tents, cable nets and grid shells.pdf

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Tents, Cable Nets and Grid Shells

Form-finding with GSA


Structural

Geotechnical

CAD

Document Management

Sustainability

Crowd simulation


Oasys Customers


WHY FORM FINDING?Geometric Optimisation…


Sagrada Familia Colonia Güell


FORM-FINDING RESULTS


Tensile Structure Shapes


Prestress

• How you achieve prestress influences concept, analysis and detail


Importance of correct pre-stress


DYNAMIC RELAXATIONGsRelax Non-linear analysis


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


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


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


Original position

Balanced position

Original position

Balanced position

Geometrical non-linearity


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


FORM FINDING


Form Finding

• Soap Film

• Force Density

• Normal Properties


Boundary Conditions


SOAP FILMForm Finding


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


Spacers

• Stress = Constant

• Young’s modulus E = 0


Acceptable & Unacceptable Spacers

=

Spacer elements

1

5 7

2

3

=

Acceptable SPACER

Unacceptable SPACER

One SPACER

Spacer elements


Form-finding with Pressures


'Water Cube' bubbles


Conics


MOMA – Liquid Skies


SOAP FILMdemonstration


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’


FORCE DENSITYForm Finding


Cable-nets


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


Cable nets


FORCE DENSITYdemonstration


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


NORMAL PROPERTYForm Finding


Normal property form finding

• Does not use form finding propertiesUses element properties

• Differences in element stiffnesses effect shape

• Creates permanent large scale deflections


Compression Structures


Grid Shells


Shells


Roof lattices


NORMAL PROPERTYdemonstration


Normal Property summary

• No spacers or Form-finding properties

• Uses section properties and large loads to achieve permanent deflections


London 2012 Olympic Main StadiumLondon 2012 Olympic Main Stadium


Soap-film cable nets


London 2012 VelodromeLondon 2012 Velodrome


London 2012 Olympic Shooting GalleryLondon 2012 Olympic Shooting Gallery


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

tents, cable nets and grid shells.pdf

Documents

oasyssoftwa resultswww

nonlinear solution method

comsoapfilm form

comwhy form finding

comequilibrium position

acceleration f

geometric optimisationwww

comform findingwww