I’ve worked out that my final bridge was made out of approximately 176 strands of spaghetti, if my bridge is to hold as much as each individual strand can hold it should support 4,602g. However as it’s made of glued together bundles I’m not sure to what extent this will be true, some of the glued bundles were stronger but some were in face weaker. We shall see…

Crud!

It only made it to 3,600g!! I really think that this is down to the glue failing as the weight was increased. I’d like to try this again at a later date using different materials and different glue.

Note 11/05/2010

Some of the other students have also been making bridges using art straws, this seems to be much more successful, the glue gun glue joint don’t fail and the bridges seem to correspond with the simulations run using West Point Bridge Designer.

Steve was testing a Warren truss and I decided to test out my analysis skills by predicting where it would fail, I chose the two central, horizontal struts as they would be under the most force from compression. 3 kilos later and I was right!

The two struts along the top edge of the bridge were being put under huge amount of compression as the weight was added, if the bridge was well made this would definitely be the fail point.

i had a go at recreating my own bridge using the program to have a look at where it should have failed if the glue joints had been good enough:

So it would probably be the same two struts that failed on Steve’s bridge even with my vertical reinforcements.

Possibly something to experiment more with over the summer?

Now if you add an extra strut…

…it becomes much stronger, this is called triangulation. The bridge structure I’ve chosen are a selection of methods of using triangulation.

Now for the bridge structures, I’m going to test a Pratt truss, a Howe truss, a Warren truss with vertical supports and an inverted Warren truss with vertical supports.

First the Pratt truss:

This structure took 250g before collapsing, It’s not looking good for my bridge building as it appears it is one of the glue joints that gave way…

Now the How truss:

This structure took 250g before collapsing, definitely a structural collapse this time.

On to the Warren:

Collapse happened after 260g

And Finally the inverted Warren:

The structure made it to 250g before collapsing in a heap.

So now I have my strongest structure; a Warren Truss :) Onward to bridge building!

To make sure that all my tests are fair I’ll be adding the weight in small increments and waiting 10 seconds between each to make sure that it is actually taking the weight. I’ll also be performing each test 5 times to try and rule out any erroneous results.

Test 1: how much weight can a strand of spaghetti take over a distance of 180mm?

Spaghetti is really flexible!! It dipped down 30mm from horizontal before it gave way on average after 26g

So what would happen if I halved the distance? Would the spaghetti take twice the weight?

Turns out it’s more than twice as strong, the spaghetti strands to an average of 66g before breaking so about 2.5 times stronger.

Next I’m going to try groups of 5 strands, are they as strong as 5 individual strands or stronger?

Ok so 5 individual strand would take approximately 130g, the results above show that 5 strands together are, in fact, a bit weaker at an average of 118g.

 Now to up it to 10 strands.

10 individual strands would take 260g so… again they work out a weaker than the sum of its parts at an average of 222g.

Now to see how they behave when the glue is added into the mix. As I’m going to be creating my structure from bundles of spaghetti glued together I want to see if gluing the bundles I have already tested effects the amount of weight they can support.

5 strands glued are, on average, 1.07 times stronger than the previous bundles of 5.

10 strands glued are, on average, 1.13 times stronger than the previous bundles of 10.

Interesting, I wonder If I can use this to try and predict how much weight my final bridge will take…

As part of my structures investigations I’ve decided to have a go at the spaghetti bridge challenge.

Sound odd doesn’t it

It involves making a bridge structure out of nothing but spaghetti and hot melt glue following a list of task parameters. The official spaghetti bridge world championships has a very stringent set of rules to follow involving a meter wide gap, a restriction in height and weight of your final structure and using only spaghetti that has not been tampered with (so no making your own and including any interesting ‘addatives’).

The current world champion bridge held over 4ooKg before it failed!!

I’m not trying to compete with this in any way, shape or form so… as I’m only doing this to investigate the forces involved in bridge building I’ve cut down the requirements to give myself something that it atchievable in the two days I have set aside for this:

1. The bridge must span a gap of 40cm
2. It must be made from only spaghetti and hot melt glue
3. it must not brace against the inner sides of the span
4. It must not exceed 20cm in height
5. it has to have a deck at least 5cm wide

To start with I’m going to have a look at the weight bearing capabilities of spaghetti and try out a couple of typical bridge structures to see which is stronger.

Here goes

Take this column for example:

What are the forces acting within it to stop it crumbling into a heap, whats keeping it in equilibrium?

The column is under compression so it pushes up against the ceiling and down against the floor to balance the forces acting on it:

As it’s under compression it is classes as a strut.

You can work out the exact forces required in a similar way to the graphical analysis I used in a previous post. As they’re acting at right angles to each other, the results are all the same, if they were at an angle you’d need to use a triangle of forces to work it out.

Dean used a really good visual example: think about a tennis ball floating on a pond, a pump is pushing the water in one direction and a breeze is blowing in another, which direction will the tennis ball move in?

The two forces, A and B, combine to form the resultant force C

When doing structural  analysis you must keep in mind that any value that has a magnitude and a direction can be drawn as a vector, so, if you put values to the forces A and B you can work out the value of the resultant force C.

By drawing the forces involves as vectors you can use graphical analysis to work out force C by drawing a parallelogram of forces. You have to draw each vector accurately and to scale, so 10N would be 10cm. By measuring the resulting vector length and angle you can state accurately the value and direction of the resultant force:

You can use a triangle of forces to work out the equilibrant force, or the force that is of the correct direction and magnitude to keep the ball where it is:

The euilibrant force is exact opposite of the resultant force, in a triangle of forces you redraw the forces so they are sitting nose to tail, by using the same method of drawing out the triangle accuratly using the correct lengths and angles it’s simple to measure the equilibrant’s magnitude and angle to the ball.

If you have many different forces acting on your tennis ball and you want it to stay still you can draw a polygon of forces:

It doesn’t matter which order you draw the forces in as long as the flow is consistent, so they move from one to the next to the next:

Simples… well I’m sure it will be simples once I’ve done a bit more work on it

Distributed -  This is also known as a UDL (universally distributed load), it’s simply a load that it distributed along a surface.

Concentrated – A load that is gathered on a point.

Static - A load that does not move.

Dynamic – A moving load.

Simples

So, a distributed load as opposed to a concentrated load:

And a static load as opposed to a dynamic load:

So when designing a structure it is important to keep in mind the nature of its use, is it a bridge that will be subject to dynamic loads or a table that will be subject to static loads?

One way to make sure that your structure can take a load is by reinforcing it using…

Triangulation!

Take this square:

It’s attached at all 4 corners using a pivot so if you push it in any direction it will deform, not a vary strong structure then.

If you add another member…

…all of a sudden you have reinforced the structure to reduce it’s tendency to deform. the material you use for this new member is important, if it is good under compression and tension you can make your square using just one, if it isn’t you’ll need to use two:

String is good under tension but no good under compression, you need the second member to completely stabilise the square.

You can easily tell what force is acting on your member by looking at the forces acting on the square:

This member is under tension. These are referred to as ties and the forces acting on them are represented by the arrows on either end facing inwards, this shows the internal forces at work within the tie.

This member is under compression. These are referred to as struts and the forces acting on them are represented by the arrows on either end facing outwards, this shows the internal forces at work within the strut.

Compression

This action is when a substance is put under force from each end, like standing on a block of foam, it is being compressed between your feet and the floor. If the material subject to compression reaches a point that it won’t return to it’s original state it is said to have buckled.

Tension

This is stretching a substance, like the steel cables that are used in suspension bridges. If the material is put under too much tension it can snap depending on it’s elasticity.

Torsion

Work shops are full of objects under torsion. Every time you use a screw or a clamp or a pillar drill you are witnessing torsion in action.

Shearing

Shearing force can be quite destructive, in scissors it just cuts the paper or fabric you’re working on but put it in the context of an earthquake and all of a sudden you’ve got huge sections of rock moving against one another. Rivits can also be put under shearing force if the metal plates it’sfixing together move towards or away from each other.

Bending

Bending is a mixture of compression and tension, the upper surface of the object being bent is under compression and the lower surface is under tension. A substance that is put under too much bending force can develop wrinkling and folds on the top surface and splits and cracks underneath, think of what happens when you bend up a cardboard box to put in the recycling.

There’s tone of information so I’m going to detail as much of it as I can here to act as a reference.

Structures can be broken down first into two categories: natural and manufactured

• Natural structures are classified as ‘not being made by people and occurring naturally in the environment.’
• Manufactures structures are classified as ‘being made by man and not occurring naturally in the environment.’

Simple really

These can be further broken down into more specific sections:

• Mass Structure – These consist of solid materials, a brick wall is a a manufactured example, or a mountain as a naturally occurring structure. Stone statues are a good example of a man made mass structure.
• Frame Structure – Some of the worlds most famous landmarks are frame structures, these consist of members fixed together e.g. welded or bolted.
• Shell Structure – These are cool, they involve a piece of material folded or shaped to give it strength like a piece of origami.
• Complex Structure – These use a combination of the types above, like the car body and roll cage of a rally car, shell and frame used together.

I’ve put together a gallery of examples of all the types of structures I’ve talked about above, hover over each to see what they are