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VisualAnalysis Advanced Tutorials: Cables

Requires Advanced leve. This example is a non-typical cable problem that demonstrates the use of cable elements in an uncommon application. In this example we will use cables as the braces in a three story braced frame structure.

Project Description:


Braced Frame Geometry

Modeling the Structure

Create two Plane Frames 40 ft. apart as shown above using the Generate Standard found on the Create Tab. Use the X - Braced Frame and delete the one of the cross members at each story.

Specify W18x35's for the columns and beams, and specify "Simple Connect" for the Connection Type (Modify tab). Also specify pin supports at the column bases.

Draw the braces in the left frame as "tension only" Nominal Rods with diameters of 1.5". (Modify Tab | Options | Action)

Draw the braces in the frame on the right using cables. Go to Model | Drawing Mode | Cables and specify a Circular Shape with a 1.5" diameter. Specify a catenary length of 230.5 inches, the same length as the actual distance between the nodes.

Set the brace material in both frames as ASTM 992 Grade 50 steel.

We will not be doing any design and therefore are not really concerned with the yield strength.


Loading the Structure

As shown on the previous page, add a lateral load of 10 kips to each level of the structure. For simplicity, place these in the Dead loads Service Case (D). Both frames should behave nearly the same. Without any slack in the cables, they should perform just like the tension only members. Performing an analysis gives forces in the members that are nearly identical. Note that the deflections are very similar as well. Picture below shows deflections under a 1.2(D+F+T)+1.6L load case.

X-Displacement Values in the Two Frames

Now select the three cables in the right frame and specify the catenary length as 230.4 inches and re-run the analysis. Pretensioning these cables 0.1 inches reduces the deflection at the top of the structure from 0.65" to 0.26". As would be expected, we still have a net tension in the cables and have reduced the deflection. Nodal Displacements can be found easily by holding the cursor over the node ("Flyover Data" will be visible at the bottom of the Project Manager) or by Double-Clicking the node in the Result.

Now, select the three cables again except this time specify the length as 231 inches. We are essentially introducing a half inch of slack in the cable. After re-running the analysis, note that the deflection at the top of our structure has now increased to 2.57 inches. This could potentially be used to model a number of situations where you may not actually have a cable in real life. For example, you might use a cable with a little bit of slack to model a web member of a truss that will have some slip or "play" at one of its connections. You could use a cable to model that slip and see what the deflection may actually be once the truss is loaded and the web member is actually fully "engaged".

Results and Reporting

In our example we have been primarily making use of graphic results and the "Flyover Data". There are some standard reports available for cables. If you double-click on a cable in a result view, you will get the Cable Results report item. Likewise with the "Report Selected Cable" item from the Context Menu (Right Click). Alternatively, you can choose Report | Report Wizard, and then Create a custom made report from scratch. Include the Cable Results and Cable Elements reports and continue with the rest of the wizard to view the report.