types of bridges

[Science Freak]

lol, i know that this is kind of silly, but our teacher has assigned us a bridge building project (its silly because we're in grade 11 ) we're going to be given 50 bendy straws and some masking tape. our goal is to create a bridge that spans (either 1 or 2 feet, i'm not completely sure but i think its gonna be 2 feet, the sheet says 2 tiles, the tiles are i think 1 foot each). its suppose to support the weight of our physics text book. the more it can handle the better it is. back in grade 7 when i did this, we had to make a bridge out of 1 Que-card to (i chose to wrap the paper into a stick diagonally as tight as possible and set the class record of a few kg or so (lol i still remember, almost all of our teacher's weight set was on my bridge before it collapsed, so i'm gonna assume somewhere from 5-10lb?)but this time, i'm not sure how i'm going to do it. there are so many choices... my current plan is to make a simple bridge with a platform and an overhead "cable" (also going to be straw) and make triangles kinda like |\|\|\|\/|/|/|/| <--- that. although it would be good to support the book's weight if it was going to be put in the middle of the 2 side structures, i think that the weight will be on the top (i don't think that there would be another way). if anyone has any suggestions as to how should i design it and what should it be like, it'd be greatly appreciated. Thank You for your time.

Edited March 30, 2011 by theunknownwonder

[imatfaal]

Not silly at all . I think it carries on throughout an engineering education - I have certainly seen undergraduate level engineers being asked to do this same task. This was in a competition that was set to schoolchildren from 8-18 yrs and students at two local universities (using spaghetti, super glue, and mars bars) a group of 16 years olds beat all competition!

 

There are real architects on this site and hopefully at least one will respond - but I guess the first thing I would want to think about is the strengths of my materials: under tension, compression, shear, twist etc. Secondly what is the weakness of my joints? What sort of anchor on the sides is allowed?

[CaptainPanic]

Not silly at all! Completely agree with imatfaal. Mechanical engineering students make bridges from spaghetti all over the world.

 

In fact, I get 4 million hits for [spaghetti bridge university]... which shows that it's a very popular, and very scientific, thing to build a small model bridge from household materials. I think that universities actually send their best teams to a world championship spaghetti bridge building.

 

Your first question must be what forces your material handle and in which directions (push, pull, sideways, etc). Then think about the bridge, and how you can use each straw most effectively.

Because I don't know the straws, I cannot answer your questions.

[Spyman]

[imatfaal]

Great site CaptainP - from a look at some of the photos - winners had over half a tonne supported by less than a kilo of spaghetti

[Science Freak]

lol i never realized that it was so famous (the bridge building i mean). well since all we have at our disposal are 50 bendy straws 2 x 75 minutes classroom periods (it MUST be done in class) and some masking tape, i doubt i'll be able to make any successful suspension bridge. the idea to build the " |\|\|\/|/|/|/| " design has been scratched off the list, although it is achievable, it'll be a pain to get the angles to a good enough degree for it to be successful. some of my ideas include over head arch bridge, like -----> http://reccs.uni-obuda.hu/en/index.php?q=gallery&g2_itemId=110 or a classic arch bridge ----> http://www.corbisimages.com/images/67/7896F387-F20F-45E4-9F10-D886612CBE8D/42-17311596.jpg but the problem with classic arch bridge is that it requires a good support to either the ground or the sides of the 2 anchors from below, so that'll be hard to achieve on tables but my favorite design is the one i came up with last night. its basic, its simple, it can be made easily, and it should work. it looks like this

 

(Side view)

________________

\/\/\/\/\/\/\/\/\/\/\/\/\/

________________ <-------- of course this line is suppose to be a bit higher up and the "\/\/\/" are 45 degrees

 

so basically, i plan on building 2-3 pairs of those, and connecting each pair with straws (bird's eye view) "| | | | |" placed like that. the point of making multiple beams like that is its 1. quicker (i have a partner so we can both be working on a separate beam at once) and also it allows us to use minimal material to gain good strength (if i was to add any suspensions or more underhand support it would end up making the 1 bridge stronger for sure, but a lot of that potential of the straws that i'd use would be wasted). our first work period is friday 2nd is monday and then i guess its prolly due at the end of the class?back in grade 7 i took the first place prize in paper Olympics bridge building (it was just a small classroom challange) and this time i plan on winning this challenge also. lol there are a lot of fancy designs out there, and although ours is prolly not gonna look like any of that, ours is prolly gonna look like a crappy piece of art, but what i do know is, whatever its gonna look like, its gonna mean business

[michel123456]

It is all about traction & compression.

I would propse this kind of structure.

 

Many bridge have 2 aspects:

1. to go from one side to another of a gap.

2. to let enough space beneath for transversal passage, like a bridge over a highway.

 

In your exercise, you don't care about point 2.

So you can span the tape from one side to the other of the gap and make in time zero the main structure of the bridge.*

Next you'll have to construct the mantel: that will take the entire time at your disposal.

Be careful to make your bridge resistant to lateral push and to inequal load: that will be the reason your bridge will collapse, if it collapses.

Good luck.

 

--------------------

*edit. i supposed the gap is stable somewhere. Otherwise, if you must make your bridge as a self-sustaining structure, it is a little bit more complicated.

Edited March 30, 2011 by michel123456

[Science Freak]

thank you everyone for their input, and right now i have 3 questions; 1. michel123456, that bridge diagram, is it drawn to scale? if no would you think it'd be possible to get a scale diagram with lengths to ratio for optimal performance?

 

2. what do you guys i recommend i do about the bendy part of the straws? should i cut it out or keep it? what i fear is that when the weight is placed onto the bridge the bendy parts will give away. since the weight is gonna be a downward force its gonna put tension to the middle of the bridge and compression towards the ends branching out towards the anchor (or so i assume)

 

3. what do you guys recommend i make the lengths of the pipes? if i make the triangles a bit bigger (according to west point bridge designer 2007) in the "/\/\/\/\/\/\/\" design then it can hold more weight but it will require more materials but if i make it a bit smaller then it takes less material but its harder to make and can't take as much weight (its harder because we would have to put together a bridge of smaller scale, so making the individual parts of the triangle is a bit harder

[michel123456]

It is not to scale. You must make a draw of your bridge (highly recommended) .The basic curve is a parabola.

 

Your structure design will depend on the available straw length and on the total bridge dimension. The width of the bridge must remain a certain percentage of its length. Try to remain above 10%.

 

The bendy part of the straw can remain as long as you use it cleverly, that is under contraction.

 

Take some time to study my diagram & Spyman's. Put in red the elements in contraction, and in green the elements in traction. Where 2 elements in contraction join together, you can use the bendy part.

 

But be carefull:

1_if the intent is to build a self sustained structure, you must be able to reverse the bridge upside-down without collapsing. In other words, your bridge would be like a beam.(like in the spaghetti bridge contest). You cannot make a suspension bridge.

2_if the intent is to build a bridge on site, the structural inventory is larger and you can imagine a suspension bridge. A suspension bridge collapse when turned upside-down, it is not a self-sustaining structure.

 

For the parts under traction, you can use only the tape. Theoretically: beware of points 1 & 2 above.

 

You could use only tape to make a bridge, without straws at all.

For example, you can roll the tape like cigarettes to make beams.

Or you can collate the tape face to face in order to make a somehow rigid ribbon, straight or curved (better)

You can increase the strength of the ribbon by cleverly shaping the tape before collating together, like origami.

Possibilities are endless.

 

---------------------------

edit:

if you are under point 1, take a look at space frame structures. Very efficient, but you'll have to deal with how to make the node.

Edited March 31, 2011 by michel123456

[Spyman]

2. what do you guys i recommend i do about the bendy part of the straws?

Can the straws be sticked into each other? If so then stick the longer part of the first straw firmly into the the shorter part of the next straw and so on, to connect a series of straws in a long row before bending each "bendy" part and creating the "/\/\/\/\/\/\/\" pattern.

[michel123456]

Also:

if you make a careful analysis of Spyman's diagram, you may notice that almost half the elements are redundant. It is an "over statical structure" (lost in translation) which means that one of the element can be damaged without provoquing the collapse of the entire structure. My diagram is more economic, but if you take away a single element a total collapse is highly probable.

[Science Freak]

update: i just found out that michel's design, there is a lot of tension downwards on the upper part of the bridge (the part that is in contact with the traffic) so when i made my actual bridge (only made 1 side, to test how it works out) it turns out that if i pull hard on both sides, i give it some horizontal tension so when that vertical tension pushes downwards on it, rather than the straws collapsing inwards the two forces counter each other for a more firm and stiff "walk way" also, i made 2 string loops and put them on both ends of my physics book, then i put the loops onto my 1 side of the bridge (by 1 side, what i mean is, there need to be 2 sides and then more straws connecting those 2 sides and making a one complete bridge, i only have 1 of those two sides made) i tried to lift the book but it didn't work, the straw upper structure of the straws was collapsing inwards, so i pulled hard on both sides and it allowed me successfully lift the book with just 1 side (although it was really hard to balance the string on just one side) i'm taking this frame to school along with the diagram so my partner can better see what we gotta do. this is going to be great. if we get it right, i'm expecting it to hold at least 2 books if not more. thank you michel123456 for the design and thank you everyone for the input

[michel123456]

I understand nothing but I am glad you are happy.

It surprises me that half the structure can stand, the design is intended to be complete (unlike Spyman's). If you have enough material, cross the straws to make your structure hyperstatic.

 

FYI if you had at your disposal many more straws, and since we are on a scientific forum, you could made a more up-to-date structure based on the chaos theory.

Something like this (the picture is copyrighted). A nest.

 

------------------------

Ah, one last thing.

Be extremely careful at both bridge ends, where the weight is transmitted to the main support (chair, table, wathever). You'll need to make another structure there that will make a good repartition of the forces.

Edited April 1, 2011 by michel123456

[Science Freak]

basically what i mean is if you apply force in <------ and ------> direction of the bridge, the top surface is under horizontal tension, so when there is weight placed on top of it pushing down, the horizontal tension stops the straws from collapsing down and it can hold much more weight than it could without this horizontal tension. the 1 side of the bridge alone can't really hold the book up, it keeps pivoting on its anchor points so its hard to balance, so after i put the book in 2 string loops and put the loops around the bridge's frame i was able to apply that same pressure of the book upon the structure to see if it can hold, and it did. and it is a pity that only 50 straws are allowed, or else i could probably make an amazing bridge with the info from this forum