DXF, SVG and STEP section properties calculator

The section properties example in the playground runs the same measurements in Python. It measures an IPE 200 and prints its values next to those of the steel table.

Section properties describe how a cross-section carries load. The second moment of area sets the bending stiffness EI of a beam, the section modulus the bending stress at its outer fiber, the radius of gyration how soon a column buckles, and the area with the density its weight per meter. Producers list these values for their standard steel and aluminum profiles. For a custom extrusion, a welded section or any profile drawn in CAD, they follow from the drawing itself, and that is what the calculator measures. For sheet parts it adds the values a laser or waterjet shop quotes by, the cut length, the number of pierces and the mass.

The section

A DXF file is read entity by entity, and SVG, STEP and BREP files by planex2d's readers. Layer by layer, two ends closer than a thousandth of the drawing's larger side are joined, and a stretch drawn twice is kept once. The closed contours of all layers then form one section, in which a contour inside another is a hole and a contour inside a hole is material again. A contour that stays open is left out of the area. When a file has more than one layer, the bar on the right lists them, so that a layer of center lines or dimensions can be left out.

Units

The values are in the unit the file names, or in the unit picked on the right. A DXF file names its unit in its header, an SVG file in its width and height, and a STEP file in the file itself. A file that names none, such as every BREP file, is taken in meters. Areas are in the unit squared, section moduli cubed and second moments to the fourth power.

Area, centroid and second moments

planex2d's Measure() integrates over the section's exact edges, so a circle or a fillet is measured as the arc it is, not as a polygon. Ix is the integral of y² over the area, about the x axis through the centroid, Iy the integral of x², and Ixy the integral of x·y. planex2d calls them Ih, Iv and Ihv, after its h and v axes. The principal moments I1 and I2 are the largest and the smallest second moment about any axis through the centroid, and the angle is that of the axis of I1 from x. A section with I1 = I2, such as a circle or a square, has every axis through its centroid as a principal axis, and no angle is given.

Section moduli and radii of gyration

• The elastic section modulus Wel = I / e divides a second moment by the distance e from its axis to the farthest point of the section, on either side.
• The plastic section modulus Wpl is the sum of the first moments of the two halves of the area about the axis that halves it. The calculator finds that axis by cutting the section with planex2d's Boolean operations.
• The radius of gyration i = √(I / A) is given about the x and y axes and the two principal axes; the smallest of them governs the flexural buckling of a column.
• The polar moment Ip = Ix + Iy is the torsion constant only for a round bar or a tube. The torsion example in the documentation computes the torsion constant of other sections in Python.

Wall thickness

The wall thickness is the diameter of the largest disc that fits inside the section and touches its boundary, at 400 points along the boundary. The thinnest and the thickest wall are the smallest and the largest of these. A sharp convex corner is infinitely thin at its tip, so the points where such a corner pinches the disc are left out. The legend entry Wall thickness colors the section by its local wall thickness, each point by the largest of these discs that covers it, as CAD thickness maps show it.

Sheet parts

The cut length is the length of every contour on the chosen layers, open ones included, and every contour needs one pierce. The mass of a sheet part is its area times the sheet thickness and the density of the material. The mass per meter is the area times the density, the weight of a bar or profile with this section per meter of its length.

1. Load

   The page loads Python and planex2d into this tab. Your browser keeps both in its cache, so later visits start faster.

2. Read

   The browser reads the files you add from your disk into memory and unpacks a zip in the tab. No request carries a file to a server.

3. Measure

   planex2d joins each file's closed contours into one section and measures it. The bar on the right picks what the results show; it changes nothing that was measured.

4. Save

   Save (csv) writes the results of the file shown, Save all (csv) one row for every open file, in the unit picked on the right.

Supported files

Run it in Python

Everything the calculator measures is available in the library. Install it and see Measuring in the docs. The example reads the IPE 200 sample in its own millimeters and measures it.

$ pip install planex2d

import planex as px

ipe = px.Face(px.FromDXF("ipe-200.dxf", scale=1))
m = ipe.Measure()
print("A", m["area"], "Iy", m["Ih"], "Iz", m["Iv"])
print("thinnest wall", ipe.Measure(px.measurements.MinThickness(ignore_corners=179.999)))

Privacy

Limits

• This tool reads planar STEP and BREP files only.
• Text, dimensions, hatches and images are not read; the calculator looks at the geometry only.
• The torsion constant needs a finite element solution, which this page does not run.
• In the browser, planex2d runs in one thread, so large drawings take longer than in Python on your machine.
• A file that fails? Open an issue on GitHub and attach the file if you can.