Working with Loads

Dr. Frame2D supports the following basic types of loads: concentrated forces; concentrated moments; and distributed loads. Examples of each are shown in the figure below. In addition, misfit loading can be applied to selected members via the Property Inspector as described in the section Working with Members. Managing load combinations is discussed in the section Working with Load Combinations.

In general:

• Loads are created by selecting the corresponding tool, clicking and then dragging at the desired location on a member or joint. Multiple joint/nodal point loads can be applied in one action by drag-selecting the desired nodes while holding down the option/alt key. This is discussed further in the Shear Wall documentation.
• Loads can be deleted by selecting them and pressing the delete or backspace key.
• Loads can be moved and modified by means of either mouse dragging, using the cursor (arrow) keys, or by direct entry of numerical data in the inspection pane. The Up/Down keys control the load's magnitude, while the Left/Right keys control the load's location (loads applied to members only). Hold the control key to decrease the effect of the cursor keys or hold the shift key to increase the effect. The increment of change is based on the settings in the Number Formatting dialog in the Options menu.
• Loads can be selected either by clicking on them or by using the Select Next/Previous Load menu commands (keyboard shortcuts: '>' and '<').
• As with all of Dr. Frame2D's objects, precise placement and value setting can be achieved via the inspection pane.
• To quickly change sign(s) of selected load magnitudes, type the '-' key.
• The scale used to plot loads can be adjusted using the Scaling Tools.

The point of application for each load tool is at the cross hairs of the cursor and NOT at the tip of the load cursor arrow. It can be helpful to keep an eye on the feedback pane to see whether the cursor is over a member or a joint.

Concentrated Forces and Moments

• Concentrated forces can be applied at joints or along members. Member loads are restricted to being normal to the member while joint loads can be oriented arbitrarily.
• To create an in-span load with arbitrary orientation:
  • first construct a member with an internal joint
  • then apply the load to that joint.
  • Note that such loads can be relocated by dragging the underlying joint with the Select Tool.
• Concentrated moments cannot be applied directly to a joint because of the ambiguity of applying a moment to a pinned or partially pinned joint. You can, however, apply a concentrated moment to a member and then slide it along to the member's end.
• Holding down the shift key while dragging a joint load will constrain the orientation to 45° increments.
• At the completion of each load application, the value of the applied force or moment is cached. The next time a similar force or moment is applied, the initial value will be set according to the cached value. This can make it convenient to apply several identical loads in succession.
• As the representative examples of the concentrated load inspector tables below show, load data can be entered either as direct values, or in terms of a tributary area.

Distributed Loads

• Distributed Loads are applied by clicking on the desired member (hold down the control key to force a vertical orientation on non-horizontal members, e.g., gravity loads; hold down the shift key to make the load extend across continuous members).

• To modify the extent of the load drag either of the load's end arrows along its member's direction.
• To modify the magnitude of either end of the load (i.e., to make a ramp load) drag the desired end's arrow perpendicular to the member's axis. Note that with the fixed load size option on, the end opposite that being dragged may shrink, which may seem strange at first.

• To move a distributed load or to change its overall magnitude, click on the load near its middle and drag along or perpendicular to the member's axis.
• To see a distributed load's resultant, choose Options>Force & Moment Display>Show Resultants or press the '7' key.
• As in the case of point loads, the most recent value of an applied distributed load is cached, and this value is used as the default for the next load application. This makes it so one can quickly apply floor loads to multiple floors, for example.
• For large displacement scales, distributed loads may appear to be floating in the air. This is simply an artifact of the drawing code, which does not attempt to track the curvature of a displaced shape.
• To set load parameters with higher precision, use the Distributed Load Info Inspector as shown below. Note that load data can be entered directly, or in terms of tributary widths and areal load intensities.

Load Multi-Selection Behavior

You can select multiple loads by shift-clicking with the appropriate load tool or the Select tool or area-select by click-dragging with the Select tool, or by invoking the Select Load commands in the Loads menu. The effects of acting on multiply selected loads are as follows:

• multiple joint loads: the same incremental modifications will be applied to the magnitudes and directions of all selected loads
• multiple member loads: a common tracking will be applied to all loads so that their magnitudes will be modified together and they can move simultaneously until any of them reaches a terminal joint.
• mixed joint and member loads: only load magnitudes will be modified by dragging. Each load's magnitude will be scaled by a common factor based on the dragging magnitude, i.e., one will get proportional loading for the selected loads.

Self-Weight Gravity Loads

Dr. Frame2D supports automatic generation of basic self weight loading for members and walls. The figure below shows the default start-up structure with self weight loading enabled (the distributed load drawing scale has been increased from the default setting in this figure). Self weight loads are drawn using a brown color, and unlike normal loads, they cannot be edited or manipulated except by changing the members themselves (cross section and/or material density).

The basic operations for working with self weight loads are outlined below:

• Self weight loading can be toggled on and off using the Enable Self Weight command in the Load menu.
• Self weight loading factoring can be toggled on and off using the Factor Self Weight command in the Load menu. By default, self weight will be factored using the same factor as the corresponding dead load factor for the current load combination. The self weight factor for a given load combination can be changed via the Load Combination Manager dialog.
• As mentioned above, automatically generated gravity loads cannot be selected or manipulated.
• The overall model summary in the top level inspector pane maintains a running total self-weight for the structure based on the current set of members.
• Self weight is calculated in straightforward fashion, multiplying volume times weight density.