Why quantities from ST-Bridge can't be your actual quantities
ST-Bridge (STB) is the standard format that Japanese structural design software exports in common. Import STB and the frame stands up; run automatic rebar and you even get rebar quantities. The common misunderstanding is "once STB is in, the quantities are done." That number covers only what the design data drew. It is not the actual quantity the project needs.
Published October 2, 2026·Last checked · October 2026
Key points
- STB is data for structural analysis. It carries the load-bearing columns, beams, slabs and walls; members the analysis does not need are treated only as loads or left out
- So Quantity A, from auto rebar on STB, often contains no rebar for parapets, non-bearing walls, interior slab steps, canopies, or sumps and pits
- Conditions decided at the construction stage, such as whether to use anchor plates, are not in the design data either. The same member gets different rebar depending on that decision
- Actual quantity B only appears after every member is modeled per the construction documents and the construction conditions are set, then rebar is placed automatically. The STB quantity is right for an early-stage estimate
1Is it really "STB in, quantities done"?
ST-Bridge is the standard format for exchanging information in Japan's building structure field, defined by the Structural Design Subcommittee of buildingSMART Japan. It bridges integrated structural analysis software and BIM software. Moving structural design results into a 3D frame without redrawing them makes it a genuinely good starting point.
The problem comes next. Import STB, stand up the frame, run automatic rebar — the screen fills with bars and a quantity table appears. Because a number comes out, it is easy to take it as the construction quantity. The video below shows how far that number actually counts, first as a diagram and then on the actual BuilderHub-R screen.
2A structural model draws the load-bearing skeleton
STB comes from structural analysis. The purpose of the analysis is to confirm the building withstands loads and earthquakes, so the model contains the members that carry force. Members that carry no force, or barely affect the results, are either added only as weight or left out of the model. That is a sound structural decision. As a result, though, some members that really do contain rebar never appear in the data.
| Member | Why it is not in STB | What happens to the quantity | |
|---|---|---|---|
| ① | Parapets | Often treated only as a load in the structural model | The rooftop perimeter wall and its rebar are missing |
| ② | Non-bearing walls | Carry no force, so they may not be on structural plans | Wall rebar and opening reinforcement are missing |
| ③ | Interior slab steps | Not in STB, so not in the quantities | Bars bent at the step and extra reinforcement are missing |
| ⑤ | Canopies | Small cantilevered members, easily omitted | Rebar for the cantilever is missing |
| ⑥ | Sumps and pits | Ancillary parts around the foundation, outside the analysis | Wall and floor rebar where the foundation slab is recessed is missing |
STB is not wrong. It is a format that carries structural analysis results accurately, and it does that well. What drops out are members that were outside the analysis to begin with. Use data with a different scope as the basis for quantities, and a gap is to be expected.
3Conditions not on the drawings — decided during construction
Item ④ in the video is different. It is not a missing member but a condition not yet decided. At beam-column joints, whether bar ends are hooked or fitted with anchor plates is often decided by the contractor based on the method and site conditions. The choice changes the development length and the shape of the bars at the anchorage.
The same goes for where splices are placed and whether they are lapped, gas pressure welded or mechanical. Until such decisions are reflected, the quantity for the same member differs from reality.
4So the actual quantity comes from the full model
In short, there are two quantities. Quantity A is the result of automatic rebar over the scope of the structural design data. Actual quantity B is the result of modeling every missing member per the construction documents (structural and architectural drawings), setting construction conditions such as anchorage and splices, and then placing rebar automatically. That is what A≠B at the end of the video means.
Quantity A is not useless. At an early stage, while design is still in progress, it is quite useful for seeing the scale of frame quantities and where clashes tend to occur. But that number is an estimate, not the actual quantity that ordering and settlement should rest on. The starting point is to call each number by what it is for.
The STB quantity is a starting point. The actual quantity comes after the missing members and construction conditions are filled in.
The difference between quantity standards (standard quantity versus net quantity) is covered separately in "One building, two quantities".
5In BuilderHub-R
From here on, this is about our product.
BuilderHub-R imports ST-Bridge (2.0.2) to build the frame, and recognizes 2D CAD drawings to add members that are not in STB. When members are added or edited, rebar is regenerated automatically to JIS and the quantity takeoff report is recalculated. It also supports settings for special members such as anchor plates.
Adding the missing members does not finish by itself, though. Modeling from the drawings takes effort for every building. In return, once the model is built, rebar and quantities can be produced again under the same conditions when the design changes. See the BuilderHub-R product page for details.
Sources
- ST-Bridge — the standard format for information exchange in Japan's building structure fieldbuildingSMART Japan, Structural Design Subcommittee
This article is a summary compiled from published material. It is not a legal interpretation and does not constitute advice. Regulation changes frequently, so please check the original documents and the latest notices from the responsible authority before relying on it in practice.