Dr. Beam Pro Quick Tour

The following simple exercise provides something more interesting to do with Dr. Beam Pro than just random clicking and dragging. It gives a quick overview of some of Dr. Beam Pro's primary functionality. Prior to doing this exercise, choose either Clear from the the Edit menu or New from the File menu to begin with a clean slate. If you get stuck at any point, there's online help available under the Help menu.

Using a Stiff Story to Control Wind-Induced Motion

An important design consideration for tall structures is to control wind-induced motion. One approach to addressing this issue is to use a relatively stiff story as illustrated below:

A cantilever beam can be used as a simple approximate model of a tall building, and so we can use Dr. Beam Pro to consider the behavior of such a system by visualizing the tall building turned on its side, and using a rotational spring to represent the stiff story:

This simple model provides a reasonable starting point to answer the following basic design questions.

What is the optimum location of the stiff story to minimize the displacement at the top of the building?

Go ahead and guess to see how good your intuition is: The stiff story should be located: (a) at the top of the structure; (b) at the mid-height of the structure; (c) below mid-height; or (d) between the top and mid-height.

Now let's use Dr. Beam Pro to answer this question:

1. Set up cantilever boundary conditions (Choose the fixed support tool and click on the left end of the beam to replace the simple support. Choose the Select tool and click on the right simple support, and hit the delete or backspace key to remove the right support)
2. Apply a uniform distributed load (Choose the distributed load tool, click on the beam. Double-click the load to enter more precise location and magnitude data if you so desire.
3. Adjust the displacement plot scale using the plot scale control. (Click on the small arrows to the right of the beam to adjust the plot. Holding the shift key down while clicking will auto-scale the plot)
4. Add a rotational spring to the beam to represent the stiff story (Choose the rotational spring tool, and click on the beam.)
5. Adjust the location of the spring to minimize the end displacement. To do this, choose the label tool and create a label the right end of the beam. Using the select tool, Click and drag the spring back and forth along the beam, using the numerical displacement feedback from the label to home in on an optimum spring location. Note the relative sensitivity of the displacement to the spring location.

Does the location change if a more realistic wind loading is used?

The uniform load applied above is a very rough approximation to a wind load. A somewhat better approximation is to use a ramp load as shown below:

Here's how to use Dr. Beam Pro to see if this changes the optimum stiff story location:

1. Choose the distributed load tool, and move the cursor to the left end of the applied load.
2. When the cursor changes to a 4-way or 2-way arrow, click and drag vertically to adjust the magnitude of this end of the distributed load to zero. Again, you can also double-click to enter precise data from the load dialog.
3. Use the select tool or the rotational spring tool to drag the spring horizontally to see if the optimum location has changed.

How does the relative stiffness of the stiff story influence things?

Double-click the rotational spring with the rotational spring tool to adjust the spring stiffness. One can either click and drag in the spring stiffness plot to update the stiffness, or enter values directly as desired. Adjust the spring stiffness from zero to near-infinity, and investigate both extremes. Does the stiffness of the story affect the optimum location of the story? How much does an infinitely stiff story reduce the maximum displacement in the structure?

Hope you learned something new and learned to use Dr. Beam Pro as well. Any ideas for other appropriate demonstration problems would be gladly recieved. Email us with your comments or ideas.