GSA basic concepts GSA Essentials

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

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

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!

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

GSA - Introduction Here is Gaudi’s hanging chain model

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.”

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

GSA – Getting Started Graphics Window Gateway Object Viewer

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

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.

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

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

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

GSA – Information Required Model Structure GSA MODEL INPUT OUTPUT

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

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

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

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

GSA – Input Data

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

GSA – Input Data SCULPT TOOLBAR

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

GSA – Input Data

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

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.

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

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

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

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)

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

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

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

GSA – Input Data Loads Beam loading Gravity ?

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

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

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?

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.

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

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

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

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

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

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

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%

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

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

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