Truss Member Force Calculator (Method of Joints)
This calculator analyses a simply supported symmetric Pratt truss under a uniform joint load using the method of joints. The truss has N equal panels, span L, and depth h. A total gravity load W is distributed equally among the top chord panel points. The calculator finds: the maximum top chord force (compression), the maximum bottom chord force (tension), and the maximum diagonal force. These are derived from the equivalent beam analogy: maximum moment M = WL/8 gives chord forces F_chord = M/h; maximum shear V = W/2 gives diagonal forces adjusted for the diagonal angle.
Truss force formulas (beam analogy)
R = W/2 (reaction)
M_max = W × L / 8 (midspan moment)
F_chord = M_max / h (top/bottom chord at midspan)
theta = arctan(h / panel_length)
F_diag = R / sin(theta) (end diagonal)
These closed-form results apply to a symmetric simply supported Pratt truss with equal panel loads. The top chord is in compression (negative), the bottom chord in tension (positive).
Design considerations for trusses
- AISC 360 Chapter E governs compression chord design (buckling) and Chapter D governs tension members (net section fracture).
- Web members change force magnitude along the span: diagonals near midspan carry low shear; end diagonals carry maximum shear.
- Connections (gusset plates, welds, bolts) are often the most complex element of truss design and require separate calculation.
- For large trusses, camber is specified to offset dead-load deflection and maintain the designed geometry under service loads.
Frequently asked questions
What is the method of joints?
The method of joints is a technique for analysing statically determinate trusses. At each pin joint, equilibrium requires sum of forces in x = 0 and sum of forces in y = 0. Solving these two equations at each joint (starting from joints with only two unknowns) determines all member forces.
What is the difference between tension and compression in a truss?
Tension members (T) are pulled apart and must not buckle. Compression members (C) are squeezed and must be checked for buckling as columns. In a simply supported Pratt truss under gravity load, all top chord members are in compression and bottom chord members are in tension.
What assumptions does the method of joints make?
Members are two-force members connected by frictionless pins, loads are applied only at joints, and members are straight. Self-weight of members is neglected. These assumptions are closely met by welded steel trusses with flexible connections analysed at the system level.
What is a Pratt truss?
A Pratt truss has vertical web members in compression and diagonal web members in tension under gravity loading. The top and bottom chords carry the primary axial forces from bending. This is the most common truss configuration in steel construction for spans of 30-200 feet.
How is a truss different from a beam?
A beam resists bending through flexural stresses distributed across its depth. A truss resists the equivalent bending action through axial forces in its chords (tension and compression). The truss is much more material-efficient for long spans because material is concentrated at the chords where stresses are highest.
Official sources
- American Institute of Steel Construction: AISC Steel Construction Manual, Part 2 General Design Considerations.
- American Society of Civil Engineers: ASCE 7 Minimum Design Loads for Buildings and Other Structures.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.