1. Raft and Soil-Structure Interaction Pdisp and GSA Raft

2. Today’s Team
Peter Debney
Senior Consultant

3. GoToWebinar Attendee Interface
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Raft and Soil-Structure Interaction
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4. Agenda
Product demonstration
Pdisp
GSA Raft
Questions and Answers
NB: this webinar is designed as an introduction to Pdisp and GSA Raft and to provide an overview of functionality. If more is required then it can be done at a later date on an individual basis.

5. Who is Oasys? Wholly owned by Arup
Formed in 1976 to develop software for in-house and external use
Most developers are engineers who have moved to programming
In recent years have added marketing and sales staff
Since 2003 have expanded the development team worldwide

6. Oasys Customers

7. Structural Geotechnical Document Management Crowd simulation CAD
GSA basic concepts
Structural
Geotechnical
Document Management
Crowd simulation
5 structural
10 geotechnical
2 document management
2 CAD
2 sustainability
1 Crowd simulation
CAD
Sustainability

8. Soil Settlement and Soil-Structure Interaction Pdisp & GSA Raft
GSA basic concepts
Soil Settlement and Soil-Structure Interaction Pdisp & GSA Raft

9. Soil settlement and Soil-Structure interaction – is it a problem?
GSA basic concepts
Soil settlement and Soil-Structure interaction – is it a problem?

10. Soil settlement and Soil-Structure interaction – is it a problem?
GSA basic concepts
Soil settlement and Soil-Structure interaction – is it a problem?

11. What structures are affected?
GSA basic concepts
What structures are affected?
Ground stiffer than structure – No

12. What structures are affected?
GSA basic concepts
What structures are affected?
Structure stiffer than ground – No

13. What structures are affected?
GSA basic concepts
What structures are affected?
Ground & structure similar stiffness – Yes Structure sensitive to movement – Yes

14. How does Pdisp and GSA Raft work?

15. What is Pdisp? Calculates soil displacements under load Uses
Boussinesq (gives stresses and displacements in soil)
Mindlin (gives displacements only)

16. What is GSA? Structural finite element analysis
Linear and non-linear static
Vibration
Buckling
Bridge loading
Form finding
Element design
Soil-structure interaction

17. What is GSA Raft? Combination of GSA and Pdisp
GSA basic concepts
What is GSA Raft?
Combination of GSA and Pdisp
Soil-structure interaction:
Vertical for rafts
Vertical & Horizontal for piles
Choice of methods
Boussinesq (rafts)
Mindlin (rafts and piles)
Iterative process
GSA includes the Pdisp engine, so that the analysis can use both engines. You do not need Pdisp but of you do then you gain additional options for
GSA 8.3 can analyse rafts
GSA 8.4 can now analyse the shear stresses necessary for piled analysis
Both methods calculate soil settlements under loads
Mindlin, Boussinesq:
Boussinesq provides the stress distribution but is in other respects inferior to Mindlin.
Boussinesq is sensitive to the user defined number of intermediate calculation layers; this can lead to significant errors, especially for concentrated localised loads. Mindlin is not sensitive to this.
Boussinesq relies on the user specifying a global PR which is not less than the local values. Mindlin has a better approximation for this (which probably could also be used in the formulation of Boussinesq).
Raft and Piled-Raft analysis are iterative processes linking GSA’s linear static solver for the structure with Pdisp’s non-linear solver for soil settlement

18. GSA – Pdisp Iteration Solution
GSA basic concepts
GSA – Pdisp Iteration Solution
GSA assumes a spring stiffness
GSA analyses to get soil pressures & displacements
Soil pressure: p = f / A
If p < pmin then p = pmin
If p > pmax then p = pmax
Pdisp analyses spring forces to get soil settlements
GSA calculates new spring stiffnesses from forces & settlements
Repeat from 2 until GSA and Pdisp movements converged
Add vertical support springs to the nodes on raft that interact with soil (default stiffness Ko = 1.0e7kN/m)
Do static analysis of the raft model only to obtain forces Fi & displacements draft at the interaction nodes
Calculate the pressure Pi for each of the interaction areas and apply them to soil
Pi = Fi/Ai
if Pi < Pmin, let Pi = 0
if Pi > Pmax, let Pi = Pmax
Do Pdisp (soil settlement) analysis only to obtain soil settlements dsoil at the interaction points
Compare dsoil & draft, If the differences between them are smaller than the preset tolerance for all the interaction nodes, STOP, otherwise go to step 6
Update support spring stiffness according to the spring forces and the soil settlements Ki+1 = Fi/ draft Go to step 2

19. Soil Data Define soil properties in the vertical direction m Etop
Top level
Etop m
Ebot
rigid level – deformation below is ignored
Layer 1
Layer 2
Layer 3

20. Soil Data Zone 2 (profile j) Zone 1 (profile i) Zone 3 (profile k)
Define Soil Zones in plan
Zone 1 (profile i)
Zone 2 (profile j)
Zone 3 (profile k)

21. Schematic view of piled-raft model
GSA basic concepts
Schematic view of piled-raft model
raft
piles
Non-linear Springs
Soil
See next slide

22. A pile section: represented by a single node on pile
GSA basic concepts
A pile section: represented by a single node on pile
sy.b Bx By H sx sy tz
sy.t

23. Non-linear soil springs
GSA basic concepts
Non-linear soil springs
Every nodes on the piles are connected by 3 non-linear springs to represent the soil non-linear behaviour immediately around the piles in x, y & z directions. The non-linear springs are then attached to the ground (soil). The soil is still considered as elastic media that the settlements are calculated using Mindlin method
Non-linear springs with interaction forces Fx, Fy & Fz
Representing soil: they all coincide with the node on pile
A node on pile

24. GSA basic concepts
Interaction Forces
The interaction forces in the non-linear springs will be the interaction forces between pile and soil
The interaction forces equal the pile-soil contact stress times the interaction area, i.e.

25. Pile-Soil Contact Stresses
GSA basic concepts
Pile-Soil Contact Stresses
The pile-soil contact stresses equal pile-soil interaction coefficient C times the soil strength, i.e.

26. Pile-Soil Interaction Coefficient
GSA basic concepts
Pile-Soil Interaction Coefficient
The pile-soil interaction coefficient C shown below, is a function of:
The relative displacements between piles and soil at the relevant pile point
The dimensions of the piles c
1.0
dx/By etc
The pile-soil interaction coefficient C needs to be defined by the users at this version of GSA
Can be got from American Petroleum Institute RP2A
-1.0

27. Pdisp and GSA Raft in action
GSA basic concepts
Pdisp and GSA Raft in action
Create Pdisp model
Import to GSA
Add raft and analyze
Export back to Pdisp
Add piles to GSA
Add soil springs to random mesh

28. GSA Raft Case study

29. Aspire Tower, Doha, Qatar
GSA basic concepts
Aspire Tower, Doha, Qatar
The centre piece of the 15th Asian Games was the Aspire Tower, shaped to represent a colossal torch, which for the duration of the Games held its symbolic flame within the lattice shell that forms the topmost section. At 300m, Aspire Tower is currently the tallest building in Qatar.
As well as functioning as a support for the Asian Games flame, the tower includes a large reception and public area on two floors; restaurants and business centre; 17 floors of five-star hotel accommodation; a sports museum on three floors; a health club on three floors with a cantilevered swimming pool 80m above ground; presidential suites; and a revolving restaurant and observation deck about 240m above ground. A 62m high lattice shell structure on top of the reinforced concrete central core frames the 15MW flame cauldron

30. Aspire Tower, Doha, Qatar
GSA basic concepts
Aspire Tower, Doha, Qatar
A geotechnical desk study indicated sound limestone near to ground level, so the initial design was based on a raft foundation. While this progressed, a site investigation was made of the deeper geology and rock types - particularly important in view of the magnitude and concentration of load arising from the building form - and this confirmed the presence of weak Rus Chalk between the limestone strata. In view of this and the assessment of soil/rock stress in this stratum based on the raft design already under way, the team decided that piling to the raft was needed to ensure adequate load capacity.
In addition to gravity loads, the raft provides resistance to overturning effects under lateral wind and seismic loads. The arrangement provided also ensures that there is no tension in the piles or uplift, as the tension effects of overturning are balanced against the vertical load from the tower's self-weight.
GSA Raft was used to analyse the foundation system as it uses an iterative analysis system between two models, one for structure and one for soil. The structure model comprises a GSA grillage model supported on springs at the underside of the raft and also at pile positions at the depth of the pile load transfer length. The soil model comprises a representation of the soil strata, with loaded areas corresponding to spring location in the structure model.
At each stage of iteration soil settlements are calculated by Vdisp, for the spring loads in the structure model. The settlements are then used to reset the spring stiffnesses in the structure model. The process is continued until vertical movements and load distributions match, providing a unique analysis for each load condition.
To assess local effects such as load spread beneath the core walls, the through thickness flexibility of the raft over an annular sector was also modelled axis metrically, using loads and spring stiffnesses output by GSA Raft. A lower/upper bound design approach allowed for the range of parameters involved - soil, concrete, design loads, etc.
Using the results of the GSA Raft analysis, bored cast in situ piles were used limited to a maximum 1.2mm diameter to suit local practice, and extending through Rus Chalk to the better limestone below. The piles were cast in grade C32/40 concrete for the necessary design strength. The 37.3m raft diameter was determined by the need to spread the entire tower load delivered by the core, so as to limit bearing pressures to appropriate levels under the raft, in the Rus Chalk and on the piles.

31. Any Questions? GSA basic concepts
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Let me know if there’s some part of the software that you’d like me to go over.
Ok well lets see what questions we have.
Sorry we don’t have time to answer all your questions right now but we will be answer all your questions by very soon. And if you didn’t get a chance to put your questions in I’ll send out a survey which you can put any additional questions in.

32. Contact us: oasys@arup.com
GSA basic concepts
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