1. Soil settlement and structure interaction with Pdisp and GSA Raft
Oasys

2. Today’s Team Nigel Rees Peter Debney Senior Consultant
Business Development Manager
Peter Debney
Senior Consultant

3. GoToWebinar Attendee Interface
1. Viewer Window
2. Control Panel
Oasys Software : Concrete Design

4. Agenda Product demonstration Questions and Answers
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. Geotechnical software
GSA basic concepts
Structural software
Geotechnical software
Document Management software
5 structural
13 geotechnical
2 document management
2 CAD
2 sustainability
Other programs produced for internal use
CAD software
Sustainability software

8. Soil Settlement and Structure Interaction with Pdisp and GSA Raft
GSA basic concepts
Soil Settlement and Structure Interaction with Pdisp and 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. What structures are affected?
GSA basic concepts
What structures are affected?
Ground stiffer than structure – No

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

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

13. How does Pdisp and GSA Raft work?

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

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

16. 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

17. 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

18. 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

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

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

21. 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

22. 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

23. 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.

24. 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.

25. 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 Institue RP2A
-1.0

26. Pdisp and GSA Raft in action
GSA basic concepts
Pdisp and GSA Raft in action

27. GSA basic concepts

28. Any Questions? GSA basic concepts
Now does anyone have any questions? Just write them in the question box in your right hand toolbar.
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.

29. Masterclass on Analysis Stages in GSA June 30
GSA basic concepts
Next Webinars
Masterclass on Analysis Stages in GSA June 30

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