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STAIRPAN LISP: Formed Steel Stair Pans in Section

STAIRPAN draws the folded sheet-metal pans that make up a steel stair, in section, to the exact fold geometry of a fabricator’s standard details — then works out the flat blank length needed to press each one.

What StairPan does

Twelve profiles, three nosing treatments crossed with four positions in the stair:

NOSING TREATMENT      POSITION IN THE STAIR
Square nosing            Riser pan   - the vertical face
Sloped nosing            Stair pan   - the tread
Sloped riser             Closing pan - closes off at the top
                            Landing pan - runs out onto the landing

These are custom profiles. They are not derived from a standard the way a rolled steel section is — every fold, return and hem is somebody’s shop practice. The geometry has therefore been carried across exactly as it was, fold for fold. Nothing has been “improved”, because a changed fold is a pan that will not fit the stair it was detailed for.

How to use it

  1. Load the routine with APPLOAD, or put it on your support file search path.
  2. Type STAIRPAN.
  3. Set the stair up once: tread width, riser height, pan fill depth, landing fill depth and sheet gage. Nose-to-nose can be picked off the drawing instead, and the tread and riser derived from it.
  4. Pick the nosing point — the front top corner of the tread, which is the point every profile is measured from.
  5. Drag to either side to choose the hand, and click.
  6. Read the stretch-out off the command line.

How it works

Every pan is drawn both ways and you drag to choose. A stair pan runs left-to-right or right-to-left depending which side of the stair you are detailing, so both mirror images are generated and shown live as you move the cursor. Move to one side and that version highlights; click to take it. There is no separate “left hand / right hand” prompt to get wrong.

The chosen profile is drawn as one closed polyline.

The stretch-out is the number the shop orders and cuts to. Getting it from the drawing rather than by hand is most of the value of a tool like this, and it is computed from the finished profile rather than by adding up folds.

The outline of a constant-thickness folded strip traces out along one face and back along the other, so its perimeter is twice the mid-thickness length plus the two square ends. Rearranged:

stretch-out = (perimeter - 2 × thickness) / 2

That is the neutral-axis length at mid-thickness — the standard developed length at a K-factor of 0.5 — and it holds for every one of the twelve profiles without needing a separate formula for each.

Sheet gage is real, not a line weight. Thickness comes from a US sheet steel gage table, 16 gage down to 8 gage. Every fold in every profile is offset by that thickness, so changing gage changes the whole section correctly.

Notes and limits

  • Twelve profiles — three nosings × four positions.
  • Both hands are generated; you drag to pick one.
  • The stretch-out assumes a K-factor of 0.5. A shop working to a different bend allowance will want to check it.
  • Gage table is US sheet steel, 16 down to 8.
  • One undo group covers the pan.

Download and details

NameStairPan — draw a formed steel stair pan in section and get its stretch-out
DownloadStairPan.lsp — 31.7 KB, plain AutoLISP source. Downloads directly; no zip to unpack.
TypeAutoCAD AutoLISP routine
CommandSTAIRPAN
Requiresnothing beyond a CAD platform with AutoLISP.
Source size31.7 KB, 740 lines
Error handlingYes — a *error* handler restores every system variable it changed, on cancel as well as on error.
UndoThe whole operation is wrapped in a single undo group, so one U reverses all of it.
CompatibilityAutoCAD (any release with AutoLISP), BricsCAD, ZWCAD, GstarCAD, ProgeCAD and other IntelliCAD-based platforms. AutoCAD LT needs a LISP enabler. No .NET, no ObjectARX, no installer.
AuthorYZ, August 2026
LicenceFree to use, supplied “as is” with no warranty.

References