Construction of a simply supported structure with central charge

Construction of a simply supported structure with central charge The structure has to be made with wooden

toothpicks and fulfill a number of

requirements

1-12-2014

Josep Mª Carbonell, Oriol Chandre, Sergi Molina and Miguel Pareja

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Contenido

1- Introduction to the project ................................................................................................... 2

Material ..................................................................................................................................... 2

Requirements ............................................................................................................................ 2

1- Design and manufacturing .................................................................................................... 3

Final outcome ............................................................................................................................ 7

2- Conclusions ......................................................................................................................... 10


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1- Introduction to the project

This is a project conducted at the ETSEIAT-UPC, in which the students had to study the

problem posed and find the design which fulfill all the requirements and, furthermore,

using only the allowed material. The problem was:

“Make a beam which can only be supported by two points and must bear 10 kg at

least.”

Material

The beam had to be composed of:

6,5 cm or shorter flat wooden toothpicks.

Glue (Freedom of choice).

Newsprint.

Requirements

The minimum length was 1m.

The maximum weight was 1Kg.

The glue proportion had to be fewer than 25% of the total material.

At the time of the choice of the wooden toothpicks that we were going to use, NOSTRUM

provided us with the harder ones we could find.


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1- Design and manufacturing

We started the design of the beam studying the distribution of charges along the structure, to

know where and how we had to strengthen it. We concluded that the most important effect

was the normal tension produced by the bending moment, and the central stretch was where

it was going to be higher. So at this moment we realized that we needed the greatest moment

of inertia we could make in the central stretch. We also decided that the length was going to

be 1,5 m.

We decided that the main structure of the beam was going to be cubes reinforced with

diagonals, cutting the toothpicks to the maximum length. After thinking about how we could

strengthen the structure with the newsprint, we designed an empty thick stick which was very

resistant to compression and a paper strap very resistant to tension. The position of both was

as can be seen in the picture:

The vertical stick was embedded in the central stretch and the strap was tied at the both ends

and compressing the stick, distributing the central charge to the ends.

1- Design of the beam


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We started making the corners of the cubes in a self-made template to optimize the time used

to make the number needed.

2- Corner template

Once done all, we began to put them together forming squares, and producing the firsts cubes.

3- Forming squares


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We finished both mid-beams structures, added two cubes at both ends to secure the stability,

and we had done the vertical paper stick.

4- Mid-beams and vertical stick


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We put together both sides, embedded the vertical stick, added two levels of cubes more on

the center and added wood toothpicks layers to improve endurance and increase the moment

of inertia.

5- Levels of cubes added

6- Wood toothpicks layers


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At this time just missing the strap, and after making and positioning it, we hooked two strips of

newsprint to spread the load and delay the bending of the beam.

Final outcome


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Finally the beam resisted 20 Kg, but it didn’t fail because the bending moment as we expected,

but failed due to the torque. The main point of failure was the section where the strap crossed

the beam.


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2- Conclusions

After the project, we can conclude some aspects which could improve its execution:

Seeing what was the point of failure, we can ensure that strengthening the section

where the strap crossed the beam, the mass which makes the beam failing would be

much higher.

Another improvement needed is to pay more attention on the stability.

Once the entire project had finished, we realized that this design was very resistant,

but it took much more time than other designs which could have similar properties. It

is a question between the results needed and the time available.

Engineering

Construction of a simply supported structure with central charge