Designs

[Bubba1960]

Please.... Sasha..... Do I have to tell you again? This has already been answred. CHECK BEFORE YOU POST!

[SLM]

What are going to use for the modulus of elasticity for your wood? I ask because everything this program calculates requires that you use the actual modulus of elasticity of your wood. The modulus of elasticity of wood, and especially balsa, varies hugely, and so will the error in the calculations in this software. I suppose you could use the program to give you a relative efficiency of one design vs. another, just keep in mind that the size of a structural member does not vary linearly with modulus of elasticity.

You are correct. There is a huge variation in the modulus of elasticity of balsa wood. I think it ranges between 1000 to 6000 Mega Pascal. I would use an average value (say 3500 MPa) for analysis purposes.

In general, the analysis of structures involves calculating member forces and nodal displacements. If you are only interested in determining axial (compression or tension) forces in the tower, them the actual value of modulus of elasticity is not that important. If, however, you want to accurately predict how much the tower is going to displace, then you need to know the modulus of elasticity of the material quite accurately. This property measure the ability of the material to deform elastically when it is subjected to loads. The more elastic the material, the more the structure is going to displace. So, to know how much your tower is going to displace, you need to know the material's modulus of elasticity.

Example: I analyzed the tower shown below using two different moduli of elasticity. The results for each analysis is shown below. Note how the tower with a higher modulus of elasticity deforms less than the one with a lower modulus of elasticity. However, no change occurs in member forces.



For this event, I would use this, or any other, structural analysis software for:

(1) deciding if the geometry of a tower is well-formed (if it is geometrically stable),
(2) comparing alternative designs (especially with regard to the deformation of the tower), and
(3) determining an "optimum" size for each truss member.

[SLM]

That prgram is really complicated.... I cant figure out how to use it.

Let me know how you find it complicated. I might be able to help.

[Bubba1960]

I dont know what anything on it mean and how to edit the tower...

[SLM]

I dont know what anything on it mean and how to edit the tower...

Okay. I’ll explain how to use the online software by means of a simple example. Suppose we have a 4-member space truss supporting a load of 100 lbs, as shown below.



Each truss member is attached to the wall using a pin connection. This means that the wall-end of each member cannot move. The members are connected together and to the load at the other end.

The first step for analyzing the truss (by that I mean calculating the force in each member and determining the displacement of the truss) is to model its geometry.

Step 1: Modeling the geometry of the truss

This step involves identifying the location of each truss node and member. To do this, we first need to establish a coordinate system. Note that I’ve already specified my coordinate system in the figure above. The origin is at the wall at the left end of the lower middle member where the three axes labeled x, y and z meet. The choice of the origin is completely arbitrary. You can pick any point as the origin.

Now, I am going to number the nodes and members of the truss, as shown below.



The nodes are numbered in red and the members are numbered in blue. Note the number of nodes and members; the truss has 5 nodes and 4 members. In what order you number the nodes, or members, does not matter. As long as the nodes are numbered 1 through 5 and the members are numbered 1 through 4, the model would work.

Next, write down the x, y and z coordinates for each of the node. Here, we have:

Node 1: x = 0, y = 0, z = 3 Or, (0,0,3)
Node 2: (0,0,0)
Node 3: (0,3,0)
Node 4: (0,0,-3)
Node 5: (6,0,0)

Then, write down the node numbers at the ends of each member.

Member 1: (1,5)
Member 2: (2,5)
Member 3: (4,5)
Member 4: (3,5)

It does not matter which node number you specify first. For example, for Member 1, you can write either (1,5) or (5,1).

Finally, access the online software, click on the “Geometry” link, and enter the above information in the data fields provided on the page.

Here is the first part of the web page:



Just click on a data field and overwrite the default data with your data. After I enter the coordinates of my nodes, the data fields should look like this:



Do the same as above with the second set of data fields at the bottom of the web page. Overwrite the values with your own data for the truss members. After you are done, this set of data fields should look like this:



That is it for Step 1. It is best that you save your data for later use. For this, you need to have a login ID. You should be able to get one if you submit the login ID request form.

See if you can get Step 1 working for you. When you are done with this step, let me know and I’ll post the next step.

Good luck!

[Bubba1960]

Thanks! Very helpful!

[reachgeek]

the weight limit isnt necisarily you do need enough supports to hold it....... i suggest if u were on last year slighty altering your bridge for the task at hand and to stand vertically, if the bridge did good that is..... I believe that should make the strongest 1...... without worrying about specific details that the rules dont cover....

[phillies413]

how do you connect the top part of the tower to the base of the tower?

[lllazar]

Well, you glue them together. If you don't understand, plz elaborate on your question.

[phillies413]

sorry, my question was a little broad, but i think i understand now.

however, the thing i dont understand is that if an 8 cm hole needs to go through the top portion of the tower, how is the 5 cm loading block going to fit on the top of the tower?

And also, it is better to divide the tower into the top vertical portion and the bottom more angled base, or just have it as one straight angled line going to the top?