Raft Analysis : Data requirements
The input data for a raft and piled-raft analysis can be divided into three categories:
Raft and piled-raft structure data
The structural model of the raft or piled-raft is modelled in GSA in the usual way as other structure models except that the nodes that will interact with soil should be kept free in vertical direction for the raft interaction nodes and kept free in all three directions for pile interaction nodes.
Vertical support stiffness for the soil interaction nodes may be specified in which case the specified stiffness are used as the initial stiffness for the interaction nodes, if the specified initial support spring stiffness are closer to the real soil stiffness, the analysis speed can be enhanced, but it has no effect on the analysis results. If nodal support stiffness are not defined, default initial nodal support stiffness will be used, the default initial nodal support stiffness can be modified on the Analysis Wizard when setting up the raft & piled-raft analysis.
Raft can be modelled by grillage using beam elements or modelled by 2D elements. For piled-raft model, the piles need to be modelled by a number of beam elements and the beams must be vertical (in global Z direction).
Soil property and soil zone data
These data are the same as those used in the standalone PDisp program. Refer to PDisp program user manual which is in GSA installation folder for guidance. The input tables for these data can be opened either from Model > Data modules > Raft > Soil > ... command or from the Data explorer using the Raft > Soil > ... option.
The loading to the soil from raft and/or piles is generated automatically during raft or piled-raft analysis and they should not be included in PDisp rectangular load table. Only additional loads, i.e. extra to the loads from raft and piles should be defined in PDisp rectangular load table.
Soil-raft interaction and soil-pile interaction data
Soil-raft and soil-pile interactions are defined by the nodes on raft and piles. The Raft Interaction table defines the nodes on the raft to interact with soil. This table is opened from the Model > Data modules > Raft > Raft interaction command or from the Data explorer using the Data > Raft > Raft interaction option. The interaction area of the raft interaction nodes can be defined on this table or choose Automatic to allow GSA to calculate the interaction areas automatically. The elevation of the interaction can also be defined explicitly or choose Automatic to take the nodal z coordinate as the interaction elevation. Minimum and maximum soil pressure can also be defined in this table. Zero minimum soil pressure means that soil will not take any tensile stresses. If a negative minimum soil pressure is defined, it will be used as the soil tensile strength. The maximum soil pressure is the compressive strength of soil. If this pressure is reached during the analysis, the soil-raft contact pressure will not increase any more to allow soil local yield effect to be considered.
The Pile interaction table defines the nodes on the piles to interact with soil. This table is opened from the Model > Data modules > Raft > Pile interaction menu command or from the Data explorer using the Data > Raft > Pile interaction option. The pile-soil interaction areas (dimensions of X, Y, Z & perimeter) of the pile interaction nodes can be defined on this table or choose Automatic to allow GSA to calculate the interaction areas automatically. The same as raft interaction nodes, the elevation of the interaction can also be defined explicitly or choose Automatic to take the nodal z coordinate as the interaction elevation.
The graphical display of raft and pile interaction nodes and/or raft and pile interaction areas are available from Graphic view label options.