Truss bridge designer: draw a truss, load it, watch a member fail

How the truss bridge designer works

Start from a preset (Warren, Pratt, Howe or K truss, four or six panels) or a blank canvas. The truss bridge designer works on a 14 m by 6 m grid with joints snapping to every half metre. Choose the Joint tool and click to place a joint, the Member tool and click two joints to join them, the Support tool to turn a joint into a pin and then a roller, and the Load tool to hang a downward load on a joint. The Size tool and the section menus in the member table set the tube size, Erase removes a joint or member, and the x and y boxes add a joint at an exact position. Every change recalculates at once.

A truss that is not a stable arrangement of triangles is reported as a mechanism rather than analysed, and that message is the quickest lesson the tool teaches: delete one diagonal from a Warren preset and the readout changes to Mechanism.

Reading the result

In this truss bridge designer, warm members are in tension, pulled. Cool members are in compression, pushed. Line weight follows the section you chose for the member. The table under the drawing lists each member’s length, force and utilisation, which is the share of its capacity the member uses at the applied load, and the readout above it gives the failure load, the first member to fail, the frame mass and the strength per kilogram.

Press Test to failure (Reset view clears the result) and the load climbs until the first member reaches its capacity. The dashed outline marks the member that failed and the message names the load, so a design always ends with a number and a cause. Any single force in the table can be checked by hand with the method of joints. A tension member fails by yielding. A compression member fails by buckling or yielding, whichever comes first, and in a slender truss it is nearly always buckling.

Three things to try

Load the Warren preset and read the failing member, then thicken only that member and test again. The failure load rises from 48.0 to 68.7 kN and moves to a diagonal, which is how a design improves one step at a time.

Compare the Pratt and Howe presets. They have the same span, height and load but the diagonals lean opposite ways, so the diagonals in one are pulled and in the other pushed. Check the result against the pages on the Pratt and Howe trusses.

Then build your own from a blank canvas in the truss bridge designer and share the link. The address contains the entire drawing, so a teacher can set a starting point and students send back what they changed.

How these figures were computed

The designer solves a two-dimensional pin-jointed truss with the direct stiffness method, using steel at 200 GPa with 250 MPa yield and four tube sections from 40 by 2.5 mm to 120 by 5 mm. Tension capacity is yield stress times area. Compression capacity is the lesser of that and the Euler buckling load of a pinned member. Self-weight, joint strength and imperfections are not modelled, so a real model will fail sooner. The solver was checked against hand calculations of a triangle, a two-panel Warren, a bracket, global equilibrium and a virtual-work deflection; the method of joints and sections pages show the same hand arithmetic.