Truss bridge design – bar forces and first failure
Design & TechnologyEngineering Drawing & DesignAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Engineering Drawing & Design
A four-panel steel truss bridge with a pin support at one end and a roller support at the other. Students choose the truss type (Warren, Pratt, Howe), the material, the bar cross-section, and place a load on any node of the bottom chord by clicking it. The simulation solves the joint equilibrium equations to find the force in every bar, colouring tension bars red and compression bars blue, drawing bar thickness by force magnitude, computing stress σ = F/A and comparing it with the allowable stress. Click Ramp load to find the load at which the first bar fails.
Lesson: Simple product design project: designing, calculating and testing a truss bridge model
What it shows
A truss is a framework of straight bars pinned together in triangles, so it keeps its shape under load. When loads act only at the joints, every bar carries a pure axial force, tension or compression, found by applying force balance (ΣFx = 0, ΣFy = 0) at each joint after calculating the support reactions. The stress in a bar is σ = F/A, so a larger cross-section lowers it. Slender bars in compression can buckle, so the model allows them only half the tension stress. Warren, Pratt and Howe trusses arrange their diagonals differently, which changes which members are in tension.
How to use
Pick a Truss type and Bar material, then click a bottom-chord node or move the Load position along the span and watch the red tension and blue compression bars change. Read which bar is most critical and what percentage of its allowable stress it uses. Press Ramp load to find the load at which the first bar fails, then change the Cross-section of each bar or the material and ramp again. Use Change truss type to compare the diagonals.
Parameters you can change
- Truss type Warren truss (zig-zag diagonals), Pratt truss (diagonals toward centre), Howe truss (diagonals toward ends)
- Bar material Wood ([σ] = 40 MPa), Aluminium ([σ] = 200 MPa), Steel ([σ] = 400 MPa)
- Load on the bridge 1–60 kN
- Cross-section of each bar 50–800 mm²
- Load position along the span 0–100 %
Questions to explore
- Is the bridge more likely to fail with the load at mid-span or near a support, and which bar fails first?
- Under the same load, which diagonals are in tension in the Pratt truss and which in the Howe truss?
- How does doubling the cross-section of each bar change the load at which the truss fails?
