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Setting Applied Loads

Specifying Boundary Loads

In LimitState:GEO loading may be specified on any external Boundary Object. To set the values select the required Boundary Object, and then using the Property Editor, click loads to expand the Loads data.

In LimitState:GEO, loads may be specified as Permanent, Variable, and Accidental. The purpose of these labels is purely to permit correct application of Partial Factors (see Scenario Manager) to obtain a design load. No additional meaning is given to these labels. Each load may have an Adequacy factor applied to it and may be designated as Favourable, Unfavourable or Neutral.

On the Viewer pane, loads are displayed as a series of coloured arrows:

1. Loads with Adequacy = false : green
2. Loads with Adequacy = true : red

Specifying Self Weight Loading

In LimitState:GEO loads also inherently arise from the self weight of Solid Objects. Each self weight load may have an Adequacy factor applied to it and may be designated as Favourable, Unfavourable or Neutral. To set these values select the required Solid Object, and modify the values in the Property Editor as depicted in the below figure.

Note that when the Adequacy factor is applied to a material self weight and this material lies below the water table, then adequacy is also applied to the water pressure. In effect this means that for static groundwater conditions the Adequacy factor is applied to the buoyant weight of the soil.

Adequacy Factors

In order that the solver can find a solution that causes collapse, it is necessary to apply an Adequacy factor to one or more of the loads or self weights. Consider a simple example of a vertically loaded footing with a permanent load of 1000kN and suppose that the soil resistance is 2000kN. To find a solution an adequacy factor must be applied to the permanent load. The solver will then find the factor that causes collapse, which in this case is 2. An adequacy factor means that the geotechnical design is safe against collapse (within the scope of the analysis technique), an adequacy factor means that the geotechnical design is unsafe.

Favourable/Unfavourable Setting

In design codes such as Eurocode 7, different partial factors are applied to loads dependent on whether they are favourable or unfavourable (i.e. help resist collapse or help cause collapse respectively). LimitState:GEO provides a facility to specify whether loads applied to a boundary are favourable or unfavourable. This specification only affects the choice of partial factor to be applied. The setting is applied to all loads (Permanent, Variable, and Accidental) applied to that boundary and may be specified under the Loading Type entry in the Property Editor. In addition to Favourable and Unfavourable, a Neutral setting may also be specified. With this latter setting no factors are applied to the load. Further information on modelling with these settings may be found in Use of Partial Factors.

For certain problem scenarios, especially those involving several soil/structure interactions, it may not be obvious whether a given load is favourable or unfavourable from the start.

LimitState:GEO assists the user in this case by carrying out a post analysis diagnostic test, checking whether a given load dissipated or added energy into the problem. In most scenarios, input of energy is indicative of an unfavourable load, dissipation of energy is indicative of a favourable load. After each analysis a message is displayed in the console indicating on which Boundary objects the favourable/unfavourable settings did not match the sign of the energy dissipation in the actual solution.

The displayed Boundary objects are hyperlinked. If a hyperlink is clicked the boundary is selected and its properties displayed in the Property Editor, where they can be edited if desired.

Note that this diagnostic assessment of favourable/unfavourable is carried out using the combined design load after partial factors have been applied. If the favourable and unfavourable partial factors are identical then no diagnostic assessment is output. Also note that it is the responsibility of the user to decide the final unfavourable/favourable setting. The output from LimitState:GEO is only provided as guidance.