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Raft analysis : Notes

Combination of raft analysis results

In general, raft analysis results cannot be combined as it is nonlinear analysis if not all raft interaction nodes are in contact with soil or piles are included. However, if it is a pure raft analysis and all the interaction nodes are in contact with soil (not separated from soil) in the raft analysis cases to be combined, the combination of those raft analysis cases can be used since in this case the results are linear.

Export and import of PDisp data

PDisp data files (.pdd) can be imported into GSA using the File > Import > PDisp (PDD file) command. The analysis results in PDisp data will be ignored in the import since they are meaningless in GSA.

The PDisp data created in GSA can also be exported to PDisp data file. The export can be done from the File > Export > PDisp (PDD file) command. If raft results exist when exporting PDisp data the equivalent soil loads will be calculated and they will be exported along with the other PDisp data.

The create new raft model tool

After the completion of a converged raft analysis it is possible to generate a new raft model in which support stiffness equivalent to the soil reactions are added to the current raft model.

This is done using the Model > Raft modelling > Create new raft model menu command.

Creating a new model in this way will modify the current model so it is recommended that the current model be saved before running this command.

The support stiffness are derived from the soil reaction forces and the nodal displacements. If raft analysis results exist for several analysis cases then an analysis case must be selected for the calculation.

Continuation runs

The Create new raft model tool can be used to restart a raft analysis since the preserved support stiffnesses will be used as initial stiffnesses in subsequent raft analyses.

It is sometimes useful to do an initial analysis with coarse convergence criteria to examine the potential for convergence. The raft analysis can then be continued by using the Create new raft model tool to generate a model with revised initial support stiffnesses and then analysing with more demanding convergence criteria.

Soil settlement (PDisp) analysis methods (Mindlin or Boussinesq) to be used

If soil stresses are not interested from raft analysis, it is recommended to use Mindlin analysis method since it is generally faster and less sensitive to number of intermediate displacement levels defined for soil layers compared with Boussinesq analysis method. However, Mindlin analysis method is not able to consider soil nonlinearity, therefore if soil nonlinearity (defined by nonlinear curves) need to be considered, Boussinesq analysis method should be used as the nonlinear curves will be ignored in Mindlin analysis. For piled-raft analysis, only Mindlin analysis method can be used since Boussinesq method is not able to consider horizontal loads and horizontal displacements in the soil that are required by pile-soil interactions. In piled-raft analysis, the soil nonlinearity is considered by the Pile-soil interaction coefficient through the nonlinear springs used to connect piles and soil. The followings are a summary of the recommended soil settlement analysis methods:

Pure raft analysis:

If soil stresses are not interested and soil nonlinearity is not considered, use Mindlin method.

If soil stresses are required or soil nonlinearity need to be considered, use Boussinesq method.

Piled-raft analysis:

Only Mindlin analysis method can be used.

See also Raft analysis guide for a how to guide and more information on performing this analysis type.