Traffic Q&A: The wild physics of bridges

Weather & Traffic Q&As

AUSTIN (KXAN) – You likely drive on a bridge every day and don’t realize it. These modern conveniences of transportation are so common that we tend to pass over them with little thought, but bridges are really cool and deserve our attention.

Let’s take a look at what they are, how they work, and a few different types that are common in Central Texas.

Bridges: The Basic

A bridge is any path that connects two points over seemingly impassable terrain. This could be a river, a canyon or just uneven land. How the bridge stays up depends on the type of bridge that’s used, but there are some general rules that apply.

Most bridges stay aloft because of the relationship between two forces: compression and tension.

Compression can best be described as materials pushing against each other. This could be the ground pushing against the steel beams holding a bridge up, or even the road pushing against itself. Compression gives a bridge it’s strength.

Tension is the opposite. It is two materials pulling apart. Tension can weaken a bridge, but can actually cause more compression if the bridge is designed properly.

Arch Bridges

In downtown Austin, the Lamar Boulevard and Congress Avenue bridges are Spandrel Arch Bridges. These bridges work by taking the weight of the road onto the top of an arch. The weight of the road then creates compression within the arch, with each stone pressing down on the one below it and then the ground pushing back.

Arch bridges are one of the oldest and most reliable types of bridges used around the world. Many arch bridges built in ancient Rome and China still stand today.

The Pennybacker Bridge, a little west of downtown on Loop 360, works in a similar way except the arch is above the street. This through arch bridge uses tension from below, created by the road, to cause the compression within the arch.

Through truss bridges

The Montopolis Bridge in east Austin used to be one of the major routes to the airport from downtown. Built in 1935, the bridge works uses a frame made of triangles to share the weight of the bridge. Each beam is experiencing either compression or tension forces, but not both at the same time.

In 2018, the bridge was closed to through traffic and converted into a pedestrian bridge.

Beam bridges

The most common type of bridge in Central Texas is a beam bridge. MoPac and Interstate 35 are both dotted with beam bridges.

These bridges places the road on giant pilons, with no other cabling or structure holding them up. Instead, tension is created on the top of the road by the materials pulling apart. While that same tension causes compression on the underside of the road. Think of it like a flatter arch.

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