Frequently Asked Questions

Q:

Can Joints be created explicitly by the user in Dr.Frame3D?

A:

No. Independently existing joints do not make sense in the Dr.Frame3D environment. To clarify this it is useful to differentiate between "nodes", which exist transparently along the span of members, and "joints", which define a member's ends. When the user applies a Point Load, for example, along the member's span, Dr.Frame3D creates an internal node which is not visible to the user.Also, when a support is created in space a joint is automatically created to define the support's location.

Q:

What is the limit of a model's size ?

A:

The size is limited by your computer's available memory. For larger models (where the definition of large depends on your computer's speed and memory) updating may become slow enough that you may want to turn off the 'Realtime Solution' option in the Modeling menu so that model updating is only done after component manipulations.

Q:

If I connect a frame member to a truss member, what kind of connection do I get?

A:

Because the end of a truss member is free to rotate, a pin connection will be formed between the truss and frame members. However, if another frame member is connected to the same joint, a rigid (fixed) joint will exist between the two frame members and drawn as a square instead of a round pin. The truss member will still be free to rotate at this point and this pinned connection is shown by a small dot on rigid (drawn as a square) connection.

Q:

How can I tell what the constraint conditions at the end of a member are?

A:

The end of a truss member is a free to rotate and thus has a pinned connected. This is drawn in one of two ways: either as a spheircal pin or as a clip-angle-like representation framing into a fixed (cube) connection.The ends of frame members typically have fixed connections which are drawn as square joints. See Working with Joints for more information.

Q:

What if I want to create a member with a pin connection at one end and fixed connection at the other?

A:

This can be done two ways:

• Use the Member Info Table for an existing member (selecting the member sets the table info to the member in question) and set the End Conditions checkboxes as desired.
• Hold the control key while clicking with the appropriate member tool near the end to be changed. For example, control-clicking near an end of a rigid ended member with the Pinned Member Tool will make that end become pinned. (Clicking near the middle of a member will change its fixity to match that of the tool.)

Q:

Sometimes the shear and moment diagrams aren't drawn even when I have them turned on. Why?

A:

When an aux diagram is on, a transparent copy of the structure will always be drawn. However, the shear and moment envelopes will only be plotted if there are no instabilities in the structure. If part or all of the structure is unstable, Dr. Statics will not be able to analyze its internal forces and support reactions. For this reason, the axial force values and coloring and support reactions will also be disabled for structures with instabilities.

Q:

How can I change the type of a support without having to delete it and then create a new one?

A:

First, select the tool for the new support. Then click on the support that you want to replace.

Q:

How can I modify loads without having to drag them and changing their direction?

A:

There are two ways this can be done:

1. Select the load vector and use the up and down arrow keys. Note that only the transverse component of a member load will be affected.
2. select the load and edit the load inspector table to make the desired changes.

Q:

The hinge tool seems to always make my structures unstable. Why?

A:

Every time a hinge is added to a structure, another equation of equilibrium can be formulated. In some cases, this may make the structure determinate if the number of equilibrium equations is equal to the number of unknown forces. However, if the number of equations becomes greater than the numbers of unknowns, the structure will become under-defined and globally unstable. Hinges can also create local instabilities in structures. An example of this is adding a hinge to a truss member. Note also in 3D, there are issues with torsional rotations that do not arise in 2D.

Q:

When I apply a load to a member and drag, it doesn't always do what I expect in terms of staying perpendicular to the member in question. What's going on?

A:

There is a consistent scheme for handling direct manipulations in Dr. Frame3D. Dragging actions are assumed to occur in the plane whose normal is closest to the viewing normal, unless the alt/option key is down, in which case horizontal dragging is interpreted as occuring out of the plane. If a member load is not dragging the way you expect, try swinging the view around so you are looking more directly at the side of the member in question.

Q:

Can I create my labels at any point on the shear and moment diagrams?

A:

Yes. In addition to the labels created automatically using the commands in the Options>Automatic Labels menu, custom labels can be created using the Label Tool. To create a shear or moment label, click with the label tool on the aux view of the structure. Drag to position it or use the inspector table to locate it precisely. New labels are automatically drawn a certain distance from the structure. To modify their position relative to where they are pointing hold the control key while dragging.

Q:

What if I want to create a point that isn't exactly on the current grid?

A:

There are two ways to do this. The first way is to refine the grid so that it contains a specific point. This can be done by using the Refine/Coarsen commands in the Options>Grid menu or by bringing up the Grid dialog in the same menu (or by using the grid settings in the overall inspector table). The second way is to turn off the grid snap in the Options:Grid menu or by holding the control key down, allowing you to place a support or member endpoint at any point.

Q:

Sometimes when I edit loads the mouse seems to stop working. What causes this?

A:

During editing, the effect of mouse movement does not correspond directly to the modifications being made to a load. When manipulating member loads Dr.Frame3D decomposes mouse movements into axial components and transverse components. Only one type of action is applied for each mouse increment so that for a slightly skewed mouse drag the effect on the load may be only to its direction or magnitude. The resulting difference between mouse location and load location can be confusing and this is the reason we don't draw the mouse cursor during dragging. One result, however, is that the mouse can reach the edge of the screen (a limiting boundary to mouse dragging) without your realizing it. In this case you should release the mouse to stop dragging and reselect the load before continuing.

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