A truss bridge is a load-bearing structure constructed from a series of straight, connected elements that form triangular units. This geometric configuration allows the bridge to distribute external forces across a span, translating them into simple internal forces within the members. In engineering, the concept of “strongest” refers to structural efficiency—the best possible use of material for the required load. Truss designs maximize this strength-to-weight ratio, allowing for longer spans with less material compared to simple beam bridges. The strongest truss design depends entirely on the specific application, span length, and construction materials.
How Truss Bridges Manage Structural Load
The fundamental action of a truss bridge is to manage two primary forces: tension and compression. Tension is a pulling force that stretches a material, while compression is a pushing force that squeezes a material together. When a load is applied to a bridge deck, the structure bends, placing the top horizontal members, or chords, in compression and the bottom chords in tension.
The angled and vertical components, known as the web members, transfer forces between the chords. These members carry loads axially, meaning the force runs directly along the member without significant bending. The triangle is the fundamental building block of a truss because it is the only geometric shape that is inherently rigid. This rigidity ensures stability and provides a predictable pathway for forces to travel.
Analyzing Major Truss Designs (Pratt, Howe, Warren)
The three most common historic truss types—Pratt, Howe, and Warren—all use the triangular web pattern but differ significantly in how they assign tension and compression to their internal members. This difference dictates their most efficient use and the materials best suited for their construction. The Pratt truss, patented in 1844, features vertical members in compression and diagonal members that are in tension as they slope down toward the center of the span. This arrangement was effective for using the emerging material of steel, which performs well in tension, allowing for slender, economical diagonal rods.
The Howe truss, developed just four years earlier, is the inverse of the Pratt design, featuring diagonal members in compression and vertical members in tension. This design was highly favored in the 19th century for use with timber construction, as wood is strong in compression, making the long diagonal timbers structurally sound. The vertical members, which are in tension, could be made of wrought iron rods, a material better suited for pulling forces.
The Warren truss is characterized by its simple pattern of equilateral or isosceles triangles, with the diagonal members alternating between tension and compression under a uniform load. This design is considered one of the most material-efficient because it uses fewer web members than the Pratt or Howe, reducing the number of connections. The Warren configuration evenly distributes the shear forces across the structure, which contributes to its simplicity and ease of fabrication.
Structural Efficiency and Determining the Strongest Design
For a modern, medium-span road or rail bridge, the most structurally efficient design is typically the Warren truss, often with the addition of vertical members. The Warren design’s alternating force pattern and minimal number of members lead to an efficient distribution of stress, minimizing the material required to support the load.
Adding vertical members to the Warren truss creates the Warren with Verticals, or N-Truss, which improves the design’s performance under concentrated loads, such as a train wheel directly over a joint. These vertical posts shorten the distance the top compression chord must span, which increases its capacity to resist buckling. This modification combines the material efficiency of the basic Warren with the improved load management needed for modern traffic.
For extremely long spans or unique loading conditions, more specialized designs are employed to manage forces precisely. The K-Truss, for example, divides the diagonal members into smaller lengths to reduce the risk of buckling in compression members, offering greater stability for longer spans. The Baltimore Truss, a variation of the Pratt, adds secondary sub-struts to break up long compression members, making the structure more rigid and efficient for very long railroad spans.