Load Patterns

Define Load Patterns

Form: Define Load Patterns

Use the Define menu > Load Patterns command to add, modify, or delete load patterns in the model file. An unlimited number of load patterns can be defined. Click here for more information about load patterns.

Loads represent actions upon the structures, such as force, pressure, support settlement, thermal effects, ground acceleration, and others. A spatial distribution of loads upon the structure is called a load pattern. An unlimited number of load patterns can be defined in ETABS. Typically, separate load patterns would be defined for dead load, live load, wind load, snow load, and so on. Loads that need to vary independently, for design purposes or because of how they are applied to the structure, should be defined as a separate load pattern. ETABS uses the type of load pattern to create design load combinations automatically.

Load patterns can include automated loads, such as self weight or code-specified wind or earthquake lateral force distributions. For all other loads on the structure, specific load values must be assigned to objects as part of a load pattern (see the topics in the Menus/Assign - Joint Loads, Frame Loads, and Shell Loads books of this Help for more information about assigning loads). The load values assigned to an object specify the type of load (e.g., force, displacement, temperature, and so forth), its magnitude, and its direction (if applicable). Different loads can be assigned to different objects as part of a single load pattern. Each object can be subjected to multiple load patterns.

Click the Define menu > Load Patterns command to access the Define Load Patterns form. This form has the following edit boxes and drop-down lists.

Load edit box. Use this edit box to specify the name of the load pattern. The previously assigned load pattern names appear in the display list below this edit box. Note that each load pattern and load combination must have a unique name. Also, the word Mode is reserved and cannot be used for a load pattern name or a load combination name.
Type drop-down list. The types of load patterns available appear in this drop-down list. The program uses these values when automatically creating design load combinations for the design postprocessors. The factors used in the design load combinations are different for the various types of loads. The load pattern types are assumed to be basically self explanatory with the following further explanations:
- Reducible Live option: This is a Reducible live load. A live load that is specified as reducible is reduced automatically by the program for use in the design postprocessors. The live load reduction parameters are specified using the Design menu > Live Load Reduction Factors command.
- Other option: Use this option when specifying a pattern that does not fit into one of the other categories or when the pattern is not intended to be included in the design load combinations that are created automatically by the program.
Self-Weight Multiplier edit box. The self-weight of the structure is determined by multiplying the weight per unit volume of each object that has structural properties times the volume of the object. The weight per unit volume is specified in the material properties. Use the edit box to specify what portion of the self-weight is to be included in the load pattern. A self-weight multiplier of 1 means to include the full self-weight of the structure in the load pattern. A self-weight multiplier of 0.5 means to include half of the self-weight of the structure in the load pattern. Normally a self-weight multiplier of 1 is specified in one load pattern only, usually the dead load pattern, with all other load patterns having a multiplier of zero. Note that if a self-weight multiplier of 1 is specified in two different load patterns, and then those two load patterns are included in the same load combination, the results for the load combination are based on an analysis where double the self-weight of the building has been applied as a load.
Auto Lateral Load drop-down list. The Auto Lateral Load drop-down list becomes active when the load type is specified as Seismic or Wind.
- Selecting a code from this list specifies that code-compliant loads be created automatically for the load case. Another form will appear that can be used to review and as necessary specify the appropriate parameters for the automatic load. (See the Modify Lateral Load bullet below for hyperlinks to topics addressing those forms.)
- When None is selected from the drop-down list, no automatic loads are used, and loads must be assigned using the commands available on the Assign menu.

Note: If a model has more than one tower, do not use a Quake type Auto Lateral Load, but perform a Response Spectrum or Time History analysis instead. Using a seismic Auto Lateral Load with multiple towers likely will result in an incorrect distribution of lateral loads.

Use the following buttons to add, modify, or delete load cases:

Add New Load button.

1. Type the name of the load pattern in the Load edit list.
2. Select a load type from the Type drop-down list.
3. Specify a self-weight multiplier in the Self-Weight Multiplier edit box. (See the Self-Weight Multiplier text above for cautions related to applying self weight.)
  - If the load Type specified is Quake or Wind, select an option from the Auto Lateral Load drop-down list.
4. Click the Add New Load button.
1. - If the load type specified is Seismic or Wind, note the text for the Modify Lateral Load button (see below).
Modify Load button.

1. Highlight an existing load pattern in the Loads area. Note that the data associated with that load pattern appears in the edit boxes and drop-down lists at the top of the Loads area.
2. Modify any of the data in the Loads area for the load pattern.
3. Click the Modify Load button.

Modify Lateral Load button. If a new load is being defined with a load Type of Seismic or Wind and a code (i.e., automatic load) has been selected in the Auto Lateral Load drop-down list, first click the Add New Load button, then click the Modify Lateral Load button to access a form specific to wind or seismic load. Use the form to specify parameters consistent with the code selected in the Auto Lateral Load drop-down list (including wind loading on an open frame structure when ASCE 7-02 is selected for the Auto Lateral Load). For example, if the load Type is Seismic and NEHRP 97 is selected in the Auto Lateral Load drop-down list, the form that appears is the NEHRP 97 Seismic Loading form. The options available on the form relate to code requirements. Thus, fill in parameters on the form as appropriate. Then click the OK button to complete the operation and return to the Define Load Patterns form. Note that selecting User Defined for the Auto Lateral Load will access the User {Seismic, Wind} Loads on Diaphragms form, respectively.

Note: After a code specific load has been defined as described here, highlight it in the list of loads and click the Modify Lateral Load button to redisplay the form if any additional changes are needed.

Note: Each automatic static lateral load must be in a separate load pattern. That is, two or more automatic static lateral loads cannot be specified in the same load pattern. However, additional user-defined loads can be added to a load pattern that includes an automatic static lateral load. A separate automatic static load pattern must be defined for each direction of wind load.

Note: Please refer to the Automated Lateral Loads manual for information about the code-specific seismic and wind loading parameters (Help menu > Documentation > Manuals > Lateral Loads Manual command).

Delete Load button. Note that when a load pattern is deleted, all of the loads that have been assigned to the model as a part of that pattern are also deleted.
- Highlight the existing load pattern in the Loads area.
- Click the Delete Load button.

After a load pattern has been defined, assign loads to the objects as part of that load pattern by selecting the objects to be loaded and using the appropriate command on the Assign menu (i.e., Frame Loads, Joint Loads, Shell Loads).