CVEN 5835 Cable Supported Structures 1

Transcription

1 CVEN 5835 Cable Supported Structures 1 Cable Structures An app for force-displacement analysis is used to set up a model of a single strand carrying three point loads. We worked with this structure in Exorcise 11. By hand + Excel we have set up a five-node, four-segment model of the structure. This document shows how to input this model to an app for cable structures. Get the App CableStructure is available as an exe file that runs on PCs with Windows operating systems. The app is posted at the course website at March 1. Right-click on the link and select Save target as... to download the app. Next, go to the folder you on selected for the download and double-click on CableStructure.exe You ll see this (Figure 1) Figure 1 Notice the Input box at the upper right. To create a model, we must input nodes and components and loads. We start with nodes. The Input box has three radio buttons. One of these is for Nodes. Click Nodes. You ll see this (Figure 2).

2 CVEN 5835 Cable Supported Structures 2 Figure 2 A grid appears at the left. There are headings to the grid. We want a copy of these headings so that we can assemble quantities in the right order. Click at the upper left corner of the grid. The grid (such as it is) will be highlighted. Now press Ctrl-c to get a copy. You can paste this into Excel. We have coordinates of nodes that we computed using joint equilibrium equations. These coordinates are balanced for external loads and for self-weight of strand. For this app, Z is the gravity axis. After the coordinates are complete in Excel, highlight the region (don t include the column headings). Go back to the app, hit the Paste button at the upper right. You ll see this (Figure 3). Figure 3

3 CVEN 5835 Cable Supported Structures 3 Now click Process. This button is at the left above the grid for coordinates of nodes. Process reads the grid and puts the information into model data. If you don't click Process, no input is retained. This is not saved to disk yet. Let's do that now (and frequently as we proceed). To Save a model at any time, click the Save button near the upper left of the window. You ll see this (Figure 4). Figure 4 Type in a File name for your model. Information is saved in an extensible markup language for cable-supported structures. You don t get a choice of format. If you are curious, the file created here can be viewed in Notepad. Do not edit this file. The format is a mix of tags and delimiters. It is easily rendered illegible. Next we input components. For our structure, there is a single component; a single strand. To begin, at the Input box at the upper right, click Component. You ll see this (Figure 5).

4 CVEN 5835 Cable Supported Structures 4 Figure 5 In earlier work, we selected 1-11/16 Bethlehem strand with Class A coating. Strands are listed in the box at the lower right. Find Strand 1-11/16 and click on that label. You ll see this (Figure 6). Figure 6 The next input is at the left, in the box labeled H. There is one input for value of force in kips, and one selection for global direction of input. For our structure and reference state of external loads, H equals kips. H is in the XY plane. You ll see this (Figure 7).

5 CVEN 5835 Cable Supported Structures 5 Figure 7 Give this component a name. Type in the box below the input for H (Figure 8). Figure 8 Now we identify the nodes that are connected by this component. Nodes are listed in the box at the right, above the list of strands. One-by-one, click a node in the list then click the next blank row in the grid labeled Node List. Do this for the five nodes connected by this strand. When complete, you ll see this (Figure 5).

6 CVEN 5835 Cable Supported Structures 6 Figure 9 Click Process to put this information into the model. Now Save the file again. Now click Load in the Input box. You ll see this (). Figure 10 Loads are input in sets. A load set for the self weight of the strand is created automatically. We create other sets. We have three node loads (20k, 30k, 40k) on our structure. We create a load set for them. Type a name for the load set in the box just below the Process button. Then type in the external loads (but not the self weight of the strand). You ll see this (Figure 11).

7 CVEN 5835 Cable Supported Structures 7 Figure 11 Click Process. This model is complete now. Save the file again. The next tab in the app is View. Click it. You ll see four views of the structure. There is one projection along each global coordinate axis and one ortho view. Figure 12 The next tab in the app is Ref State. We use this to check on our reference state; to see if we have a structure at equilibrium (Figure 13).

8 CVEN 5835 Cable Supported Structures 8 Figure 13 On this tab, we can include/exclude load sets in a check of the reference state. We can update H for the strand, if we are modifying the reference state. The grid at the left shows results from computation of joint equilibrium relations. Our reference state is self weight plus external loads. Be sure both load sets are checked, then press Compute at the upper right. You ll see this (Figure 14). Figure 14 The app reports net loads at nodes. We see (negative) reaction values at nodes a and e. We see no net forces, that is we see equilibrium, at nodes b, c and d. This is a valid reference state. Now go to analysis tab (Figure 15).

9 CVEN 5835 Cable Supported Structures 9 Figure 15 Load sets are listed in the grid on the left. Each load set gets a load factor. Initial values of factors are zero. If we put a 1 in as a load factor, we get 1x that load set. If we enter 2.2 as a load factor we get 2.2x that load set. For now, enter 1 for self weight and for external loads. Then press Go. A load vector is formed, the system stiffness matrix is formed and a single iteration is made. Results appear in the grid on the right. The Net forces at nodes shows whether a structure is at equilibrium. This structure is. Let's try a transient load analysis. At the Input tab we create a load set called Transient. It is a downward 4 kip load at joint d. The load input looks like this (Figure 16). Figure 16

10 CVEN 5835 Cable Supported Structures 10 At the analysis tab, enter a 1 load factor for the transient load and press Go. After a single iteration the set of net forces show that the structure is not at equilibrium (Figure 17). Figure 17 Press Iterate at the upper right. And keep pressing iterate. After several iterations, you ll see zero net forces at nodes b, c and d (Figure 18). The structure is at equilibrium in a new position and with new member forces. You can see deflections and member forces in the grid at the left. Use the radio buttons to select the results to view. Figure 18 Save your model file before you exit the app.