Truss Force Calculator Formula

Understand the math behind the truss force calculator. Each variable explained with a worked example.

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Formulas Used

Bottom Chord Force (tension)

chord_force = total_load * span_l / (8 * depth_h)

Support Reaction

reaction = total_load / 2

Variables

VariableDescriptionDefault
total_loadTotal Applied Load (W)(kN)120
span_lTruss Span (L)(m)12
depth_hTruss Depth (h)(m)2

How It Works

Truss Chord Forces

For a parallel-chord truss carrying uniform load, the midspan chord forces can be approximated from the equivalent beam moment.

Derivation

The midspan moment for equivalent simply supported beam: M = W L / 8.

The chord force equals the moment divided by the lever arm (truss depth): F_chord = W L / (8 h)

The bottom chord is in tension and the top chord is in compression with approximately equal magnitude at midspan.

Worked Example

A 12 m span Pratt truss, 2 m deep, carrying a total load of 120 kN.

total_load = 120span_l = 12depth_h = 2
  1. 01Equivalent beam moment: M = 120 x 12 / 8 = 180 kN-m
  2. 02Chord force = 180 / 2 = 90 kN (tension in bottom chord)
  3. 03Support reaction = 120 / 2 = 60 kN

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