1 GSA Raft & Piled-raft Analysis Raft & Piled-raft analysis (Soil-structure interaction analysis)

1GSA Raft & Piled-raft Analysis

Raft & Piled-raft analysis

(Soil-structure interaction analysis)


1. Introduction

2. Data required for raft & piled-raft analysis

3. How GSA raft & piled-raft analysis works

4. Running GSA raft & piled-raft analysis

5. Inspecting raft & piled-raft analysis results



1. Introduction1. In theory, soil-structure interaction should always be

considered in structure analysis

2. In practice, structure analysis and soil settlement analyses are sometimes done separately If soil settlements are very small (its effects on top structure are

negligible), structure can be analysed assuming supports are pinned or fixed

If soil settlements are relatively large, iterative procedures may be used, i.e. do structure analysis using spring support to represent soil , do soil settlement analysis to estimate soil stiffness

If soil settlements are relatively large and its effects on structure are significant, soil structure integrated model may need to be built and analysed

– Including the whole structure in the soil-structure model (rare)

– Including only the foundation parts of the structure model (common)



1. Introduction3. Soil region need to be considered

The larger the better, but not unnecessarily too large Assuming soil is rigid beyond some level when soil settlements

are negligible (reducing soil region)



2. Data required for raft & piled-raft analysis1. Raft & piled-raft data – structural model

2. Soil data – soil properties etc

3. Interaction data – define how soil & raft (pile) interact



2. Data required for raft & piled-raft analysis1. Raft & piled-raft data - structural model

The same as other GSA models, but the top structure may be ignored in a raft or piled-raft model

The vertical direction should be free for raft-soil interaction nodes

All three directions should be free for pile-soil interaction nodes

The spring supports representing soil during the analysis will be generated and deleted automatically, so it is not necessary to define them in raft/piled-raft model



2. Data required for raft & piled-raft analysis2. Soil data - soil properties etc

1. Raft analysis specification

– Rigid boundary level – soil deformation below this level will be ignored

– Maximum E ratio – sub-layer will be used if the E ratio of this layer is larger than this value

– Global Poisson’s ratio (Boussinesq only) – used in calculating soil stress (note: Young’s modulus & Poisson’s ratio of each layer will be used to calculate the vertical strains and the settlements)

– Mindlin analysis method (only used in pile analysis) Use weighted average Use greatest stiffness Use stiffness at displacement point




2. Soil profiles – define soil properties in vertical direction


Top levelEtop Ebot

rigid level – deformation below is ignored

Etop

Top level

Ebot

Etop

Top level

Ebot

Layer 1

Layer 2

Layer 3



3. Soil zones – assign soil profiles (property) to areas in plan, soil profile defined latter will be used for overlapped areas


Zone 1 (profile i)Zone 1 (profile i)

Zone 2 (profile j)Zone 2 (profile j)

Zone 3 (profile k)Zone 3 (profile k)



4. Pile-soil interaction properties (used by soil profiles), define: Maximum soil stresses for each layer in different directions Assign Pile Soil Interaction Coefficient (PSIC) curve




5. Pile-soil interaction coefficients (PSIC) curve, define: the variations of pile-soil interaction forces along with the

normalised relative displacement between pile & soil




6. Other data – normally not required and do not need to be defined Load data – define the extra loads directly on the soil in

addition to the loads from raft & piles

Displacement data – extra displacement output points, lines or grids

Non-linear curve – to define the variations of soil Young’s modulus along with strains (only used in Boussinesq analysis and ignored by Mindlin analysis)



2. Data required for raft & piled-raft analysis3. Interaction data – define where soil & raft (pile)

interacts1. Node list – soil-raft interaction is through GSA nodes, the nodes

list defines the interaction nodes2. Dimension of interaction area – rectangular area defined explicitly

by X & Y dimensions or generated automatically3. Elevation of interaction – the level of the soil-raft (pile) interaction

– it is relative to the elevation in soil profile or it can be generated automatically

4. Minimum pressure (raft interaction only)1. = 0, soil will take compression only2. < 0, defined as soil tensile strength3. > 0, not allowed

5. Maximum pressure (raft interaction only)1. define the maximum pressure that soil can sustain



3.1 How soil-raft interaction works1. Add vertical support springs to the nodes on raft that interact

with soil (default stiffness Ko = 1.0e7kN/m)

2. Do static analysis of the raft model only to obtain forces Fi & displacements draft at the interaction nodes

3. Calculate soil pressure load Pi for each of the interaction areas and apply them to soil

1. Pi = Fi/Ai

2. if Pi < Pmin, let Pi = 0

3. if Pi > Pmax, let Pi = Pmax

4. Do Pdisp (soil settlement) analysis to obtain soil settlements dsoil at the interaction points

5. 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

6. Update support spring stiffness according to the spring forces and the soil settlements Ki+1 = Fi/ draft Go to step 2



3.2 How soil-pile interaction works1. Add three non-linear springs to each pile interaction node in x,

y & z directions

2. The forces in these spring are calculated from the normalised relative displacements between soil and pile at the same points according to the PSIC curve, maximum soil stresses & interaction areas

3. The maximum forces in the nonlinear spring are controlled by the interaction area and the maximum soil stresses defined in pile-soil interaction properties table

4. In the converged state, the forces in the nonlinear springs make the pile in equilibrium with the rest of the structure.

5. See separate PPT file for details



4. Running GSA raft analysis1. Analysis option

1. Mindlin – faster, no soil stress results, results are not sensitive to the number of soil layers, linear analysis only

2. Boussinesq – relatively slow, give soil stress results, results are sensitive to the number of soil layers (only for pure raft analysis), soil nonlinearity can be considered

2. Convergence criteria1. Tolerance – the differences between soil settlements and raft

displacements and the differences of the force in the nonlinear spring and the soil reaction force for pile interaction points

2. Absolute – define the absolute tolerance of displacement & force

3. Relative – define a percentage of tolerances and the program will calculate the tolerance automatically based on the largest soil displacements and pile-soil interaction forces



4. Running GSA raft analysis3. Damping & initial soil stiffness

1. Damping is not normally required for raft analysis unless it is difficult to converge, e.g. there are heaves

2. Damping is normally required for piled-raft analysis and the damping ratio can be as high as 99%

3. Apply damping only affects the analysis speed, not the analysis results

4. Initial support stiffness – used only for raft interaction nodes, it only affects the analysis speed and not the results. If the given initial support stiffness is close to real soil stiffness, the analysis will converge quicker. Normally default value is used as we do not know soil stiffness beforehand



5. Inspecting GSA raft analysis results1. Raft results – the same as the results from other

analyses, e.g. nodal displacements, element forces & moments etc

2. Soil results1. Soil settlements

2. Soil stress (Boussinesq analysis only)

3. Soil-raft interaction1. Soil – raft contact pressure

2. Pile – soil contact pressures


Documents

1 GSA Raft & Piled-raft Analysis Raft & Piled-raft analysis (Soil-structure interaction analysis)