Plan underlay: drawing to column layout

The plan underlay puts the architect's drawing behind your work, so you can read spans and bay widths straight off it instead of retyping dimensions from a printout, and mark the columns where the building actually has them. You give it one real distance; it works out every other distance from the same picture.

That single number is why this page exists. A scale set from the wrong dimension does not look wrong afterwards — the spans it produces are plausible, the calc sheet is arithmetically perfect, and every load, moment and bar in the design is out by the same factor with nothing to give it away. Ten minutes of care here is worth more than any check further down.

The image never leaves your device. It is display-only: not uploaded, not stored with the project, and never seen by the calculation engine, which receives only the numbers you hand it. (Structura can also read a plan for you — that is a separate, paid feature that does upload the drawing, and it asks first. Everything on this page stays free and stays on your device.)

Step 1 — Getting the plan in

Open Plan underlay from the home screen, or Fit on the architect's plan from a grid or takedown form. Three ways in, in order of how much they preserve:

• PDF or image file — the drawing as issued. A PDF's first page is rasterised at full quality, so this is always the best source. If the file is a multi-sheet set, export the floor plan page on its own first: only page one is read.
• From gallery — the plan somebody sent on WhatsApp. Messaging apps recompress images hard. Zoom in and check you can still read the dimension figures before trusting it; if they are mush, ask for the PDF.
• Take a photo — a paper drawing. Lay it flat, stand over the middle of it rather than off to one side, get the whole plan in frame, and keep your shadow off it.

Shooting at an angle is the one fault calibration cannot undo. The underlay applies a single uniform scale — it does not correct perspective. On a skewed photo the near edge reads short and the far edge reads long, and no calibration line can be right for both. If the plan comes back inconsistent, retake the photo square-on rather than splitting the difference.

Step 2 — Calibrating the scale

In Calibrate mode, tap the two ends of something whose real length you know, then type that length in metres — 3.6, not 3600. Structura divides one by the other and keeps the result:

Do not use the paper scale

You never enter 1:100, and you should not want to. A drawing that has been photocopied, "shrunk to fit" or photographed is no longer at its printed scale, and a photo has no fixed relationship to paper at all. Calibrating from a written dimension sidesteps the whole question — it is true whatever happened to the image on its way to you.

What makes a good calibration line

• Long. Your tap is off by a fixed few pixels whatever you tap, so the longer the line, the smaller that error's share. Prefer an overall building dimension to a single door.
• Written on the drawing. Tap the two extension lines of a printed dimension string and enter exactly the figure printed on it — not a figure you remember or expect.
• Unambiguous at both ends. Wall face to wall face, or grid line to grid line. Decide which and keep to it: face-to-centreline is out by half a wall thickness, twice.
• Tapped zoomed in. Pinch in until the two points are clearly separated on screen. The overlay zooms with the plan, so a point placed at high zoom stays exactly where you put it.

Why "long" is worth a paragraph

Say your tap lands 4 px from where you meant. On the 900 px overall dimension above that is 0.4%. On a 90 px door opening it is 4.4%. Spans do not stay put either — bending moment goes with L²:

Nine per cent of moment is a bar size. It buys nothing to calibrate off the nearest convenient thing when a dimension string across the whole plan is a pinch-zoom away.

If the two taps land almost on top of each other, Structura refuses the calibration rather than accepting a wild scale — that is the "points are too close together" message. Zoom in and pick two points further apart.

Step 3 — Prove it before you trust it

This is the step people skip, and it is the one that pays. Switch to Measure and measure a different dimension you know — ideally running the other way across the plan. Then read what comes back:

Measure is not only a check. It reads spans, bay widths, room sizes and overall extents straight off the plan for the single-member forms — the slab panel you are about to design, the beam span you need, the footing spacing.

Step 4 — Fitting the array

In Grid mode, with the panel on Fit array, tap the plan where the first column sits — the top-left one, which becomes grid A/1. Tapping again moves it, so place it roughly and refine. Then set the spacings and bay counts in the panel underneath.

This is a starting point, not the answer. It is the quickest way to put twenty columns on a plan at roughly the right centres; step 5 is where you move them to where the building actually has them.

• Grid X and Grid Y step in 50 mm, because that is a setting-out increment rather than an arbitrary one.
• Bays X and Bays Y count the spaces between grid lines, so 3 bays means 4 lines and 16 columns on a 3×3 grid.
• The red squares are columns, drawn at their real 230 mm size. They are your feedback: when they land on the columns drawn on the plan, both the grid and the scale are right.

Read the way they miss, because it names the problem. Markers that drift further out the further you go from A/1 mean the spacing is wrong. Markers all off by the same amount in the same direction mean the origin is — tap to move it. Markers that fit across the plan but not down it mean the two spacings are not equal, which is normal: set each independently.

Irregular bays: type them instead

The two steppers move every line at once, which is only what you want while the bays really are equal. Where they are not, tap Type bay dimensions. It lists every bay by name — A–B, B–C, 1–2 — and you type what the drawing says against each one. A typed figure moves the lines beyond it and leaves every column you have already placed exactly where it is, so it can be used at any point in the job. The steppers are the ones that rebuild the array and clear your edits.

That is also the way to set out an irregular building with no plan on screen at all. Holding the architect's dimensions and no image, fit a rough array, then type the real bays down both chains. Anything outside 0.5 to 15 m is refused rather than rounded into range — that is the span range this engine designs, and under half a metre the beam between two lines is a deep beam, which BS 8110 does not cover — and a typed figure sets out to 25 mm.

Repeating and mirroring

Two more tools live in that same sheet, because both are about repetition rather than dimensions:

• Repeat this bay — the icon beside a bay adds a setting-out line one bay-width further on, carrying the same column pattern, gaps included, and moves everything past it out to make room. Twenty taps become one.
• Mirror left to right / top to bottom — flips the whole layout about its own centre, without moving it on the drawing. That is how a plan fitted from the wrong end is put right, and how the second half of a symmetrical house gets built.

This page used to tell you to fit the largest bay in each direction and accept the oversizing. That was the honest answer while the grid had to stay regular; it is no longer the best one, because every column is now designed on its own tributary rather than on a single worst-case spacing.

Step 5 — Editing the columns

Real plans have a stairwell where a column would be, an open bay across a living room, an L-shaped footprint, or a span too long to leave unpropped. Switch the panel to Edit columns and change the array to match the drawing:

• Tap a red marker to take that column out. It becomes a hollow marker, so you can see the node you emptied — and tap it again to put the column back.
• Tap open ground to add a column. It snaps onto a nearby setting-out line, and inserts a new one where there is none, because a column has to line up with something for a beam to reach it. A line the layout invents is set out to 25 mm rather than to wherever your finger landed — that figure ends up in the bay chains and on site.
• Press and hold a column to pick it up, then drag. A ghost shows where it will land and what that comes to: the line it has caught, or the new one it would insert, and how far that is from the line beside it. It only moves when you let go. This is the edit for a column that is close to right rather than wrong, and it is one operation — not a delete and an add, which would pass through a layout the engine refuses and lose the column's grid reference on the way.

Snapping reaches exactly as far as the shortest bay the engine designs — half a metre — so a column tapped closer than that to a setting-out line goes on the line, at any zoom, and the ghost says how far it moved. It is not tidiness: two lines that close make a beam whose clear span is under twice its depth, which is a deep beam, and BS 8110 has no rules for one. A setting-out line runs the whole way across the plan, so it would be that beam at every frame. If your drawing really has twin columns a few hundred millimetres apart — either side of a movement joint — they are two structures, and each half is designed on its own.

The panel keeps up as you edit

Below the counts it says everything it has to say about the layout at once: in red what the engine would refuse, in amber what it will design happily but you should know about. Both name the column by its grid reference — B/2, not a pair of coordinates in metres — and ring the same column on the plan. Design grid stays dead until the red is gone, so a refusal is never something you find out by asking for a design and waiting. Tap any message and the plan moves to the columns it names, which is the half that takes the time on a layout of twenty.

Then read the two spans in the panel, because the difference between them is the whole point. The widest panel is measured between setting-out lines and is what the slab is designed for. The longest beam span is measured column to column — so taking a column out doubles it, while the panels either side stay the size they were, because the beam is still there. On a 3.6 × 4.5 m grid that puts the typical beam on a 9.00 m span, and a 230 × 450 section fails it loudly. That is the engine noticing what you just did.

What it does to the design

Each column is designed on its own tributary: half the distance to the nearest column each way. The neighbours of an empty node each pick up the whole of the widened strip, so their shares overlap and add to more than the plan area — the overview reports that percentage rather than hiding it, and every one of them is designed for the larger share. What is not calculated is how a two-way slab really redistributes around a missing support, or how two beams crossing over an empty node share the load. Both are conservative here; check them by analysis before building to it.

Changing a spacing or a bay count moves every line, so it clears the column edits and says so. Fit the array first, edit second — and if you do it by accident, undo puts them back.

Step 5b — A building that steps in

Plenty of Nigerian houses are wider at the bottom than at the top: a ground floor covering the plot, a first floor set back off one edge, a penthouse smaller again. Those columns stop where the building stops with them, and the ones that carry on take a larger share of the floors above.

Under Edit columns the panel carries a row of storeys — Ground, First floor, Second floor — with the storey count beside it. Pick the storey, then edit it exactly as you edited the ground floor: tapping a column out on the first floor stops it there and leaves it standing below. A column carries a height, not a tick per storey, so a column starting half way up the building cannot be drawn at all — that is a transfer beam, and the engine does not design one.

Set the storey count before you draw the setback. A setback is drawn against the number of storeys the plan believes it has, so editing "the second floor" of a building the app thinks is a bungalow puts the step on the wrong level, and nothing downstream can notice. Shortening the building afterwards straightens it rather than refusing it.

What the engine does with it

• Each floor is framed by the storey underneath it, and each column accumulates against the tributary that storey gives it. A storey with fewer columns hands the survivors a larger share, so carrying the ground-floor tributary up would understate the load on exactly the columns holding the setback up.
• The imposed-load reduction still counts total floors. BS 6399-1 Table 2 is about how many floors a column carries, not how big any one of them is.
• The bill is counted level by level — beam runs, panels and lifts of column — rather than the ground floor multiplied by the storey count, which would price a building nobody is putting up.
• The drawing set gives a column application plan per storey. A column that has stopped below is drawn dashed and unmarked rather than left blank, because a gap where the sheet below shows a column is indistinguishable from one somebody forgot to draw. The note saying one plan applies at every storey is dropped from those sheets, since it would no longer be true.

The two refusals, and the one you will meet

Wind cannot be designed for a building that steps in. The lateral method here applies one pressure over the full height to one plan extent, resisted by the frames of one layout — and a setback changes the exposed face, the frames and the lever arm at every level. So a stepped building is gravity-only, which caps it at 3 suspended floors — four storeys counting the ground. Design it that way, or carry the columns full height and design the building with wind. It is refused outright rather than warned about, because there is no version of "roughly right" available here.

The other refusal is the transfer beam: a column that stops while the floor above still spans over it. The engine has no transfer element and would design that beam as an ordinary span, on a sheet saying PASS. The panel names the column and asks you to carry it up, or to set the storey above back so nothing lands on its head. Alongside those, every storey has to keep at least four columns of its own, and at least one column has to reach the top — otherwise the setback is drawn on the wrong storeys, or the building is shorter than it says it is.

Undoing a step, and putting it back

The undo arrow at the top of the underlay reverses the last thing you did, whichever mode you are in: a calibration point, a measurement, the grid placed or moved, a column taken out, put back, added or dragged, a bay retyped, a bay repeated, the layout mirrored, a spacing or bay count changed. It names what it undid rather than leaving you to spot the difference on the plan, and tapping it again keeps walking back — as far as the scale itself, and past it to the bare drawing.

Redo sits beside it, and it is there for a specific reason: the way to find out what a step actually did is to undo it and look, and looking should not cost you the step. Anything you do afresh clears the redo, because that is a new branch — offering to put back a layout you have since edited away from would be worse than not offering at all.

The change most worth taking back is the accidental one. Changing a spacing after you have edited the columns rebuilds the array and throws those edits away; that is several minutes of reading a drawing, and undo is the only thing that brings them back. (Typing a bay dimension does not do that — it keeps every column.)

Neither arrow touches the plan itself — the image, the zoom and where you are on it all stay where they are. Importing a different drawing starts the history again, because every step up to that point was measured in the old image's pixels. To drop the scale and the grid outright rather than a step at a time, use Clear calibration & grid from the menu — which is itself one step, so undo brings it all back.

Step 6 — Into the design

Design grid takes the column layout into the column grid form. If you came from a form via Fit on the architect's plan, Use grid hands it back to that form instead — the takedown designs a single column, so only the spacings return to it.

Once you have edited the columns, the form shows that a layout from the plan is attached, with the option to remove it. The spacing fields still read as what the layout works out to; the layout is what the engine designs from, and the foundation drawing puts a pad under every column in it and nowhere else. Where the building steps in, the storey count you set on the plan comes across to the form with it, so the setback is designed on the storeys it was drawn on.

Everything else is still yours to enter: storey heights, dead and imposed loads, concrete grade, cover, soil bearing capacity, and wind if the building calls for it. The plan tells the app how far apart the columns are. It says nothing about what is standing on them.

What the underlay is not

• It is not saved. The image is display-only and is not stored with the project or backed up, so re-import it when you come back to a job. Nothing on this page uploads a drawing: that is what keeps the promise that your clients' plans never leave your device. The one thing that does upload a plan is a scan, which is a different feature, costs credits, and asks before the first one.
• Nothing is detected off the drawing. No walls, no columns, no dimension text, no AI reading the plan. You place every column; you decide it is right. Taking one out is a structural decision, not a drafting one — the engine will design the longer beam it creates, but it will not tell you the building wanted a column there.
• It does not check the architect. Calibrate from a wrongly written dimension and the error goes straight into the design. If a dimension string and the drawing disagree, that is a question for the architect, not for the app.
• It does not change what the engine will design. The same ceilings apply to a grid fitted on a plan as to one typed in by hand — four storeys on gravity alone, eight with a wind path, and four for a building that steps in, since a setback is gravity-only. See what it will not do.

Next

• Scanning a plan
• Load takedown
• Slab design
• Beam design
• Column design
• Bar bending schedules
• Pad footing design
• Approval drawings
• How Structura works