Part 1 · 2017 — 2023

Making invisible quantities visible

Why were rebar quantities always off, and why could no one explain it?

1.1Formulas gave no evidence

The problem with the old method was not accuracy — it was traceability. When quantities came out higher than planned, there was no way to tell whether the increase was legitimate or over-calculated. Every number existed only as the output of a formula.

The continuity of the building is not reflected — calculating member by member makes connections and anchorages diverge from reality.
Contingency is added by convention — how much was added, and why, is buried inside the formula.
Quantity can be hidden in the drawings — reviewing drawings by eye rarely finds it.

1.2Someone benefits when the quantity grows

The deeper problem is that the incentives run in opposite directions. Rebar quantities are fixed as they pass from head office to the site, to the rebar shop drawing firm, to the fabrication yard — and along that chain, more quantity means more money for someone.

How rebar quantity gets fixed — and where the incentives collide

The same quantity, incentives pointing opposite ways

Head office

Sets execution budget

Site

Decides placement rules

Rebar shop drawing firm

Prepares bar schedule

Fabrication yard

Fabricates and delivers

Rebar tons ↑ → margin ↓

Buyer side — more quantity means higher cost

Rebar tons ↑ → revenue ↑

Supply side — more quantity means more sales

The point — the incentive to inflate quantities is structural, and formula-based estimating offered no basis to verify it. The problem was not the counterparty; it was the absence of a means of verification.

1.3Putting three numbers side by side

Contractor K's approach was simple: produce three quantities for the same building and compare them — then chase down the “why” behind every difference.

Three-way comparison — from plan to actual placement

STEP 1 · Execution budget stage

Conventional 2D formula estimate

Drawings + Excel formulas + customary contingency

BuilderHub quantity

Taken from the 3D model · calculation basis included

1st comparison — is the execution quantity sound?

Design changes · site requests → model regenerated

Baseline quantity reflecting actual pre-construction conditions

STEP 2 · Actual placement verification

BuilderHub recalculated quantity

Latest model with all changes reflected

Actual rebar shop drawing quantity

Fabricated quantity actually placed on site

2nd comparison — what causes the gap?

When the second comparison shows a gap, the cause is traced. BuilderHub quantities exist visibly in a 3D model and come with the underlying calculation, so you can point and say “the difference is here, this much, and this is why”.

A case that was actually caught — for short walls under 2 m, the 3D quantity was set to calculate horizontal bars and then add U-bars at both ends. When the estimating rule and the modeling rule differ, the same wall yields different quantities. Differences like this were resolved into standards, one at a time.

1.4Where it is used — the workflow in the early adoption phase

“Where do we slot BuilderHub in?” is the first question every new customer asks. Here is the actual flow at the pilot site in the first year.

Early adoption workflow — pilot site

① BIM modeling · in-house

Site opened · CAD staff assigned

BIM training

Detailed design documents delivered

Structural frame 3D BIM created

· Supported by one head office BIM specialist
· About 5 months to modeling
· Done by site staff, not outsourced

② Using the BIM model

Construction detail quantity analysis

Rebar shop drawings prepared

Rebar cost monitoring

VE items identified

Site consultation

Rebar quantity down approx. 2%

Site BIM training → model in use

Constructability review · quantity review by work package

③ Analysing results

Analysis of BIM outcomes

Fed into the next site

BuilderHub enters at two points — creating the structural frame 3D BIM, and the quantities taken from that model

Everything else stays as it was. You are not building a new process; you start by changing two points in the existing flow.

In the first year this was enough. No database, no conceptual estimating, no continuous monitoring yet.

In the early adoption phase it works at the single-site level only. There is no company-wide system yet — this is the stage of confirming that it works on one project.

1.5Rebar is not one thing

The most important fact the repeated comparisons revealed was that rebar splits into two categories. This distinction is not well known outside Korea, but a large share of the quantity gap originates here.

Structural rebar and assembly rebar

Structural rebar

Determined by structural calculation · shown on structural drawings

92~95%

5~8%

Structural rebar · 26 types

Main bars · reinforcing bars · vertical bars · horizontal bars · stirrups · U-bars ···

Set by structural calculation, so the standard is clear and it can be verified

Assembly rebar · 15 types

Chairs · purlin bars · dowels · spacing bars · auxiliary stirrups ···

Relies on subcontractor experience — there was no standard

Structural rebar appears on the structural drawings, so it can be verified. The problem is assembly rebar. It is not shown on drawings and is decided by the subcontractor's experience, leaving no basis to judge what amount is reasonable.

5–8% sounds small, but on a site with several thousand tons of rebar it is hundreds of tons. And no standard means it becomes a matter of negotiation.

1.6Why the gap appears — cost monitoring

Comparing model quantities against rebar shop drawing quantities is not enough. Control only begins when you can name the cause of a difference. So a rebar cost monitoring report — reconciling rebar ordered after groundbreaking against the 3D BIM model — was issued monthly.

Overall and per-building comparison tables of BIM quantities against the estimate
Ordered-quantity comparison charts and tables against BIM quantities
Comparative analysis tables by total / building / floor
Detailed analysis of the major differences

Repeating this work, the causes of the differences resolved into four categories.

Cause of difference	Nature
Change in design documents	Unpredictable — handled by updating the model
Site detail changes for constructability lap length adjustments · purlin bars · long site bars	Legitimate difference — absorbed into the standard
Items not decided or not modeled in advance sleeves · sumps · MEP boxes, etc.	A scope issue — stated in the take-off rules
Modeler error	A quality issue — solved by standards and training

What matters is that quantity remains that fits none of the four. In the review results these are classified as “unexplained quantity present in the rebar shop drawing”. Under the old method such quantity existed but never surfaced — there was no baseline to compare against.

What a single floor's difference weighs — the review is done member by member. Wall support bars, spacing bars, slab main bar lap lengths: individually each difference is well under a ton. But typical floors repeat. A difference let through on one floor is multiplied by the number of floors and buildings. That is why cost monitoring has to repeat every month.

1.7Building the standard

Contractor K established company-wide shop drawing guidelines for assembly rebar — putting numeric standards on items that had been settled by convention.

Item	Before (convention)	After (standard)
Support bar spacing, foundation top bars	@ 1,000 ~ 1,200	@ 1,500
Wall connection bars	8-HD10	4-HD10
Wall–foundation connection bars	Minimization rule applied	—

At the same time, the shop drawing stage was made to review over-reinforcement and shop drawing omissions together. Once the standard existed, training and consultation with subcontractors followed, so that it actually worked on site.

1.8At first they did it themselves

In the first year, Contractor K did the BIM modeling with site staff, without any outsourced service.

“So that our own site staff can operate BIM data themselves”
— the goal at the start

BIM training was run for site CAD staff, BIM was built into the new-hire curriculum, and videos and user manuals were produced and distributed internally.

But this did not mean they intended to keep modeling in-house forever. The aim was to have the eye to judge. Why did the quantity come out this way, which standard should apply, is the deliverable correct — if you cannot judge that yourself, nothing you hand out gets verified.

And once they had confirmed how the process should run, they moved to outsourcing. Today a specialist firm models for four weeks, and the in-house BIM group and the site review the model and quantities for one week before feeding them into the final execution budget.

The order matters: build in-house capability → establish the process → move to outsourcing. Reverse it and you receive deliverables you cannot verify. Outsourcing works only once you have a standard for what to ask and how to judge what comes back.

Structural frame BIM model generated with BuilderHub — A structural frame BIM model generated at the site. Models like this, project after project, became the asset behind Part 2.

Part 1 conclusion — they did not reduce quantity. They became able to explain why the quantity is what it is. The savings followed as a result of transparency.

The customer and project names are anonymized, and figures are cited only from publicly announced material and within a pre-agreed scope. Diagrams and charts were newly produced from the source data.