The bar bending schedule, and steel by weight

Reinforcement is drawn as lines on a drawing and bought by weight. The bar bending schedule is the document that gets you from one to the other.

For every member it lists the bar mark, the diameter, the shape, the cut length, the number of bars, the total length and the weight. Added up, it is the steel order. Handed to a bender, it is the cutting list. Compared against what was delivered and what was fixed, it is the reconciliation.

One document doing three jobs is why errors in it are expensive: they do not show up once, they show up in the order, in the yard and in the measurement, and by the time the third one contradicts the first, three weeks have passed.

Weight from diameter, and where the number comes from#

Steel is ordered in kilograms and drawn in millimetres, so every schedule contains one conversion.

It is not a magic constant. Take a round bar of diameter d millimetres. Its cross-sectional area is π·d²/4 square millimetres. Steel has a density of about 7,850 kilograms per cubic metre. Convert the units and the mass of one metre of bar comes out as roughly d² divided by 162 kilograms.

That is the whole derivation, and it is worth doing once rather than memorising, because then you know two things the constant alone does not tell you.

First, it assumes plain round section and nominal diameter. Deformed bars have ribs, and their actual mass per metre is governed by standards that permit a tolerance either side of nominal. Your bar may legitimately weigh slightly more or less than the arithmetic says.

Second, the tolerance is a band, and a supplier who consistently sits at one end of it is delivering consistently less steel per kilogram of invoice than one who sits at the other. Nothing improper has happened. But if you are reconciling fixed steel against purchased steel, this is a real component of the difference and it is not wastage.

The lengths that are not on the drawing#

The drawing shows where steel goes. The schedule has to show how it is cut, and several lengths appear at that step that never appeared on the drawing.

Laps. Bars come in standard lengths and members are longer than bars. Where two bars overlap, both are consumed for the overlap distance. Lap length is a design quantity, governed by the code and by the grade of concrete and steel, and it is where a lot of steel quantity lives.

Bends and hooks. A bar bent round a radius follows a longer path on the outside than the inside. The cut length is not the sum of the straight dimensions; standard bend deductions and hook allowances apply.

Cover. Dimensions on a drawing are usually to the face of the concrete. The bar stops short of that face by the cover. Every dimension has to be reduced, at both ends, and forgetting it is one of the most common schedule errors — consistently producing bars slightly too long, which fit badly and quietly increase consumption.

Chairs, spacers and support steel. Real, necessary, frequently absent from schedules, and then the site finds them from the offcut pile and the reconciliation never balances.

Where the schedule and the site disagree#

Even a perfect schedule will not match consumption, for reasons that are ordinary rather than suspicious.

Bars are supplied in standard lengths. Cutting a standard length into required lengths leaves an offcut. Whether that offcut is usable depends on whether a shorter bar is needed nearby, which depends on sequencing and on how tidily the yard is run. Two sites with identical schedules can differ substantially in purchased steel purely on cutting strategy, which is why that loss belongs inside the rate rather than being discovered in it — quoting a rate you can live with.

Then there is revision. Drawings change. If the schedule is revised and the already-cut steel is not, you have material that fits nothing and a reconciliation that will never close.

This is the same category of problem as the gap between measured and docketed quantities: an ordinary, explicable difference that becomes an argument only because nobody recorded the explanation at the time.

The version control problem#

A bar bending schedule is a live document on a live project, and it is very often the worst-controlled document on the site.

It exists as a spreadsheet. It gets emailed. Somebody at the yard has a printed copy from three weeks ago. The consultant has issued a drawing revision that the schedule has not caught up with. There is no single answer to "how much steel does this floor need", because there are four answers and they are all somebody's current file.

The practical minimum: every schedule carries the drawing number and revision it was prepared from, the schedule's own revision, and a date. If a bar is cut against a superseded schedule, that fact is discoverable afterwards rather than mysterious. The identifiers cost nothing and they are what makes a reconciliation possible at all.

The general form of this rule — that a record without a handle back to what it came from stops being evidence — is the same reason a concrete cube result needs its pour reference.

Reconciling steel, properly#

Three quantities, per diameter, per period:

• Purchased, from invoices and weighbridge slips, in kilograms.
• Issued from store to yard, in kilograms.
• Fixed in the work, from the schedule, for the members actually completed.

The difference between the first two is a store question. The difference between the last two is a consumption question. Keep them apart — running them together produces a single unexplainable figure, which is exactly the trap described in wastage on site.

Do it per diameter. A site-wide steel figure hides the case that matters most, which is one diameter being over-consumed because a schedule error is repeating in every member of the same type.

The short version#

A bar bending schedule is a cutting list, a purchase order and a measurement basis wearing one set of clothes.

The weight conversion is arithmetic you can derive rather than a constant you have to trust. The lengths that cause trouble are the ones not on the drawing — laps, bends, cover. And the errors that cost most are systematic ones, repeated in every member of a type, which is why reconciling by diameter finds things that reconciling by total never will.