Systems for Lightweight Structure Design:the State-of-the-Art and Current Developments Erik Moncrieff! Kurvenbau Emdener Str.39 D-10551 Berlin,Germany erik.moncrieff@kurvenbau.com http://www.kurvenbau.com Summary.This paper deals with the design of lightweight structures.In partic- ular the role of computational modelling software in this process is discussed.The state-of-the-art is first described paying close attention to the requirements for indus- trially effective solutions.Some of the less well understood aspects of the modelling processes are discussed.In particular the load analysis,form-finding and cutting pat- tern generation processes are covered.The modelling of tertile is addressed in detail. Approaches to the design of software design systems for lightweight structure design are discussed in the contert of system fleribility and effectiveness.Finally,inter- esting applications in the field of lightweight structures arising from design system developments are highlighted. Key words:design,lightweight structure,modelling,simulation,textile,element type,form-finding,geometrically non-linear structural analysis,elastically non-linear structural analysis,cutting pattern generation,crimp,pneumatic structure,hybrid structure,adaptive design 1 Introduction To most structural engineers and architects the design of lightweight structures is mysterious.The objective of this paper is to summarise the state-of-the-art in lightweight structure design systems in order to highlight several important concepts. Emphasis is directed to the requirements of industrial procedures. 2 Lightweight Structure Design 2.1 Design Process As with conventional structural engineering projects,the design process for lightweight structures involves three key players.These are the Client,the Architect and the 17 E.Onate and B.Kroplin (eds.).Textile Composites and Inflatable Structures,17-28. 2005 Springer.Printed in the Netherlands
Systems for Lightweight Structure Design: the State-of-the-Art and Current Developments Erik Moncrieff1 Kurvenbau Emdener Str. 39 D-10551 Berlin, Germany erik.moncrieff@kurvenbau.com http://www.kurvenbau.com Summary. This paper deals with the design of lightweight structures. In particular the role of computational modelling software in this process is discussed. The state-of-the-art is first described paying close attention to the requirements for industrially effective solutions. Some of the less well understood aspects of the modelling processes are discussed. In particular the load analysis, form-finding and cutting pattern generation processes are covered. The modelling of textile is addressed in detail. Approaches to the design of software design systems for lightweight structure design are discussed in the context of system flexibility and effectiveness. Finally, interesting applications in the field of lightweight structures arising from design system developments are highlighted. Key words: design, lightweight structure, modelling, simulation, textile, element type, form-finding, geometrically non-linear structural analysis, elastically non-linear structural analysis, cutting pattern generation, crimp, pneumatic structure, hybrid structure, adaptive design 1 Introduction To most structural engineers and architects the design of lightweight structures is mysterious. The objective of this paper is to summarise the state-of-the-art in lightweight structure design systems in order to highlight several important concepts. Emphasis is directed to the requirements of industrial procedures. 2 Lightweight Structure Design 2.1 Design Process As with conventional structural engineering projects, the design process for lightweight structures involves three key players. These are the Client, the Architect and the 17 E. Oñate and B. Kröplin (eds.), Textile Composites and Inflatable Structures, 17–28. © 2005 Springer. Printed in the Netherlands
18 Erik Moncrieff Structural Engineer.The client commissions the project and invites tenders from architects.The architects prepare conceptual designs working in collaboration with structural engineers.The client chooses a conceptual design and appoints the archi- tect.The architect,again working in close collaboration with a structural engineer proceeds to refine the conceptual design into a production design.Finally the design is fabricated and installed.The critical path of this design process is shown in Fig.1. In reality there are several design modification cycles operating. Conceptual design Form-finding/oad anal sis Patterning/etailing nstallation Fig.1.The phases of the design process critical path 2.2 Design deliverables The deliverables can be conveniently divided between those for the conceptual and production designs. Conceptual design Pre-stress surface geometry Form-finding Reaction,support and cable forces Load Analysis ●Textile stresses Production design Pre-stress surface geometry Form-finding Reaction,support and cable forces Load Analysis ●Textile stresses Cloth pattern system line geometry Cutting Pattern Generation Support structure design Detailing ●Cable dimensions ·Connection design Cloth and reinforcement cutting patterns Detailing is a critical process and highly integrated with the other processes.It will,however be the Form-finding,Load Analysis and Cutting Pattern Generation processes which will be mainly considered here. 2.3 Load Analysis and Form-Finding The tasks Load analysis and form-finding require the determination of Force Equilibrant mod- els.In a Force Equilibrant model the residual forces acting on any degree of freedom
18 Erik Moncrieff Structural Engineer. The client commissions the project and invites tenders from architects. The architects prepare conceptual designs working in collaboration with structural engineers. The client chooses a conceptual design and appoints the architect. The architect, again working in close collaboration with a structural engineer proceeds to refine the conceptual design into a production design. Finally the design is fabricated and installed. The critical path of this design process is shown in Fig. 1. In reality there are several design modification cycles operating. Conceptual design Form-finding/ oad anal sis Patterning/ etailing nstallation Fig. 1. The phases of the design process critical path 2.2 Design deliverables The deliverables can be conveniently divided between those for the conceptual and production designs. Conceptual design • Pre-stress surface geometry F orm F − f inding f d ining • Reaction, support and cable forces Load Analysis Load Analysis Load Analysis oad Analysis nalysis • Textile stresses Production design • Pre-stress surface geometry F orm F − f inding f d ining • Reaction, support and cable forces Load Analysis Load Analysis Load Analysis oad Analysis • Textile stresses • Cloth pattern system line geometry Cutting P attern Generation Cutting P attern Generation C Gutting P attern Generation utting P attern Generation • Support structure design Detailing D etailing etailing • Cable dimensions • Connection design • Cloth and reinforcement cutting patterns Detailing is a critical process and highly integrated with the other processes. It will, however be the Form-finding, Load Analysis and Cutting Pattern Generation processes which will be mainly considered here. 2.3 Load Analysis and Form-Finding The tasks Load analysis and form-finding require the determination of Force Equilibrant models. In a Force Equilibrant model the residual forces acting on any degree of freedom
Systems for Lightweight Structure Design 19 after summing the internal and external loads acting there is zero.In the case of computational load analysis the elemental forces may be calculated using several elastic models.Similarly several methods may be used to define the elemental forces in computational form-finding.The load intensity distribution must be estimated. Historical development Before the development of computational structural modelling,textile structures were form-found using physical models and load analysis was performed using hand calculations.The development of linear structural analysis software had little appli- cability for the design of textile roofs due to their strong geometrical non-linearity Non-linear systems have been developed since the 1970's and are now routinely used. Current system configurations Today industrial systems are broadly based on three main solver algorithms. Conjugate Gradient (CG)/Force Density (FD) Dynamic Relaration (DR) Modified Stiffness (MS) Developments in mainland Europe have mostly used CG/FD solvers,Britain has concentrated on DR,and Japan and the USA have mainly used the MS method. Two element types are commonly used to model textile roofs.Cable net models using link elements have been popular in CG/FD systems,while triangular contin- uum elements have been typically used in DR and MS systems.It is important to highlight that the prevalence of using particular elements with particular solver al- gorithms does not have a theoretical or computational basis.CG/FD systems with triangular continuum elements are used when appropriate,and MS and DR sys- tems can also use link elements to model textile.Appropriate element types for the modelling of lightweight structures will be discussed in Section 3.2 below. 2.4 Cutting Pattern Generation The tasks Cutting Pattern Generation is the process where two dimensional unstressed cloth polygons are created from three-dimensional doubly curved stressed surfaces.This involves the specification of seam line locations,transformation of the stressed 3D surfaces to stressed 2D surfaces,and compensating the stressed 2D surfaces to un- stressed 2D surfaces. Historical development Before the advent of computer modelling,textile roofs were patterned using physical models.Simple triangle strip development between computer model seam lines were next implemented and have been used successfully for medium to large structures. Distortion minimisation techniques have been adapted from map making to cope with the demands of smaller and more sensitive configurations. Seam generation Regardless of whether physical or computational modelling is used,patterning based on geodesic seam lines is the preferred approach.This is because geodesic lines
Systems for Lightweight Structure Design 19 after summing the internal and external loads acting there is zero. In the case of computational load analysis the elemental forces may be calculated using several elastic models. Similarly several methods may be used to define the elemental forces in computational form-finding. The load intensity distribution must be estimated. Historical development Before the development of computational structural modelling, textile structures were form-found using physical models and load analysis was performed using hand calculations. The development of linear structural analysis software had little applicability for the design of textile roofs due to their strong geometrical non-linearity Non-linear systems have been developed since the 1970’s and are now routinely used. Current system configurations Today industrial systems are broadly based on three main solver algorithms. • Conjugate Gradient (CG)/Force Density (FD) • Dynamic Relaxation (DR) • Modified Stiffness (MS) Developments in mainland Europe have mostly used CG/FD solvers, Britain has concentrated on DR, and Japan and the USA have mainly used the MS method. Two element types are commonly used to model textile roofs. Cable net models using link elements have been popular in CG/FD systems, while triangular continuum elements have been typically used in DR and MS systems. It is important to highlight that the prevalence of using particular elements with particular solver algorithms does not have a theoretical or computational basis. CG/FD systems with triangular continuum elements are used when appropriate, and MS and DR systems can also use link elements to model textile. Appropriate element types for the modelling of lightweight structures will be discussed in Section 3.2 below. 2.4 Cutting Pattern Generation The tasks Cutting Pattern Generation is the process where two dimensional unstressed cloth polygons are created from three-dimensional doubly curved stressed surfaces. This involves the specification of seam line locations, transformation of the stressed 3D surfaces to stressed 2D surfaces, and compensating the stressed 2D surfaces to unstressed 2D surfaces. Historical development Before the advent of computer modelling, textile roofs were patterned using physical models. Simple triangle strip development between computer model seam lines were next implemented and have been used successfully for medium to large structures. Distortion minimisation techniques have been adapted from map making to cope with the demands of smaller and more sensitive configurations. Seam generation Regardless of whether physical or computational modelling is used, patterning based on geodesic seam lines is the preferred approach. This is because geodesic lines
20 Erik Moncrieff are,by definition,straight when developed to a plane.Cloths patterned between geodesic seam lines will therefore be straighter than non-geodesic patterns.Non- geodesic patterns typically have banana or "S"shaped cloths which cause larger cloth wastage. Fig.2.Non-geodesic (lighter cloth),and geodesic (darker cloth)patterns Fig.3.Architecturally mandated semi-geodesic seam pattern for the German Chan- cellory In some situations architectural requirements mandate non-geodesic seam lines. A prominent example of such a situation is the German Chancellory [1].In such situations the use of line generation algorithms which are curvature based rather than length-minimising is helpful [2]. Cloth planarisation The process of transforming a doubly curved surface into planar cloth sub-surfaces requires the introduction of distortion.The most basic approach taken to solve this problem is to define the cloths in terms of developable triangle strips.This works entirely adequately for large structures,but small structures are more difficult because cloth roll width does not limit pattern widths.This results in a wish to have fewer cloths for economic reasons.Meshes for small structures based on simple triangle development fail to reliably model the surface [3].This can be seen in Figure 4.In such cases the use of more sophisticated distortion minimisation algorithms is very effective
20 Erik Moncrieff are, by definition, straight when developed to a plane. Cloths patterned between geodesic seam lines will therefore be straighter than non-geodesic patterns. Nongeodesic patterns typically have banana or “S” shaped cloths which cause larger cloth wastage. Fig. 2. Non-geodesic (lighter cloth), and geodesic (darker cloth) patterns Fig. 3. Architecturally mandated semi-geodesic seam pattern for the German Chancellory In some situations architectural requirements mandate non-geodesic seam lines. A prominent example of such a situation is the German Chancellory [1]. In such situations the use of line generation algorithms which are curvature based rather than length-minimising is helpful [2]. Cloth planarisation The process of transforming a doubly curved surface into planar cloth sub-surfaces requires the introduction of distortion. The most basic approach taken to solve this problem is to define the cloths in terms of developable triangle strips. This works entirely adequately for large structures, but small structures are more difficult because cloth roll width does not limit pattern widths. This results in a wish to have fewer cloths for economic reasons. Meshes for small structures based on simple triangle development fail to reliably model the surface [3]. This can be seen in Figure 4. In such cases the use of more sophisticated distortion minimisation algorithms is very effective
Systems for Lightweight Structure Design 21 Fig.4.Planarisation:(a)Large structure simple triangle development,(b)Small structure triangle development,(c)Small structure deformation minimising flatten- ing 2.5 Design Methodologies Textile structures have been designed in three general ways. Non-computational:Physical models are used to form-find the pre-stress sur- face geometry and create the cutting patterns.Simplified "hand calculations" are used to predict structural response. Non-specialised software:Non-equilibrium computational modelling soft- ware,such as 3ds mar,is used to generate the pre-stress surface geometry and cutting pattern generation.Standard FE structural analysis software is used to perform load analysis. Specialised software:Lightweight structure task-specific equilibrium based computational modelling software is used to perform form-finding,load anal- ysis and cutting pattern generation. The non-computational method has the advantages that it is intuitive,the form can be realised,it can be implemented with low initial investment,and modification of conceptual forms is quick and simple.It suffers from its lack of computational non-linear structural analysis,low precision and lack of computational mesh for rendering.Its slowness,particularly with respect to making modifications to the production form and cutting patterns,makes it operationally expensive. Using the non-specialised software method leverages existing CAD and analysis software skills and provides many sophisticated geometric tools.With few excep- tions,the forms generated are,however,not force equilibrant.Consequently they can not necessarily be realised with a tensile surface.Lack of integration between the mesh generation and analysis leads to slow design modification cycles.Conven- tional FE software is often inappropriate for use with textile models.In particular, convergence problems are usually experienced by standard FE systems when dealing with textile slackening on-off non-linearity. Specialist textile structure software systems quickly provides high confidence, high precision,integrated solutions.Initial investment is higher but when design volume is adequate,per-design costs are low.It is therefore the recommended method for production design.Having said that it is important to stress that the continued use of physical modelling during the conceptual modelling phase should always be encouraged
Systems for Lightweight Structure Design 21 Fig. 4. Planarisation: (a) Large structure simple triangle development, (b) Small structure triangle development, (c) Small structure deformation minimising flattening 2.5 Design Methodologies Textile structures have been designed in three general ways. Non-computational: Physical models are used to form-find the pre-stress surface geometry and create the cutting patterns. Simplified “hand calculations” are used to predict structural response. Non-specialised software: Non-equilibrium computational modelling software, such as 3ds max, is used to generate the pre-stress surface geometry and cutting pattern generation. Standard FE structural analysis software is used to perform load analysis. Specialised software: Lightweight structure task-specific equilibrium based computational modelling software is used to perform form-finding, load analysis and cutting pattern generation. The non-computational method has the advantages that it is intuitive, the form can be realised, it can be implemented with low initial investment, and modification of conceptual forms is quick and simple. It suffers from its lack of computational non-linear structural analysis, low precision and lack of computational mesh for rendering. Its slowness, particularly with respect to making modifications to the production form and cutting patterns, makes it operationally expensive. Using the non-specialised software method leverages existing CAD and analysis software skills and provides many sophisticated geometric tools. With few exceptions, the forms generated are, however, not force equilibrant. Consequently they can not necessarily be realised with a tensile surface. Lack of integration between the mesh generation and analysis leads to slow design modification cycles. Conventional FE software is often inappropriate for use with textile models. In particular, convergence problems are usually experienced by standard FE systems when dealing with textile slackening on-off non-linearity. Specialist textile structure software systems quickly provides high confidence, high precision, integrated solutions. Initial investment is higher but when design volume is adequate, per-design costs are low. It is therefore the recommended method for production design. Having said that it is important to stress that the continued use of physical modelling during the conceptual modelling phase should always be encouraged