1. GSA basic concepts
GSA Essentials

2. GSA – Using Analytical Software
Introduction
Getting Started
Structural Model Types
Information Required
Input Data
Realistic Input
Tutorial example

3. Definition of Structural Engineering
Structural Engineering is the art of designing structures
to withstand loads that we cannot predict
using materials whose properties we cannot measure
by methods of analysis that we cannot prove
and to do so in a manner that ensures that the public and client are ignorant of our shortcomings

4. GSA - Introduction
In the "good old days", structures could be "designed" by feel and experience. The medieval cathedrals - this is Exeter, compared with Ove Arup’s Coventry…, and masonry structures generally, were designed and built by masons. Some times unforeseen full scale testing (or what we now call collapse) was used…!
More recently, Gaudi used inverted models to study the flow of forces when designing the Sagrada Familia in Barcelona, an amazing structure, but still a long way from completion.
More recently still, structural engineers used hand calculation methods that were simple, safe approximations of behaviour for routine structures. They used moment distribution, slide rules etc. - who has heard of these? [Ask for show of hands.] Who has used them? [Another show of hands - fewer this time!]
Times are changing. Computers are quicker and cheaper than hand calculations, so why shouldn't we use them? There is less time for checking now. Are we thinking enough? As a firm we seem to be making more mistakes than we used to when Arup started around 50 years ago.
This course is one of a number of measures that are being taken to tackle this problem. Looking specifically at computers, engineers often set up quite complex and detailed computer models early on in the design process. They then get bogged down with large quantities of output, and lose the clarity of understanding and the ability to carry out "what if" parametric studies on the design because of this. We can't see the design wood for the analysis trees!

5. GSA - Introduction
What is this?
Here is Gaudi’s hanging chain model

6. GSA - Introduction
Here is Gaudi’s hanging chain model

7. GSA - Introduction
What is GSA? “GSA has developed from a program for the static analysis of three-dimensional structures composed of skeletal elements, to become a complete analysis package with connection to spreadsheet, CAD and design programs.”

8. GSA - Introduction What is GSA?
Written by Oasys for Arup (but also commercially available).
Based on Stiffness Matrix and Dynamic Relaxation Philosophies
3 dimensional - 6 degrees of freedom
Not material specific

9. GSA – Getting Started
Graphics Window
Gateway
Object Viewer

10. GSA – Getting Started
Notes are very useful for version numbers and memory prompts

11. GSA – Getting Started Structural Type: Space Grid Plane Plane Stress
Plane Strain
Axisymmetric
The degrees of freedom available varies for each structural type of model.

12. GSA - Structural Model Types
Space
6 degrees of freedom:
x,y,z translation
xx,yy,zz rotation
Example: Any structural with 3D load paths and loading (Stadia!)
Note: Most common
Grid
3 degrees of freedom:
z translation
xx,yy rotation
Example: Floor plate or Bridge Deck where we are interested in out of plane forces and displacements

13. GSA - Structural Model Types
Plane
3 degrees of freedom:
x,z translation
yy rotation
Example: This is used for modelling for example a 2D frame.
Plane Stress
3 degrees of freedom:
x,y translation
zz rotation
Example: Any 2D analysis such as a frame or support system where we are interested in in plane forces

14. GSA - Structural Model Types
Plane Strain
2 degrees of freedom:
x,y translation
no rotation
Example: This is a 2D representation (slice) of a long problem such as a tunnel. The slice is constrained in its section unlike plane stress
Axisymmetric
2 degrees of freedom:
x,y translation
no rotation
Example: A continuous problem with an axis of rotation such as a cylindrical tank or vessel

15. GSA – Information Required
Model Structure
GSA MODEL
INPUT
OUTPUT

16. GSA – Information Required
Model Structure
OUTPUT
Forces
Deflections
Stresses
Graphics
INPUT
Geometry
Nodes
Elements
Properties
Restraints
Materials
Loads
Permanent
Variable
GSA MODEL

17. GSA – Input Data Sequence
You start to create a model in the following sequence:
Define the nodes – Nodes are points in space represented by co-ordinates
Define the elements - Elements are the items that are analysed. Their position in space is determined by the nodes which they are connected to.
Assign properties – Both nodes and elements have properties.
Define the loads – Loads can by applied to both nodes and elements

18. GSA – Input Data All menus are duplicated in the Gateway
For simple models follow the order on the bottom of the main screen

19. GSA – Input Data
Method
There are three ways of inputting data into GSA to define the nodes and elements:
Manually – inserting data in tabulated format

20. GSA – Input Data

21. GSA – Input Data
Method
There are three ways of inputting data into GSA to define the nodes and elements:
Manually – inserting data in tabulated format
Sculpting – using the sculpt toolbar you can define the geometry in the graphics window

22. GSA – Input Data
SCULPT TOOLBAR

23. GSA – Input Data
Method
There are three ways of inputting data into GSA to define the nodes and elements:
Manually – inserting data in tabulated format
Sculpting – using the sculpt toolbar you can define the geometry in the graphics window
Copying – alternatively duplicate existing geometry to extend the model

24. GSA – Input Data

25. GSA – Input Data Properties Node Properties Restraints
Free Pin Encastre
Six degrees of freedom:
Translational: X Y Z
Rotational: XX YY ZZ

26. GSA – Input Data Properties Node Properties
Restraints
Constraint axis
The default constraint axis for a node is the GLOBAL axis.
An user defined axis can be created and used to constrain a node so it can have support conditions related to the new axis.
GSA uses the right-hand rule to define axes.

27. GSA – Input Data User Axis
- created to constrain nodes for section of building that is at an angle on plan
Global Axes

28. GSA – Input Data Properties Element Properties Element Type
Main Types:
Beam – bending, torsion and axial
Bar – axial only
Tie – tension only (non-linear)
Strut – compression only (non-linear)
Other types:
Links
Mass
2D (Quad 4, Quad 8, Tri 3 and Tri 6)
Cable
Spacer
Struts
Beam
Bar

29. GSA – Input Data Properties Element Properties Element Type
Section property
The section property defines the element’s material, cross-section, etc

30. GSA – Input Data Element Properties Properties
Element Type
Section property
End releases
End releases define the restraints at the end of the element in term of its local axis
As with nodes there are six degrees of freedom.
Simply supported connection
Rotation released about the local yy and zz axis of the element (indicated by cross-hairs)

31. GSA – Input Data - Properties
Element Properties
Element Type
Section property
End releases
Orientation
Orientation is used to alter the local axis of an element.
The element y and z axes are rotated from their default positions about the element x axis by the orientation angle (Beta).
Revised Local Axis
Global axis
Default Local Axis

32. GSA – Input Data Element Properties Properties Element Type
Section property
End releases
Orientation
Alternative to Beta angle is to orientate elements to a node ŷ x z y

33. GSA – Input Data Loads Beam Loading Beam Load Prestress Distortion
Thermal Load

34. GSA – Input Data
Loads
Beam loading
Gravity ?

35. GSA – Input Data Loads Beam loading Gravity Node Loading
Node Loads – Fx, Fy, Fz, Mxx, Myy, Mzz applied at node in node axis directions
Settlement which is a displacement or rotation applied at node.
Note that a node should be restrained in a direction before a settlement is applied

36. GSA – Appropriate Modelling
Model Structure
RUBBISH IN
GSA MODEL
RUBBISH
OUT

37. GSA – Appropriate Modelling
Why did the
columns crack
on site?
Here is a little example of a problem on site which could occur not far from here…
The engineer set up a model as at the top sketch, got some answers and sized the members.
A panic ‘phone call from site during a water leakage test of the roof described the cracks.
What do you think went wrong?

38. GSA – Appropriate Modelling
If the analysis model doesn’t reflect what’s actually built in terms of DETAILING then it may be meaningless. Analysis should never be carried out in a vacuum, oblivious to the practicalities of construction, which in themselves may vary from region to region.

39. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Model based on a real foundation

40. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Design was on the bending moments
Can you see what might have been forgotten?

41. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Moments are vanishing from the Myy chart, but they must be going somewhere

42. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
The moments went into under-designed for torsion, causing failure of the foundations during construction

43. GSA – Appropriate Modelling
Make sure that you design for all the induced forces

44. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Sleipner A Offshore Platform
Gravity base North Sea platform designed for depth of 82 m
24 cells forming a total base are of 16,000 m2

45. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Sleipner A Offshore Platform
The structure failed at a depth of 65 m
The impact on the sea bed measured 3 on the Rictor scale
Cost $700 million
Failure due to inappropriate FEA meshing plus poor concrete detailing
Shear stresses underestimated by 47%

46. GSA – Appropriate Modelling
GSA basic concepts
GSA – Appropriate Modelling
Sleipner A Offshore Platform
Error also caused by bad detailing

47. GSA – Appropriate Modelling
Ensure that you model is sufficiently detailed to get accuracy Make sure that you model is not over-complicated so that mistakes are not missed i.e. Design your model to give you the required results

48. K.I.S.S. It is easy to make something difficult
It is difficult to make something easy
“Things should be made as simple as possible, but no simpler”
Albert Einstein