Home / Truss Analysis: Axial Forces and Support Reactions

Truss Analysis: Axial Forces and Support Reactions

A truss model assumes its members transmit axial force only. Under ideal pin-jointed action, each bar carries either tension or compression, not bending. FramePad includes a 2D truss element type and an example three-member triangular truss.

Example: symmetric triangular truss

Consider three nodes: A = (0, 0) m, B = (3, 2.5) m, C = (6, 0) m. Members AB and BC form the sloping sides, and AC is the horizontal bottom tie. Place a pin at A, a vertical roller at C, and apply a 40 kN downward load at B.

Equilibrium check

Symmetry gives equal vertical support reactions:

For one inclined member, let sin θ = 2.5 / √(3² + 2.5²). Joint equilibrium at the apex gives equal compressive forces in AB and BC:

Horizontal equilibrium at the support yields the bottom chord's tension force:

These forces do not depend on bar axial rigidity for this statically determinate configuration; displacements do depend on the EA values of the members.

Try it in FramePad

1. Open FramePad and click the Truss template.
2. Confirm the three-member geometry, support symbols and the 40 kN downward nodal load at B.
3. Select the Axial result tab and compare each member force magnitude with the values above.
4. Switch to the Deflected diagram to explore how axial stiffness affects displacement.

Scope and limitations

The axial-only truss formulation does not represent flexural stiffness, rigid joints, member bending from distributed loads, imperfections or buckling resistance. Apply loads at nodes in the truss model. A member showing compression is not automatically safe against buckling; slenderness, effective length and design codes must be checked separately.