Parametric Structural Design and beyondpapers.cumincad.org/data/works/att/ijac20108305.pdfdesign of a cantilevering truss,a parametric setup was essential in generating quick construction

319

Parametric StructuralDesign and beyondAnke Rolvink, Roel van de Straat and JeroenCoenders

issue 03, volume 08international journal of architectural computing

320

Parametric Structural Design and beyondAnke Rolvink, Roel van de Straat and Jeroen Coenders

Abstract

In order to directly make insightful which implications follow fromstructural design changes and to be able to adapt a structural designquickly to geometrical design changes made by the architect, thestructural engineer may embed a parametric and associative designapproach in the structural design process.This approach focuses onparametric modelling and the development of parametric tools whichserve specific needs in the structural design process, allowing designersfor instance to quickly communicate and discuss alternatives or toinform design team members of structural results of changing designparameters.

The paper presents multiple projects within these categories ofparametric approaches.They are concentrated on design and analysiswith the goal of presenting practical examples of these approaches instructural design which were integrated in the full design process inorder to benefit from the qualities of a multi-disciplinary parametricand associative design process.

1. Introduction

Design of the built environment requires the collaboration of a team ofdifferent roles and disciplines:The client, the architect, the structuralengineer, the MEP consultant, etc. However, most parametric and associativedesign systems and research do not focus on a multi-disciplinary approach,but mainly part of the architectural domain: the geometry. However aparametric approach from the architectural perspective alone does notserve the collaborative possibilities of a parametric and associative designprocess.Working within a multi-disciplinary design team, the project canbenefit if the structural engineer adopts a structural parametric andassociative design approach that follows the general design intentions of theproject and that provides insight, shows possibilities and presents boundaryconditions which have to be taken into account by other members of thedesign team.

This paper presents a number of research and development projects aswell as case studies of real buildings and structures, within Arup around theglobe which all exemplify the influence of structural parameters in thedesign process.The experiences by the engineers and computationaldesigners will be discussed alongside some of the technical details of theapproaches.

The paper will be subdivided in two main components: design projectswhich are based on parametric modelling and research and developmentprojects that employ and enhance the possibilities of parametric technology.

All projects made use of Bentley’s GenerativeComponents [1] and/orMcNeel’s Grasshopper plug-in for Rhinoceros [2] as parametric andassociative modelling system or as a base for custom tool development.

2. Structural Parametric Modelling

Over the past years,Arup has more and more experienced the need tohave full control over the complete design to improve the behaviour andperformance of the design by design variations and optimisation. Especiallyin the case of adaptation of the structural design to the geometricarchitectural design (usually with a complex geometry) the project anddesigners can greatly benefit from parametric control. If constructionmodels and analysis model have to be remodelled manually, especially inearly stages of the design where the architectural design is usually liable tomany influential design changes, the parametric approach delivers furtherbenefits of easier change and exploration.The projects below show howparametric modelling approaches are embedded in the structural designprocess to quickly set up structural alternatives, generate constructionmodels and to reanalyse the structure.

321Parametric Structural Design and beyond

2.1. Competition entry for the Austrian Pavilion of theEXPO2010

Viennese architects, SPAN and Zeytinoglu Architects designed ageometrically complex surface model for the competition entry of theAustrian Pavilion for the EXPO2010 in Shanghai, China. For the structuralengineers of Arup the main challenge was, next to proving a soundstructural concept, to convince the jury of the buildability of the projectwithin a limited timeframe and within a tight budget. Based on initial handsketches, the structural design was set up parametrically inGenerativeComponents in a way that a number of key structural elementscould be analysed individually, but also were associated into an overallparametric model that could be communicated with the architects, Figure 1.

The parametric model proofed its value in allowing for quick designupdates when the geometric surface model was edited by the architects orwhen structural alternatives had to be examined. For example for thedesign of a cantilevering truss, a parametric setup was essential in generatingquick construction and analysis models.The truss, with a height of 10marranged for the 18m long cantilever at the south-west side of the building.The complexity in the design of the truss was related to the bad soilconditions, urging the designers to avoid tensile forces in the foundation.Asa result, the tuning of the downward counter loads from the first floor androof structure with the upward loading from the rotational moment of thecantilever was matter of constantly changing the number and location of thefloor beams as well as their support locations which determined the floorloads that were transferred to the backside of the truss.

Employing this parametric approach to model and analyse structuralelements based on structural parameters allowed for a quick setup of thestructural design and a proposed building sequence, Figure 2.The mainbenefit however was that the parametric model could easily demonstratethat a complex architectural model could be simplified to a fairlystraightforward structure, consisting of mainly standard elements whichcould be easily assembled, convincing the jury of the buildability of theproject within the given boundary conditions.

! Figure 1. Left:The parametric modelof the structural elements. Middle:Therendered structural model includingthe profile sections. Right:Thearchitectural image render of thecompetition entry. Image (c) SPAN andZeytinoglu Architects.

322 Anke Rolvink, Roel van de Straat and Jeroen Coenders

2.2. Scheme Design for Coastal Canopies

The second project aimed to design a series of coastal canopies with acomplex geometry. Having considered typical structural systems for a seriesof freeform canopies, the limitations (long spans, material constraints, tightbudget, ease of construction and specific architectural details) gave rise to asingle acceptable solution: a steel structure following a rationalisedapproximation of the original geometry.Together with the architect asystem was set out in Grasshopper to interpret the architectural geometryusing simple geometrical surfaces, such as spheres and cones and settledupon a system of interconnected tori, Figure 3.

Four patches of four tori would be connected tangentially together, allmeeting at a single point, forming the basis of the geometry of each canopy.These tori were generated parametrically so that their base radii, their


! Figure 2.The proposed buildingsequence of the main structuralelements of the Austrian Pavilion

" Figure 3. Geometry logic based onfour tori

inclinations and translations relative to global coordinates could all becontrolled based on parameters.The idea being that the engineer woulddefine the rule-set and the architect would determine which geometry theypreferred based on the rules was agreed upon.

The base geometry permitted the development of a parametricallydefined structural grid upon the surfaces of the tori, so that the maximumlength of any element could be fixed.This created structural elements basedon arc geometry, with much repetition in the structural nodes andelements, with only a handful of different node types per canopy, creating acost-effective solution, Figure 4. Considering that there were upwards of2000 elements per canopy, this would facilitate prefabrication of the steelarc members.

Additionally, the base geometry has been used to generate the claddingpanelisation system, which inherits the repetitious quality of the toroidalgeometry.This creates a set of panel types that only vary where drainage is

required.These panels also vary at the perimeter of the surfaces where thethickness between the top of bottom cladding surfaces taper to give theillusion of a very slender volume.

The final geometry is that of the tori with the original architecturalperimeter (in plan) slicing through the base geometry giving the edges afluid “random” flow, thus presenting to the naked eye what appears to be afreeform surface, but is in fact a highly rationalised surface.


# Figure 4.Arc based structural gridsystem for one of the canopies

2.3. NSP Arnhem transfer hall

The NSP Arnhem transfer hall project, designed by UNStudio includes alarge freeform concrete shell with a complex geometry, Figure 5.Thegeometry of the shell has been defined by the architect in Rhinoceros astwo free form surfaces, consisting of NURBS surfaces. However, thearchitectural geometry was not directly usable for structural analysis , sinceit only comprised geometrical surfaces and for engineering purposes ananalysis model was required.

Since Rhinoceros’ meshing tools did not provide the opportunity togenerate a centre or offset mesh following the demands of the structuralengineering team and to directly transfer the mesh data to a FEM softwareapplication, a toolbox for the modelling of complex concrete shells fromfree form surfaces has been developed by the Arup team.The customdeveloped Grasshopper plug-in supported the engineers in generating FEMmodels which allowed for the analysis of the design.

The workflow of the toolbox asks for single surfaces on which meshpoints and face edges are tensioned based on user input, such as thenumber of elements in U- and V-direction of the surface. Subsequently, theuser can for instance pull the vertices to a surfaces edge or any otherlocation on the surface or add or delete vertices and point connections,Figure 6.

" Figure 5.The NSPArnhem transfer hall.Image (c) UNStudio


Finally, the generated mesh geometry can be exported to the FE analysissoftware application Infograph [3] via the toolbox’ interoperability interface,Figure 7.As such, the toolbox provides for the connection between theanalytical power of the FE analysis software application and the modellingcapabilities of Rhinoceros.

2.4.The Kurilpa tensegrity bridge

Another project which has been based on a parametric and associativemodelling approach is the Kurilpa tensegrity bridge in Brisbane,Australia.The competition design brief called for an architecturally striking rivercrossing to link Brisbane’s central business district with the newly developedarts and cultural precinct on the city’s South Bank and a regenerated andrapidly growing West End.The concept of the design, a multi-mast, cable-staystructure, based on the principles of tensegrity has resulted in a bridge that

! Figure 7. Screenshot from the FEManalysis software with the centre meshof the NSP Arnhem transfer hall


! Figure 6. Left:The original twosurfaces from the architect. Middle:The meshes modelled on the surfaces.Right: Generation of a centre meshfrom the two meshes constrained tothe original surfaces.

is both lightweight and incredibly strong. In dimensions the bridge is 470mlong with a main span of 120m and features two large viewing andrelaxation platforms, two rest areas, and a continuous all-weather canopyfor the entire length of the bridge, Figure 8.

During the design process various tasks have been performedsimultaneously.The use of GenerativeComponents provided the ability tostart modelling the complex tensegrity superstructure, even whilst theimportant bridge centreline geometry was still being finalised.This allowed acompression of the critical path, by enabling simultaneously working onlinked design tasks, Figure 9.

Different challenges appeared during the design process, such as the sitegeometry, existing structure and the complex sculptural tensegritysuperstructure. Bentley’s MX [4] design package has been used to modelsite geometry, including the horizontal and vertical alignments, whilebalancing with functional requirements and property boundaries.After thesite geometry was finalised it was imported into GenerativeComponents.Using the software’s flexible, associative modelling technology, key set-outpoints from the final MX centreline drove the model of the superstructuregeometry that had been prepared with project specific components alongthe centreline driven by those key points.These components had built in‘solvers’ to meet the design criteria.The final geometry was the resultant ofthe relationships between the components and the allowed clearances.Thegeometrical model was subsequently imported fromGenerativeComponents into analysis software for structural analysis andoptimisation. Bentley’s extraction technology has been used to complete thepre-assembled Bentley Triforma model with documentation and steelworkdrawings.The final 3D model has also been used to create a 4D model inNavisworks [5].


! Figure 8.A picture of the Kurilpatensegrity bridge, taken from theBicentennial bikeway


! Figure 9.The workflow ofinformation from concept model toconstruction model

# Figure 10.The parametric setup ofthe bridge’s superstructure inGenerativeComponents

The parametric setup of the project has been proven successful when atthe end of the project the steel detailer came up with some issuesconcerning the connection geometry that resulted as an artefact of thesuperstructure geometry.The GenerativeComponents model could then beutilised again to quickly resolve the geometry in an aesthetic solution. Itinvolved reversing the orientation of four masts to rectify the detail at thebase of the mast to achieve a more aesthetic solution.This actually meantchanging the orientation of 17 of the 20 masts due to the arrangement ofcables and clearances, but was completed in a matter of hours due to theparametric setup of the project.

2.5. Cable stay bridge option study

A parametric approach was adopted for the design option study of a curvedhighway bridge in the UK.The general form of the bridge was a single pylonand cable plane on the inside of the highway curve.TheGenerativeComponents model that was used to parametrically asses themultiple cable arrangements and tower location proposals was set up basedon:

• The 3D inroads alignment curve generated by the highwayalignment design team.

• The existing topography model

These were referenced into GenerativeComponents as spline curvesand formed the basis of the parametric model.

Pylon and abutment locations were defined by free points along thealignment which allowed the locations to be “dragged” along the alignmentand provided a high degree of real-time interaction with the structure.Background mapping was also referenced into the model to assist inlocating the structure.Additional free points defined the top of the pylonand upper and lower cable points which allowed the locations to be movedon screen. Graph variables were used to define cable spacing and structuralwidths.

As part of the model, a simplified set of solids were added to indicatethe approximate size of the deck, pylon, cables and parapet.At a designworkshop the model was then displayed on a large screen and used by thearchitect and structural engineers to investigate various arrangements inreal-time. Multiple views were set up in Bentley’s Microstation to quicklyshow the anticipated form of the structure from several viewpoints. Keyaspects like headroom at abutments and vertical clearances to the riverwere easily reviewed.Text labels were added to display key dimensions suchas span and pylon height in real-time without needing to refer to thedimension tool.


This approach enabled the general arrangement to be agreed in a singleworkshop rather than through prolonged discussions involving the issuing ofsketches and responding to review comments.At the end of the workshopa .DGN file of the agreed model was exported fromGenerativeComponents and issued to the architect for preparation of thedetailed visualisations and rendered images, Figure 11.Additional (hidden)lines were added to the model which were subsequently exported tostructural analysis software Oasys GSA [6] to enable the initial loading anddesign checks.

Once the GenerativeComponents model had been set up, theparametric approach led to considerable time savings being achieved in theinvestigation of bridge options, Figure 12.The ability to sit around the table


! Figure 11.The final result of thecable stay bridge option study

! Figure 12.The parametric model ofthe cable stay bridge

and review options in real-time enabled a holistic approach to be utilisedthat accommodated the requirements of varying parties at the same timeand maximised value for the client.Any ongoing revisions to the highwayalignment were simply re-imported into the GenerativeComponents modeland a revised .DGN and analysis model generated with a few clicks of themouse.The fully parametric nature of the model and use of alignmentcurves as direct references allow the .GCT file to be re-used on similarfuture projects with only minimal changes required.

3. Structural Parametric Tool Development

The previous chapter showed examples of design projects where structuralprinciples were embedded in the design process by creating parametricmodels.The following three research projects focus on an approach wherespecific design systems or tools have been build for the design developmentduring early stage structural design and structural analysis.These tools servespecific needs for both engineers as architects, but are generically applicable,mainly in early stages of the multi-disciplinary design process.

3.1. Salamander for Rhinoceros

Salamander is a plug-in for Rhinoceros and Grasshopper, designed to addnew features to the program in order to help engineers with creating andmodifying structural analysis models within Rhino.The plug-in extends thefunctionality of Rhino with the ability to store structural data(section/material properties, loads etc.) linked to the geometry andmanipulate it via a customised interface for viewing and editing this data.The model can then be exported to a finite element package such as GSAfor analysis. In other words, Salamander links geometry in the 3D Rhinomodel to structural data and keeps them in-sync.

As an example, points in Rhino represent nodes, lines represent 1Delements and so on. Structural modelling logic is imposed, such that if anode is moved any elements attached to that node will also have theirgeometry updated.This allows the engineer to edit the model in Rhino asthey would in an analysis package, with the added advantage of theavailability of Rhino’s geometric toolkit and user-friendly modellingenvironment. Data can be displayed in a graphical form in real-time in theRhino viewport where extruded section profiles can be rendered in 3D,points of support and releases can be labelled etc.The structural data canalso be browsed through a GSA-like ‘data tree’ and edited via pop-upwindows. Salamander also includes a set of tools to eliminate time-consuming and repetitive manual work such as algorithms to align elementsto surface normals, set releases between differing sections and tools tocheck the viability of the data before it is exported for analysis.


Salamander has its own built in manual interface, but it also has thecapability to be controlled through Grasshopper allowing the structuralmodel to be created parametrically, see Figure 13. Once the model’s nodesand elements have been created via a Grasshopper component, they remainbound to the Grasshopper model and will automatically update themselvesto match any change, with structural data remaining intact.This makes itpossible to change the geometry of the structural model very rapidlymerely by adjusting a slider or moving some control geometry, allowingengineers to investigate a range of options very rapidly. Furthermore, thegeneration of the structural model can be integrated with the generation ofthe architectural model, meaning that the structural engineer does not haveto spend time recreating a structural model from scratch every time thearchitect makes a small change to the design and the structural implicationsof these changes can be more quickly and easily understood.

3.2.Tall Building Simulation Tool

The Tall Building Simulation Tool is an example of a multi-disciplinary virtualdesign environment.This project was a collaborative effort betweenarchitects, structural engineers, mechanical engineers and cost consultants.The tool provides a dashboard interface for parametric design ofstandardised cases of high- rise buildings and measures various key designdrivers, such as cost, environmental performance, energy, etc, Figure 14.Theuser can design a building by adjusting a large number of parameters on thesystem.

The tool provides the ability for the multi-disciplinary optioneering of ahigh-rise building from 15 to 60 storeys.The structural plug-in for thisproject is based on assumptions and simplified calculations and is essentiallycapable of modelling standard stability cores with concrete floors andconcrete columns or standard stability cores with composite floors, steelbeams and steel columns.The tool serves as a tool for early discussions onkey design drivers and allows for improved communication betweendifferent project participants working cooperatively in an integrated high-rise project.


! Figure 13.Two different 3Dengineering models modelled inSalamander.The right model isparametrically controlled entirelythrough Grasshopper

To improve performances, the design team needs to understandinfluence of parameter changes to the design constraints and performances.Furthermore, when more disciplines are involved, knowledge of how designvariables and performances interact becomes increasingly important.Parametric studies can be carried out to explore the influence of structurallimits, allowing the design team to make better informed decisions aboutwhich design performance is governing.

3.3. StructuralComponents

Another development towards (parametric) design technology whichsupports the designing structural engineer is StructuralComponents.Thistoolbox focuses on employing the parametric and associative approach inthe conceptual design stages of a building; when the design concept of abuilding is conceived and studied.The current design process incorporatedin the toolbox allows the engineer to quickly compose various concepts ona dashboard, resulting in structural design models, which can be judgedbased on various structural performances.The toolbox allows for conceptsto be adjusted and analysed relatively quickly to be able to study theinfluence of parameter changes and alternative concepts.


! Figure 14.The interface of the TallBuilding Simulation Tool showing thebuilding outline, core structuralelements and the performance

One of the challenges of StructuralComponents is to support andaugment the creativity of the engineer.This is especially important duringthe early design stages since the impact of choices made during these stagesis often high and of high influence during the rest of the design process.Little information is known at these stages to base decisions on, making itdesirable to access a wide range of options.Another challenge is the uniquenature of design projects, since each project brings its own hurdles toovercome.The aim of StructuralComponents is therefore not to present asingle solution for design problems or workflow, but to provide theengineer a toolbox that provides parts of solutions that can be easily becomposed to a total solution and adapted to the design challenges of eachunique situation in design.

A prototype of the toolbox has been developed which is based on thestructural design of tall buildings.The structural engineer can use theprototype to compose different concepts by interrelating predefined andcustom components.These components can be loaded into a parametricsoftware application. Figure 15 shows the interface ofStructuralComponents for Grasshopper.A structural model comprisesstructural components, which are predefined elements, such as cores,outriggers, columns and frames.These elements are pre-programmed blocksof differential equations and have the ability to perform real-time structuralanalysis.

# Figure 15.A model in the currentversion of StructuralComponents,modelled in Grasshopper


The information on the building models performance is presented on adashboard and visible to the engineer in a single view, Figure 16.Theseresults are given in the form of dials and graphs.When a model is adapted,the output changes accordingly.The graphic output of the structuralbehaviour of the building structure give a clear impression not only to thestructural engineers, but it also allows the architect to get a feeling of theworking methodology of the engineers.

4. Discussion

The various projects described in this paper showed some of thepossibilities for a parametric and associative design approach in thestructural engineering practice of Arup. However, it is important to notethat adopting a full parametric and associative design approach requires achange in design culture for all the parties involved in the design process.Quick design changes imposed solely on for instance the architecturalsurface geometry may not be beneficial to the parametric and associativedesign process when structural, environmental or financial implicationscannot be interrelated directly to these design changes.The papershowcases a number of possibilities of which the authors think that theyserve the full parametric and associative design process as these parametricapproaches are able to follow the iterative process in the early stages of thedesign and take into account relationships with other disciplines.

5. Conclusion

With the growing number of developments in parametric and associativedesign for structural modelling and analysis, new possibilities arise allowing a


! Figure 16.A structural modelvisualised in Rhinoceros

more integrated design process, where amongst others the architect andengineer can communicate via parametric models.Assessing an extensivevariety of design options in a short time supports the creative process thatis often bounded by time limitations and greatly increases the designflexibility throughout the entire process.

The presented projects showed that integrating structural designintelligence based on the parametric and associative design approachenables the engineer to make better-informed decisions and to bettercommunicate them.Additionally, it allows the structural designer to get in-sync with the constant changing geometric definitions and variable designrequirements.

Acknowledgements

The authors of the paper would like to thank the many contributors to thispaper, who provided information on the various projects; Jan-Peter Koppitzand Kayin Dawoodi (Scheme design for coastal canopies), Christopher Pynnand Ken Enright (Kurilpa tensegrity bridge),Antony Schofield (Cable staybridge option study), and Paul Jeffries (Salamander).

References1. Aish, R., Introduction to GenerativeComponents, a parametric and associative design

system for architecture, building engineering and digital fabrication, white paper,http://www.bentley.com [15-05-2010].

2. McNeel, Grasshopper - generative modelling for Rhino,http://www.grasshopper3d.com [15-05-2010].

3. Infograph, InfoGraph GmbH - Software for structural design, http://www.infograph.eu/[15-05-2010].

4. Bentley Systems, Bentley Microstation, http://www.bentley.com [15-05-2010].

5. Autodesk, Autodesk Navisworks Products, http://www.autodesk.com [15-05-2010].

6. Oasys, Structural software, http://www.oasys-software.com [15-05-2010].


Anke Rolvink, Roel van de Straat and Jeroen CoendersArupNetherlands

Email: [email protected]; [email protected];[email protected]

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Parametric Structural Design and beyondpapers.cumincad.org/data/works/att/ijac20108305.pdfdesign of a cantilevering truss,a parametric setup was essential in generating quick construction