Raft analysis : Analysis steps

The following are the main steps required for doing a raft analysis.

1. Build up a raft model, the raft can be modelled by a grillage using beam elements or by a horizontal plate using 2D elements.
2. Define loads on the raft in the same way as for doing other GSA analysis.
3. Open Raft analysis specification dialog box from the Data explorer or from menu Model > Data modules > Specification > Raft analysis specification... to define rigid boundary level etc soil settlement analysis parameters
4. Open Soil profiles table from the Data explorer or from menu Model > Data modules > Raft > Soil > Soil profile to define soil properties. One soil profile defines the soil properties in a vertical line from the top surface of the soil to the rigid boundary level below. One soil profile may include soil properties of many layers depending on the real soil conditions. There is no limit for the number of soil profiles to be defined and the number of soil profiles depends on the actual site requirements. Soil profiles are then assigned to rectangular areas called soil zones.
5. Open Soil zones table from the Data explorer or from menu Model > Data modules > Raft > Soil > Soil zones to assign soil profiles (properties) to relevant rectangular areas called soil zones. If soil zones are overlapped, the overlapped areas will use the soil-profile defined later in the soil zone table.
6. Open Raft interaction table from the Data explorer or from menu Model > Data modules > Raft > Raft interaction to define the nodes on raft to interact with soil. The interaction areas 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 here 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.
7. Now we are ready to do raft analysis by going through the analysis wizard start from menu Analysis > New analysis task... in the same way as doing other GSA analysis.

Note: The data described above are the data always required for doing raft analysis. There are also some other PDisp data which are not always required and necessary in most of the raft analysis. If they are required, they can be defined as explained below:

1. Non-linear curve – If soil non-linearity needs to be considered and the soil strain - Young’s modulus (or Young’s modulus reduction factor) relationship is known, the non-linear curve defining the strain - Young’s modulus reduction factor can be defined and then referred to by the relevant soil profile. Once this has been done, the soil Young’s modulus will be changing according to the current soil strain so non-linear properties is considered. This is non-linear curve is only used when Boussinesq is selected for soil settlement analysis.
2. Soil rectangular load – The loads defined in the Soil rectangular load table are the loads acting directly on the soil in addition to the loads on soil come from the raft. Normally, these loads are not required as the soil loads from raft are defined as raft loads. The loads on raft will be transferred to soil through the interaction nodes during the analysis. If there are really some extra loads directly on the soil, e.g. construction loads acting on directly on soil outside the building, they can be defined here to increase the flexibility of GSA raft analysis.

See also:

Notes on raft analysis

Iteration scheme in Raft analysis (Theory).