Define Load Cases

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 case. An unlimited number of load cases can be defined in SAP2000. Typically, separate load cases would be defined for dead load, live load, wind load, snow load, thermal loads, 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 case. SAP2000 uses the type of load to create design load combinations automatically.

Load cases 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 case (see the topics in the Menus/Assign - Joint Loads, Frame/Cable/Tendon Loads, Area Loads, Solid Loads and Link/Support 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 case. Each object can be subjected to multiple load cases.

Define Load Cases

The Define Loads form also display when the Analysis Cases item is selected and the Define/Show Loads button is clicked on the Bridge Wizard. It also displays when the + (plus symbol) is clicked on any of the forms used to assign loads; see the topics in the Assign - Area Loads, Frame Loads, Cable Loads, Tendon Loads, Joint Loads, Link/Support Loads and Solids Loads books of this Help.

Note: To calculate any response of the structure resulting from the load cases, you must define and run analysis cases that specify how the load cases are to be applied (e.g., statically, dynamically) and how the structure is to be analyzed (e.g., linearly, nonlinearly). The same load case can be applied differently in different analysis cases.

Tip: Every time a new load case is defined, the program automatically creates a corresponding linear static analysis case of the same name that applies the load. For a new model, the program creates a default load case called DEAD that applies the self weight of the structure. The corresponding linear static analysis case is also called DEAD.

Use the buttons in the Click To area of the form to complete the actions described below:

Note: If nothing happens when the Modify Load button is clicked, the load case is an automatic load. As described in the Note below, highlight the load case and the click the Modify Lateral Load button.

If a new load is being defined with a load Type of Quake, Wave 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 wave, wind or seismic loading. Use the form to specify parameters consistent with the code selected in the Auto Lateral Load drop-down list. For example, if the load type is Quake 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 that 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 Loads form.

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.

If a new load is being defined with a load Type of Bridge Live, first click the Add New Load button, then click the Modify Bridge Live Load button to access the Multi Step Bridge Live Load Generation form. Use that form to refine the lane loading for a moving load analysis. Note that a bridge layout line, a lane(s) and a vehicle(s) must be defined before the bridge live load can be defined.

Note that when a static load case is deleted, all of the loads that have been assigned to the model as a part of that static load case are also deleted.

After a load case has been defined, assign loads to the objects as part of that load case by selecting the objects to be loaded and using the appropriate command from the Assign menu.