HOW TO RUN VISUAL ANALYSIS - TRUSSES

Main Screen

To build a new project:

The truss we will solve is the truss shown in problem 4-9 on the top of page 220, Matrix Structural Analysis by L. P. Felton. We will use P = 10 kips, L = 20 feet, A = 1 in², and E = 29,000 ksi. A sketch of this truss is shown below:

 
 Visual Analysis will now present the following screen upon which to build your model:

Since the default setting for grid spacing is 1 foot, and since our truss is too big to be seen at that scale, change the grid setting to 5 feet.

Now zoom out until you can see about 80 feet of structure. You can also do this by clicking on the "-" magnifying glass button, or hit ctrl -

Now start drawing members. Put the crosshair on the coordinate 0,0 and click the left mouse button, thereby starting the member at that point. Still holding the mouse button, drag the member into final position by moving it to coordinate 15,20. The coordinates of your mouse location are listed on the bottom right of the screen. Release the mouse button, and VA will generate member M1, starting at node N1 and ending at node N2. NOTE that the node numbering on each member is very important. When you put uniformly distributed loads on members (not permitted for trusses), it matters whether the member goes FROM node 63 TO node 64, or FROM node 64 TO node 63.

Note that the member ends "snap" to the nearest 5 foot grid mark. Also note that neither the member or the node names are shown, by default. We will correct this shortly.
 
Now draw another member from coordinates 0,0 to 0,20.

 
And then a member from 0,0 to 30,0. Note that I turned on the member names - I will show you how to do this shortly.

I now have generated three members of the truss.
 
I will now use the View / View Manager to label the members and the nodes, so they will be easier to find and work with.

 
The View Manager allows you to toggle on/off various identifiers on the structure. If you "X" in "Nodes", red dots will be shown where you have nodes, and nothing else. If you also "X" in "Names, Filter by name", VA will give you the node names. If you "X" in the "Location" he will list the coordinate location of the node. Etc. So, "Xing" in the following, produces a picture:

 
Humm. What went wrong? He didn't give me the node names. Oh yea. I forgot to hit "Apply." I will do that in a minute. Or perhaps I hit "cancel" by mistake. But now I see an even bigger problem. Member M2 isn't long enough. I accidentally made it 20 feet long, and it's supposed to be 40 feet long. So, let's just edit the Node to which this member is attached.

Well phooey. Now I'm stuck. I don't know which node to edit, since I didn't finish making him list the node names. So, let's go to the view manager and add a request for numbering the nodes using the "View Manager."
 
Now we have:

Ah yea. The name of the erroneous node is N3.
 
After clicking Model, Edit, Edit Nodes, we see the following. Click on the bad node N3 to highlight it, and then click on the "Move" tab.

Now click on the field "Move Y by:" and enter the fact that you want node 3 moved 20 feet up from its present position:

And you will see the node moved. Note that an easier way to reposition a node is to simply double click it, and enter the corrected coordinates.
 
Now add a member from N3 to N4:

Whoops! What's that all about? Well, he is telling you that if you add a member from N3 to N4, it will cross member M1, and he wants to know if you are actually trying to add TWO new members (one from N3 to N2 and another member from N2 to N4) or are you really trying to add a single unconnected member from N3 to N4? This gives you the ability to add several members at once, or to have unconnected members crossing each other.
 
Now add all the remaining members in the truss. Then, to see your truss:

 
Now that's a nasty mess. Too much information to tell what's going on. So back to the view manager:

 
Using the view manager let's take off some of the information which we don't need. Note that the extra information still appears, since I haven't clicked on "Apply."

 
Now note that after clicking on "Apply" the picture changes. You can tell I clicked "Apply" since that button is now grayed out. This allows you to try out various settings without leaving the View Manager.

Hummm. Why is he not showing me the supports? Don't I have them checked in the "Nodes" "Supports" box? (See next figure.)

Yes, but you have not yet told him how the truss is to be supported. Let's do that now.
 
First we click on node N1, highlighting it. Then we go to Model/Edit Selected Node. See how the node has a highlighting box around it?

 

 
Now we highlight node N4 and make it a roller:

Note that he shows reaction forces at the supports. Check these forces carefully to insure that you have placed the proper support at the joint.
 
Now we must add loads to the structure.

In the real world, the loads are made up of combinations of some dead load, some live load, some wind load, etc. These load combinations are specified by controlling codes such as the Standard Building Code, ACI Building Code, International Building Code, ASCE Standard Building Code, etc. In general, the applicable code for your structure will ask that you separate the different types of loads into Service Loads like "Dead", "Live", "Wind", "Earthquake", "Snow", etc. However, the code doesn't believe your structure is ever likely to experience a full blown max Dead + max Live + max Wind + max Earthquake + ... all at the same time. So they ask you to apply the loads in several different "Combinations" of likely loadings. For example, they may ask you to study the structure under the loading:

1.2 Dead + 1.6 Live + 0.5 Snow,

or:

1.2 Dead + 1.3 Wind + 0.5 Live + 0.5 Snow,

stuff like that. Therefore, you usually tell your computer program what kind of Service Load you are discussing so he can "factor" it appropriately and code-check the structure. However, that's pretty well beyond what we will be doing. Still, you must tell VA something. He won't let you proceed if he doesn't think he understande what kind of load you are talking about, even though for us it doesn't matter.

We will just pretend that all of our loads are dead loads. Also, to make our life easy we will omit the self-weight of all members. First, we will create a New Service Load Case. Note that in the real world you will have a separate "Service Case" for each load required by the code.

 
Just leave his default name "Service Case 1." Then go to "Source" of the loads:

 
We will pretend that all loads are dead loads, not that it would matter. Also, make sure the "structure weight" is not added:

 
Although it was already on "Dead Loads" I clicked on the drop down menu just to show what other load types are available.

 
Next we specify the nodal loads to be applied as "new" loads. Note that on a truss you cannot put distributed member loads, so the drop down menu only says "Nodal Loads." Unfortunately, before I opened this menu I see that node N4 was already highlighted (for some reason - I don't remember), so he will probably think I want to load node N4, which I don't. We'll see.

Add a new load:

 
Of magnitude 10k in the X direction:

 
Now go to "Placement." Yep, node N4 says it is "pre-selected", and it is highlighted. However, that's not a problem. When we click on the proper node, that one will un-highlight. Now since we want to add a 10 kip load to several nodes, first click on node N5. The highlighting on node N4 disappears, and node N5 is highlighted. Then press the CTRL key and "add" more joints to your selection by clicking on (and highlighting) nodes N6, N7, and N8.

 
Now click OK.

 
He goes back to the load summary screen:

 
To get answers for forces in the members, and nodal displacements:

 
 
 

 

 
 He will now analyze your structure with the selected load case applied:

 
Now, I like the answers, but I can't see the bottom of the truss. So:

 
And now cover an area over the truss, and extending down even lower than the bottom of the screen to get the reactions in the coverage (see magnifying glass at bottom of screen):

 

 
 Now, get a quick report of the answers:

Now "close" the quick report with the "x" in the upper right hand corner:

Now let's get some individual member answers. Double click on member M15 with the mouse, and highlight it, thereby giving you a report on that member:

 

 
Now when you are happy with your analysis you save your project:

 
Now I am going to add a new node and 2 new members:

 

 
Whoops. Looking at that node, you will note it has a reaction on it. So, the default must have been "add a roller support whenever you add a new node." I could undo that and go track down where that default was and change it to "not a support" but I think I will instead just edit that new node and take the support off of it.

 
There is also a "button" for "Edit Selected" (node or member). Click that. Or you can simply edit the node by double-clicking it.

 
Now change the support to "Free from external restraints". Note that you can add any of the restraints you like from the table shown:

 
  Now create the new members. I am going to remove the members from the picture so it won't be so cluttered and I can read the node numbers. I will do that by going to the View Manager and de-selecting the "Members" box:

To see the effect immediately, hit the "Apply" button. You can then make other changes in the view and see if you like what you have selected.

 
 Hit OK.

To add the new member:

 

 

 
Add the next member:

What the? Oh. This new member must have kept the same node numbering as the previous member.
 
Go back and change the nodes on the last member:

 
Now let's see what the new members look like.

Good enough. Now let's put a vertical 60 kip gravity concentrated load on the new node.

 
 
 
 
 
Final Structure looks:

Now run it as before.

What a great program!

NOTE that if you don't get the member internal forces, or something else like that it is because you did not include them in your report request. Hit "Options", "Edit Report Style", highlight the "Basic Model Report Style" and click on the "Edit Style" button, then highlight "Member Extreme Results" or whatever you are missing, and add it to the right hand window for "Reported Items."