Elevated Bridge B/C

[blue cobra]

Hello, im new to this forum and i had a few questions for u " pros"

1. What's a fixed node and rolling node?

2. How do i solve compression and tension (is there anything i can do to make the bridge stronger if i face this)

3. Is bass better for points of tension?

How much more does bass cost than balsa?

Thank you

I wouldn't consider myself a pro, but I hope I can still help you.

1. I don't know what they are to be honest. You really won't face them except for the JHU bridge designer.

2. You don't really "solve" it. Those are the two major forces on your bridge. Compression is like squeezing the piece between your fingers, and tension is like grabbing both ends and pulling. Generally, increasing cross sectional size increased compressional strength, and (someone please correct me if I am wrong) increasing density increases tensional strength. Sometimes people will use a pair of high-density balsa as thin as 1/64 for tension members.

3. As I said before, higher density is beter for tension. Bass is higher density than balsa, so it could be stronger. However if you have balsa that will be strong enough, you can save a lot of weight.

4. Bass is still pretty cheap. Generally, bass sticks are sold in 2 ft sticks for about the same price as a balsa 3 ft stick.

If you haven't already, read the wiki page.

[andrewwski]

A fixed node is fixed in position. It's anchored in all directions. In the context of this 2-dimensional applet, it's constrained into place in both the X and Y directions.

A rolling node is only fixed in one direction, thus it can move in the other. It is only fixed in the X or Y direction, depending on which you choose, and can move freely in the other direction - thus it is only constrained into the path of a line.

For the purposes of designing the bridge, you'll need one support node to be fixed and one to be rolling.

[SLM]

Hello, im new to this forum and i had a few questions for u " pros"

1. What's a fixed node and rolling node?

2. How do i solve compression and tension (is there anything i can do to make the bridge stronger if i face this)

"Fixed" and "roller" are types of supports. If I took my elevated bridge and glued one of its legs to the test table, then I could say the bridge has a fixed support. It basically means the fixed node (the glued end of the leg) cannot move (in either x or y directions) and it cannot rotate. On the other hand, if I placed the leg of the bridge on the table, without gluing it to the surface, so that the leg is free to roll on the table (move in the horizontal direction), then I have a "roller" support. A basic understanding of how your bridge is supported becomes important when you want to analyze your bridge in order to determine its member forces.

The function of your elevated bridge is to carry a 15 kg load. The bridge does this by transferring the load from the load block to the table. So, in a way, the 15 kg load flows through the members of the bridge to get to the table. In this transfer process, each member carries certain amount of load (force). If you view your bridge as a truss, then some of the members carry a tension force and some carry a compression force. You can use a software such as JHU bridge designer or the Structural Analysis Notebook (http://nexote.net/nexote/Structural%20A ... 20Notebook) to calculate these forces. If the tensile, or compressive force, in a member becomes too large, then the member would fail. To prevent failure due to too much tension force, you need to increase the cross-sectional area of the member. To prevent failure due to too much compressive force, you may have to either increase the area of the member or reduce its length. Alternatively, if you don't want to change the size of the members, you can play around with the geometry of the bridge to see if you can reduce its member forces.

[lllazar]

I think i understand the fixed and rolling nodes now, but do they allow you to glue a leg to the table in the actual competetion?

Thank you guys.

[sr243]

Just wondering if anyone did a test, but what are you getting (percentage-wise of ratios) with the same design from last year? Like last year i got around 1100, but right now with the nearly the same design i just got past 900. My ratio is only around 85% of last year's. Anyone want to share their results?

[croman74]

I think i understand the fixed and rolling nodes now, but do they allow you to glue a leg to the table in the actual competetion?

Thank you guys.

No, I don't think you're allowed to glue it to the table. You just place it as it is.

[SLM]

I think i understand the fixed and rolling nodes now, but do they allow you to glue a leg to the table in the actual competetion?

Thank you guys.

No, I don't think you're allowed to glue it to the table. You just place it as it is.

That is right, you are not allowed to attach the bridge to the table by gluing or pinning. I was just trying to explain "fixed" support, I did not mean to imply that you can actually glue the bridge to the test table in the competition.

Further, I know that JHU bridge designer uses the term "fixed node." But, that is not accurate. You cannot have a fixed node (support) in trusses, since by definition, truss joints are free to rotate (unlike a fixed support where rotation is prohibited.) Truss structures are generally supported using a pin support and a roller one.

[lllazar]

I went to my local hobby store today to get some wood to start makin test bridges. There were NOOOOO 1/4 by 1/4 sticks of balsa, which i believe is the max dimensions of any wood used in the bridge rules....

So in the end i got some bass, i just thought wat the heck i might as well try it out, its always an option.

I was just wondering, do bass bridges generally hold more weight due to their strength and flexibility (bend)

And also, is the increased weight in bass worth it compared to balsa?

I am not makin an arch bridge....just soo not worth it....

[fmtiger124]

I went to my local hobby store today to get some wood to start makin test bridges. There were NOOOOO 1/4 by 1/4 sticks of balsa, which i believe is the max dimensions of any wood used in the bridge rules.....

Actually you can use any size you want, the rules specifically say that that restriction has been removed.

[croman74]

There's no size limit for wood this year. And even then, you probably don't want to use that big of a piece in a bridge. It makes the bridge heavier and in my opinion don't really have much more strength.