BIM Based Geometry Control in Cable Stayed Bridge Construction

Atte Mikkonnen, SOFiN Consulting Ltd, Espoo, Finland

The Kruunuvuori Bridge, slated for completion in 2025, stands as a forthcoming landmark in Helsinki, Finland. It is a cable-stayed bridge with two main spans of 260 meters and with a total length of 1160 meters (Figure 1). The project embraces full building information modeling (BIM), a standard practice in Helsinki’s public infrastructure construction. The geometry control of the cable-stayed bridge construction is also wholly model-based, marking an unprecedented implementation of BIM in major bridge construction globally. This paper elucidates the workflow of full model-based erection control, shedding light on the innovative methodologies employed in the Kruunuvuori Bridge construction.

BIM in Finland

Building Information Modeling (BIM) has been part of Finland’s architectural and construction practices for decades, reflecting a gradual shift towards digital innovation. This journey from two-dimensional drafting to the utilization of three-dimensional digital models showcases Finland’s role in embracing and shaping the future of construction technology. Initially, the application of 3D data was limited to the design phase, particularly valuable in the planning of industrial buildings and factories where precision across different disciplines was crucial. The introduction of BIM expanded these capabilities significantly, moving beyond mere geometry to include metadata enriching models with detailed information about materials, condition, costs and schedules. This transition signifies not just a change in tools but a broader shift towards integrated project management. The Finnish government and municipal authorities have played a significant role in this evolution. By requiring BIM in public projects, they have not only improved the management and maintenance of infrastructure but also propelled the industry forward, encouraging the adoption of advanced technology and practices. These requirements have fostered innovation, with the industry continually adapting to incorporate the latest technological advances. The early days of BIM in Finland were characterized by a learning curve, as the absence of standardized practices meant navigating through uncertainties. Collaboration and flexibility were essential as clients, designers and contractors worked towards finding common ground in the absence of established guidelines. This period was crucial in a country like Finland, where the close relationships within the industry emphasized the importance of mutual understanding and cooperation.

The Critical Role of IFC in Finnish Infrastructure Projects

In Finland, where the vast majority of infrastructure assets are publicly owned by either municipalities or the national government, there exists a fundamental requirement for transparency and fairness in the procurement process. Public administration authorities are obligated to avoid favoring any specific operator or supplier to ensure a competitive and open market. This imperative extends to the digital realm, particularly in the selection of file formats used in infrastructure projects.

The Industry Foundation Classes (IFC) file format, maintained by buildingSMART®, has emerged as the neutral platform of choice. Its adoption ensures that no single commercial supplier’s interests are prioritized over another, thus fostering a competitive environment that benefits all stakeholders. The IFC format provides a universal interoperability solution among diverse software applications, enabling the seamless import and export of building objects and their associated properties. This capability ensures that a wide array of BIM software can read and edit IFC files, making it a linchpin for collaboration and efficiency in project delivery.

Over time, IFC has become the standard file format for Building Information Modeling projects not only in Finland but across Scandinavia and increasingly worldwide. Its widespread support across numerous software platforms and the development of applications leveraging IFC for design, construction, and surveying underscore its critical role (Figures 2, 3). The format’s versatility and neutrality facilitate a broad range of applications, from the initial design phases through to construction and ongoing maintenance, illustrating its foundational role in modern infrastructure development.

For additional details on the IFC file format, buildingSMART® offers comprehensive resources at (http://www.buildingsmart-tech.org/).

Integrating Design Models in Construction

The City of Helsinki, one of the major infrastructure owners in Finland, is fully dedicated to adopting model-based construction practices for all their projects, including Major Bridge projects, like Kruunuvuori Bridge. Information modeling has been implemented from the early stages, and the commitment to extend this approach through the construction period to the in-service stage is a crucial aspect of the project’s overall strategy. For BIM to realize its full benefits for construction works, it is essential to model all designs, including those of the contractors, covering temporary structures and shop drawings. In the project, contractors’ project management and quality systems are intricately tied to the data connected and integrated into these models. This wealth of data serves as the foundation for real-time situational awareness, combining orthophotos, 360 photos, and up-to-date designs. The result is a sophisticated system for real-time schedule tracking, as well as monitoring safety and quality measures. To complete the situational picture, there is an additional requirement for up-to-date as-built models. This holistic integration of BIM not only streamlines the construction process but also ensures that the project maintains the highest standards of accuracy, efficiency, and transparency.

The case of the Kruunuvuori Bridge exemplifies how clients continuously push the boundaries of construction practices, prompting innovative responses from the industry. Design requirements mandated that all earlier design stages be delivered in BIM models, which were subsequently utilized in the tender process. Furthermore, construction and as-built documentation also required BIM integration, either as models or embedded data. Contractors were incentivized with bonuses for achieving ambitious applications of BIM during the construction process.

The Kruunuvuori Bridge project underscores the City of Helsinki’s commitment to embracing modern construction technology as a guiding principle. Here, new methods are always considered without prejudice, ensuring that innovative approaches are explored and implemented. This commitment to pushing technological boundaries is best exemplified by practices such as “geometry control,” which are integral to the project’s success and demonstrate the feasibility of adopting advanced construction methodologies.

Information Model-Based Geometry Control, Enhancing Construction Efficiency

Information models used for construction projects typically exhibit geometric stability, remaining static over time. These models, often referred to as system geometry, depict the expected structure configuration at the time of completion or after a designated period post-completion.

For complex structures like cable-supported bridges, the construction process involves several stages before finalization. Given the inherent flexibility of such structures, precise control of their geometry becomes mandatory to ensure adherence to the planned construction sequence. Geometry checks are conducted at specific construction stages, with necessary adjustments made as required. These adjustments are rigorously scrutinized and verified before proceeding with subsequent construction phases. Traditionally, geometry control involves referencing the structure’s position and coordinates relative to the system geometry, which are then translated into construction coordinates. Surveyors subsequently validate these coordinates against predetermined control points, a process that entails post-processing of data and is susceptible to human error. Moreover, this approach provides a limited amount of information, which may be challenging to interpret visually.

The concept of Information Model-Based Geometry Control emerged during the Thu Thiem 2 bridge project, where the fabrication shape of the steel structure was provided in IFC models (Mikkonen 2020 [1]). The fabrication shape, representing the structure’s initial geometry with a camber to compensate for deformations during erection, served as the foundation for this innovative approach. It was realized during software development that automating the production of this mandatory part of the geometry could streamline the export of any stage included in the analysis into the IFC format. Additionally, these models could be directly utilized for surveys, eliminating the need for manual translation and post-processing of data.

This workflow revolutionized data transfer from analysis to construction, significantly reducing time and potential errors associated with traditional geometry control methods. By leveraging Information Model-Based Geometry Control, construction projects can achieve heightened efficiency and accuracy while enhancing communication and understanding across project stakeholders.

Geometry Control Models in Construction Projects

The principle of model-based geometry control revolves around the creation of separate geometry models to accurately represent the necessary data for construction works. These models are defined in global coordinates, aligning with those used in the construction process. They can be seamlessly integrated into collaboration platforms for visualization and directly utilized for on-site surveys. The objective is to maintain data separation and refrain from enriching it into other models or digital twins. Geometry control models generate independent sets that portray the progression of bridge construction over time, showcasing the structure’s geometry at each stage. This approach was successfully demonstrated in the Kruunuvuori Bridge project, where models were exported directly from analysis without requiring postprocessing.

Prior to exporting data from analysis to Building Accuracy is paramount in geometry control models, necessitating alignment with the coordinate system used in construction. Large coordinate numbers in analytical and BIM models may pose challenges, often resolved by translating models into a local coordinate system. Model rotation is discouraged due to numerical challenges associated with large numbers.

Modern bridge designs provide initial geometry information in real coordinates, facilitating seamless integration with analysis software for Finite Element Analysis (FEA). Analytical models comprise discretized finite elements, representing physical geometry with selected accuracy limitations. Non-structural components may not be fully modeled in structural analysis, but additional data can be included as needed, provided it is understood that not all data is intended for surveys.

In summary, geometry control models play a pivotal role in construction projects by providing accurate geometric data essential for monitoring and managing construction progress.

Information Models, careful selection is essential to ensure that relevant data intended for site use is included and correct. Understanding the survey methods and the type of data measured (geometry objects) is crucial for accurate representation.

Leveraging IFC Models and Erection Engineering

For the Kruunuvuori Bridge construction project, the client provided designs for construction in IFC format models. These models encompassed all the necessary geometry and material information for the completed structure, supplemented by text documents and specific drawings outlining additional specifications and requirements. To ensure clarity and accountability, certain drawings extracted from the model or model data were stamped with the statement “this drawing is a printout from the model for filing, provided model is the governing design.” These models were provided in the system geometry within the global coordinate system used for construction works.

Notably, the client’s designs presented the erection method only in principle through drawings, without providing detailed geometry information, including pre-camber specifications. As a result, the responsibility for erection engineering with geometry control fell entirely within the contractor’s scope. This approach aimed to maximize the contractor’s input for developing feasible bridge construction plans.

Initially, the project adopted a balanced cantilever erection method, which necessitated the completion of pylons before main span segments could be installed. However, a revised method was later introduced, wherein the superstructure would be constructed on temporary supports, enabling parallel construction of the pylon (Figure 4, 5). This concurrent construction strategy streamlined the initiation of cable works for the entire bridge upon pylon completion, eliminating the time-consuming cantilever construction process. Despite incurring additional costs for temporary supports, the decision was justified by the substantial time savings offered by this method.

However, these changes necessitated a comprehensive redesign or re-evaluation of the client’s superstructure design for the main spans. To address this challenge, the contractor, TYL Kruunusillat (a joint venture of Kreate Ltd and YIT Ltd), enlisted the expertise of Ramboll Finland Ltd and SOFiN Consulting Ltd. Ramboll oversaw the shop drawing designs and conducted capacity checks, while SOFiN Consulting was tasked with geometry control, including analysis for the shop form, and provided Structural Health Monitoring (SHM) analysis services.

Modeling for Geometry Control Analysis

The modeling for geometry control analysis in the Kruunuvuori Bridge project was conducted using SOFiSTiK Finite Element Analysis (FEA) software, initially with version 2020 and later updated to versions 2022 and 2023 throughout the project’s progression. These updates were motivated by the introduction of new features available only in the latest software versions. Additionally, parametric modeling workflows utilizing Rhino-Grasshopper software were implemented from the project’s inception.

To align with the local coordinate system used for construction, the models were prepared by simply eliminating the first 4 to 5 digits from the global coordinate values, facilitating a straightforward translation without rotation (Figure 6).

The superstructure of the bridge comprises an open steel-concrete composite structure, characterized by a concrete deck supported by steel girders in a ladder deck configuration. Cross beams spaced approximately 4 meters apart support the concrete deck, while independent cable cross beams, spaced approximately 12 meters apart, connect the cables to the main girders. The modeling approach prioritized accurate representation of the steel girders, cable formwork tubes, pylon geometry, and concrete deck edge-beam geometry to ensure the availability of essential data for geometry control.

In modeling the steel girders, the centerline of the girder top flanges was selected as the element axis, consistent with the axis provided in the client’s design (Figure 7, 8). The analysis for geometry control purposes was conducted with the same principles as the design checks, ensuring consistency across modeling approaches. Cable forces in the final stage were adjusted to maintain the original pylon action forces, obviating the need for pylon redesign. The structure was modeled with higher accuracy for geometry control analysis, with detailed inclusion of construction steps in the staged construction simulation.

While structural design utilized beam elements and capacities with an effective width concept, geometry control analysis prioritized geometric nonlinearities and realistic stiffness distribution. Shell elements were employed to model the deck, providing realistic structural behavior but posing challenges for composite section design checks. Despite minor differences in results stemming from varying modeling approaches, the concept of “one source of truth” was adopted, as both models were generated from the same data source, facilitating cross-checking and verification.

Analysis for Geometry Control: Staged Construction Simulation

Analysis for geometry control, often referred to as staged construction analysis or construction simulation, involves describing and analyzing all erection steps in chronological order. This comprehensive approach ensures that all effects of element activation, temporary supports, loads (both temporary and permanent), creep, shrinkage, and other relevant factors are considered and controlled.

The analysis typically incorporates selected material and geometric nonlinearities, such as creep, shrinkage, cable geometric nonlinearity, support uplift, and concrete cracking. While including all these complexities in a single analysis can be challenging due to limitations in available software, the process is iterative and performed in forward calculation to capture nonlinear behavior accurately.

During the iterative process, the initial geometry of the structure (installation geometry) is updated to achieve the system geometry at the specified construction stage, typically at the time of opening. As a result, the analysis provides both the geometry and action forces for all selected construction stages, offering a theoretical plan for construction execution (Figure 11, 12).

For the Kruunuvuori Bridge project, geometry control models included a global model for overall geometry and local models for approach bridge launching. These local models incorporate the real geometry with camber as fabricated, which is crucial for geometry control and design (realistic load and support reaction distribution). Additionally, local models are designed to be quick and easy to use, enabling prompt responses during construction works at the site.

Fabrication and installation geometry were defined using the global models, as were the models for segment assembly at the assembly yard. Protocols were developed for model translation, ensuring minimal numerical errors in coordinate calculations during the model transition from the site (system geometry) to the assembly yard (Figure 9).

As-Built Simulation and Geometry Control

In bridge construction, deviations from the ideal plan are inevitable. Tolerances, variations in deformations, applied forces, loads, weather effects, and temperatures all contribute to the dynamic nature of construction. Recognizing these challenges, a new workflow has emerged, incorporating the concept of as-built simulation. This approach maintains an analytical model representing the current status of construction with the required accuracy, allowing for analysis and adjustment in response to on-site events or deviations from the ideal plan (Figure 10).

The staged construction analysis for the Kruunuvuori Bridge project included simulation of the steel structure’s incremental launching, particularly for the approach bridges. This launching process presented unique challenges due to the bridge’s curved plan geometry and the presence of an S-curve at the abutment. Guides were employed to control the plan geometry, enabling the polygonal shape of the main girders to navigate through. Additionally, as new segments were aligned at site, consideration was given to the deformation of already launched segments, which were no longer temporarily supported.

During the launching process, the structure exhibited movement outward from the theoretical axis within allowable tolerances, while the skewed last support induced warping in the cross-section. To address these effects, as-built adjustments were integrated into the analysis after each launch. This included incorporating realized geometry from surveys and making small jacking adjustments to compensate for warping. The streamlined workflow facilitated swift updates to the IFC model, which were then used for segment assembly on the embankment without the need for post-processing.

This streamlined workflow proved highly effective. The analysis, performed on the same day after the superstructure launching, facilitated the swift upload of the new IFC model to the cloud-based databank for the next segment installation. Upon arrival, the new segments were unloaded directly to their planned position based on the IFC geometry downloaded from the cloud. The efficiency of this workflow was attributed to the absence of post-processing, as the models used for surveys were directly generated from the analysis software.

As-Built Models for Planning

In the Kruunuvuori Bridge project, forward-thinking planning based on survey results and as-built models plays a pivotal role in ensuring construction aligns with the intended design. Adjustment updates are meticulously planned in advance, with a focus on critical elements such as cable works.

Cable stressing is a phased process, where each step is surveyed and re-analyzed to guarantee adherence to the project plans. The initial stages are particularly critical to prevent overstressing, necessitating a thorough comparison between surveyed on-site geometry and analytical models. This comparison enables precise interpretation of results, encompassing changes in both elevation and plan with ease.

Furthermore, the final cable stressing lengths are determined post-application of surface wear coats and rail structures, leveraging survey data for accuracy. Plans are meticulously finalized based on this data, aiming to align superstructure geometry as closely as possible with the planned design while ensuring that final cable forces remain within permissible values.

The maintenance of as-built models throughout the project proves invaluable in this process, serving as a reliable reference point for planning adjustments and ensuring construction progresses in accordance with design specifications. This proactive approach underscores the importance of leveraging real-time survey data and as-built models to optimize construction efficiency and maintain structural integrity.

Conclusion: Advantages of BIM-Based Workflow

Throughout the Kruunuvuori Bridge project, the adoption of a BIM-based workflow has proven to be a game-changer, offering significant advantages over traditional methods. This new approach doesn’t replace existing practices but enhances them, providing substantial benefits across various facets of construction management.

One of the standout advantages is the transparency afforded by the BIM-based workflow. All stakeholders have access to comprehensive documentation and real-time data through a cloud-based collaboration platform. This transparency enables effective oversight and management, empowering stakeholders to make informed decisions based on a clear understanding of project details.

Moreover, the efficiency and speed of the workflow are unparalleled. With direct data flow from analysis to execution, modeling becomes a seamless process that eliminates the need for post-processing. This not only saves time but also ensures data accuracy while minimizing human errors. The visual nature of model-based viewing further enhances comprehension and simplifies quality assurance and inspection checks.

Flexibility is another key strength of the BIM-based workflow. As-built information can be easily incorporated, allowing for real-time analysis and predictive modeling to anticipate the effects of construction changes on the final structure. Maintaining an up-to-date model throughout the construction process provides a reliable plan that accounts for evolving geometry, facilitating efficient decision-making and project management.

The success of this workflow validates its applicability to future projects, particularly those where time-dependent geometry changes are significant. Contractors are likely to increasingly demand such capabilities, not only for main structural components but also for secondary and non-structural items. Future workflows may involve integrating non-structural geometric objects into analysis software or further developing geometry data exchange via IFC formats or parametric modeling tools, ensuring accuracy and alignment with surveyed data.

In essence, the experience gained from the Kruunuvuori Bridge project underscores the transformative potential of BIM-based workflows in modern construction practices. By embracing these methodologies and leveraging advanced technologies, the industry can drive efficiency, accuracy, and innovation to new heights.

[1] Mikkonen, Infra BIM open 2020, BIM for Complex Bridges. BIM_for_Complex_Bridges.pdf

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