The Power of Triangulation
A truss transforms bending into pure axial forces through geometric triangulation. Where a solid beam must resist bending across its entire cross-section, a truss concentrates material into discrete members that carry only tension or compression. This principle, understood since ancient Roman timber bridges, makes trusses extraordinarily efficient for spanning large distances — from highway bridges to stadium roofs spanning over 100 meters.
Method of Joints
At each pin joint of an ideal truss, the sum of all forces must equal zero. Starting from a support where the reaction is known, you can solve for two unknown member forces at each joint, progressing through the entire structure. This simulation computes all member forces for a Pratt truss and color-codes them: cyan for tension, red for compression. The thickness of each member scales with force magnitude, immediately revealing the load path through the structure.
Chord Forces & Height
The top and bottom chords of a truss act like the flanges of a deep beam — the top chord compresses while the bottom chord stretches in tension. Their forces are inversely proportional to the truss height: F = M/H. This relationship is why doubling truss depth halves the chord forces and roughly halves the required steel tonnage. Practical trusses balance structural efficiency against headroom constraints, typically targeting a height-to-span ratio between 1:8 and 1:12.
Diagonal Members & Buckling
Diagonal members carry shear forces as axial loads. In a Pratt truss, diagonals are oriented so they carry tension under gravity loading — a deliberate choice because tension members never buckle and can be made from slender rods or cables. Compression diagonals and the top chord require stockier sections to resist Euler buckling, making member orientation a critical design decision that affects both weight and cost.