Progress Billing of Executed Linear Construction Project Based on Information Modeling
The following article shows the process of how to use an information model as a basis to certify and invoice the quantities of work executed in a period and thus obtain the necessary liquidity to continue working. Specifically, this BIM use case has been developed for a linear construction project located in Bremen-Germany, using the information model of a retaining wall in IFC4 format and the cost and time management software RIB iTWO 2024.
How have we done it until now?
During the execution of the work, the technical team (engineers and/or surveyors) makes an on-site sketch of what has been executed, including details such as the location, dimensions and characteristics of the retaining walls built. This preliminary document serves as the basis for the preparation of the official measurement sheet. It is reviewed and approved by the site supervisor (representative of the client or contractor) and, in some cases, by a representative of the construction supervision authority. Approval confirms that the sketch accurately reflects what has been built.
Based on the approved sketch, a detailed measurement sheet is prepared that includes the exact quantities of the work performed, such as linear meters of wall, volume of concrete and reinforcing steel used. The measurements are entered into the RIB iTWO software, which allows managing and documenting the quantities of work in a digital measurement sheet. This measurement sheet is sent to the client in *.D11 format, along with sketches and supporting photos for review and validation. In some cases, an onsite co-inspection is performed to verify that the measurements match what was executed.
For progress billing, the public works customer in Germany only accepts measurement sheets that can be calculated according to REB 23.003, which describes calculation methods and data exchange
methods. In addition, it has a collection of geometric formulas for calculating the required quantities.
Once the measurements are approved, a partial certification is issued confirming that the work has been executed as agreed and complies with quality standards and regulations. Based on the approved measurement sheet and the partial certification, the corresponding progress billing is generated using RIB iTWO, which automatically calculates the costs according to the certified quantities and the contract unit prices. All documentation (sketches, measurement sheets, certifications and invoices) is digitally archived in RIB iTWO to ensure traceability and future access to the information. Copies are provided to the client and competent authorities as required.
How will we do it from now on?
For the progress billing based on the information model of a retaining wall of the linear construction project located in Bremen-Germany, the following requirements were considered:
| Requirements | |
| 3D Information Model | Retaining Wall |
| Information for filtering objects and calculating quantities | Object characteristics, classification system |
| Files in a specific format/scheme | *.ifc | IFC4, Information model *.D83 | Bill of Quantities (BoQ) *X84 | Bill of Quantities Price specifications |
| Software/OS | RIB iTWO 2024 Simplebim 10 S3 Windows 10 Enterprise |
| Hardware | Processor: AMD EPYC 7763 64-Core Processor 2.44 GHz RAM: 32.0 GB Operating System Type: 64-Bit |
| Staff | Billing > 01 Person Planning coordination > 01 Person BIM Specialist > 01 Person |
Each object in the information model must include specification data that facilitates its grouping and assignment to billable partial services. This process is essential for generating selection groups in RIB iTWO 2024. Since in this particular software quantities from the geometry of an infrastructure object cannot be reliably determined automatically, the Simplebim tool, an IFC file editor, has been used, which allows to efficiently calculate quantities directly from the geometry of the model objects, such as volumes, lengths, areas, surfaces, among other parameters, and which also offers the possibility to save these quantities in the form of attributes that can be read in RIB iTWO 2024.
The procedure for linking objects with their attributes and partial services within the iTWO RIB software is as follows:
Next, we proceed to create the certification of quantities executed in the work in a determined period, as well as the respective deliverables, through the following steps:
Outlook for model-based billing
There is a legal complexity in the use of BIM models for invoicing, as progress billing requires legal precision, and BIM models do not always capture contractual details (e.g., unforeseen work variants).
There are proposals for a standardized format for exchanging data between BIM models and billing documents (DIN 18290 Part 4, Germany). This approach aims to eliminate manual errors, accelerate certification of progress and ensure transparency between contractors, clients and authorities. Sector agreements, BIM training and legal frameworks that support interoperability are needed to drive this format. Pilot projects in public infrastructure (such as railways, highways, tunnels and bridges) could serve as initiators or “triggers”. However, their success will depend on collaboration between construction companies, software developers, regulatory bodies and public sector construction authorities.
Author: Mariel R. Dapena, Project Engineer in the BIM Department of the Civil Engineering and Special Foundations Division in Germany, Implenia Civil Engineering GmbH. Zigurat Ambassador 2024-2026.
