Truss Force Calculator

Formula used

This tool assumes a symmetric triangular truss with a single apex joint load. Uniform load is converted to an equivalent total vertical load:

• P_total = P + w·L
• R_left = R_right = P_total / 2
• θ = arctan(h / (L/2))
• F_diagonal = P_total / (2·sinθ) (compression for downward load)
• T_bottom = F_diagonal·cosθ (tension)

Optional checks use Force_design = Force × SF, then stress σ = Force_design / Area. With allowable stress in MPa (N/mm²), the required area in mm² is A_req = Force_design(N) / σ_allow.

How to use this calculator

1. Enter the truss span L and rise h in meters.
2. Provide a downward apex point load P in kN, and optionally a uniform load w in kN/m.
3. Set a safety factor if you want conservative sizing checks.
4. Optionally enter allowable stresses or member areas for quick stress/area estimates.
5. Click Calculate. Results appear above the form, with CSV and PDF downloads.

Example data table

Important notes

• This is a fast estimator for a basic triangular truss. Multi-panel trusses need full joint or matrix analysis.
• Uniform load is simplified as an equivalent vertical total load for this three-joint model.
• Compression members may require buckling checks based on section, length, and end conditions.

Professional article

1) Truss forces in everyday construction

Roof trusses convert distributed gravity loads into axial tension and compression, helping you span longer distances with lighter members. Early force estimates reveal whether a proposed span and rise are sensible, and they highlight which members and joints will govern detailing. Typical light construction trusses often span about 4–12 m, depending on material and bracing.

2) What this calculator represents

The model is a symmetric, three-joint triangular truss: two equal diagonals meeting at an apex and one bottom chord between supports. It is commonly used for simple canopies and small roofs. Multi-panel trusses require a full joint or matrix analysis.

3) Loads and the equivalent total load

Enter an apex point load P and an optional uniform load w over the span. For quick planning, the uniform load is converted to a total vertical load w·L, giving P_total = P + w·L. Include dead, live, and equipment load estimates as needed.

4) Support reactions for symmetry

With symmetric geometry and centered loading, reactions split equally: R_left = R_right = P_total/2. Reactions are useful for bearing design, anchor planning, and verifying that supporting walls or columns can receive the expected vertical demand.

5) Geometry: why rise changes forces

The diagonal angle is set by θ = arctan(h/(L/2)). Increasing rise generally increases sinθ, reducing the diagonal compression required to balance the same vertical load. Shallow trusses can create high axial forces and larger connection demands. Rises near 0.20–0.35 of span are common starting points for roof work.

6) Member force equations used

For a downward apex load, each diagonal typically carries compression and the bottom chord carries tension. The key relations are F_diagonal = P_total/(2·sinθ) and T_bottom = F_diagonal·cosθ. The calculator reports both magnitude and force type.

7) Practical sizing checks

Apply a safety factor for conservative trial sizing, then estimate stress from σ = Force_design/Area. If you provide allowable stress in MPa (N/mm²), the required area is A_req = Force_design(N)/σ_allow. Use this to shortlist sections before detailed connection and code checks.

8) Buckling awareness and documentation

Compression members may buckle before reaching material limits. If you enter K and radius of gyration r, the tool reports KL/r as a quick stability indicator. If forces are high, increase rise or shorten members with more panels, then export CSV or PDF to document assumptions.

FAQs

1) Which truss configuration is supported?

A symmetric triangular truss with one bottom chord and two equal diagonals meeting at the apex. It is ideal for quick checks on simple roof or canopy forms.

2) How is uniform load treated?

The uniform load is converted to an equivalent total vertical load w·L and added to P. This simplifies early planning but is not a substitute for panel-point loading.

3) Why are diagonals often in compression?

With a downward apex load, diagonals provide upward components at the apex joint by pushing along their length, producing compressive axial force in this symmetric three-member model.

4) Can I use this for Pratt, Howe, or Warren trusses?

Not directly. Those trusses have multiple panels and web members, so internal forces depend on layout and load positions. Use a joint method or truss analysis software for those cases.

5) What does the safety factor affect?

The safety factor scales forces only for the optional sizing checks. The reported base member forces are from statics; the SF helps you apply conservative trial stresses and areas.

6) What units should I enter for stress and area?

Allowable stresses are MPa (N/mm²) and areas are mm². Forces are computed in kN and internally converted to N for stress and required-area calculations.

7) What should I do after getting these forces?

Select preliminary member sizes, check compression stability, and design connections. Then confirm final forces using a detailed truss model with realistic load distribution and code-required combinations.