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Raft and Soil-Structure InteractionPdisp and GSA Raft
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Peter Debney
Senior Consultant
Today’s Team
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1. Viewer Window 2. Control Panel
Raft and Soil-Structure Interaction
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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.
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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
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Oasys Customers
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Structural
Geotechnical
CAD
Document Management
Sustainability
Crowd simulation
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Soil Settlement and Soil-Structure Interaction Pdisp & GSA Raft
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Soil settlement and Soil-Structure interaction – is it a problem?
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Soil settlement and Soil-Structure interaction – is it a problem?
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What structures are affected?
Ground stiffer than structure – No
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What structures are affected?
Structure stiffer than ground – No
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What structures are affected?
Ground & structure similar stiffness– Yes
Structure sensitive to movement– Yes
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How does Pdisp and GSA Raft work?
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What is Pdisp?
• Calculates soil displacements under load
• Uses• Boussinesq (gives
stresses and displacements in soil)
• Mindlin (gives displacements only)
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What is GSA?
• Structural finite element analysis
• Linear and non-linear static
• Vibration• Buckling• Bridge loading• Form finding• Element design• Soil-structure interaction
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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
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GSA – Pdisp Iteration Solution
1. GSA assumes a spring stiffness
2. GSA analyses to get soil pressures & displacements
Soil pressure: p = f / AIf p < pmin then p = pmin
If p > pmax then p = pmax
3. Pdisp analyses spring forces to get soil settlements
4. GSA calculates new spring stiffnesses from forces & settlements
5. Repeat from 2 until GSA and Pdisp movements converged
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Soil Data
• Define soil properties in the vertical direction
Top levelEtop mEbot
rigid level – deformation below is ignored
Etop
mTop level
Ebot
Etop
m
Top level
Ebot
Layer 1
Layer 2
Layer 3
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Soil Data
• Define Soil Zones in plan
Zone 1 (profile i)
Zone 2 (profile j)
Zone 3 (profile k)
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Schematic view of piled-raft model
raft
piles
Non
-line
ar S
prin
gs
Soil
Soil
See next slide
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sy.b
Bx
By
Hsx
sy
tz
tz
sy.t
A pile section:represented by a single node on pile
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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
A node on pile
Representing soil: they all coincide with the node on pile
Non-linear springs with interaction
forces Fx, Fy & Fz
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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.
zzz
yyy
xxx
AF
AF
AF
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Pile-Soil Contact Stresses
• The pile-soil contact stresses equal pile-soil
interaction coefficient C times the soil strength, i.e.
zzz
yyy
xxx
fc
fc
fc
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Pile-Soil Interaction Coefficient
• The pile-soil interaction coefficient C shown below, is a function of:1. The relative displacements between piles and soil at
the relevant pile point2. The dimensions of the piles
1.0
-1.0
dx/By etc
c
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Pdisp and GSA Raft in action
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GSA Raft Case study
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Aspire Tower, Doha, Qatar
http://www.oasys-software.com/casestudies?id=14/
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Aspire Tower, Doha, Qatar
http://www.oasys-software.com/casestudies?id=14/
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Any Questions?