Download - Tents, Cable Nets and Grid Shells.pdf
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Tents, Cable Nets and Grid Shells
Form-finding with GSA
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Structural
Geotechnical
CAD
Document Management
Sustainability
Crowd simulation
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Oasys Customers
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WHY FORM FINDING?Geometric Optimisation…
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Sagrada Familia Colonia Güell
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FORM-FINDING RESULTS
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Tensile Structure Shapes
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Prestress
• How you achieve prestress influences concept, analysis and detail
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Importance of correct pre-stress
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DYNAMIC RELAXATIONGsRelax Non-linear analysis
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What is GsRelax?• A non-linear structure analysis program embedded in GSA
as a Solver (previously called – Fablon)
• It uses a non-linear solution method called DYNAMIC RELAXATION
• It specialises in doing: • Geometrically non-linear analysis
• Material non-linear analysis
• Non-linear buckling analysis
• Form-finding analysis
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The model moves under the action of out of balance forces until it is in equilibrium
Equilibrium position:No out of balance force (‘residual’ ) - system ‘converged’
Unbalanced position: There is an out of balance force (‘residual’) which will generate an acceleration F=ma
oscillation
Dynamic Relaxation
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Equilibrium position:maximum speed,zero acceleration,
Most unbalanced position:zero speed,acceleration in peak,
Dynamic relaxation is not a matrix method
1. F = mGSA applies a mass to each mode to enable the motion equations
2. DampingGSA applies damping to the system to prevent continuous oscillation
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Original position
Balanced position
Original position
Balanced position
Geometrical non-linearity
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Advantages of using GSA
• Automatic consideration of geometric non-linear effects
• No limit on type of structure analysed.For example:
• Rigid structures• Flexible structures• Fabric structures• Cable structures• Mechanisms
• Non-linear behaviour• Material non-linearity• Geometric non-linearity• Buckling
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FORM FINDING
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Form Finding
• Soap Film
• Force Density
• Normal Properties
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Boundary Conditions
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SOAP FILMForm Finding
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Soap-film form finding
• The fabric will take up a shape similar to a soap film which fits with the boundary conditions.
• Curvature is achieved by changing boundaries and prestress
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Spacers
• Stress = Constant
• Young’s modulus E = 0
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Acceptable & Unacceptable Spacers
=
Spacer elements
1
5 7
2
3
=
Acceptable SPACER
Unacceptable SPACER
One SPACER
Spacer elements
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Form-finding with Pressures
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'Water Cube' bubbles
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'Water Cube' bubbles
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Conics
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MOMA – Liquid Skies
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SOAP FILMdemonstration
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Soap Film summary
• Include ‘spacer’ elements to control nodes within the surface
• Define edge cable prestress in ‘1D soap film properties’
• Define fabric prestress in ‘2D soap filmproperties’
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FORCE DENSITYForm Finding
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Cable-nets
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Force Density
• No spacers
• During the analysis:• 2D element stress is set to proportional to its area (= A*FD)• Bar element force is set to proportional to its length (= L*FD)
FD = 1 FD = 2 FD = 3 F =30
FD = 1 FD = 2 FD = 3
1030 15
F =30
15 1515
The Model
The Result
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Cable nets
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FORCE DENSITYdemonstration
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Force Density summary
• No need for Spacer elements
• Define edge cable prestress in ‘Force Density 1D’ properties
• Define fabric prestress in ‘Force Density 2D’ properties
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NORMAL PROPERTYForm Finding
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Normal property form finding
• Does not use form finding propertiesUses element properties
• Differences in element stiffnesses effect shape
• Creates permanent large scale deflections
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Compression Structures
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Grid Shells
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Shells
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Roof lattices
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NORMAL PROPERTYdemonstration
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Normal Property summary
• No spacers or Form-finding properties
• Uses section properties and large loads to achieve permanent deflections
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London 2012 Olympic Main StadiumLondon 2012 Olympic Main Stadium
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Soap-film cable nets
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London 2012 VelodromeLondon 2012 Velodrome
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London 2012 VelodromeLondon 2012 Velodrome
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London 2012 Olympic Shooting GalleryLondon 2012 Olympic Shooting Gallery
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Where next?
• Arup and GSA Suite users• Activate Form Finding in Preferences
• Other GSA users• Request GSA Suite trial
• Non-GSA users• Download trial from www.oasys-software.com/GSA• Request GSA Suite trial
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