Creating a Model Truss Bridge with AI

We would like to get 3D printer design for a modelTrussBridge

We want a design combining a Warren truss pattern on the side with an X Pattern on the top and bottom of the bridge.

We will use the nicescad webinterface to OpenScad for our design and

• ChatGPT
• Claude AI
• DeepSeek
• Mistral

as the AI chat tools.

First we need a list of prompts that will lead to a proper scad module. Note how there area ready made models available at https://www.yeggi.com/q/n+scale+truss+bridge/ and still the AIs behave like stochastical parrots and have no clue what they are doing - see Talk page.

Given the notorious incapability of AI Tools for this kind of task as of 2025-06 we need to be very careful in what to expect from the AI and what to do our selves.

Steps

Step 1: Create polygonAt and beamAt Modules

Prompt: Create an OpenSCAD module named beamAt that places a cube beam at a specified 3D position x,y,z, applies a 3D rotation rx,ry,rz, and uses given dimensions sx,sy,sz. Add an example usage creating a horizontal and vertical beam. Add a header comment with a link to https://wiki.bitplan.com/index.php/Creating_a_Model_Truss_Bridget_with_AI and // Copyright Wolfgang Fahl with current iso date

// Prompt:
// Create an OpenSCAD module named `polygonAt` that places a polygon-based beam at a 3D position `x,y,z`,
// applies 3D rotation `rx,ry,rz`, and extrudes the polygon with `linear_extrude(height=length)`.
// The polygon is defined via `points` and should be extruded symmetrically around its local origin.
// Define a second module `beamAt(x, y, z, rx, ry, rz, sx, sy, sz)` that creates a centered rectangular beam 
// cross-section of width `sy`, height `sz` and extrudes it along its axis with length `sx`.
// Add example usage for a horizontal beam (extruded along X) and vertical beam (extruded along Z).
// Add a header comment with a link to https://wiki.bitplan.com/index.php/Creating_a_Model_Truss_Bridget_with_AI
// and // Copyright Wolfgang Fahl with current iso date

// https://wiki.bitplan.com/index.php/Creating_a_Model_Truss_Bridget_with_AI
// Copyright Wolfgang Fahl 2025-06-15

// place and extrude a 2D polygon at given 3D position and rotation
module polygonAt(x, y, z, rx, ry, rz, length, points) {
    translate([x, y, z])
        rotate([rx, ry, rz])
            translate([0, 0, -length/2])  // center extrusion symmetrically
                linear_extrude(height=length)
                    polygon(points);
}

// place a rectangular beam at 3D position with given rotation and dimensions
module beamAt(x, y, z, rx, ry, rz, sx, sy, sz) {
    polygonAt(x, y, z, rx, ry, rz, sx, [
        [-sy/2, -sz/2],
        [ sy/2, -sz/2],
        [ sy/2,  sz/2],
        [-sy/2,  sz/2]
    ]);
}

l=10; // length
h=1; // height
w=1; // width
// Example usage
// Horizontal beam along X → rotate Z→X
ry=90; 
beamAt(
    0, 0, 0,
    0, ry, 0,
    l, w, h
);

// Vertical beam 
beamAt(
    -l/2+w/2, 0, l/2,
    0, 0, 0,
    l, w, h   
);

• Example answer by ChatGPT

Note that this answer was never created by ChatGPT - there was a loop of a dozen hallucinations and the variables and calculations for the example have been introduced manually after giving up.

Step 2: right triangle

Prompt: Based on module

beamAt(x, y, z, rx, ry, rz, sx, sy, sz) {
    translate([x, y, z])
        rotate([rx, ry, rz])
            cube([sx, sy, sz]);
}

// Example usage
// horizontal beam
beamAt(  
   0, 0, 0, 
   0, 0, 0, 
  10, 1, 1);   
// vertical beam 
beamAt(
    0, 0, 0,
    0, 270, 0, 
   10, 1, 1); 

create a rightTriangle function consisting of three beams and allow a positioned/rotated version rightTriangleAt. Add an example usage creating a warren truss pattern. Add a header comment with a link to https://wiki.bitplan.com/index.php/Creating_a_Model_Truss_Bridget_with_AI and // Copyright Wolfgang Fahl with current iso date

Example

Step 3: Warren Truss pattern

Prompt: For a european single track railway bridge - Length: 19.1m - Width: 3.3m - Height: 4.5m - Top beam: 16.2m - we want the Model scale: H0 (1:87) - open scad design. Use the existing beamAt to create a WarrenTruss pattern function. The base of the pattern are pairs of right angled triangles which in turn form isosceles triangles with a support beam in the middle.

Add a header comment with a link to https://wiki.bitplan.com/index.php/Creating_a_Model_Truss_Bridget_with_AI and // Copyright Wolfgang Fahl with current iso date

References

HAER-Truss-Poster

see HISTORIC AMERICAN EINGINEERIING RECORD