Set up minimum lateral loads

Many design codes require consideration of a minimum lateral design load (MLL) as a proportion of the gravity loads.

In some codes MLL will be a minimum wind load case, in others a robustness check - some examples of the application of MLL are:

The USA code (ASCE 7-16 clause 1.4.2) recommends a value of 1% of total dead load as a minimum lateral design force. For more on this see Minimum lateral loads (MLL) (ASCE 7)
Eurocode - the Singapore National Annex to EN 1991-1-4 Wind Actions requires that buildings should be capable of resisting, as a minimum, a design ultimate horizontal load equal to 1.5% of the characteristic dead weight of the structure. For more on this see Minimum lateral loads (MLL) (Eurocode)
British Standard (BS 5950) recommends a value of 1% of factored dead load as a minimum lateral load replacing wind load For more on this see Minimum lateral loads (MLL) (British Standards)

Minimum lateral loads settings

MLLs are set up for the current model via the Minimum lateral loads page in Model Settings.

The default MLLs depend on the code being worked to:

For the USA, Singapore (Eurocode) and Malaysia (British Standards) MLL is switched on and set to Case A.
For other Design Code regions MLLs are switched off i.e. set to None.

The MLL load is a percentage of gravity loading, with the gravity load to be considered configured as follows:

Gravity Loads - either Dead or Dead & Live (Dead & Imposed outside of the US) can be selected.
Using Combination - either Unfactored or Factored Loads can be selected.

There are potentially two sets of loading that can be applied - Case A and Case B - each of which creates four loadcases acting on the structure in the +ve and -ve Building Directions.

When Case A or Case A & B are enabled, you specify the percentage loading for both directions (Dir 1 & Dir 2) as shown below.

Create minimum lateral loadcases

If the regional code being worked to has a minimum lateral loads requirement, these loads can be set up as follows:

On the Home tab, click Model Settings.
In the Model Settings dialog box, click Minimum Lateral Loads.
Select the number of cases to set up by clicking Case A, or Cases A & B.
Select the gravity load type to be used for the calculation.
Select whether the gravity load used for the calculation should be factored or unfactored.
Specify the percentage of the gravity load to be applied as a minimum lateral load in each direction.
Click OK.

Minimum lateral load loadcases are created and added to the list of loadcases in the Loading dialog box.

Create minimum lateral load combinations

If you are manually creating combinations:

You must nominate a gravity load combination to be used for the minimum lateral load calculations. To do this, first select an existing gravity load combination, and then on the Parameters tab select 'Minimum Lateral Loads'.
You can then manually create your MLL combinations, selecting the appropriate MLL loadcase for each combination as required.

If you are automatically generating combinations:

Additional “+MLL” combinations are included for all Gravity combinations listed on the second page of the Combination Generator. You can then select which of these you wish to include.
As you proceed through the Combination Generator, a Minimum Lateral Loads page is displayed - as shown below. On this page you specify,
1. in the list at the top the combination to be used for minimum lateral load calculations, and
2. in the table of combinations the applied load direction(s) for each active MLL combination.

Review minimum lateral loads

The MLLs are automatically calculated and applied during analysis. Their magnitude and direction can be displayed graphically in a results view by selecting MLL in the Notional/ Seismic Loads group of the ribbon, as shown below. The Min Lateral Loadcases are listed together with the other loadcases in the Loading list, and the MLL combinations are included with the other combinations.

The Min Lateral loadcases are also listed in the Loading window of the Project Workspace where the total MLL applied can be seen as shown below.