Digital Technology Impacts on the Arnhem Transfer Hall
Structural Design
Roel VAN DE STRAAT*, Sander HOFMANa, Jeroen COENDERSb, Joop PAULc , *Arup – Computational Design
Naritaweg 118, 1043 CA, Amsterdam, the Netherlands Roel.van-de-Straat@arup.com
a Arup – Building Design
b White Lioness technologies and Delft University of Technology c Arup and Delft University of Technology
Abstract
The new Transfer Hall in Arnhem is one of the key projects to prepare the Dutch railways for the increased future demands for capacity. UNStudio developed a master plan in 1996 for the station area of which the completion of the Transfer Hall in 2015 will be a final milestone. The Transfer Hall is a merging point of passengers, commercial and social interchanges, containing a multi-use development integrating program and flows of people and vehicles. The design includes a complex geometrical, double-curved shell roof where many functions are combined as well as many other geometrically challenging structural elements.
This paper forms the fourth and final paper of a series [1][2][3] and focuses on the developments in digital technology during the project’s design and construction phase and how these developments could impact the structural design of a special project like this.
Keywords: building information modeling, advanced geometry, parametric design, structural design,
digital fabrication, free form shell, transfer hall, architecture, master planning.
1. Introduction
The master plan for the area around the station dates back from 1996 and has been a significant new development for the city. The station’s terminal integrates train, bus, bike and car travel with direct pedestrian access to the city in one transport hub. The design of the Transfer Hall presented both the architects and the structural engineers with interesting challenges. Arup has been involved from the earliest stages delivering multiple technical design activities as the structural design forms a direct and essential part of the overall design with a combination of innovative underground and large span structures [4].
During the structural design phases different building materials were applied for the distinguishing sections of the structure. Besides the office building, which was designed as a large cantilevering steel frame with composite floors, most elements of the terminal were designed in in-situ concrete. During a Design and Built tender the selected contractor proposed to change the material of the roof to steel [6] to improve buildability in order to stay within time and budget constraints, but compromising some of the architectural design intent which included the use of exposed concrete, see Figure 1.
Figure 1: Architect’s render of the master plan (top – courtesy of UNStudio) and a photograph of the construction site in Spring 2015 (bottom – courtesy of Jacq Top).
The free form shape of many structural elements drove the project teams to develop and apply computational tools within the design workflow. Examples of these tools have been discussed in previous papers [2][3]. In relation to these tools, this paper focuses on possible ways of applying new and future digital technologies on the translation from architectural surface geometry to structural geometry and on communication of BIM models for a project like NSP Arnhem Transfer Hall.
2. Applied digital workflows and tools
At the end of the 20th century, new computational and technical developments allowed for new ways of approaching structural and architectural forms using form finding techniques. Amongst these developments are certain form finding techniques, use of custom developed parametric design plug-ins, and advanced analysis methods.
2.1. Complex surface geometry definition and form finding
It was early recognized that it was not possible to design such a building in a traditional manner. Although the structural design dates back almost two decades, sophisticated computational design strategies were already employed including the computational form finding of certain structural elements. In the early design phases, structural form finding was performed through Oasys GSA software fabric analysis. The study was based on Seifert’s algorithm defining the process of generating an orientable surface with one boundary component such that the boundary component of the surface is given by a knot [7], see Figure 2.
Figure 2: Seifert’s algorithm and a resulting curve diagram for NSP Arnhem Transfer Hall. By tuning the surface and edge cable pre-stress and applying surface pressure at specific locations, the geometry was shaped to arrive at favorable load bearing behavior for the complex form. Although the final shape was not the result of a pure structural form finding process, its form finding process helped smoothen out bending stress peaks and provided the desired smooth surface.
Through several form iterations together with UNStudio, the geometry was further refined based on definitions shared with NURBS surfaces in Rhino, form finding with GSA Fablon and FE analysis. One of the more significant elements of the Transfer Hall, called the Twist –a structural element via which the double curved roof is smoothly connected to the Balcony–, was defined via this topological surface study. As such, this structural element is formed as one single surface which twists and folds to obtain its stability.
2.2. Meshing strategy tools for structural design and analysis
An innovative mesh strategy was applied in order to generate a structural FEM mesh geometry that accurately fitted the free form architectural geometry. In this strategy, several custom developed tools were used. Several tools were developed that parametrically meshed a surface, optimized the mesh and generates a thickness driven offset mesh and one tool to check the location of a FEM mesh compared to the top and bottom surface of a double curved concrete surface. The latest generations of these tools are briefly discussed below.
2.2.1. Parametric meshing modeling and optimization toolbox
The early tool used a three-dimensional particle-spring system approach in Rhinoceros3D to interactively and parametrically model and negotiate the mesh geometry given the architectural constraints. Meshes could either follow surface constraints, curve or point constraints or could negotiate a space between surfaces based on the algorithm while continuously optimising for their equilibrium location. Although this worked well for the early analysis models, refinement made the algorithms too computationally intensive to reliably use in the remainder of the design process.
2.2.2. Parametric meshing and offsetting toolbox
This toolbox was used to rationalize and gain geometric control over the free modelled surfaces and transform these into parametric mesh definitions. Based on FEM requirements, such as mesh size, minimum angle, maximum edge length, etc. a mesh was be created based on the Delaunay triangulation algorithm [3]. The tool has also features to offset the mesh over a constant distance or over a variable offsetting based on predefined offset distances, see Figure 3.
Figure 3: Discrete offset curves indicating the shell thickness of the roof structure [left] and the resulting FEM mesh based on a Delaunay triangulation algorithm with color indicated offset thicknesses [right].
2.2.3. Automated mesh validation tool
This custom developed Rhino plug-in enables the validation of a finite element mesh with respect to an architectural model (top and bottom surface). It checks both thickness and position of the elements and indicates deviations according to a predefined tolerance, see Figure 4. It has proven to be a very successful tool in the analyses and validation of the calculation meshes for several geometrically complex parts of the NSP Arnhem Transfer Hall project.
Figure 4: Initial mesh validated against updated surface geometry of the twist [top]. Updated mesh geometry and element thickness [middle]. Revalidated mesh geometry indicating that all tolerances
2.2.4. Physical and geometric non-linear analysis
Due to the significantly improved capacity and functionalities of FE software the different structural building elements could be studied in detail appropriate for the development of the design as well as their interaction in an overall – more coarse – model, see Figure 5.
Important aspects included the stiffness and capacity of the already build supports. The three and four levels deep parking, built in 2004, supports the superstructure and has a limited capacity. The interaction of shell behavior and bending moments is influenced by horizontal stiffness of the different building elements. Being able to model such a large structure and preform physical none linear calculations was a significant benefit to determine realistic results.
Figure 5: Impression of the structural overall model.
In order to study critical parts of the building in more detail, advanced finite element models of the structure were made [5]. These include the possible buckling effect of the roof including the non-linear effect of the concrete as cracking, yielding of reinforcement and creep, see Figure 6. Including these effects helped secure a less conservative approach and enabled to predict realistic deflections and reinforcement quantities. Additionally many studies were performed to study and rationalize the differences and uncertainty in stiffness of the various parts of the structure and their influence on the load bearing behavior. Much effort was put in rationalizing the building in such a way that the influence from this uncertainty was minimized.
3. Assessment of impact of recent and future developments
Given the time frame from the initial design phase in 1996 to the construction phase planned to be finished end of 2015, different digital techniques and methods have become available or have become common practice in the structural engineering industry. This chapter discusses the possible impact of recent and future digital developments on the structural design of the NSP Arnhem Transfer Hall, focusing on meshing strategies, structural analysis, and communication and collaboration methods.
3.1. Improved meshing strategies
One important aspect of the structural analysis of the Transfer Hall, was the definition of the FEM models, and in particular the mesh definition for these analysis models. Over the course of the various design phases, different meshing strategies have been used to develop a satisfactory set of meshed surfaces. One version allowed for a mesh relaxation based on a particle spring algorithm, whereas the final mesh strategy included the option to create thickness offsets following predefined rules.
Ideally, a mesh strategy would follow a certain set of characteristics, including i) a parametric setup in order to quickly process mesh densities, ii) efficient algorithms to refine mesh definition (subdivision of surfaces), iii) a certain way of mesh relaxation in order to optimally follow predefined geometry, iv) fast negotiation of modelled architectural and structural constraints while maintaining a proper analysis mesh for complex three-dimensional structures and v) capable of incorporating FEM analysis requirements, such as a preference for equally sized square elements.
Figure 7: Application of a simple meshing strategy for the free form stairs structure. Although the mesh was set up parametrically, no driver was in place to force the mesh towards equally sized
quadrilateral mesh elements.
Currently, parametric software such as Grasshopper for Rhino incorporate freely available plugins that support designers in defining mesh geometry – examples are the Weaverbird and Kangaroo plugins for Grasshopper. Using these relatively new plugins would allow for improved meshing strategies. However, custom developed components which perform specific functionalities, such as the offsetting of meshes to pre-defined thicknesses, are necessary additions to the functionalities of these plugins.
3.2. Developments for analysis of geometrically complex concrete shells
3.2.1. Control over geometry definition
One of the most challenging features of the design, was the complex geometry and specifically the control over the geometry definitions. The definition of the NURBS geometry posed the structural engineers with challenges in for instance extracting fully accurate centre surfaces between architectural top and bottom surfaces of structural elements. For complex geometric designs such as that of the Transfer Hall, a clear understanding of NURBS surface definitions is essential, especially when other geometries, such as the mesh geometry, are extracted from this. Such projects highly benefit from a good understanding of NURBS geometry from both the architects and the engineers. During the design phases, experts in the field of NURBS geometry focused on the geometric definitions of the Transfer Hall design from which it was eminent that centrally coordinated geometric model improved collaboration between architects and engineers.
3.2.2. Analysis software and tooling
During the final design phases Arup was involved in, the majority of structural elements were planned to be constructed in concrete. Due to the complexity of geometry and the related load distribution in the structural elements, the placing of rebars in the concrete geometry was a challenging matter. Arup developed a conceptual reinforcement tool, capable of modelling reinforcement bars and elements in complex free form concrete shapes [8], see Figure 7. The development of such tools may hugely benefit the reinforcement design process as general solutions for rebar layout are not trivial for projects like the Transfer Hall. As the project was finally designed as a steel structure, the practical development of the tool is currently on hold.
Figure 7: Subsequent steps of the computational strategy for modelling complex reinforcement: a) Input of a curved surface (b) Surface rationalization (c) Creation of a solid model (d) Visualization of
From an architectural stand, the goal was to design a monolith concrete structure without visible cracks. Arup engineers adopted Infograph as the primary analysis software. Some difficulties with the software were the convergence issues and models which could not run did not show the actual issues at hand. Over the last few years, various analysis software packages have proven to be able perform even better than the Infograph versions used at the moment of the FEM analysis phases. And in addition, calculation power – either within a single desktop or laptop, or via analysis clusters in the cloud – has increased significantly, allowing from quicker and more accurate results, see Figure 8.
Figure 8: Over the recent years Arup has adopted cloud analysis processes for projects with highly demanding computation power. Example: MST Enschede (arch: IAA) project, where light analysis calculations were performed in Dialux in the cloud to optimise room daylighting for a large hospital.
Concerning the stiffness studies, the team of Arup engineers would have been able to assess the structures responses in much more detail, given the time. Computational methods, such as a Monte Carlo analysis were not adopted due to the computational intensive nature, although this would have provided the engineers with additional control over the structural behavior of the design.
Another type of optimization could be found in the optimization of the structural thicknesses. As mentioned before, the initial form finding process, was not a purely structural form finding process aiming towards an optimized structural shape definition. An additional step, focusing on adapting the structural geometry to a more optimized shape was one which was not taken. In addition, a digital driven optimization routine was desired over the adopted manual ‘optimization’ of the thicknesses of the main structural elements. Arup engineers feel that it was a missed opportunity to not have the thicknesses optimized within the given geometry. Embedding structural optimization algorithms, capable of taking single or multiple objectives, would have driven the design to a more efficient structure and helping with assessing engineering judgment.
3.3. Increase of detail and modes of communication
The complexity of the geometry posed another challenge: understanding the actual shape of the design and communicating this clearly in different formats. Nowadays, making quick 3D printed models are almost common practice in the AEC [Architecture, Engineering, and Construction] industry. Although various 3D printed models were developed during the design phases, design alternatives are currently more easily presented via 3D printed models. Figure 9 shows a 3D printed model of the twist, dating from the earliest days of 3D printing.
Figure 9: One of the preliminary 3D printed models of the twist. Printed at the Arup in London.
Obviously, a large impact in communication of designs the AEC industry is coming from the field of BIM. Recent developments show that BIM software is better capable of dealing with complex geometry. Current and near future developments might allow for geometrically complex models such as those of the Transfer Hall to be defined in BIM software. Also, in the use of automated connections between parametric software and BIM software interesting developments have been seen and will be developed further in the near future. For the NSP Arnhem Transfer Hall project, no fully coordinated BIM model was used by the design team. The collaborative aspect of BIM models would have been a great asset to the design process, especially because of the many interfaces with existing buildings. Based on the BIM models, building sequencing in relation to temporally measures to keep the train station operational could also be an improved asset to the design process.
In order for BIM to have a solid place in the design process, a BIM execution plan would have to be in place. Such BIM plan would have to communicate the most important requirements for the project and incorporate questions such as i) how to collaborate?, ii) how to deal with clashes in the design?, and iii) how data is exchanged between the different design partners and the client?. Recent collaborations have shown the benefit of such a plan, see Figure 10.
Figure 10: Workflow BIM models as part of the BIM plan for Theatre De Stoep (arch: UNStudio) [left] and the resulting integrated structures and MEP BIM model [right].
4. Discussion
Working on highly complex designs demand well integrated processes in order to optimize the design process. For the NSP Arnhem Transfer Hall project, the design team used various complex models for different end results; geometry definition, analysis, BIM, and communication.
Most of these models proved to be complex to perform iterative loops as a result of intensive remodeling required and the structural analysis setup at that time. For instance, loading definitions in the analysis models were not parametrically set up and these models were based on geometric input following from initial structural input, rendering changes to the design a laborious process. As a result, optimization of various aspects in the design and the design process were hard to accomplish.
Discussions on how the Transfer Hall project would be approached in the current and near future give interesting food for thought, regarding optimization of the design and design processes of such complex structures.
5. Conclusion
This paper discusses a number of design and process requirements which have to be in place in order to successfully complete highly complex projects. Projects like the NSP Arnhem Transfer Hall require a trade-off between standard tools, (custom developed) computational tools, high computational power, the latest specifications in analysis modelling, and well-defined BIM plans and strategies. With the latest developments in the AEC industry, more tools and methods are available in order to tackle the challenges the design team have been facing since the start of the project. Most likely, the design process for the NSP Arnhem Transfer Hall when executed with the current state of technical developments would include the 1) parametrical definition of geometry and analysis models and 2) the extensive integration of geometry and BIM models and the automated link between those two model types.
With the completion of the NSP Arnhem Transfer Hall in 2015, the design team may look back on a very interesting and rewarding design process, showing that besides available tools, most importantly, working on these projects require an intensive and good collaboration between architects, engineers, and clients, and demand a high level of knowledge, eye for innovation, and a matching ambition from the design team partners and the client.
Acknowledgement
The final outcome of the series of IASS papers on the NSP Arnhem Transfer Hall required positive input from all authors and ProRail. The authors of the final paper in the series would like to acknowledge all partners in the process of designing and constructing this project, amongst others ProRail, the municipality of Arnhem, UNStudio, Arcadis, ABT, BBB and Bouwcombinatie OVT and its subcontractors, Movares, NS, VROM, Rijkswaterstaat, Van Rossum, Grontmij, Fugro, and Van der Werf & Lankhorst. In addition, the authors would like to express their gratitude towards all project team members who have worked on this project in the past.
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