Truss (structure)
A truss is a structural framework composed of straight members connected at their ends by joints (nodes), forming a series of triangular units that collectively carry loads by developing only axial tension or compression in each member, with no bending. The triangular geometry makes the truss one of the most efficient structural forms because it converts bending into purely axial forces, allowing slender members to span large distances with minimal material. Trusses are widely used in roof structures, bridges, transmission towers, and space frames in both steel and timber construction.
Key Formula
Members + Reactions = 2 × Joints (condition for a statically determinate truss)
LaTeX: m + r = 2j
| Symbol | Meaning | Unit |
|---|---|---|
| m | Number of truss members | (count) |
| r | Number of external reaction components | (count) |
| j | Number of joints (nodes) | (count) |
Worked Example
Problem
A simply supported Pratt truss has 7 joints. Verify whether the truss with 11 members and 3 reaction components (2 vertical + 1 horizontal) is statically determinate.
Solution
Step 1: Apply the determinacy condition: m + r = 2j. Step 2: Left side: m + r = 11 + 3 = 14. Step 3: Right side: 2j = 2 × 7 = 14. Step 4: 14 = 14 ✓ — the truss is statically determinate.
Answer
The truss is statically determinate (m + r = 2j = 14)
Common Truss Types and Their Applications
| Truss Type | Chord Arrangement | Typical Span (m) | Common Application |
|---|---|---|---|
| Pratt Truss | Vertical members in tension | 20–60 | Railway bridges, roof frames |
| Howe Truss | Vertical members in compression | 10–30 | Timber roofs, light bridges |
| Warren Truss | No verticals; equilateral triangles | 15–50 | Highway bridges, gantries |
| Fink Truss | Fan-like diagonals to ridge | 10–25 | Residential and industrial roofs |
| K-Truss | K-shaped diagonals | 30–80 | Long-span bridges |
| Vierendeel Truss | No diagonals; moment joints | 10–40 | Architectural openings |
Interactive Tools
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Related Terms
Structural Load
A structural load is any force or collection of forces that acts on a structure, causing internal stresses, deformations, or displacements within the members. Loads are classified by their nature (static or dynamic), their source (gravity, wind, seismic), and their duration (permanent or transient). Accurate load estimation is the foundation of structural design, ensuring that every member can safely resist the demands placed on it throughout the life of the structure.
Structural Beam
A structural beam is a horizontal or inclined load-bearing member that resists transverse loads primarily through bending and shear, transferring forces from the loaded surface to the supports at its ends or along its length. Beams develop internal bending moments and shear forces that determine the distribution of tensile and compressive stresses across the cross-section, with the neutral axis experiencing zero direct stress. Beams are among the most fundamental structural elements and are constructed from steel, reinforced concrete, prestressed concrete, timber, or aluminium depending on the application.
Structural Column
A structural column is a vertical compression member that transmits axial compressive loads from beams, slabs, and other upper-structure elements down to the foundation, and may also resist bending moments arising from lateral loads or eccentric loading. Columns are classified by their slenderness ratio (effective length divided by radius of gyration) into short columns, which fail by material crushing, and long (slender) columns, which fail by elastic or inelastic buckling before the material reaches its yield stress. In reinforced concrete design to IS 456, columns are also classified as axially loaded, uniaxially bent, or biaxially bent based on the combination of forces they carry.
The word "truss" derives from Old French "trousse" (a bundle, package), later used in the sense of tying or binding together — apt for a structure whose strength comes from binding members into rigid triangles. The engineering use of the term was established in the early 19th century alongside the rapid development of iron railway bridges.