# Seismic Loading

The **Seismic Loading** option allows you to include a pseudo-static seismic (earthquake) loading in the finite element analysis. To define seismic loading:

- Select the
**Loads**workflow tab. - Select
**Seismic Loading**from the**Loading**menu. - In the Seismic Load dialog, enter the Seismic Coefficient, Orientation and staging information (see below for details) and select OK.

TIP: you can also right-click on the viewing screen and select **Seismic Loading** from the **Loading** sub-menu.

## Seismic Coefficient

The **Seismic Coefficient** is a dimensionless coefficient that represents the (maximum) earthquake acceleration as a fraction of the acceleration due to gravity. Typical values are in the range of 0.1 to 0.3.

When a seismic coefficient is defined, an additional Body Force will be applied to each finite element in the mesh, as follows:

Seismic Force = Seismic Coefficient * Body Force (due to gravity)

= Seismic Coefficient * (volume of element * Unit Weight of element material)

Body Force (due to gravity) is simply the self-weight of a finite element. The Seismic body force is vectorially added to the (downward) Body Force that exists due to gravity, to obtain the total body force acting on the element.

NOTE: acceleration due to gravity (g = 9.81 m/s2) is already incorporated into the Unit Weight of Material (remember that the dimensions of Unit Weight are force/volume), and therefore does not explicitly appear in the above equation. The Unit Weight of material is the Unit Weight that is defined for each material in the Define Material Properties dialog, when Body Force is included with the Initial Element Loading (see below).

### SEISMIC FORCE AND INITIAL ELEMENT LOADING

It is very important to note that Seismic Force only applies to materials that have an **Initial Element Loading** that includes **Body Force**. Initial Element Loading is specified in the Define Material Properties dialog. See the Initial Element Loading topic for more information.

If Body Force is not included in the Initial Element Loading, then the applied Seismic Force will be zero, even if a non-zero seismic coefficient is defined in the Seismic Load dialog. You must remember this if you want to use Seismic Load in combination with Constant Field Stress, since by default, the Initial Element Loading for Constant Field Stress is Field Stress Only.

## Orientation

There are two options for defining the **Orientation** of a Seismic Load:

- Trend/Plunge
**Vector**(enter XYZ vector components to define direction)

It is important to remember that the *RS3* seismic load option is a pseudo-static analysis, and the seismic force will be simultaneously applied to all solid elements at the specified Orientation. If you wish to consider seismic force at other orientations, you will have to manually carry out a sensitivity analysis (i.e. re-run the analysis for a range of different orientations in order to evaluate the effect of seismic force direction on the analysis results).

## Application of Seismic Coefficient

The **Application of Seismic Coefficient** option allows you to specify the stage(s) at which the seismic load is applied.

### APPLY IN STAGE

The **Apply in Stage** option allows you to select the stage at which the seismic load will be applied. Strictly speaking, the seismic load should be applied at the final stage of excavation. If seismic load is applied at an intermediate stage of excavation, the effect of the load will propagate to all subsequent stages (i.e. the seismic load is not removed at subsequent stages, after the initial stage of application).

### STAGE FACTORS

You can also define seismic staging by using Stage Factors. Select the **Stage Load** checkbox and select the **Add Stage ** button for each stage. As each stage is added, a row will appear that allows you to define a stage factor at each stage. The stage factor is multiplied by the seismic coefficient to obtain the actual coefficient used at each stage (e.g. if the seismic coefficient = 0.2 and the stage factor = 0.5, then the actual seismic coefficient applied at that stage will be 0.2 * 0.5 = 0.1). This allows you to increase or decrease the applied seismic load at any stage. To remove the seismic load at a stage enter stage factor = 0.