## Arches - how strong are they?

Complexity level:
6
Project cost (\$):
70
Time required:
1 day for preparation 1 day for science fair project
Material availability:
May be obtained from a hardware store
Safety concerns:

Sawing should be performed by an adult wearing proper safety gear.

### Abstract

This science fair project aims to investigate the strength of an arch. The experiment is carried out using plywoodof different lengths bent into an arch and fixed to a test stand.

### Hypothesis

A larger arch is able to withstand a heavier load.

### Background

Arch

Arches are semicircle structures supported at both ends. Arch structures are used in the construction of entrances, windows and bridges.

The building of an arch will normally start with a wooden frame that will support the underside of the arch. Scaffolding is normally used together with the wooden frame for taller arches. The main component of the arch will be the keystone which is located in the middle and at the highest point of the arch  An arch which is not build properly will collapse when the frame is being removed.

Most materials that are used in the construction of arches are weak under tensile stress but they are strong in withstanding compressive stress. Such materials are  concrete, stone, iron and wood. The design of an arch is such that it is able to minimize  tensile stress . and withstand as much compressive stress as possible.

### Scientific Terms

Tensile stress, compressive stress, arch

### Materials

The materials required for this science fair project are:

-    4 plywood sheets (1 meter x 1 meter)
-    1 large pail
-    Tap water
-    1 weighing machine
-    2 fixed pillars 500 mm apart
-    1 saw
-    1 ruler
-    14 bricks (add more bricks as required)
-    1 marker pen

### Procedure

1.    The independent variable of this science fair project is the length of the arch. The dependent variable is the amount of weight place on  the arch before it breaks. This is determined by measuring the weight of the load using the weighing machine. The constants (control variables) are the distance between the pillar, the type of wood used and the dimension (width and thickness) of the wood.

2.    Cut the plywood  into smaller pieces, of width 100 mm. Cut 4 pieces of plywood into  the following lengths – 500mm, 550mm, 600mm, 650mm, 700mm, 750mm, 800mm and 850mm. Draw a line at the center of the plywood.

3.    Two pillars 500mm apart are used as a test table. See figure 1 below. Such a pillar can also be created by arranging the bricks on the sides of a door as shown in figure 1.

4.    Position the 500mm peice of plywood as shown in figure 1  Hang the pail on the plywoodwith a string. Use the line marked earlier as a guide for the center. Add water to the pail until the plywood breaks. Record the final volume of the water used.

5.    Repeat step 4 on the remaining 7 sizes of plywood. Record the measurements  in the table  provided below.

### Observation

The strength of the arch peaks at  a plywood length of 650mm  I don’t understand what this means.

 Plywood length Weight applied to the plywood arch before it breaks 500mm 550mm 600mm 650mm 700mm 750mm 800mm 850mm Weight (kg) 7.3 8.9 12.4 16.5 16.4 16.2 15.9 15.5

The graph below represents the results of our experiment

### Conclusion

The hypothesis that larger arches are able to withstand a heavier load is proven to be true to a certain extent only. As the curve of the arch increases, so does the surface tension on the upper surface of the plywood, leading to breakage.

Arches have been used in construction since the 2nd millennium BC by the Mesopotamians. Ancient arch bridges built by the Romans are still structurally sound and some of them are still standing today.

### Also consider

The science fair project can be repeated by varying the type of wood used.

The experiment can be repeated by varying the width of the plywood used.

### References

Arch - http://en.wikipedia.org/wiki/Arch

How bridges work? - http://science.howstuffworks.com/bridge6.htm