A Pratt truss is a bridge and building framework where diagonal members slope downward toward the center, placing them in tension under normal loads while the vertical members handle compression. Patented in 1844 by architect Caleb Pratt and his engineer son Thomas, this design became one of the most widely used truss configurations in structural engineering because it makes exceptionally efficient use of materials.
How the Pratt Truss Works
The defining feature of a Pratt truss is the orientation of its diagonal members. In every panel of the truss, the diagonals angle downward from the outer supports toward the center of the span. When weight pushes down on the structure (gravity loads from traffic, snow, or the structure’s own weight), those diagonals are pulled taut rather than squeezed. The vertical members, running straight up and down between the top and bottom chords, are the ones that get compressed.
This matters because steel and iron are far better at resisting tension (pulling forces) than compression (squeezing forces). A long, thin steel rod can carry enormous pulling loads without issue, but that same rod will buckle if you try to compress it. By routing tension through the longer diagonal members and confining compression to the shorter vertical ones, the Pratt truss lets engineers use less material overall. Shorter compression members are less prone to buckling, so they don’t need to be as heavy or bulky.
How It Differs From a Howe Truss
The Howe truss is essentially the Pratt flipped. In a Howe truss, the diagonals slope outward from the center, putting them in compression under gravity loads while the verticals carry tension. This arrangement worked well in the era of wooden bridges, since timber handles compression reasonably well and iron rods could be used for the vertical tension members. The Pratt truss required more iron than the Howe, and because of the higher cost and less rigid construction in wood, builders initially avoided it for timber structures.
Once iron and steel became cheaper and more available in the late 1800s, the Pratt truss overtook the Howe. Its ability to keep the longer members in tension and the shorter ones in compression simply uses metal more efficiently. Today, nearly all modern truss bridges and long-span roof structures favor the Pratt configuration or one of its variations.
Common Variations
Several modified versions of the Pratt truss address specific engineering challenges:
- Baltimore truss: Adds extra bracing in the lower portion of each panel to prevent the compression members from buckling. This reinforcement makes it suitable for heavier loads and longer spans than the basic Pratt.
- Parker truss: Uses a curved top chord (polygonal shape) instead of parallel top and bottom chords. The arch-like profile distributes forces more evenly, reducing material needs for medium to long spans.
- Warren-style Pratt: Combines alternating diagonal directions with some Pratt principles, sometimes omitting vertical members entirely in favor of a zigzag pattern of diagonals.
Each variation keeps the core Pratt principle intact: diagonal members carry tension, vertical members carry compression.
Where Pratt Trusses Are Used
Pratt trusses show up in two main categories of structures: bridges and long-span buildings. In building construction, Pratt trusses are commonly used for spans ranging from 20 to 100 meters, covering everything from warehouse roofs to aircraft hangars to convention centers. For spans under about 20 meters, simpler beam structures are usually more economical, so trusses generally make financial sense only beyond that threshold.
In bridge engineering, the Pratt truss has been a standard since the late 19th century. Highway overpasses, railroad bridges, and pedestrian crossings all use variations of the design. The straightforward geometry makes fabrication relatively simple compared to more complex designs, and the predictable load paths make structural analysis more reliable.
Why It Remains Popular
The Pratt truss has survived for nearly 180 years because its geometry naturally aligns with how steel behaves under load. Three practical advantages keep it relevant. First, the material savings are significant: keeping longer members in tension means they can be lighter, which reduces both cost and dead weight. Second, the design is highly adaptable. Engineers can adjust the number of panels, the depth of the truss, and the angle of the diagonals to suit specific span lengths and load requirements. Research on optimal Pratt truss proportions shows that as span length increases, the ratio of actual weight to minimum possible weight grows, meaning careful panel selection becomes more important for longer spans.
Third, the Pratt truss is easy to analyze and construct. Each member has a clearly defined role (tension or compression), which simplifies both the engineering calculations and the connections between members. Tension connections can use simple bolted or pinned joints, while compression members need stiffer connections but are short enough to keep those details manageable.
How to Identify a Pratt Truss
If you’re looking at a truss bridge or roof structure and want to identify whether it’s a Pratt, focus on the diagonals. Stand at one end and trace the diagonal members: in a Pratt truss, they form a “V” or “N” pattern, with diagonals angling down from the top chord toward the center of the span. The vertical members stand upright between the top and bottom horizontal chords. If the diagonals slope the opposite way, angling up toward the center, you’re looking at a Howe truss. If they alternate direction in a zigzag with no verticals, that’s a Warren truss.
On older bridges, the Pratt pattern is sometimes easier to spot from the side. The repeated “N” shape across the length of the span, with verticals and diagonals clearly distinct, is the visual signature of this design.