Boomilever B/C

[iwonder]

I believe there's a picture of the testing rig in the NSO website, www.soinc.org

[chinesesushi]

Or a more direct link, http://www.soinc.org/sites/default/file ... lation.pdf.

[chinesesushi]

If my tension members are not completely in line with my compression members (for example if the base is smaller than the width of the boomilever) then do tension-compression members still help? Also, if one is using a "tube boom" design, how would one correctly line it up with the wall? It seems that it would inevitably be pointing one way or the other.

[nxtscholar]

This is my first year doing boomilever, and while i have done build events before, I've never done any involving weights (boomilever, towers, etc)

I've done some research and my question that I hope can be answered is as follows:

Would truss designs that are commonly seen in bridges i.e. Warren still have the same effect if applied to the boomilever as a compression-tension bracing? From what I've seen, bridges and cantilevers are similar, but still different so...

[Balsa Man]

If my tension members are not completely in line with my compression members (for example if the base is smaller than the width of the boomilever) then do tension-compression members still help? Also, if one is using a "tube boom" design, how would one correctly line it up with the wall? It seems that it would inevitably be pointing one way or the other.

First question- There was pretty extensive discussion of this exact question last year; I’d highly recommend you go back and read. To understand the answer, you need to understand how buckling works, and how bracing of a long, thin member under compression works. The short answer is, ‘helps’, yeah, some, but it is very inefficient- there are ways to get the buckling strength in the vertical plane that weigh less. This is true if the compression and tension members are are aligned, and even more true if they are not.
Second question- not sure what you’re asking; ‘pointing one way or the other’ – as in the vertical plane (pointing up or down from perpendicular), or in the horizontal plane (pointing one side or the other from perpendicular)? While you refer to a tubular compression member, is the question really about a single compression member- there’s nothing special about a tube (as opposed to a square) cross section, in terms of this sort of stability. If, as I’m guessing, the question is, wouldn’t a single compression member/single tension member be unstable; swing side to side? At first consideration, it would seem so, wouldn’t it? But then, factor in the fact that ‘single/single’ booms won a number of State tournaments, and placed in the top 10 at Nationals – that says there is not a side to side stability problem. The why lies in looking at the 3-dimensional geometry of the point where compression member, tension member, and load eyebolt intersect. To see this, take a stick; glue a string to it. On a vertical surface, glue the string and position the stick, so that the stick is horizontal- so that you have a simple model of a boom- a right triangle. Now move the end of the stick side to side a bit. What happens to the point the string joins the stick? Specifically, what happens vertically? As the stick moves side to side, the point moves up. To do that, the load on the boom has to move up-be lifted. The more load, the more force it takes to lift the load. With ’’hands off’, it sits ‘centered’; perpendicular in vertical and horizontal planes- it is ‘self stabilizing.’ At first, with little load, it doesn’t take much force to swing it to one side or the other. As the load increases, the force required to get side movement goes up. By the time you get to 15kg, it takes a substantial force to move it. So, you need to be careful at first when you start loading to control swing, but then as you get some load on, it gets more and more stable.

This is my first year doing boomilever, and while i have done build events before, I've never done any involving weights (boomilever, towers, etc)

I've done some research and my question that I hope can be answered is as follows:

Would truss designs that are commonly seen in bridges i.e. Warren still have the same effect if applied to the boomilever as a compression-tension bracing? From what I've seen, bridges and cantilevers are similar, but still different so...

Same advice as above; go back into the archived posts and read. You’ll need to understand the forces on compression member(s) and tension member(s), and how buckling, and ‘column’ bracing work- which is all discussed in great detail, then you’ll have the basis for understanding the issues and trade-offs in tension member to compression member bracing.

[dholdgreve]

This is my first year doing boomilever, and while i have done build events before, I've never done any involving weights (boomilever, towers, etc)

I've done some research and my question that I hope can be answered is as follows:

Would truss designs that are commonly seen in bridges i.e. Warren still have the same effect if applied to the boomilever as a compression-tension bracing? From what I've seen, bridges and cantilevers are similar, but still different so...

In addition to what Len said, a typical truss like you would see in a home would be configured with tension chords on the bottom, and compression chords on top(Stop and think about it for a minute... the bottom chords are already as straight as they can be... the only way for them to go is down in the middle, causing the members to be stretched (tension)... The top chords are also exposed to the gravitational forces, but this time, the members would need to get shorter, or be compressed)... So to answer your question further... the same forces are involved, but in a substantially different orientation.

[randomguy1234]

Does anybody have any good ideas of how to build a light and strong boomilever or a picture of a design THanks

[UQOnyx]

Read the forums starting from page 1 on this thread. If there is not enough information, you might want to go to the archives of boomilever from last year. Also read the wiki and I would recommend using aia's boomilever guide. It's very helpful in learning building techniques and has good information about wood. Once you have enough information and are educated you should be able to make a basic design of your own. I would recommend looking at pictures and try to copy the ones you think you like. The problem with asking a question like how do I make a good boomilever is that it is too general and tends to annoy people. Once you have a good idea of the event then ask if you have any specific questions and you should get satisfactory answers.


3 Days Later...
Man, what a day. I finally tested the boomilever. Before testing, I had noticed a fault in gluing of one tension cord on the distal end, so I was not expecting excellent scores. I put the boomilever up, mounted onto the testing wall with the J-hook and everything. I started testing, things were going well. The base was fantastic (I described it a few pages back) and working well. The distal end was also fine, et cetra, et cetra. There were no pops or cracks and the boom was probably going to hold all the weight. Then around 11 or 12 kg. of pouring the sand, I heard a noise. I looked up, and saw that the boom was falling. There was so much weight on the boom that the testing wall which was not well constructed was tipping over. I put my hands up to stop it from falling, my hands hit the boomilever, and bam, the boom broke. the boom was about 17 grams, and held a bit less than 12 kg, so the score was around 700. Still, I am fairly sure that it could hold the full weight, so it would be a bit less than 900 efficiency. So summing it up, since I can't be sure, I would say that the score is between 700 and 900.

[fanjiatian]

I'm really frustrated b/c I can't find sticks or sheets that are long enough.
If you use a 36 inch long stick, you can only cut 1 compression member (assuming 47.5 cm), barely falling short of getting 2. If you use a 48 inch long stick, you can cut out 2 compression members, but you waste a good 10-11 inches. It gets even worse with tension members, because a lot of places sell bass wood that is 24 inches long.

Anyone else having this problem?

[chinesesushi]

I remember reading somewhere that it is best if the loading block's center is directly above the distal end. Is this true? Also, would it be better if the loading block was closer to the testing wall than the distal end? Obviously if it was farther it would be bad.