The four most basic categories in the truss bridge arena are the Warren, Pratt, Howe and K Truss. Each utilises the basic ‘triangle’ design, characteristic of the truss bridge, however, each varies slightly in the way they distribute compression and tension.
In our earlier article, Truss Bridges: Advantages and Disadvantages we looked at the merits of their design, cost, and versatility versus their disadvantages. In this article, we’ll take a closer look at each of the basic truss design variations.
Woops…wait, wrong Warren Truss! (How many can there be?) Hang on a minute…
There we go, that’s better!
The Warren truss, despite what the name suggests, was not created by the Australian National Party leader; in fact it earned its moniker from James Warren who patented the design in 1848, in England. The Warren truss is most easily recognised by its use of equilateral triangles (where all three sides of the triangle are equal in length). For this reason, it is considered the simplest of truss designs. Because the forces move quite substantially to different members (or parts) of the bridge depending on where the load is sitting, it is important to understand the kinds of traffic your Warren truss bridge will be subject to—and to compensate with stronger members in the relevant areas.
Because it can be hard to predict traffic conditions (and because they change so regularly), the Warren truss is often considered insufficient in versatility, and engineers often opt for newer, more reliable designs. The Warren truss is still a popular bridge design however, as it is quick and easy to construct and requires a minimal amount of materials.
Originally designed in 1844 by railway engineers Thomas and Caleb Pratt, the Pratt truss bridge is one of the most common bridge designs. Its diagonals are usually parallel and slope towards the centre of the bridge, as demonstrated in the illustration below.
This bridge design is more complex than the Warren truss variation, and is also more expensive due to the extra materials needed to build it. The Pratt truss is a hot favourite among engineers (and those nerdy science folk) because when you crunch a whole heap of numbers (better them than me!), it turns out that the Pratt truss dissipates force more effectively than other designs.
The Parker, camelback, Pennsylvania and Baltimore are each variations on the Pratt truss design.
The Howe truss, patented in 1840 by William Howe, a millwright from Massachusetts, is very similar to its friend the Pratt truss; except that its diagonals face the opposite direction (away from the centre). The small change simply means a change in direction of where the force is applied. Originally, because their larger parts were constructed from wood, it was a popular design because it was cheaper to build. However, in recent years the Pratt design tends to be more popular and is slightly better at distributing force.
If you want to get technical about it: in the Howe truss design, the longer diagonal members are in compression, while the shorter vertical members are in tension. This is in contrast to the Pratt truss design.
The K truss bridge design was a variation on another Pratt truss style, called the Parker truss. The idea of the K truss is to break up the diagonal members into smaller lengths, in the hope of reducing their likelihood of buckling under pressure. The reason they never really took off, according to Garrett’s Bridges, is because they’re so complex in design. Complex = time consuming and heavy on materials, which in turn means—expensive.
Tip: If you’re looking at building your own real-life truss bridge, or simply want to do some hobby building, Garrett’s Bridges have done the math for you on all your favourite bridge designs!
Ok, so what are Pony truss, Through truss and Deck truss bridges?
Truss bridges, unlike other bridge designs, are able to carry roadway within their structure. This is perhaps why they’re such a popular option for bridge builders. Their versatility doesn’t end there—you can also choose whether your roadway (for cars or trains, for example) runs through your bridge (through truss), or on top of your bridge (deck truss). Neat, right?! Furthermore, the pony truss allows traffic to travel through the structure of the bridge, however the two sides of the bridge are not joined at the top (or cross-braced).
Whether you’re an engineer, a handyman or a hobbyist looking to build your ideal bridge, you can’t go past a truss bridge for versatility (not to mention their snazzy appearance!) The type of truss you choose will depend on the budget, time and engineering nous you’ve got access to. Happy bridge-building!