crash if the rider isn’t prepared. Motorcycles are dangerous, according to NHTSA data, so providing riders with a heads-up that there will be small breaks in the pavement helps prevent accidents.

Although bridges are built to be incredibly strong, they are also subject to certain forces. Even the biggest bridges ever built must contend with temperature changes. Depending on the weather, the materials that make up the bridge can either expand or contract. Expansion joints are used to allow the sections of the bridge a bit of room to move with this thermal activity. Without this flexibility, the concrete of a bridge can crack, causing structural issues. 

Open joints are just one type of expansion joint, and their design allows water to drain off of the road surface. There are also finger joints, sliding steel plate joints, and plug joints, to name a few. According to bridgematerials.com, a 328-foot (100-meter) concrete bridge deck can see just over 2 inches (55 mm) of thermal movement on a hot summer’s day. The extent of expansion and contraction also increases with the bridge’s length.

Expansion joints for managing thermal movement aren’t without their problems. They can account for up to 20% of the total cost of maintaining a bridge, as they require continual attention. Open joints, for example, allow water from the bridge deck to oversaturate the ground under the bridge, potentially affecting the foundation. Ensuring a solid foundation is one of the most significant challenges for bridges that are built over water.

However, not all bridges have expansion joints; some use other methods to provide flexibility. For instance, a semi-integral bridge design uses a system in which the columns remain rigid, while the deck above can move slightly back and forth, eliminating the need for traditional expansion joints. This design prevents drainage from running down the supports of the bridge (one of the issues with open joints). However, this option is only recommended for spans of under 197 feet (60 meters).

Another bridge design that replaces traditional expansion joints is being used on a section of the Austrian Autobahn. It features sections at either end of the bridge that are much wider than the expansion joints, allowing the structure to move. It also includes many separate pieces in sequence, which are attached via specialized glass-fiber cabling with elastic properties. As the concrete expands, the separate bits of the bridge remain evenly distributed across the span.