Structural Truss & Space Frame

Triangulated framework of straight two-force members connected at nodes that resists bending through axial tension and compression.

Specifications & Metrics

type: Load-Bearing Structural System
load mechanism: Converts external bending moments and shear into pure axial tension and compression in chords and webs
governing relation: m = 2j − 3 (statically determinate planar truss with m members and j pin joints)
classic configurations: Pratt (1844, tension diagonals), Warren (equilateral triangles), Howe, Bowstring, Octet space frame
exemplars: Southern Pacific Tempe Bridge, Bank of China Tower (Hong Kong), International Space Station Integrated Truss

Because a triangle is the only polygon whose geometry cannot deform without changing the length of its sides, a pin-jointed triangulated truss eliminates bending moments in its members, carrying loads entirely through axial tension in bottom chords and compression in top chords.
A planar truss with j joints and 3 support reactions is statically determinate when it has exactly m = 2j − 3 members, allowing every internal member force to be solved from static equilibrium (∑F_x = 0, ∑F_y = 0) via the method of joints, method of sections, or Cremona diagrams.
Patented in 1844 by Caleb and Thomas Willis Pratt, the Pratt truss places its longer diagonal web members in tension and shorter vertical members in compression to resist buckling, a principle adopted in steel railway bridges and the biplane wing cellules of the 1903 Wright Flyer.