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VisualAnalysis Tutorials: Steel Floor Vibration Modes

Project Description:

In this example we will analyze a steel floor system to see if vibrations are going to be a problem. It is necessary to know the different fundamental frequencies of vibration. The floor system carries a mechanical unit, modeled as point loads.

Setting Up the Project

When you start up VisualAnalysis, you are presented with the VisualAnalysis Startup Wizard. The wizard presents you with a number of options. We want to start fresh with a new project, so choose the "Start a New project" radio button and click Start!. Select the structure type as Space Frame (Modify Tab), set the unit style to Kips & Feet (Drop Menu above Model View), and change the project Title to "Steel Floor System" (Modify Tab).

Note: The floor itself all lies within a single plane however loading and displacements occur out of plane. To do this we must use the three dimensional Space Frame model type.

Modeling the Structure

Under the "Create" Tab select Girders(XZ) from the Floors & Beams area.

Leave the origin point at the default location (0,0,0) and select Y-axis for rotate generation models about. Choose Next. Specify the X Bay Length as 30 ft, the Z Bay Length 5 ft, and leave the Elevation Height equal to 0 ft. Choose Next to move to the next dialog. Enter 1 for Bays, M, 5 for Perpendicular Bays, O, and leave the Stories, L equal to zero. Click Finish.

On the Filter Tab of the Project Manager, type BmZ in the Name Filter field under Members. You should now only see the end beams. Holding the Shift key, click one of the beams. All of the beams should now be selected. Specify them as W21x68. Following the same process, filter the BmX members and specify them as W16x36. With the girders still selected, use the split member command to split each girder into ten equal parts. Lastly, make the four corner nodes pin supported.

Loading the Structure

Under the dead load service case apply 5 kip nodal loads at the locations shown to simulate the dead load of mechanical equipment. It may be necessary to turn off the "High Quality Drawing" under the Filter tab.

We also need to apply extra nodal mass to all of the nodes to account for a 6 inch concrete slab.

In the calculation above, the unit weight of concrete for a 6 inch slab is 75 psf and the tributary widths for each node are 3 and 5ft. For this example we will assume that the floor section we modeled is part of a larger floor system. If the floor section ended the tributary area would be smaller for the perimeter nodes. This nodal mass can be applied by selecting all of the nodes and specifying the translational mass on the Modify tab of the Project Manager. In the analysis of this floor system, the self weight of the structure and the 5k point loads are internally converted to mass for the analysis.

Analysis and Results

Now that we have our model created and loaded, we should check it for errors before setting the analysis options and actually analyzing the structure. Select Analyze | Check Model for Errors to make sure there are no problems. Before analyzing, set the advanced analysis options as shown on the right. We must include mass from the dead load case. The dynamic response analysis is highlighted but will not be performed since we have not created any response cases. With the options set, select Analyze | Analyze Now or click the lightning bolt button on the toolbar.