Surface load (Load)

Surface Load

Property	Value
Default Short Command	LS
Icon

In all FEM-Design module constant or variable  Surface load (force) can be defined   in the oridinary way by giving the load region, direction and load intensity positions, if necessary. In the 3D modules, there is an additional option to define   Soil/hydrostatic pressure-like load by giving the load region, direction, the soil/water level, the intensity at the soil/water level and the increment of the intensity.

The load properties and definition tools are available on the tool palette of the Surface load.

Figure: Tool palette of Surface load

Ordinary, Constant Surface Load

Figure: Constant surface load on a slab and on a wall

Definition steps

1. Select the load case from the load case drop-down list from Loads tabmenu, which you would like to add the new surface load(s) to.
   If you have not defined a load case yet, you can also define and set at the  Default settings by giving a load case name. Here you can type any comment, which will be displayed next to the force.
   
   Figure: Load case definition
   
   The load will be displayed on the layer assigned to the selected load case and in the layer’s color.
2. Set the intensity value in the q1 field according to the current unit (that can be set at Settings > Units) in the tool palette or at  Default settings. You may also choose intensity from the predefined load values from the list appears after clicking on the   button.
3. Define the load direction (FEM-Design module dependent).
4. Choose a method for the action surface definition (geometry).
5. In the 3D modules only, you can set the “intensity mode”, that means the connection between applied intensity and the action plane  at  Default settings, if the new load will be in skew position (not vertical or horizontal):
   • “Intensity meant along action plane”
     The intensity will be meant along the action plane, so the resultant will be calculated from the intensity and the area of the action surface. Apply this option for dead load-type loads.
   • “Intensity meant perpendicular to direction of load”
     The intensity will be meant perpendicular to the load direction, so the resultant will be calculated from the intensity and the area of the horizontal projection of the action surface. Apply this option for example to model snow load.
     
     
     Figure: Intensity difference at skew bars
6. Place the surface load in the model view based on the chosen geometry method (4^th step).
   With the special tool called Object’s local system () you can add surface load directly to a selected planar object in a direction set parallel with the one of the local system axis of the planar object. So, with this tool, you can merge 3^rd, 4^th and 6^th steps to one step, if the required load direction is equal to the direction of the assigned planar object’s local axis.
7. If the load is acting on the structure’s eccentric axis/surface select the Apply on the eccentric axis/surface option in the Default settings.

Ordinary, Variable Surface Load

Linearly variable surface loads can be defined with Surface load command. Three intensity values define the linearly variable load as a “skew plane”.

Figure: Variable surface loads

Definition steps

1. Define first a constant surface load with the required direction and geometry as written at Constant Surface Load.
2. Use the  Variable intensity tool of the Surface load command.
3. Set the new intensity values (q1, q2 and q3). If you inactivate the arrows (   ) next to the value fields, you can type different values by the q fields.
4. Select the constant surface load that you would like to modify being variable.
5. Give the position of the intensity values. You may click outside the surface load’s action plane too.

Optional steps for constant/variable surface loads

1. Place a hole/opening in a surface load. Use the  Hole tool of the Surface load.
2. Modify the load properties (the host load case, intensity values, “intensity mode”) with the  Properties tool of the Surface load tool palette.
3. Modify the load direction with the editing tools (Editing Loads).
4. Information (the resultant force value and the position of its action point) about a selected surface load can be inquired with the  Info tool. A drawing point can be placed in the resultant’s action point, if needed.
   
   Figure: The resultant force value and position calculated by Info tool
5. Set the display settings of the load (Load Display Settings).

Soil/hydrostatic pressure

Soil/Hydrostatic pressure-like load can be defined in a convenient way by choosing “Soil/Hydrostatic pressure” option.

The load intensities are automatically calculated based on three user-defined values:

• z0: the surface level of soil/water (on the global Z axis)
• q0: the intensity of load at the surface level
• qh: the increment of load intensity (along the global Z axis)

The intensity of the load at a certain point of an element is calculated with the formula below:

                                                        q=q₀+(z₀-z)∙q_h                                                                       (2.1)

                             q: the intensity of the load at the particular point

                             z: the Z coordinate of the point

The direction of the soil/hydrostatic pressure load can be determined either in the same way as in case of the ordinary surface load.

For the fastest and easiest load definition it is recommended to use the Object’s local system option with the “Direction is parallel with the local z’ axis” mode.