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VisualAnalysis Advanced Tutorials: Rigid Diaphragms

Project Description:

This tutorial applies to Visual Analysis Advanced with Design only. This example will use three multi-story structures to compare the rigid diaphragm feature with modeling plates. If you intend to design the diaphragms using VisualAnalysis along with the structural frames, you should model the diaphragm using plate elements rather than using the rigid diaphragm feature. The rigid diaphragm feature is meant to force rigid diaphragm behavior on a structure without having to model all the plates.

Modeling the Structure

In this example we will use the multi-story structure shown below. Create a new Space Frame project and quickly draw the model by going to the "Create" tab and choosing the Moment Frame (Unbraced) Generate Standard.

Specify the story height as 12 ft, X bay spacing as 20 ft, and Y frame spacing as 25 ft and click "Next".

Specify 3 Bays (M) as, 4 Stories (N), and 3 Perpendicular bays (O).

After clicking Finish, the structure should look something like that shown below. Note you may have to rotate the model some to get more of a "perspective" view.

Multi-Story Structure Geometry

Specify the columns and beams as W12x30 members.

Create pinned supports at the base of each column. Try selecting the nodes with the help of the Toggle Members button on the Tool Bar. Or by using the Cut Tab in the Project Manager to show only the bottom nodes.

When you are finished creating the model, put a 15 kip seismic load in the negative Z-direction (E-Z) acting on the corner at the 24 ft level. This is shown above as well. Lastly, make 2 copies of the structure in the X direction spaced at 125 ft. When you are finished, you should have three identical, separate structures. Next, select either the ASCE 7-05 LRFD or ASD load combination in the Load Case Manager.

Creating the Diaphragms

We will now use different modeling techniques for diaphragms as a basis for comparison. The first structure will stay as it is to model a basic moment frame with a flexible diaphragm. The second structure will have a rigid diaphragm modeled with plate elements consisting of a 4" thick concrete floor slab at the 24ft level. For the third structure, we will utilize the rigid diaphragm feature.

Zoom in on the middle structure and from the Cut Views Tab select Auto-Cut Planes, Y Plane, and Yplane = 23.8 ft. You should now be looking at the second floor level. Set the Drawing Mode to Draw Plates (there is a button for this on the Tool Bar) and draw a plate in each of the bays. Edit the plates so that they are concrete with f'c = 4000 psi and have a thickness equal to 4 inches. (Select the plate and the Material setting will be located on the Modify tab of the Project Manager.) The plan view should look something like the picture on the right when you are finished.

Now we want to switch our focus to the third structure on the right. In the same cut plane, Toggle the Members off and select the nodes with shift + click + drag (Do not hold the Shift key and click on one of the nodes as this selects all the nodes at this level in all three of our individual structures). Deselect the load using ctrl + click if it was selected. Once the nodes for the third structure have been selected, go to Model | Create Rigid Diaphragm. Since we pre-selected our nodes, the plane will be automatically selected and the default type will be an "Advanced diaphragm". It will also report the number of nodes included in the diaphragm. Click OK.

Rigid Diaphragm Dialog

When you are done the model should look something like the following:

Plan View of the second floor of the third structure; the squares signify the rigid diaphragm

Analyzing the Structures

Now that we have defined our rigid diaphragms, we are almost ready to analyze. Set the Static Method under the Analysis heading on the Modify tab of the Project Manager to "First Order" analysis. Now, check the model for errors (Analyze | Check Model for Errors), and analyze the models (Analyze | Analyze Now). Once the analysis finishes, change the view to a Top (ZX) view and set the Cut Plane to y = 23.8 ft. Your result view should now look like the following under a load case including the seismic load.

Result View; Left | no diaphragm, Middle | plate rigid diaphragm, Right | rigid diaphragm feature.

It is clear that the structure on the left has no rigid diaphragm because there are evident bending type displacements in the plane of the floor. The other two floor levels look very similar and we can see the characteristic translation and rotation of a rigid diaphragm. Now compare the displacements for the top right node of each structure. The values for middle diaphragm are dx = -0.44, dy = -0.01, and dz = -2.60 inches. The values for the right diaphragm are dx = -0.46, dy = -0.00, and dz = -2.63 inches. As we can see, the values agree very well. Specifying a rigid diaphragm with the diaphragm feature was quicker and easier than drawing in the plate elements; however, this does not allow you to design the diaphragm itself.

As mentioned before, different modeling techniques are available with Visual Analysis. Choosing which method is the best depends on what you, the engineer, need to design. The rigid diaphragm feature only imposes the rigid diaphragm behavior on the rest of the structure. If you need to design the diaphragm as well, the plates will have to be included. Also note that contrary to what we did here, for accurate plate results both the plates and the beams would need to be "meshed". For information on meshed plates, consult the User's Guide.

In this tutorial we pre-selected a group of nodes and defined an "Advanced diaphragm" when we created a rigid diaphragm at the second story level. You may have noticed that a "Simple diaphragm" is also available. The simple diaphragm only requires a plane and an elevation as input parameters. Everything in the plane at that elevation then is forced to act as one large diaphragm. Now on your own, save this project with another name such as "Simple Diaphragm". Delete the structure in the middle and on the right. If you do not do this, all three structures will be linked by a single diaphragm in the step that follows. Create a simple diaphragm on the same floor and analyze the model. Check the displacement of the same top right node that you did before. How do these compare to the displacements determined above? For more information on rigid diaphragms, go to Help | Contents to view the VisualAnalysis User's Guide.

Congratulations, you have now completed this tutorial!