Lateral Force Method

For structures which conform to certain Code defined limits on regularity and fundamental period, the Codes permit the use a simplified method of seismic analysis, referred to as lateral force analysis. The lateral force method typical involves the determination of the base shear, calculated from the structural mass and earthquake intensity. The base shear is then used to estimate a set of lateral forces distributed spatially throughout the structure. Since the base shear is calculated on the basis of the fundamental period, the application of the equivalent force method is therefore restricted to structures whose dynamic response is governed by the fundamental mode.

Lateral Force Loading

The Lateral Force method is added to a loadcase directly in Masterframe. Although the Lateral Force Method does not require the use of the dynamic module, it is part of the seismic module as so requires a user to have the seismic module available on their license. To access the Lateral Force set-up, go to Loads>Dynamic and Seismic Loads>Seismic Loads. This will open the Dynamic Loading pane on the right hand side of the screen, with the seismic tab active.

To add seismic loads to a loadcase, select the required loadcase from the drop down and then check the Apply Seismic Loading checkbox. For the lateral force method, ensure the Lateral force radio button is selected. Once done, the layout of the screen is as shown below.

Common Static Options

The common static options area contains inputs which are common to all available design codes. The required code is selected from the Code drop down. The mass can be calculated from the current loadcase or from a previously analysed load case. To calculate the mass from another load case, therefore, the load case used to calculate the mass from needs to occur before the selected load case, so that the mass can be calculated in each analysis.

The U and V settings control the two orthogonal sets of loads. An appropriate load factor for each load are input in the text boxes. A negative value can be used to reverse the direction of the load. For guidance on the appropriate factors to use for seismic load combinations, refer to the appropriate design code.

The angle input determines the orientation of the U-component force relative to the global x-axis.

Lateral Force Method

The Lateral Force Method area displays options relevant to the selected code. The Eurocode 8 area is shown below.

The EC8 options allow for the fundamental period T1 of the structure to be calculated using the displayed formulae based on the C_t factor and the structure height, or, alternatively, a user calculated value.

Checking the Design Response Spectrum option will use the response spectrum to calculate the fundamental period of vibration S_D(T₁). This option also allows the use of the behaviour factor q to modify S_D(T₁).

For further guidance on the inputs, refer to Eurocode 8.

UBC 1997

When the UBC 1997 is selected, the layout of the screen is as shown below.

Similarly to the EC8 method, the fundamental period of the structure can be calculated using a Code formula, or by inputting a user calculated value. The remaining inputs determine various parameters related to the structure bracing and type along with the occupancy of the structure. For further information of the required inputs, reference should be made to the UBC.

IBC 2000

When the IBC 2000 is selected, the layout of the screen is as shown below.

The IBC inputs are similar to the UBC. For further details of the required inputs, refer to the Universal Building Code.

Analysis

Once the seismic loads have been applied to the required loadcases, the static analysis can be run by going to Analysis>Static Analysis. The Analysis Type and License window will indicate which loadcases have seismic loads applied. Running the static analysis will then include the lateral forces from the seismic load in combination with the selected Masterframe loads. The results of the analysis can then be viewed as for a normal Static Analysis.