Truss Bridges: Advantages and Disadvantages

Thanks to the industrial revolution, the tradition of building bridges from stone became a thing of the past—reserved for the English countryside, Lord of the Rings novels and ancient European monuments. Readily available wood and soon thereafter, wrought iron, saw the arrival of truss bridges onto the 19^th century engineering scene.

Truss bridge design

In 1820, the first truss bridge design was patented, and the concept quickly took off, with engineers beginning to experiment with different truss styles. Before long there were dozens of different types of truss to excite the keen bridge-builder! The Warren, Pratt, Howe and K Truss remain some of the most widely used truss designs to this day.

Truss bridges are characterised by their interconnecting triangular structures, which give them the strength to withstand more heavy and dynamic loads than the bridges of old. Nowadays however, there are an abundance of bridge designs available to us. Suspension, beam, arch and cantilever designs are all able to cope with our modern-day heavy loads of traffic—so why choose a truss bridge? Well, despite the fact that they are a hobbyist model-builder’s dream, they pose a number of advantages and disadvantages. Let’s take a look at a few of them.

Advantages

Extremely strong

The structure of interconnecting triangles means that the load-bearing capacity of truss bridges is huge. The structure effectively manages both compression and tension, by spreading out the load from the roadway throughout its intricate structure. This means that no one part of the structure is carrying a disproportionate amount of weight. Sort of a ‘chain is only as strong as its weakest link’ type of situation.

Uses materials effectively

While the truss bridge has many, many linked parts to make up its structure—its use of materials is extremely effective. Materials such as wood, iron and steel are all utilised to their highest potential, and every piece plays a role. The building of a large truss bridge can be a very economical option, when compared to other bridge designs.

Withstands extreme conditions

Where other bridges such as beam and arch bridges may not be a viable option, truss bridges come into their own. They are able to span great lengths, and often used in precarious locations such as deep ravines between mountaintops. You’ll regularly see truss bridges in use throughout mountainous areas to carry railways.

Roadways built on to the structure

Unlike other bridge designs, the truss bridge is able to carry its roadway on its structure. The load can be carried above (deck truss), along the middle (through truss) or on a bottom truss, which sits below the major truss structure. The options make the truss bridge both versatile and economical to build.

Fun fact: the longest truss bridge in the world is the Ikitsuki Bridge in Japan, which stretches for 400 metres.  

Disadvantages

Requires a lot of space

The structure of a truss bridge is, by design, large. The interconnecting triangular components need to be large in order to bear and distribute heavy loads. This means that in certain restricted spaces, the truss bridge may not be the best option.

High maintenance costs

Economies of scale! The truss bridge uses a LOT of parts. Each of these are relatively light, and used effectively within the design, which means that if you’re building a huge truss bridge it is economically sensible. However—the maintenance costs of so many parts can be expensive. A truss bridge, like any load-bearing structure, will require regular and detailed maintenance. So many parts to look after can mean that this is expensive—not to mention time consuming!

How good is your engineer?

Truss bridges are intricate, complex structures. Each and every piece needs to fit perfectly in order to perform its function, and anything less will mean that the bridge simply does not hold a load. A truss bridge requires detailed engineering and specialist construction—this does not come cheap. You know—anyone can build a bridge, but it takes an engineer to get you over it.

They’re heavy

Because truss bridges are so large, and use a lot of materials, they are heavy. Depending on the landscape supporting the bridge, some reinforcement may be necessary to cope with the weight. There may be other bridge options such as suspension or beam bridges, which might be more suitable, if your landscape can’t support a truss bridge.

As one of the oldest styles of ‘modern day’ bridges, the truss has been a trusty go-to design for many an engineer, since the early 19th century. Whether you’re building over steep ravines, crossing vast lakes or planning a creepy B-grade railway horror movie—there’s a truss bridge to suit your needs.