A section properties calculator: draw or assemble a cross-section and read its area, centroid, moment of inertia (Ix, Iy), principal axes, radii of gyration, and elastic + plastic section moduli — updated live as you draw. Place a standard steel section, or draw plates and cut holes. It draws both centers — the elastic centroid and the plastic neutral axis — which coincide only on a doubly symmetric shape. It also handles composite sections by the transformed-section method.
Every command in the toolbar, what it is for, and how to use it — with before-and-after drawings produced by the tools themselves. Open a section, or search for a tool by name or by what you are trying to do.
The four shapes every cross-section is built from, plus the dimension tool. Everything you draw is a region: a closed outline the engine integrates over. You never have to close a shape by hand — each command closes its own.
Draws a rectangle from two opposite corners. It is the workhorse — every plate, flange, web and slab is one.
Tip. Build up a plate girder as three rectangles: bottom flange, web, top flange. Draw each at its true position and the properties are correct with no further work.
Draws a circle from its center and a radius — a bar, a pin, a bolt hole, or a round hollow section.
Watch out. The prompt asks for the RADIUS, not the diameter. A 20 mm bar is radius 10.
Draws a circular arc through three points, for a curved edge that is part of a larger outline.
When to use it. Rarely needed on its own — most curves in a steel section are fillets between straight edges, and FILLET makes those. Reach for ARC when you are tracing a shape that is genuinely curved, such as a haunch or a tapered nose.
Draws a run of connected straight segments and closes it into one region — the tool for any shape that is not a rectangle or a circle.
When to use it. An angle, a tee, a channel, a cold-formed lipped section, or a profile you are tracing from a drawing.
Watch out. A polyline that does not close is not a region and cannot be integrated. Pressing Enter closes it for you — do not draw the last segment back to the start by hand, or you will leave a zero-length edge.
Tip. Turn ORTHO on before tracing a section with square corners. Every segment then comes out truly horizontal or vertical, which matters because a web 0.3° off vertical changes Iy.
Adds a dimension line between two points you pick.
When to use it. For the report. A dimension is an ANNOTATION — it is drawn on the figure and it can never change a computed property, which is why you can add as many as the drawing needs without worrying about the numbers.
Tip. Drag a placed dimension to move it clear of the geometry; Alt-drag rotates it. Double-click resets it to where the tool put it.
These act on what is SELECTED. Click an entity to select it, drag a box to take several, or press Escape to let go. If nothing is selected, the modify commands that need a selection say so rather than guessing.
Replaces a sharp corner with a circular arc of the radius you type, tangent to both edges.
When to use it. Every rolled steel section has a root radius where the web meets the flange, and it carries real area. Fillet the re-entrant corners of a built-up section and the properties stop being slightly conservative.
Watch out. A radius larger than either edge can accommodate is refused — the arc would run past the end of the edge it is supposed to be tangent to. Use a smaller radius, or lengthen the edge first.
Cuts a corner off flat, a set distance back along each edge.
When to use it. A flame-cut or machined corner, and the usual way a plate edge is prepared for welding.
Moves the selection from a base point to a second point.
When to use it. Positioning a flange relative to a web. Note that MOVING a region changes the centroid and the second moment of the whole section — that is not a side effect, it is the point.
The same gesture as MOVE, but the original stays where it is.
Turns the selection about a base point you pick, by an angle you type.
When to use it. A raking member, a skewed plate, or checking a section about a non-principal axis.
Watch out. The base point is the pivot, not the origin. Picking the wrong one turns the shape AND moves it somewhere unexpected.
Tip. You rarely need to rotate a section to find its worst axis. Turn on the principal axes instead — the tool computes θp and draws them, which is the same answer without touching the geometry.
Reflects the selection about a line you pick, keeping the original — so you get a symmetric pair from one drawn half.
When to use it. Back-to-back angles, a pair of channels, the second flange of a symmetric girder, or any section you would rather draw half of. It is the single biggest time-saver in the tool.
Tip. Turn ORTHO on before picking the two axis points. Otherwise the axis is a degree or two off vertical and the "symmetric" section that results has a small Ixy and a rotated set of principal axes — a subtle wrong answer rather than an obvious one.
Copies the selection into a rectangular grid — a number of columns and rows at a spacing you give.
When to use it. Bolt holes, stiffeners, a run of identical voids. Six holes in one command instead of six copies.
Changes a drawn entity by typing an exact value, instead of dragging it.
When to use it. When the drawing is nearly right and the number has to be exact — "that plate is 300 long, not 297".
Watch out. Center means the center of the bounding BOX, not the centroid. On an angle or a channel those are different points, and it is the box center that moves to where you type.
Tip. The same four fields sit in the Selected panel under the drawing, as boxes you can type straight into. Same code, same result — use whichever is closer to your hand.
Deletes the selected entities. Undo brings them back.
Every region is either SOLID (it adds material) or VOID (it removes it). The mode is set BEFORE you draw and applies to the next shape, which is how a hole, a duct opening or a hollow box gets drawn.
The default. A shape drawn in solid mode adds its area, and its own second moment about the section axes, to the totals.
A shape drawn in void mode SUBTRACTS. Its area comes off, and so does its contribution to I — correctly weighted by how far it sits from the centroid.
When to use it. A bolt hole, a service penetration, or the inside of a hollow section: draw the outside solid, then the inside as a void.
Watch out. A void that is not fully inside a solid removes area that was never there, and the properties become meaningless. Keep every void within the material it is cutting.
Tip. This is how you check the cost of a penetration: note Ix, add the void, and read Ix again. The difference is what the hole cost you, and it is far larger for a hole in the flange than the same hole at the neutral axis.
A section made of two materials cannot be added up directly, because a square millimeter of concrete does not carry what a square millimeter of steel carries. The transformed-section method scales each material by its stiffness ratio before summing — and that ratio is the only number you have to supply.
The stiffness of this material divided by the stiffness of the reference material: n = E_this / E_reference. The first material in the list is the reference, so its n is exactly 1.
When to use it. A composite steel-and-concrete deck, a timber flitch beam with steel plates, or an aluminum section checked against a steel one.
Watch out. Codes quote this ratio THE OTHER WAY UP. AS 3600, Eurocode 2 and AISC all write n = E_steel / E_concrete ≈ 7 to 15; this box wants E_concrete / E_steel ≈ 0.143, which is 1/7. Type 7 here and you have claimed your concrete is seven times stiffer than steel. The value beside the box shows the reciprocal live, so the two readings are visible at once.
Tip. Use n ≈ 1/7 for short-term loading and n ≈ 1/15 for sustained load, where creep has softened the concrete. Running the section both ways takes ten seconds and brackets the real behavior — that is the standard check, not an optional refinement.
Makes every selected shape the active material. Until you assign, everything is the reference material.
Rather than draw a rolled section, place one: 2,686 sections across six standards (AS/NZS, AISC, EN, BS, JIS and IS), each with its published dimensions. Placed sections draw at their real size and can be modified, cut or combined like anything you drew yourself.
Chooses a standard, a type and a size, and draws that section at the origin.
Tip. Place a rolled section, then add a cover plate as a rectangle on top. That is the classic strengthening check, and the properties update as soon as the plate lands.
These do not change the drawing — they change where your clicks LAND. Every one of them exists so a picked point is exact rather than nearly right, which matters because the properties are computed from the geometry, not from what you meant.
Makes a click land exactly on a real feature: an endpoint, a midpoint, a center, an intersection.
When to use it. Always on. It is the difference between a web that meets the flange and a web that ends 0.4 mm short, leaving a sliver the engine faithfully integrates.
Tip. Press F3 to toggle it mid-command. Choose WHICH features it catches in Drafting Settings — turning off the ones you do not want stops the snap fighting you on a crowded drawing.
Locks the next point to horizontal or vertical.
When to use it. Tracing any section with square corners, and — importantly — picking a MIRROR axis that is truly vertical.
Tip. F8 toggles it. ORTHO is POLAR fixed at 90°; turning one on turns the other off.
Locks the direction to fixed angular steps, so a raking edge comes out at exactly 30° rather than 29.7°.
Shows the prompt and the live length and angle at the crosshair, so you can type an exact value without looking away from the drawing.
A background rule for judging size. It never affects where a point lands — that is OSNAP's job.
Draws the equal-AREA axes, which split the section into two halves of equal area.
When to use it. The plastic neutral axis governs the plastic section modulus and therefore the plastic moment capacity. It coincides with the centroidal axis only on a doubly symmetric section — on a tee, a channel or an angle the two are visibly apart, and seeing that is the fastest way to understand why the plastic and elastic moduli differ. (⚠ the letters for those two are reversed between AS 4100 and AISC — the panel prints whichever your design code uses.)