How to use the truss member force calculator
This tool solves a simply supported parallel chord truss with equal loads at the interior top chord panel points, using the method of joints. Enter the span, the truss depth, the number of panels and the load at each panel point, and you get the panel length, the diagonal angle, the support reactions, and a panel by panel table of shear with diagonal, top chord and bottom chord forces. The table shows the left half because the truss is symmetric.
Diagonals follow shear and chords follow moment. Each diagonal force is the panel shear divided by the sine of the diagonal angle, and each chord force is the bending moment at that section divided by the truss depth. That is why the end panel diagonals and the midspan chords control the design.
Depth is the cheapest way to lighten the chords. Chord force is inversely proportional to depth, so doubling the depth halves the chord forces. The cost is longer, steeper diagonals and a deeper structural zone. Common steel trusses run between one tenth and one fifteenth of the span in depth.
Only axial forces appear here. Secondary moments from eccentric joints, compression buckling capacity, bolt and weld design at the panel points, deflection and camber are all separate checks, so have a licensed structural engineer confirm the truss before the design is finalized.
Frequently asked questions
Diagonal force follows the panel shear and chord force follows the bending moment at that section. In a simply supported truss shear peaks at the supports while moment peaks at midspan, so the end diagonals and the center chords control.
Chord force equals moment divided by depth, so doubling the depth halves both chord forces. The trade-off is longer, steeper diagonals with tighter buckling limits and a deeper floor-to-floor allowance.
The magnitudes are identical and only the sense flips. A Pratt truss puts the long diagonals in tension and the verticals in compression, and a Howe truss does the opposite, which is why Pratt layouts dominate in steel.