NationStates Jolt Archive

I'm going to make this bridge out of posicle sticks and I have to talk to a civil engineer about how I can quantify when my bridge will break. My teacher said he knows somebody and 2 other students said the same thing but so far no one has received a response from them. So, this is pretty much my best shot... NS General forum... yeah...

So, this is what my bridge is going to look like:
http://img157.imageshack.us/img157/3889/printscr6cb.png

That's the side view. I would like to know if I hang a mass from the middle of the bridge, how is the force going to distribute itself if all of those triangles are equilateral? This mass will be increasing systematically (at a constant rate).
So, basically, I need my dependent variable that will change with my independent variable (which is the mass hanged from the bridge). Can any engineers help me?

I'm going to make this bridge out of posicle sticks and I have to talk to a civil engineer about how I can quantify when my bridge will break. My teacher said he knows somebody and 2 other students said the same thing but so far no one has received a response from them. So, this is pretty much my best shot... NS General forum... yeah...

So, this is what my bridge is going to look like:
http://img157.imageshack.us/img157/3889/printscr6cb.png

That's the side view. I would like to know if I hang a mass from the middle of the bridge, how is the force going to distribute itself if all of those triangles are equilateral? This mass will be increasing systematically (at a constant rate).
So, basically, I need my dependent variable that will change with my independent variable (which is the mass hanged from the bridge). Can any engineers help me?

How is the bridge suspended? I have access to a bunch of engineers that actually build bridges, but you aren't pointing out exactly what the suspension will be. There is more to the bridge than the bridge itself.

My bridge is 57 cm long and 10 cm wide; it will be placed between 2 school desks that are 40 cm apart. A chain or rope will be tied to its middle and from that a huge mass holder will be hooked. Is that enough info?

My bridge is 57 cm long and 10 cm wide; it will be placed between 2 school desks that are 40 cm apart. A chain or rope will be tied to its middle and from that a huge mass holder will be hooked. Is that enough info?

What's the mass of the bridge? I'll assume the weight of the bridge is equally distributed

The maximum weight of the bridge is 300 grams and I'm going to come as close to that as possible.

What's the mass of the bridge? I'll assume the weight of the bridge is equally distributedAnd the strenght & flexibility of the posicle sticks?

Also, bear in mind that cross-bracing becomes an issue so one would need to know that structure. After all, wood is very compressive but will shear easier under twist. Chemical fastenings such as glue also are often directionally sensitive to torque.

It is also important to be aware that failure is expected at the weakest point. So if the glue or other fastening system has a lessor tensile strength than the physical structures, then you are into a whole other scenario to deal with.

Indeed, how your wieght is suspended may also be key. If hung from a loop around the bridge, then the mechanical structure of the whole bridge is in play. If hung from a single cross-member or beam, then that sub-component's failure may preceed the failure of the whole bridge.


So you 'll need to consider a lot more info on your plans before you talk to an engineer

There is no y=mx+b kind of solution. You need to find all the forces in all the members. Then, you need to decide if they are going to fail in tension, compression, or buckling. You can do that by several different methods. Look for method of sections or method of joints for starters.

How are the popsicle sticks connected. This is important as it will determine wheather or not it is a truss, or a compound truss.

That said, assuming you apply the load in the middle and simply support it at both ends, without other loads, it will break at the top chord in the middle.

You can thank me later.

Oh, and helpful hint - if allowed - put in a couple of diagonal cross-braces in the center of the bridge near where the weight will be hung. This should help mitigate crush-failure from a rope looped around the structure that is pushing the corners in.

"You need to find all the forces in all the members. Then, you need to decide if they are going to fail in tension, compression, or buckling."

I'll be using hot glue which is incredibly strong, therefore my bridge will definetly not break from any kind of twisting or where the sticks are glued.
Also, I know exactly where tension and compression will occur and I know that my bridge wil break from compression.

"How are the popsicle sticks connected."
You don't need to know that. I could explain but it would take too much time and it would be for nothing.

"put in a couple of diagonal cross-braces in the center of the bridge near where the weight will be hung"
No, I can't do that.

I really appreciate all this help you guys are giving me, but all I want to know is how is the force distributed from a point, the top middle of the bridge, to everywhere else. I have thought about everything else and I can assure you that I'm not a newbie at this. I just need some advanced tips.

"How are the popsicle sticks connected."
You don't need to know that. I could explain but it would take too much time and it would be for nothing.


Err, yes, you do. If the connection allows free rotation around the joint, it's a truss. If it doesn't then it's not, and the analysis would change.

In full size trusses this is not so much of an issue, but with a little bitty one like this it will be.

How are the popsicle sticks connected. This is important as it will determine wheather or not it is a truss, or a compound truss.

That said, assuming you apply the load in the middle and simply support it at both ends, without other loads, it will break at the top chord in the middle.

You can thank me later.

Yeah, that's what I was just going to say. The pressure is at the joints and it depends on how they are connected on what the strength of the structure is.

To the OP, you need to look at the bridge in parts and calculate the forces on the joints. You do this by looking at how the forces are formed to follow along the popsicle sticks. Make sure to make the horizontal forces cancel out and that should help you figure out your forces since the initial verticle force is a known quantity.

Can any engineers help me?

Not an engineer, but I've always wanted to drive a train. That'd be awesome.

I really appreciate all this help you guys are giving me, but all I want to know is how is the force distributed from a point, the top middle of the bridge, to everywhere else. I have thought about everything else and I can assure you that I'm not a newbie at this. I just need some advanced tips.

The load in the top chord is this:

Top Chord Load = (L/2H)*applied load.

Where L = truss length to support, and H= truss height to CL of top member, applied load = mass * acceleration due to gravity.

You can find this using the method of sections.

Err, yes, you do. If the connection allows free rotation around the joint, it's a truss. If it doesn't then it's not, and the analysis would change.

In full size trusses this is not so much of an issue, but with a little bitty one like this it will be.
You know what I'm thinking? Treat this like a simple beam, then use a fudge factor to account for the truss design. That might get a ballpark answer without having to teach the guy how to figure reactions at each end, and how to sum moments and forces when he takes a section through the truss. The question is how much of a fudge factor to apply.

The load in the top chord is this:

Top Chord Load = (L/2H)*applied load.

Where L = truss length to support, and H= truss height to CL of top member, applied load = mass * acceleration due to gravity.

You can find this using the method of sections.

Thanks for saving me the typing. I don't do this type of engineering so I'm translating for one of our bridge engineers, but he said once you've accounted for crush you calculate from the outside in or the inside out assuming the bridge and support is symmetrical.

You know what I'm thinking? Treat this like a simple beam, then use a fudge factor to account for the truss design. That might get a ballpark answer without having to teach the guy how to figure reactions at each end, and how to sum moments and forces when he takes a section through the truss. The question is how much of a fudge factor to apply.

If it's a truss, simply connected that is, the internal moment on the top chord is zero, and force in the top chord becomes L/2H*applied load. (L=truss length, H= Height).

If its not, the top chord is going to carry an internal moment, as will the lower joint. However, you could estimate it by treating it as a beam, which would give a midpoint moment of 0.25*applied load*truss length, and then using MC/I to find the stress profile in the top chord, leading to a guesstimate. But that's a bit more complex. Or instead of MC/I you could approximate with a couple a the joint and the top chord, but I think that would be way off.

Thanks for saving me the typing. I don't do this type of engineering so I'm translating for one of our bridge engineers, but he said once you've accounted for crush you calculate from the outside in or the inside out assuming the bridge and support is symmetrical.

Yah, the loads should show symmetry around the CL. Actually, if we are talking about the same crush (internal chords buckling), it shouldn't really be an issue in an itty bitty popsicle stick bridge like this. Anyway, buckling analysis is well beyond the scope of this project.

I haven't thought about this stuff for years.

If the connection allows free rotation around the joint, it's a truss.
There is no free rotation; the popsicle are all glued together with a very strong type of glue.

you need to look at the bridge in parts and calculate the forces on the joints. You do this by looking at how the forces are formed to follow along the popsicle sticks.
Yep, that's what I'm asking. How do I calculate those forces if I know all the angles and lenghts in the bridge, and the initial force which will be applied on its top?

To Lacadaemon: In that formula you gave, what is "L"?; the bottom chord is 57 cm long, the top chord is 45.5 cm, and it is placed on 2 desks which are 40 cm apart. So, what's my "L"? 45.5 cm?

How about using torque for my calculations? A friend of mine said that wouldn't work and I'm not sure how to do the calculations but I would eventually figure it out by myself... do you think that could work?

Yah, the loads should show symmetry around the CL. Actually, if we are talking about the same crush (internal chords buckling), it shouldn't really be an issue in an itty bitty popsicle stick bridge like this. Anyway, buckling analysis is well beyond the scope of this project.

I haven't thought about this stuff for years.

My bridge engineer said the same thing. He's in his fifties. His response, "Well, you just punch all of the information into the computer and it calculates it for you."

For the record, I don't think the design of this bridge is going to work out well, but that's just my two cents. This is a pretty common task in civil engineering and I've seen this design fail numerous times.

There is no free rotation; the popsicle are all glued together with a very strong type of glue.


Yep, that's what I'm asking. How do I calculate those forces if I know all the angles and lenghts in the bridge, and the initial force which will be applied on its top?

To Lacadaemon: In that formula you gave, what is "L"?; the bottom chord is 57 cm long, the top chord is 45.5 cm, and it is placed on 2 desks which are 40 cm apart. So, what's my "L"? 45.5 cm?

How about using torque for my calculations? A friend of mine said that wouldn't work and I'm not sure how to do the calculations but I would eventually figure it out by myself... do you think that could work?

If you wanted to work from the outside in you would take the half the force (the weight) on the structure at each of the sides. Then slice the structure apart mathematically and calculate your forces on each of the parts of the structure, working your way across.

EDIT: And when I showed the engineer he said you're right to assume no rotation. If you put it together even remotely well you can assume that it won't rotate.

To Lacadaemon: In that formula you gave, what is "L"?; the bottom chord is 57 cm long, the top chord is 45.5 cm, and it is placed on 2 desks which are 40 cm apart. So, what's my "L"? 45.5 cm?


We'll assume simple support, for the sake of ease. The L is the unsupported length, i.e. the length between the supports. So that would make it 40 cm.

First of all, thank you all very much for all this help you're giving me. I really appreciate it.

slice the structure apart mathematically and calculate your forces on each of the parts of the structure, working your way across.
Do you mean that I should treat the forces like the lenth of the sticks and just calculate them with sines, cosines and so on?

Is it "L/(2H)" or "(L/2)*H"?

EDIT: And when I showed the engineer he said you're right to assume no rotation. If you put it together even remotely well you can assume that it won't rotate.

Well, ideally, is a stable truss, members are free to rotate around the joints in the XY plane, because the truss geometry acts as bracing - so they don't actually rotate. And it's an assumption you have to make, because otherwise the chords of the truss will carry a moment, which will mess up the method of joints.

Of course, to justift this you have to check wether or not the truss is stable by counting the nodes and members, but that's again beyond the scope of this.

Strong glue, does mess up the assumptions somewhat, but it probably doesn't matter.

Ok, so this Top Chord Load is going to be focus only in the middle top of the bridge?

Oh, and is it "L/(2H)" or "(L/2)*H"?

It's the load in the truss member at the top, directly above the middle joint at the bottom. But only between the two joints. Obiously, the other members (i.e. the lengths of popsicle sticks between the other joints), have different loads on them.


It's L/(2H).

*quietly backs out of the thread*

Disclaimer: I don't have a background in civil and I spend a large portion of my time making fun of our civils and how hard they have to work. On that note - HAHAHAHAHA!

So, um... any thoughts on using torque for my calculations?...

So, um... any thoughts on using torque for my calculations?...

There's no torque. Forget it.

Ok, thank you. I'll see what I can do with all this infomation that you've given me. In a week or two, I'll post some pictures of my bridge and the other 3 bridges in the competition, with their respective stats.

Ok, thank you. I'll see what I can do with all this infomation that you've given me. In a week or two, I'll post some pictures of my bridge and the other 3 bridges in the competition, with their respective stats.

May the bridges of your competitors suffer from woefully inadequate design and materials, resulting in the regretable loss of severl popsicle-people, whose families launch a devastating tort claim against your competitors, forcing them into insolvency, whereafter you achieve global domination of popsicle stick industrial applications, until you go mad and seal yourself into a sterile clean room and grow your finger nails long.

I have one last question: what if instead of putting the bridge between 2 desks, I put it between 2 weighting scales? The purpose of a bridge is to spread the load from its center to its extremities which are supported and cannot fall or break. So, if I do that, the more succesful a bridge is at spreading the load, the more weight it will read on its extremities compared to the weight that is hanged from the center of the bridge.

Is this mathematically true?

I have one last question: what if instead of putting the bridge between 2 desks, I put it between 2 weighting scales? The purpose of a bridge is to spread the load from its center to its extremities which are supported and cannot fall or break. So, if I do that, the more succesful a bridge is at spreading the load, the more weight it will read on its extremities compared to the weight that is hanged from the center of the bridge.

Is this mathematically true?

No. Basic physics. Gravity is exerting a force on the bridge due to its mass and the mass of the weight plus the tether. The scales are exerting an equal and opposite force that keeps the bridge from moving (or at least continuing to move since scales tend to move a bit at first).

I have one last question: what if instead of putting the bridge between 2 desks, I put it between 2 weighting scales? The purpose of a bridge is to spread the load from its center to its extremities which are supported and cannot fall or break. So, if I do that, the more succesful a bridge is at spreading the load, the more weight it will read on its extremities compared to the weight that is hanged from the center of the bridge.

Is this mathematically true?

What jocabia said. Also, you want the bridge to be supported at the joints, not along the chord, so it develops support as a truss. But don't use the very end joints of your bridge, uses the second ones in, because the outer section (the parralelogram at the end) is statically unstable.

Wait around and I'll post a picture of where the supports should go.

http://img.photobucket.com/albums/v719/Lacadaemon/Bridge.gif

You want to support the bridge on the joints where the two arrows are. (For all intents and purposes this should be where the desk ends. Also the L in the calculation I gave you becomes the distance between these two joints.)

The red area is internally unstable if not attached to the rest of the truss, so you don't want to use the end joints.

Hope this helps.

Yes, that was what I was going to do. That's why I said the desks are 40 cm apart but my bridge is 57 cm long at the bottom and 45.5 cm long at the top. And I was saying that instead of puting the bridge on 2 desks, I should put it on 2 weighting scales that are 40 cm apart.

I think you misunderstood me somehow... or did you?...

To Jocabia: What's a "tether"?

http://img.photobucket.com/albums/v719/Lacadaemon/Bridge.gif

You want to support the bridge on the joints where the two arrows are. (For all intents and purposes this should be where the desk ends. Also the L in the calculation I gave you becomes the distance between these two joints.)

The red area is internally unstable if not attached to the rest of the truss, so you don't want to use the end joints.

Hope this helps.

If he uses the desks 40 centimeters apart as he said he'll have roughly 8 centimeters of support on either side and that should go well into covering your arrows.

Yes, that was what I was going to do. That's why I said the desks are 40 cm apart but my bridge is 57 cm long at the bottom and 45.5 cm long at the top. And I was saying that instead of puting the bridge on 2 desks, I should put it on 2 weighting scales that are 40 cm apart.

I think you misunderstood me somehow... or did you?...

To Jocabia: What's a "tether"?

Whatever ties the weight to the structure. It's just a more generic term than rope or chain.

If he uses the desks 40 centimeters apart as he said he'll have roughly 8 centimeters of support on either side and that should go well into covering your arrows.

Oh I forgot about that.

Well he wants to put it at the joint, not between them, otherwise the member will develop an internal moment.

You should only load a truss at the joints. In other words, exactly where the arrows are, you don't want to overlap and have the desk ending between the two joints. Othewise the bottom chord will run into internal shear and moment issues. (Non truss action).

I just presumed that they were 40 cm apart yesterday. Now, the question has been brought up, I am just clarifying.

No. Basic physics. Gravity is exerting a force on the bridge due to its mass and the mass of the weight plus the tether. The scales are exerting an equal and opposite force that keeps the bridge from moving.

I'm sorry for asking you to repeat yourself but did you check this with your friend, the civil engineer? It's just that I need to be absolutely certain if this works or not.

Jump one *bump* and now I'm off to bed. I hope I'll get a response later.

I'm sorry for asking you to repeat yourself but did you check this with your friend, the civil engineer? It's just that I need to be absolutely certain if this works or not.

He's right. Forget about it.

Oh I forgot about that.

Well he wants to put it at the joint, not between them, otherwise the member will develop an internal moment.

You should only load a truss at the joints. In other words, exactly where the arrows are, you don't want to overlap and have the desk ending between the two joints. Othewise the bottom chord will run into internal shear and moment issues. (Non truss action).

I just presumed that they were 40 cm apart yesterday. Now, the question has been brought up, I am just clarifying.

I totally agree with this.

Ok, thanks guys.

I'm sorry for asking you to repeat yourself but did you check this with your friend, the civil engineer? It's just that I need to be absolutely certain if this works or not.

I'll explain it this way. Consider yourself to be a bridge. Now someone is picking you up. No matter where they put their hands (under your knees and around your back or they throw you over their shoulder or if they pick you up under the armpits) you will always weight what you weigh. You're weight (the force) does not change by where you are supported.

Now, conduct another experiment. Have to friends hold you up. One holds your head and one holds your feet. The pressure that you feel and the difficult that you have in staying stiff, that's the same forces that a bridge experiences more or less. Your efforts on your bridge and bolstering the weak points is kind of like you working out your abs in this situation. You are trying to make it so that the bridge (or you) does not buckle. However, no matter what forces you are experience that make it difficult for you to stay stiff, so long as you aren't moving, your exact weight is what your two friends are supporting.

In a symmetrical bridge, the force on each side is half the overall weight of the bridge, tether and additional weights.

I have yet another question: Should I use a 60 degrees angle or a different measurement?

Btw, I'm going to construct the bridge today :)

At the risk of appearing to be somewhat primitve in my approach to all this: why not build two bridges and test the first one to destruction by slowly adding greater and greater weights to it? Assuming that the construction quality and materials in both bridges are effectively identical, you will have your answer.

Here are some pics. Our bridge weights about 275 grams, and the maximum is 300 grams, so we'll be changing it a bit. We'll test it after a few weeks.

http://img362.imageshack.us/img362/2918/bridge11kv.jpg
http://img362.imageshack.us/img362/6593/bridge23gn.jpg
http://img101.imageshack.us/img101/3695/bridge35la.jpg
http://img101.imageshack.us/img101/5185/bridge42ky.jpg
http://img524.imageshack.us/img524/4610/bridge55uw.jpg

Tell me what you think of it.

http://boards1.wizards.com/images/smilies/Bump.gif

I'll be an engineer one day. I hope...

Here are some pics. Our bridge weights about 275 grams, and the maximum is 300 grams, so we'll be changing it a bit. We'll test it after a few weeks.

http://img362.imageshack.us/img362/2918/bridge11kv.jpg
http://img362.imageshack.us/img362/6593/bridge23gn.jpg
http://img101.imageshack.us/img101/3695/bridge35la.jpg
http://img101.imageshack.us/img101/5185/bridge42ky.jpg
http://img524.imageshack.us/img524/4610/bridge55uw.jpg

Tell me what you think of it.

Is your bridge going to be supported right at the point where the second bar connects to the bottom? That would be the best place for it to be supported.

We did this in high school, except with toothpicks. Whoever's bridge held the most weight without breaking won. The winning bridge had an arch on either side, like this one (http://danny.oz.au/travel/scotland/p/4422-tyne-bridge.jpg). When the suspended weight was added to the middle of the bridge, the arch was a good counterbalance, helping to keep the bridge from bowing and breaking too soon.

The joints of the bridge you drew up are not simple pinned joints, they transmit both tension and moment therefore this structure should be treated as a space frame type beam and not a simple truss. The photos of the bridges you presented earlier do not have any transverse bracing and will almost always fail by folding and then collapsing. The only thing resisting load in this mode is the strength of the glued joints at the top and bottom corners. To improve this design without blocking the drive through area, you will need to glue pieces perpendicular to the exterior surface with lap joints on the corners so that when you look axially down the bridge the pieces you just glued in place resemble a picture frame. Your effiency will go down but this added weight should be offset by a significant increase in folding strength.

The "competition" is over. My group's bridge weighted 275 grams and held 69 kg, and the other 3 bridges held below 40 kg each. These results surprised me greatly because the other groups had people who were quite smarter than me but, for some reason, they screwed up.
Meh, just thought some of you might like to know this.

i had to make something similar

any structure that would hold the largest weight over a bin, using only 25 4mm paper art straws and glue guns.

I won it, :) with it holding 5.4kg before crippling, although it weighed alot.

And i know it didnt support 69kg or whatever but you try and do better.

i hope i too become an engineer one day - aeronautical though