Designs B/C

[noobforce]

Hi,

I am having some trouble with my bridges.

I am making my bridges smaller than 5cm width to save mass, yet I think this leads to another issue. Whenever I test my bridges they frequently just tip over.
I've been thinking of angling my bridge to a trapezoid when viewed on its short end. However I think this would be more difficult to make because it introduces new angles and leads to the compression members sliding away from each other from the bottom. Would this cause more stress to the bracings?

Is this method any useful? What else could I do to fix the problem?

[chalker]

Hi,

I am having some trouble with my bridges.

I am making my bridges smaller than 5cm width to save mass, yet I think this leads to another issue. Whenever I test my bridges they frequently just tip over.
I've been thinking of angling my bridge to a trapezoid when viewed on its short end. However I think this would be more difficult to make because it introduces new angles and leads to the compression members sliding away from each other from the bottom. Would this cause more stress to the bracings?

Is this method any useful? What else could I do to fix the problem?

Have you read the rules carefully? Rule 3.e. says the bridge must be at least 5cm wide.

[Friedoyster3]

Hi,

I am having some trouble with my bridges.

I am making my bridges smaller than 5cm width to save mass, yet I think this leads to another issue. Whenever I test my bridges they frequently just tip over.
I've been thinking of angling my bridge to a trapezoid when viewed on its short end. However I think this would be more difficult to make because it introduces new angles and leads to the compression members sliding away from each other from the bottom. Would this cause more stress to the bracings?

Is this method any useful? What else could I do to fix the problem?

Have you read the rules carefully? Rule 3.e. says the bridge must be at least 5cm wide.

But if noobforce here is using a so-called "useless outrigger" then the load bearing structure can be less than 5cm, as per an FAQ if I recall correctly. In fact, my bridge at Wright State used a useless outrigger and we did just fine.

To your original question, noobforce, personally, I am drifting away the "less than 5cm + Useless outrigger" designs. That's not to say they aren't viable, I have won some fairly significant tournaments using that mentality. With the trapezoid that you are talking about, I think, but I'm not positive, that it would marginally increase the compression forces. It may also increase the bending moment along those members. You're going to have to weigh (metaphorically and physically) whether or not the tiny about of wood you save is worth the extra stability by just making your bridge 5cm.

[noobforce]

Hi,

I am having some trouble with my bridges.

I am making my bridges smaller than 5cm width to save mass, yet I think this leads to another issue. Whenever I test my bridges they frequently just tip over.
I've been thinking of angling my bridge to a trapezoid when viewed on its short end. However I think this would be more difficult to make because it introduces new angles and leads to the compression members sliding away from each other from the bottom. Would this cause more stress to the bracings?

Is this method any useful? What else could I do to fix the problem?

Have you read the rules carefully? Rule 3.e. says the bridge must be at least 5cm wide.

But if noobforce here is using a so-called "useless outrigger" then the load bearing structure can be less than 5cm, as per an FAQ if I recall correctly. In fact, my bridge at Wright State used a useless outrigger and we did just fine.

To your original question, noobforce, personally, I am drifting away the "less than 5cm + Useless outrigger" designs. That's not to say they aren't viable, I have won some fairly significant tournaments using that mentality. With the trapezoid that you are talking about, I think, but I'm not positive, that it would marginally increase the compression forces. It may also increase the bending moment along those members. You're going to have to weigh (metaphorically and physically) whether or not the tiny about of wood you save is worth the extra stability by just making your bridge 5cm.

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

[bernard]

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has to do with the density of the tension you are using.

[noobforce]

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has to do with the density of the tension you are using.

My bridges are only holding about 7kg with 1/18 x 1/16...
In fact, I don't even know the density of my wood. I am ordering from specialized balsa under the general balsa stick selection section and the local hobby shop closed last year. In general, the wood I am using seems to be easily squished. Is that okay?
The locations of where the members fail are in the common spot of about 2cm away from the joints. What could be the cause of this?

[bernard]

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has towith the density of the tension you are using.

My bridges are only holding about 7kg with 1/18 x 1/16...
In fact, I don't know the density of my wood. I am ordering from specialized balsa under the general balsa stick selection section and the local hobby shop closed last year. In general, the wood I am using seems to be easily squished. Is that okay?
The locations of where the members fail are in the common spot of about 2cm away from the joints. What could be the cause of this?

If it is easily squished, it is probably light or very light balsa. I ordered from Specialized Balsa for the first time and my wood just arrived yesterday. My light balsa n't seem to be squishy. If you provide an picture of your bridge, I could see if there are other reasons for why your tension is failing. Feel free to private message pictures if you don't want to publicly post them.

Edit: What is the mass of your entire bridge?

[chinesesushi]

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has to do with the density of the tension you are using.

My bridges are only holding about 7kg with 1/18 x 1/16...
In fact, I don't even know the density of my wood. I am ordering from specialized balsa under the general balsa stick selection section and the local hobby shop closed last year. In general, the wood I am using seems to be easily squished. Is that okay?
The locations of where the members fail are in the common spot of about 2cm away from the joints. What could be the cause of this?

Wood that can be easily squished is in general very bad under tension.

[someusername]

I tend to find that bass wood does better in tension than balsa. Even though it is a little heavier, it has much more flexibility and strength than balsa. You don't need a very big piece of bass wood to hold a lot of force in tension either.

[bearasmith]

Okay. I'll start building some "normal" bridges and see how they go.

Also, what kind of tension members are you using? if you are running single tension members? So far I've been experimenting with 1/8x1/16 balsa and they seem to fail also a lot, way before the compression members' max load. Maybe it's because I am cutting them by hand. My shipment for machine-cut balsa will not come for a few days so I will be pressed on building/testing time.

How muchyour bridge hold when your tension fails? For one of my best bridges so far, it failed at 21 kg with 1/16" x 1/16" balsa tension. I don't think its because you're cutting your tension by hand. It probably has to do with the density of the tension you are using.

How light were the 1/16" x 1/16" tension members?