Working with Load Combinations

Dr. Frame provides various facilities for managing load sets and combinations. In the presentation to follow, load set refers to a set of loads applied to a structure with a typical designation such as live load, dead load, wind load, and so on, while load combination refers to a combination of load sets, e.g., dead load + live load, dead + live + wind, etc... In general, the load sets in a load combination are factored.

The most direct interface to the load combination settings is via the load combination tool bar located at the top of each window:

The leftmost pop-down menu is used to select the desired load combination:

The second pop-down menu selects the active load set, i.e., the particular load set that will be modified by user actions:

The "Load Factor On" checkbox is used to toggle on and off the load factor applied to the current active load set. This makes it convenient to enter and manipulate loads without load factors.

Pressing the "Load Mgr..." button causes the load combination manager dialog to pop up. As shown below, this dialog provides controls for customizing load sets and load factors for any particular load combination:

Load combinations and load sets can be created, deleted, and modified using the corresponding buttons. Load factors can be set using the Load Factor box. The list box shows the load sets with non-zero load factors for the selected Load Combination.To remove a load set from a given load combination, set the load factor to zero.

	This dialog can be used also to apply proportionally varying loads by modifiying the desired load factor(s) and then hitting the Preview button.
	The "Scratch" load set and load combination can be renamed, but not deleted.

Menu Interface

An alternative means for accessing the load combination functionality is via the Loads menu:

In analogy with the toolbar, one can choose the currently active load set:

and the current load combination:

One can also control the load factor toggle with the Factor Current Load Set command. The final command in the menu brings up the Load Combination Manager dialog shown earlier.

The Load Factor toggle only effects the currently edited load set. It does not remove the load factor from all visible load sets.

 

Example

Consider the set-up for a simple 1-bay frame that carries a dead load of 0.5 k.ft, a live load of 2.0 k/ft, and a transverse wind load of 10 kips.

1. Use the Auto Frame command to create the structure (member properties could be set now or later):
2. Choose D + W + L + [R or S] and turn off the load factor on the Dead Load, which will be the first load edited.
3. Apply the Dead Load.
4. Switch the edited load set to the Live Load. (Note that dead load previously applied will become factored)
5. Apply the Live Load. (Note that the distributed load scale has been adjusted to avoid overlapping labels):
6. Switch the edited load set to the Wind Load.
7. Apply the Wind Load:
8. Turn on the Load Factor. We now have the desired 1.2 D + 1.3 W + 0.5 L load combination, and we are displaying strong axis moments:
9. Additional load combinations can be considered. Shown below is the 1.2 D + 1.6 L case (again, the load scale has been adjusted):
10. Different load factors can be applied. Here the Load Combo Manager dialog has been used to create a 1.4 D + 1.3 L combination.

To avoid having to deal with overlapping loads during the creation process, use special load combinations with single loads and load factors = 1.0. By default, the User/Custom settings are pre-defined in this manner.

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