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c o v e r t h e w o r l d t mUser Manual
Release R 2.0
by Gerry D'AnzaF o r T e n 3 0 0 0
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All rights reserved. No parts of this work may be reproduced in any form or by any means - graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems - without the written permission of the publisher.
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While every precaution has been taken in the preparation of this document, the publisher and the author assume no responsibility for errors or omissions, or for damages resulting from the use of information contained in this document or from the use of programs and source code that may accompany it. In no event shall the publisher and the author be liable for any loss of profit or any other commercial damage caused or alleged to have been caused directly or indirectly by this document.
Printed: ottobre 2010 in Naples (Italy)
ixForTen 4000
© 2010 Gerry D'Anza
Publisher Special thanks to:
Many Thank s to My Wife Rita and my Son Andrea that leave me work at any hour and any day with a huge amount of patience
Managing Editor Technical Editors Cover Designer TSI s.r.l Gerry D'Anza Loredana Di Benedetto Shehzad Irani Gerry D'Anza Production TSI s.r.l.
Architecture & engineering
Team Coordinator
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c o v e r t h e w o r l d t m ixForTen 4000 4Table of Contents
Foreword 11Part I Introduction
13
... 13 1 Where do I begin ? ... 14 Project Setup ... 16 Model Setup ... 17 Form Finding ... 18 Analysis ... 18 2 FAQ ... 18 What are tensile structures?... 20 After Form Find I dont see anything. What Happens ?
... 20 I am not able to find desired shape. What can I do ?
... 21 What loads are acting on the structure?
... 22 How to calculate w ind loads from Wind speed ?
... 24 How can I add a m ast w ith stay cables ?
... 24 Analysis stops w ith a m essage MATRIX Error . What to do ?
... 26 How do I add fixed length links ?
... 26 How can I check if pre-stresses are Ok ?
... 28 I am not able to cut the surface. What to do ?
... 28 How to I calculate com pensation ?
... 29 How can I check if patterns are correct ?
... 30 3 Modelling Fabric Structures
... 31 Form Finding
... 31 Static Non linear Analysis
... 32 Patterning
... 32 4 Application Interface
Part II Fabric Structure FormFinding 35
... 35 1 Groups ... 37 2 Nodes ... 39 3 Linear elements ... 39 Linear Elem ent Properties
... 43 Seeds or Named properties
... 43 Cross Section
... 44 Material
Part III Structural Analysis with ixForTen 4000
48
... 53 1 Newton Raphson
... 54 2 Newton Raphson Modified
... 55 3 Incremental Method
... 55 4 Incremental Iterative method
... 56 5 Static Nonlinear Analysis
... 56 Loading the structure
... 58 Add Load ... 59 Nodal Loads ... 60 Cable-Beam loads ... 61 Pressure loads ... 62 Vector loads
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c o v e r t h e w o r l d t m 5 Contents ... 63 Add Self Weight... 64 Add Stress Multiplier
... 64 Add Zero Load
... 64 Creating Load Cases
... 66 Running the analysis
... 67 View ing Results
Part IV Patterning with ixForten 4000 72
... 72 1 Patterning
... 73 2 The Patterner Module
... 75 Cutting The surface
... 78 Single Cutter ... 79 Multi Cutter ... 80 Helpers ... 82 Importing selection & cutting curves
... 82 Make Patterns
... 84 Making Patterns
... 86 Changing Pattern parameters
... 86 Compensating
... 88 Offsets & Markers
... 91 Flip upside dow n
... 91 Notes on patterning ... 92 Production ... 93 Detailing pane ... 93 Pattern w elding offsets pane
... 95 Pattern Compensation Pane
... 97 Edge Decompensation ... 99 Layout ... 101 Decimation Pane ... 102 Export Pane ... 102 Option Pane ... 103 Layout ... 105 Text Pane ... 107 Colors
Part V Structure of the software
110
... 110 1 Main Groups ... 111 2 Entity Specification ... 112 Nodes ... 113 Structural Entities ... 115 Boundary entities ... 116 2D Pattern entities ... 116 Special Graphical entities
... 117 Graphical entities
Part VI Modeler
119
... 119 1 File Menu ... 119 New ... 119 Open ... 119 Im port ... 121 Last opened Files... 121 Save
... 121 Save As
... 121 Save as Tem plate
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c o v e r t h e w o r l d t m ixForTen 4000 6 ... 123 Capture View ... 123 Exit ... 123 2 Create ... 123 Quad Surface ... 126 Cone Surface ... 128 Cushion ... 129 Cushion_grid ... 130 Edge Elem ent... 130 Project to surface ... 130 Boundary Group ... 131 Tenso Group ... 132 Graphic Group ... 132 Copy selected to new Tenso
... 133 Copy selected to current tenso group
... 133 Line ... 133 Poly line ... 133 Polygon ... 134 Circle CR ... 134 Circle 3P ... 134 Triangle ... 135 Footings ... 139 3 Edit ... 139 Undo ... 139 Conical Control ... 140 Check clean invalid objects
... 140 Change Behaviour ... 141 Flip orientation ... 142 Move ... 142 Rotate ... 142 Weld ... 142 Join A to B ... 143 Delete ... 143 4 Select ... 144 Selection A-B ... 144 Clear ... 144 All ... 144 Single / Painting ... 144 Window ... 145 Fence / poly select
... 145 Circle ... 145 Warp ... 145 Weft ... 145 Filter ... 146 Property ... 148 Child / Tenso / Boundary
... 148 5 Tensile Structure
... 148 Form Find
... 150 Pneum atic Options
... 152 Save as reference state
... 152 Reload reference state
... 153 Non Linear Analysis
... 154 Anim ate ... 154 Check DOFS ... 154 Check Model ... 155 6 Tables ... 155 Data Base Explorer
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c o v e r t h e w o r l d t m 7 Contents ... 156 Material DataBase ... 159 Section DataBase ... 163 Seed (Named property) DataBase... 166 Show Materials and Sections
... 166 7 Loading
... 167 Add load condition
... 167 Add self-w eight condition
... 168 Add Pre-stress condition
... 168 Add Zero-Load condition
... 168 Add Load ... 168 Nodal loads ... 169 Cable-Beam loads ... 170 Pressure loads ... 170 Vector loads ... 171 Thermal loads ... 172 8 Info ... 172 Project info ... 172 Report Manager ... 175 Model Item ... 175 Nodes ... 176 Elements ... 176 Cables ... 177 Steel ... 177 Membrane ... 178 Membrane Mesh ... 178 Seeds ... 179 Materials ... 179 Cross Sections ... 180 Load Conditions ... 180 Bill of Materials ... 180 Membrane Area ... 181 Cable List ... 182 Steel List ... 182 FF Response ... 182 Pretension Reactions ... 183 FF El.Results ... 183 FF Membrane-Cable-Steel ... 184 FF Tri-Mesh ... 184 Analysis Response ... 184 Node Displacements ... 185 Query Distance ... 185 Selected Objects ... 186 Form Find Info
... 186 Analysis Info ... 187 9 Visibility ... 187 Hide/show Nodes ... 188 Hide/Show Entities ... 188 Hide/Show Mesh ... 188 Hide Selected ... 188 Hide Not-Selected ... 188 Flip Visible/Hidden ... 188 Un Hide ... 189 10 UCS ... 189 UCS Store/Recall ... 189 UCS World
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c o v e r t h e w o r l d t m ixForTen 4000 8 ... 190 UCS Place ... 190 UCS Norm al X ... 190 UCS Norm al Y ... 190 11 Scripting ... 190 12 Settings ... 191 Preferences ... 191 Editor Pane ... 193 Patterner Pane ... 194 Metrics Pane ... 196 Miscellaneous Pane ... 197 Auto Save Pane... 198 Precision Pane ... 200 HPGL Pane ... 201 Units Pane ... 202 Form Finder Pane
... 203 Report Setup Pane
... 205 Dark UI ... 205 Plastique UI ... 205 Window s UI ... 206 13 Toolbar ... 206 14 Help ... 207 License Key ... 207 Contents ... 207 ForTen On the Web
... 207 About ... 207 15 Views ... 208 Set Layout ... 208 Render Shaded/Wirefram e ... 208 Zoom Lim its
... 208 Zoom current ... 209 Zoom selected ... 209 Zoom w indow ... 209 Hide/Unhide Grid ... 209 Align to UCS ... 209 Maxim ize/Minim ize
... 209 Change background
... 210 16 Plot options
... 211 Plot Options Pane
... 214 Plot Options Labels
... 216 Response Plot
Part VII Properties Tab
227
Part VIII Browser Menu Commands
234
... 234 1 Root Commands ... 234 Create Group ... 236 Form Finding ... 236 Analysis ... 237 Reports ... 238 FEM Export ... 238 2 Common Local Commands
... 240 3 Boundary Local Commands
... 240 Boundary
... 242 Find Parts
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c o v e r t h e w o r l d t m 9 Contents ... 242 Tenso Groups ... 243 Modify ... 244 Extended Export ... 244 Win Rete ... 244 DXF Polylines ... 244 WaveFront Obj ... 244 Im port ... 245 4 Tenso Local Commands... 245 Set Fram e as Current UCS
... 246 Make boundary Edge
... 246 Make Mesh
... 248 Make Iso Curves
... 248 Make Section at UCS
... 249 Mesh Param eters
... 250 Grid Mesh Parameters
... 251 Conical Mesh Parameters
... 254 Make Grid/Polar Mesh
... 254 Extended Export
... 254 Winrete Format
... 254 Wave Front obj
... 254 5 Patterns Local Commands
... 254 Export HPGL
Part IX Step by Step Tutorials
256
... 256 1 Simple Saddle Shape
... 256 Setup
... 257 Step 3: Zoom ing & Panning
... 258 Step 4 :Node properties
... 259 Step 5 : Checking Dim ensions.
... 260 Step 6 : Find Parts
... 260 Step 7 : Meshing
... 262 Step 8 : Elem ent properties
... 264 Step 9 : Form Finding
... 265 Step 10 : Query Results
... 272 Step 11 : Printed Reports
... 273 Step 11 : Scale Factors
Part X Video Tutorials
277
... 277 1 N°1 : Simple Saddle shape
... 277 2 N°2 : Making a pagoda
... 277 3 N°3 : Model a cone in 3 steps
... 277 4 N°4 : Model mangement
... 277 5 N°5 : Making a vault model
... 277 6 N°6 : Making a double cone
... 277 7 N°7 : Adding steel support
... 277 8 N°8 : Using Gaps
... 278 9 N°9 :Example of nonlinear analysis
... 278 Wind Load Analysis
... 280 Cp factors
... 281 Loading the m odel
... 281 Analysis & Results
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c o v e r t h e w o r l d t m ixForTen 4000 10 ... 282 11 N°11:Patterning A cone ... 282 12 No12: Making a hexagonal headringPart XI Bibliography
284
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Introduction 13
1
Introduction
ixForTen 4000 is the new system developed over For
Ten 3000 and 2000 for the design, structural analysis and
pattern making of tensiles, membranes and cable nets.
After 20 years of work thousands of tensile structures
have been built world wide using our system and we are
proud to say that the community is growing day by day.
Our slogan is quite simple "Your success is our
success" since we believe in teamwork and strong
relationship between users of a software and who
develops it - Welcome - to our world of endless
possibilities.
Thanks to All
Gerry D'Anza Architect
For Ten developer manager
1.1
Where do I begin ?
If this is the first time you are looking at the software
don't get scared by all the options and parameters seen
in the interface. Most of them are for advanced, special
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features, and if you don't understand them all at the
beginning, do not worry, go ahead to make your first
Membrane, keep it simple, do not add the steel work or
supporting structure in the
beginning to
avoid
complicating your work.
Look at the video tutorials to have a clear idea on how
the system works.
ForTen is a software developed in 15 years and cannot
get learned in 10 minutes but surely you will be able to
make your first model in a short time after looking at the
videos.
If you are already a ForTen owner - then you can be
re-assured that this is the fastest, most-stable,accurate and
yet the easiest to learn ForTen ever! Dispensing off with
the earlier user interface of 2000 and 3000 - ixForten
comes with a completely fresh and versatile platform
that makes re-sizing, ordering and flexibility a new way
of working. This interface still maintains some of the
earlier features - so previous users are comfortable - but
this comes with a brand-new shell. Also - the solvers, the
mesh creation and the interface is more powerful than
ever before. Check out the new features.
1.1.1
Project Setup
If you are using this system for the first time follow these
simple steps and avoid loosing a huge amount of time in
attempt to find a way to make a model. Tensile Structure
design is a complex task by itself so a initial planning will
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Introduction 15
help to avoid mistakes.
Some basic suggestions for making quick
models:
a) Make a sketch on your idea on paper
b) Keep the system simple - and then build it up. Don't put
all links, tensile elements, fabrics and steel at one go.
c) Draw in a Cad system the boundary of your model,
made of simple 3d lines. Avoid using splines and
curves as much as possible, as they eventually get
converted to lines when they are imported into For
Ten. The end points of these lines will be the fixed
nodes for the final model. In the cad file draw any help
graphic entity and any structure element taking care to
organize them in layers.
d) Have a rough idea of materials and their cross
sections you are going to use, type of membrane,
steel,wood or concrete parts, and import the same
before you start your model.
e) If you are creating new materials - check if the units
are in co-relation with your model - or you may result
with a very stiff / very flexible member.
f) Always draw near the global origin (0,0,0). These
prevents problems in round off errors when running
non-linear analysis. Drawing near the origin also
keeps the co-ordinates small, the equations smaller
and eventually reduces the overall calculation time.
This also helps the graphics to run faster - and
speeding up the overall response time of the software.
g) Since For Ten DEFAULT parameters are based on
meters and Kg, we suggest to keep these units in the
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has to set all variables accordingly, so to avoid
mistakes while in a learning phase keep it simple. Use
meters and daN , when you feel comfortable with the
software you can set up your desired units.
Next step
1.1.2
Model Setup
Now that you have a clear idea of the model
a) Import the Cad file from File -> Import Cad option the
button looks like this
b) choose appropriate groups for each layer in the CAD
File. If not sure what to use, import minimal data, you
can always add later new groups to your work adding
complexity to the model in sequential way. At least one
boundary poly-line has to be imported.
c) Call the data base explorer
and import in the
model the seeds (material+section ) you want to use
(membrane,cables steel tubes)
d) From the local popup menu of the boundary call the
find parts command.
e) A first check of errors is simple, the number of
Tenso-Groups under the boundary has to be equal to the
number of closed regions. If this is not true there is a
problem in the boundary with pending elements or
not-closed poly lines. The weld tool
can help to fix
these.
f) Call the command Boundary: Tenso Groups : Set
Params & Build
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Introduction 17
g) Select Boundary entities and assign property data (
cable,seed etc)
h) Select mesh elements and assign properties
i) Check boundary restraints
Go to Next Step
1.1.3
Form Finding
A. Call the Form Finder.
If this works you will have
your first model on video.
B. Check pre stresses and shape.
C. if you are not satisfied change C Values (pre-stress)
and/or geometric positions of fixed nodes and recall
the form finder
D. You can also do this by selecting edit -> change
C-value from the pull down menu.
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Analysis
The Analysis is done after following steps:
Create as many Load Conditions as required
Select surface elements,nodes or any element to load
and Add Loads
Call the Non Linear dialog box and create load
combinations
Run the Analysis and see results or plot deformed
shape,stresses etc
Look at the Video Tutorial to have a idea
1.2
FAQ
A few common questions and problems :
1.2.1
What are tensile structures?
Structures to sustain loads by tensions of soft materials,
such as wire and membrane, is called tensile structures.
Familiar examples are the tent, the suspension bridge,
the spider net, the heat balloon, etc. There are two types
of tensile structures, one to sustain themselves under
the gravity and the other without gravity.
M echanism of tensile structures to sustain
loads under Gravity:
Let us observe a suspension bridge, as an example. The
gravity W of a part of girder (in the figure below) is
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suspended by the vertical wire, which is connected to
the thick curved wire. The gravity W balances with two
tension forces F1 and F2 on both sides. It can be proved
that the rough shape of the thick wire is a parabolla, (i.e.
a quadratic curve), if the girder has a uniform weight
distribution.
Note that, if we have no girder and the wire sustains its
own load, the wire curve is not a parabolla but the
catenary. This is the basic principle of a doubly curved
(anti-clastic) surface that describes a single layer tensile
structure.
Another example is the membrane reservoir of water,
where the tension of membrane should sustain the
hydrostatic pressure of the water.
If the water has a density and the gravitational
acceleration is g ( = 9.8 m/s2), the hydrostatic pressure p
of the water at a depth z is expressed as
p = g z,
i.e. the pressure increases in proportion to the depth.
Then, in order to balance with this pressure, the shape
of the membrane at the deeper point must have the
larger curvature. It comes from the fact that the pressure
difference produced on both sides of a membrane is the
product of the tension strength and the mean curvature
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of the membrane. Hence, the curvature is inversely
proportional to the depth.
Of course, you can choose a spherical shape (uniform
curvature). But, the tension at the bottom becomes large
to balance with the hydrostatic pressure and the
membrane has a danger to break.
This ideal shape of reservoir is similar to a liquid drop on
a non-wetting floor.
This is the basic principle of double layer membrane
surfaces like cushions / foils.
1.2.2
After Form Find I dont see anything. What Happens ?
1. A common problem is node RESTRAINTS : If there are
no fixed nodes these will collapse to a single node and
your model simply disappears.
2. Did you do "find parts" to find the boundary first? You
need to first "find parts" make mesh and then run a
form-find.
1.2.3
I am not able to find desired shape. What can I do ?
Tensile structures work on force intensive forms and as
a result, not all shapes are possible with tensile
structures. The rules of equilibrium under tension are the
basis and more generally speaking, one needs to have
sufficient double curvature (anticlastic shape) for any
good tensile structure.
Start thinking in terms of these rules and then try to
make your model. Synclastic (doubly curved, but bubble
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Introduction 21
shaped) shapes are possible only if the surfaces are
internally pressurized.
After that generally we have 2 main ways to control
shape :
A) Fixed points - fix some points, edges, beam elements
so that the form can be dictated by them.
B) Assigning different Pretension ( C value or Force
Density Value ) to different parts of the structure.
1.2.4
What loads are acting on the structure?
A tensile structure's shape is maintained by the balance
of two axes stressing along the principle directions of
curvature. Which means that they need to be stressed to
attain a particular form. This impregnated stress is
known a pre-stress which has to exist in all types of
tension - active structures and they are impregnated
using various techniques like compensation, stressing
the supporting structure etc. These forces have to be
included in all other load cases.
As the surface of the tensile structure gets loaded, due to
a variety of factors like wind suction, snow load, rain
loads, loads of people working on the surface etc, they
cause one of the stresses to increase, and the other to
decrease. This change in stress is capable of changing
the overall shape of the structure and completely change
the behaviour of the structure.
At some load value the structure will loose all stress in
one direction, and will then behave locally as an
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Synclastic structure. This can be the point where large
deflections start to be observed in the structure - which
then leads to the eventual collapse of the structure.
So the pre-stress value is the main value that determines
how the structure reacts to loads. A high pre stress gives
a stiff structure that will resist applied loads with low
defections, but will need a material resistant to high
tensile stresses, without any great elongation and strong
supporting structure. A low pre stress gives a structure
that will deflect more with loads, and one that can be built
with a lesser resilient material and weaker supporting
structure.
Hence a tensile structure has to be designed for
Pre-stress - which will then determine the behaviour of the
structure under other loads. Like any building a tensile
structure is susceptible to exactly the same type of loads
as mass structures, but due to their unique geometry and
their force-active behaviour, their response to loads is
completely different.
Due to this behaviour, one has to be very careful in
designing such structures, and sufficient amount of
investigations have to be made to be able to safely
construct and design such structures.
1.2.5
How to calculate wind loads from Wind speed ?
Due to the minimal mass of tensile structures and the
fact that the ratio of applied load to self weight is usually
many times larger than the ratios for conventional
buildings, they tend to be easily agitated by wind forces,
and extremely susceptible to small changes in the snow
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Introduction 23
and water loading, which can have a large impact on the
service life of the structure, if they are not carefully taken
to account.
Unfortunately the amount of research done in this area
of building is scarce, and as a result most codes written
around the world are for standardized building shapes,
materials and behaviour, often neglecting these type of
lightweight structures. As a result, more time and effort
needs to be spent by designers in studying the
behaviour of such structures to simulate the precise
behaviour and specifying load cases.
Consequently large
scale
tensile
structures
like
stadiums, arenas and building roofs, require accurate
information on static and dynamic effects of wind loads
so as to reduce over-design and improve the overall
safety of the structure. In this respect the closest answer
to finding the effects of wind speeds would be to put the
structure through a wind-tunnel test, which, for larger
scales can invariably justified as the effect of
conservatively derived wind loads on the structure would
cost the client considerably more than the test itself.
However, with the ever decreasing time-scales and ever
increasing competitive bidding's, there is seldom scope
or sufficient budget outlay for a wind tunnel test. Also
-the dominance of -the standard design procedures over
tensile structures, causes most design engineers to
design the tensile structure in accordance with the codes
of their country.
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Here you can find a few formulas from Italian codes.
1.2.6
How can I add a mast with stay cables ?
Contrary to their earlier version, ixForTen has one
straightforward way to process sub-structure ( or
supporting structures ) by directly importing elements
and inserting them into a tenso-structure.
So to add a mast with stay cables with Non-Linear
behavior:
a) Import your mast and cable geometry in a
Tenso-Group
b) Set mast with truss or beam type and a valid cross
section
c) Set cables as cable type and assign a initial pretension
(not C Value) enabling the KEEP PRETENSION
WHILE FORM FINDING option to TRUE
d) set the deformability to NL-Deformable
Look at this tutorial to have an example.
1.2.7
Analysis stops with a message MATRIX Error . What to do ?
Non-Linear analysis is a complex topic. The failure can
be connected to many reasons but we suggest to look
first here :
1) Your model is placed near the global origin ?
If not move it near the global 0,0,0 origin.
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Introduction 25
5685412788.0E+2,8560001E02
are of course legal but when it comes to make a
stiffness matrix hundreds of times the accuracy of
numbers is lost and can cause instability problems.
2) Your model has enough curvature and pretension ?
Generally the process stops because of instability
under load. Around a certain point all element
become compressed and so the stiffness matrix row
connected to that node becomes zero.
This problem can be fixed only with a revision of the
model. In other words the system is advising that the
model has potentially some problems.
3) Do the supporting elements in your model have
enough stiffness?
If you assign a very small cross-section to an
element susceptible to a lot of stress, then, ForTen
instead of crashing through, gives an error message
"FEM error".
Try increasing the stiffness of your elements. Change
the cross sections, look into the material property
-verify if they are rightly defined. Take care when you
are opening an old (For Ten 3000 / For Ten 2000 )
model - as they have different property assignments
than ix For Ten 4000.
4) Have you assigned rightly the node constraints?
Node constraints are important to the stability of the
structure. If they are wrongly assigned - the model can
either collapse or give a wrong result. Check node
assignments / constraints again.
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1.2.8
How do I add fixed length links ?
To create a fixed length link :
1) Add a link with DESIRED length into a tenso-group
2) set the property as cable or truss and assign a pretension ( if required by design )
3) Set the deformability to NL-Deformable
4) Fix one end of the link if not attached to other structural parts
5) Run the formfinder
To avoid high non-linear instability set the link near to the final position
this will let the converge in a fast and accurate way
1.2.9
How can I check if pre-stresses are Ok ?
The level of prestress in a membrane structure affects all
the elements within the supporting structure. Prestress
is an inherent part of the form of the structure and hence
a part of it's behaviour. The prestress levels are chosen
as a result of the form-finding process - and they have to
be sustained through the installation and the service life
of the structure. Long term effects such as creep of the
membrane material - deflection in the supporting
structure - even settlement of foundations (although
rarely) may alter the the prestress levels.
However,
for
PVC
coated
polyester
membrane
structures, a 'rule of the thumb' is that the prestress
should not be less than 1.3% of the average tensile strip
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Introduction 27
capacity of the material in both warp and weft directions*.
The prestress values for PTFE coated glass fibre
membrane structures tend to be higher as the material is
stiffer. For PTFE fabrics, a 'rule of the thumb' is that the
prestress should not be less than 2.5% of the average
tensile strip capacity of the material in both warp and weft
directions. Although there is no rule to this due to the
wide range of shapes and size of tensile structures and
the expanse of methods in their design, installation and
maintenance.
Prestress can be chosen with higher values too to
minimize the deflections of inefficiently curved membrane
forms with the increased prestress marginally reducing
the allowable working stress range. Temporary or special
case membrane structures can even be designed with
lower or considerably higher prestress values.
Different prestress values while finding the form of the
structure alters the form and shape of the structure and
this may change to a certain extent the structural
behaviour of the structure. This strategy is in fact a
fine-tuning for structures that have different loading
behaviours in different directions - however choosing
more suitable geometric boundary conditions and more
curvature will always be a more successful way of
improving structural behaviour. Generally prestress
ratios for perpendicular directions should not vary more
than 4:1 or 1:4.
Stresses under load have to be checked against
admissible working stresses, so any pretension where
stresses under load that do not go higher than
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*This is from the European Tensile Guide pg. 192
-Marcus Balz, Mike Dencher
1.2.10 I am not able to cut the surface. What to do ?
The patterner will work only if :
The current Group is a Tenso-Group with a valid Mesh
From inside the patterner click on the tenso-group to
pattern and then in the graphic window to
update, you will see a light rectangle over the current
tenso-group and the boundary edges .
If no edges are visible it is likely true that the tenso-group
has a bad mesh and needs to be fixed from within the
editor.
From the editor check if a valid Triangle mesh is available
for that group ( a shaded visualization that shows a nice
surface with no holes or black areas is necessary )
1.2.11 How to I calculate compensation ?
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Introduction 29
the fabric will achieve the predefined prestress levels at
the correct geometry once the creep of the membrane
has occured. The compensation process accounts for
the the elastic stretch and creep of the membrane and
cables. compensation is computed in the following way :
For a number of points over the surface Warp & Weft
pretension are assigned.
For these stress states biaxial tests are made on the
material to be used in the final structure, so that we know
the exact expansion / contraction of the fabric in their
primary and secondary direction.
The biaxial tests will report elongation values for the
desired stress state. These values can then be directly
used in compensating our patterns. Although this is a
simplistic outlook many times the compensation values
may vary even along the length of a pattern - Although
ixForten offers the possibility of de-compensating edges
and overall compensation of patterns, differential
compensation along the length of the panel needs to be
done manually.
1.2.12 How can I check if patterns are correct ?
In ix For Ten 4000 we always suggest to :
Make all patterns for each Tenso-Group of the model
Go in the production pane
Activate in the options pane "Display Pattern Edge
length"
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With this option active look at the pattern assembly
edges and check that all edges welded have the same
length for the 2 sides. This will give a quite absolute
check that patterning is well done.
ixForten also gives you the 2d-lengths and the 3d lengths
(in parenthesis) in each patterns this can be matched
against the 3D actual lengths to get a perfect check.
1.3
Modelling Fabric Structures
A fabric structure is modeled using a mesh of linear
elements connected to nodes.There is no limit to the size
or connectivity of the mesh or the number of nodes.
Each node has X,Y,Z coordinates and six degrees of
freedom tx,ty,tz,rx,ry,rz .Each linear element has a
number of properties (material, C Coefficient, cross
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section etc.) which will be discussed later.
1.3.1
Form Finding
Be sure to check the list of actions for a
correct Form finding:
1. Make the mesh model
2. Restrain nodes with a fixed position in space
3. Define cable properties and C Coefficients
4. Do the form find process
5. Go back to step 3 if the model does not satisfy your
needs
6. Check the final model against boundary models
imported via DXF or 3DS
1.3.2
Static Non linear Analysis
For Non Linear static analysis follow these
steps :
Make a Mesh Model (if not already done)
Create one or more load conditions
Activate a condition and apply nodal or wind loads
Define a load combination
Define analysis parameters
Do the analysis
Look at the analysis results
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Patterning
Check these steps for a correct patterning
process:
Make a Mesh Model (if not already done)
Enter the patterner module
Make all necessary seams with geodesic cuts or plane
cuts
Enter process 2 of the patterning module with the
option automatic patches
For each patch define seam cut 1 and seam cut 2 and
make the pattern
Optimize, stretch, rotate or offset sides of the pattern as
required.
When all patterns are done go to the production module
to see final work.
Export to CAD for plotting and producing
Look at the video for a simple patterning example:
The steps shown will be discussed in more detail later in
the documentation and in the tutorials
1.4
Application Interface
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2
Fabric Structure FormFinding
Modeling complex fabric structures, is one of the main
goals of ixFor Ten 4000, to understand all features and
links between elements, experience and trial and error is
needed.
ix For Ten relies on groups to store information on fabric
parts, cable parts and steel or concrete parts.
These groups then, can have several sub-groups where
other kinds of mathematical models are kept like
Patterns, mesh , quad mesh, 3D patches etc.
ix For Ten 4000 behaves in a quite different to ForTen 2xxx so users of previous versions should fully understand how these improvements can change their work.
It is not an easy task to explain how to use all features correctly so we will use a different approach in documenting the software.
Explaining basic features
Examples and tutorials to understand the basics.
2.1
Groups
ixForTen works with a hierarchical group system - where
elements are grouped together for matter of organization
and convenience.
A
description
of
the
groups and how they
connect is necessary and
will be discussed here.
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ixForTen relies on master groups for the mathematical
model that will be processed by the form finder, static
nonlinear analysis and linear analysis.
The Master groups are :
Boundary Groups They store information on
boundaries which are generally one or more closed
lines in 3D space. For ForTen to identify a boundary,
it needs to be closed. This is a master group under
which tenso groups, mesh groups and other groups
can be created.
Tenso Groups They store information on fabric,
steel elements, cable nets or any structural
components. This is a special group as it can come
up as a child group under Boundary Groups or alone
as a Master group. Unlike earlier versions of ForTen,
where these groups were usually used to store
information for cables and tension-active structures
like mast-tie down systems, now these groups are
used to store all structural types including truss and
beam elements.
Cushion Groups these store information on
boundaries of cushions and are generally processed
like boundary groups. Cushion Groups can have up
to 3 layers (Tenso Sub Groups) respectively Top,
Bottom and Mid layer
Graphic Groups these store useful geometric
information for snapping and design control,
elements in a graphic group will never interfere with
the analysis, design or form-finding processes.
Mesh Groups these store information on surfaces
that are to be used as shells / FEM meshes for
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Fabric Structure FormFinding 37
design and analysis.
Each master group always has two database’s :
Nodes
Linear Elements
Of course there are many other databases but here we
will focus our attention on the essentials.
2.2
Nodes
Nodes are the interface between elements, a free node
without any element attached is a problem during form
find and analysis so pay attention to nodes created,
when deleting elements it is better to delete nodes, as
elements connected
to
nodes
will be
deleted
automatically .
Node Restraints
ixForTen 4000 nodes have a single restraint set unlike
earlier versions of ForTen. A node restraint controls 6
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degrees of freedom and are called Stiffness Restraints
D1 = Deflection in Direction X
D2 = Deflection in Direction Y
D3 = Deflection in Direction Z
R1 = Rotation around X
R2 = Rotation around Y
R3 = Rotation around Z
symbol for a node fixed for 3 rotations
symbol for a node fixed for 3 displacements
Symbol for a free node
Color and size of the node is controlled by the Settings:
Preferences Dialogue box under the Editor panel :
Nodes Color for the color of free nodes Restraints Color for restrained nodes
Node Symbol size in pixel units for the size of the symbol
Understanding of the restraint conditions is essential
for using many features especially when we have rigid
borders or structures.
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Like conventional FEA packages the stiff restraint will fix
nodes
for a particular movement and represent the real
structure restraints
and manages to link this process with a standard non
linear stiffness analysis.
2.3
Linear elements
Linear elements are simple geometric lines between two nodes there is no curved element so curved cables are implemented as piecewise linear elements.
Linear elements have a type property :
CableMembrane Beam Truss Gap
Many linear element properties have a different meaning
that depends on the type.
2.3.1
Linear Element Properties
Linear elements are simple geometric lines between two nodes there is no curved element so curved cables are implemented as piecewise linear elements.
Linear elements have these properties :
Code : a number to identify it in the reports Type : cable,membrane,beam,truss and gap
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c o v e r t h e w o r l d t m ixForTen 4000 40 Deformability : FDM-Deformable,NL-Deformable,L-Deformable,Fixed.C Value : [0 +INF ] the force density value
Seed : a Cross Section geometry and Material definition
Group Code : a user defined number useful for grouping
elements
Rotation Angle : angle in degrees of the first principle axis End restraint A,B : Only for beam elements
Warp-Weft direction : Used only by membrane elements Keep Pretension flag : Keep user defined pretension
while formfinding, valid only for NL-deformable elements
Pretension : User defined pretension ( valid only for
NL-deformable elements )
Constraints : Fixed Length, undeformed length and force
Code: This is a numeric value used to identify them.
Type : The FEA type used by this element.
Cable : only tension element, non-linear
Membrane : only tension element used to model membrane nets
Beam : compression,tension and bending stiffness. Can be linear or non-linear
Truss : compression, tension (called also Strut ). Pinned element with axial stiffness only. Can be linear or
non-linear.
Gap : compression only element, non-linear .
Behaviour : for beams and trusses only. We can specify a linear
or a non-linear FEA element.
Deformability: This is VERY IMPORTANT flag to specify.
Membrane and boundary cable elements are generally flagged as FDM deformable ( they find a geometry from the form-finding
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process) while stay cables, beams,truss and gap elements are generally NL-deformable ( they have a predefined geometry but deform under FEA stiffness analysis ).
C Value is the force density value, the ratio of the Force and the
Length of a element in the final shape position.
Form Finding a shape where no constraints are used will be processed in a single linear system of equations where the equilibrium equations on node j in a net of elements with connections ij are :
With :
Nij = Force in the element connecting nodes i - j Lij = Length of element i-j
The value Nij/Lij non linear in the above equation is replaced by Cij and solved in a single step.
Good starting values for Cij are not difficult to find out when the initial prestress in the membrane is known.
Generally, we do form finding many times to agree not only membrane initial prestress values but even other aspects like geometry , surface curvature etc.
It is easy to understand that higher values of Cij will shorten the element and increase its internal force while lower values will elongate it and result in lower forces.
C values depends on the units we are using as it is a Force on a Length ratio, so changing system units generally needs an update of C values too.
In the tutorials C values will be widely used to model fabric structures and to establish the range of prestress. We must always keep in mind that overall structure stiffness depends on prestress and geometry , so if under loads our structure undergoes large displacements even with high internal prestress, geometry should be checked and maybe changed due to bad initial design.
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Group Code : this is a user specified code. Elements with the
same group code get grouped and in any reports we will find them in a unified row.
Rotation Angle : This is the angle of rotation of the local axis 2
around its default position. Rotation angles have a meaning only for beam elements.
End restraint A,B : Beam elements are by default fully fixed to
their end nodes. We can release any degree of freedom to simulate pinned or any special behaviour.
Warp-Weft direction : this flag is meaningful only for membrane
elements in a net grid. The warp/weft properties specified in the material will be applied accordingly to elements that have the warp/weft flag assigned. Even selection of warp-weft curves uses this flag to identify elements.
Keep Pretension flag : This flag enables user defined pretension
Pretension : user defined pretension. Valid only for cable,truss
elements flagged as NL-Deformable and keep pretension flag set to true. When running the Form-Find process these elements will start with the value assigned in this field. The real pretension will then get computed after the form-finding step. This is a useful starting value to give stiffness and avoid instability while form-finding the structure.
Constraints : Various constraints applied on cables. These
constraints will enable the so called non-linear FDM solver.
Do not use if not sure on how they behave. Generally these are used to form-find complex cable systems. they are NOT intended for fixed links between a point and a membrane, for these links we use a normal cable set as NL-Deformable drawn at the
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Fabric Structure FormFinding 43
desired length.
2.3.1.1 Seeds or Named properties
The seed ( or named property ) is a named data base
object created from Tables : Data Base explorer and is
made of two entities :
A cross section A material
2.3.1.1.1 Cross Section
The cross section is used by all types of elements but not by membrane elements. Geometry of membrane elements is calculated automatically after a first form find step and always updated after each form find, this will be explained in detail later .
Other types of entities will rely on the cross section for :
Area A : Cross section area of the element Inertia J1 : Moment of Inertia first axis
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c o v e r t h e w o r l d t m ixForTen 4000 44 Torsional Inertia T Shear Area S1 Shear Area S2Note: Truss,Cables and Gap elements will only use the
Area value while beams will use all of them. This is
because truss and cable elements are supposed to be
subjected to only axial loads ( i.e. pure compression/tension) ,
while for beam elements ixForten uses the other values to
calculate bending, shear, torsion etc. for that particular section.
2.3.1.1.2 Material
A Material definition (see Tables Data Base explorer) is
made of two sets of properties
Homogeneous material Membrane material
Homogeneous material has :
E modulus : Young modulus (F/L^2) Poisson ratio
Density : weight per unit volume (F/L^3) Thermal expansion coefficient
Membrane Material
Membrane material is defined by :
E warp : Membrane young modulus in warp direction F/L E Weft : Membrane young modulus in weft direction F/L Weight x unit area : F/L^2
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is just part of the element definition so we should be careful when using it to simulate a real membrane.
This is because after patterning warp and weft direction of the patches can be quite different from warp and weft of the mathematical grid used for form finding and non linear analysis, this is under the control of the user and not the software.
A element typed as a warp element will get its E value from the warp table entry while a weft typed element will get it from the weft entry.
It is clear from the definition that thickness of the membrane is not taken into account.
Generally manufacturers of fabrics do not specify E modulus in warp and weft direction but elongation ratio under a specified stress for a strip of fixed width.
To use these values we must convert them to E modulus and pay attention to units used by fabric manufacturer and those used in the software.
Membrane Conversion example
Say we are using Kn and meters and have a PVC specification :
Tensile strength = 660 N/cm (warp and weft) Elongation ratio = 20 %
The tensile strength in Kn and meters Tr = 66 Kn / m
Te = 66/S
Tp= about 10% Te
where :
Tr = ultimate stress
Te = maximum permitted stress during the life of the structure Tp = membrane pre stress
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using a safety factor of 5 we get
Te = 13 Kn/ m Tp= 1.3 Kn / m
The E modulus for the membrane should be
E = 66/20 * 100 = 330 Kn/m
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3
Structural Analysis with ixForTen 4000
ForTen 4000 modeling techniques are quite different from previous version so we need to focus here on how to model tensile's with supporting structures.
When we click on the Form-Find icon ForTen makes several tasks to relax the shape.
This process ensembles what happens in a real structure where a membrane is pre-stressed over a stiff supporting structure.
We can simplify these several steps and say that there are 2 main processes:
· FDM process
· Non-Linear Stiffness analysis
FDM process or Force Density method relaxes the membrane mesh using the well known Force density method. In this process the C values ( or force densities ) will control shape and level of prestress in the membrane and boundary edges. This is not unlike the earlier versions of ForTen.
At the end of the FDM process if the membrane is connected to a supporting structure we have to transfer the prestress forces to the structure. This step is solved by a conventional non-linear finite element analysis kernel. It is clear that we have to communicate to the software which part has to relax following the rules of the FDM method and which part is the supporting structure. All this is done with 2 main properties:
· Node restraints
· Element Deformability
