3. EXPERIMENTAL WORK

3.1Experiment 1 – Compare the Strength of Square Tube

and Triangle Tube

3.1.1 Introduction

The purpose of this experiment is to decide which shaped tube will be used in the specified section. After long-time selection, the principal shapes of the tube’s bottom surface were chosen to be square and triangle. The main problem was that where these two types of tubes could play their best role. The places where tubes were designed were vertical supporters and horizontal girders. The difference between these two places was that the tubes’ stress states were totally different. Under this background, this experiment was launched and the results would be compared to place the tubes appropriately.

3.1.2 Experimental Arrangement & Procedure

This experiment had two parts. The first part was to test the tubes’ ability of taking horizontal stress. The equipment used is listed in the following table:

Object DescriptionTwo tables Places for tested tubes

Plastic bucket To load the pebblePebble To add weight to the tested tube

Electronic weigher

To measure the maximum load of the tested tube

The schematic diagram of the experimental apparatus is shown:

Figure 1 Experiment 1 first part

The procedure was followed by the following steps strictly. Firstly, the tested tubes were placed between the tables. The location was illustrated by Figure 1. Next, the bucket was hanged at the middle of the test bar, remaining empty. The string connecting the bucket was kept in steady so that the bucket would not rock. Then, the pebble was sent to the bucket by shovel slowly. The amount of pebble sent in one time was small, in order to increase the accuracy of the result. When the pebble was poured into the bucket, the bucket was kept in balance. This step continued until the tube could not bear the load, for example, appearing deflection significantly. Finally, the bucket loaded was taken to the electronic weigher to be weighed.

There were three square tube and three triangular tube tested in this experiment.

The second part of this experiment was to test the tubes’ ability of taking vertical stress. The equipment was changed by some items. Two tables were not required in this experiment. However, there was a new wooden trestle added. The diagram of facilitate was shown:

Figure 2 Experiment 1 Second Part

In terms of the procedure, first, the loading arm was correct and clamped by the pivot. The standard was that the loading arm should at the same plate of the base. After that, the tested tube was placed beneath the C-line. The tubes were kept in vertical direction strictly. Then, the bucket was placed right at the notch. The requirement was the same with what was referred to in in the first part of Experiment 1. Next, the pebble was transferred by the shovel to the bucket slowly, same with the step in the first part. Finally, after the tube was out of shape, the loaded bucket was weighed by the electronic weigher.

There were also three square tube and three triangular tube tested in the second part of Experiment 1

3.1.3 Results

The result of the maximum load in the first part experiment is shown in Table 1. The average for square tube is 3.24kg, while that for triangular tube is 2.52kg.Definitely square tube is more appropriate to take the column place.Table 1. The result of the maximum load in the first part experiment

Number of Tube Type Maximum Load/kg1 Square 3.252 Square 3.303 Square 3.184 Triangle 2.615 Triangle 2.506 Triangle 2.47

The maximum loads for tubes in vertical direction are shown in Table 2. In comparison, it is obvious that triangular tubes can hold more vertical stress than square tube.

Table 2. The maximum loads for tubes in vertical direction

Number of Tube Type Maximum Load/kg1 Square 2.672 Square 2.543 Square 2.724 Triangle 3.515 Triangle 3.436 Triangle 3.41

3.1.4 Discussion

It is clear that square tube is good for holding horizontal stress and triangular tube is good for holding vertical stress. In theory, triangle is more stable than square because triangle is more difficult to be out of shape. Moreover, it is more difficult to let triangular tube have deflection, as square tube can be bent easily. Therefore, as a supporter, triangular tube is stronger than square tube. However, if triangular beam is put on the top of the bridge, there is nearly only a line to support the stress. Therefore, the stress for the triangular tube is much larger than the real stress. That could be the reason why square tubes can bear more horizontal stress.

3.1.5 ConclusionsAccording to the outcomes of Experiment 1, triangular tube is suitable for supporters and square tube is fit for columns. Thanks to this experiment, the design of the whole bridge was basically decided.

3.2 Experiment 2 –Test the Strength of Each Members and Calculate the Factor of Safety

3.2.1 Introduction

The purpose of this experiment is to decide the rightful size of the tube which can be the smallest to reach the requirement of load-bearing. On one hand, the principal factor of the tube which is quite significant is length; on the other hand, the factor of bars which should be seceded is the thickness. Therefore, this experiment was supposed to determine the suitable length of tubes and thickness of bars.

3.2.2 Experimental Arrangement & Procedure

The equipment needed in Experiment 2 was the same with the second part of Experiment 1. That was because the principals of these two experiments were

the same. The schematic diagram of the experimental apparatus was also the same with Figure 2. During the operation of Experiment 2, the procedure was similar with that in the second part of Experiment 1. The only difference was that the tested bars should be placed beneath the T-Line, as the bars should take tensile stress. Before the bar was tested, bars should be stuck with some scraps of paper in order to fix them on the arms. At first, usually the bar would be broken at the agglutinate place. This situation was regarded as the meaning of the failure of the test. The reason was that too much glue weakened the strength of the bar. This condition could not be avoided so that the data for bars should have some error.

3.2.3 Results

Test # Cross-Section Length

Number of

Specimens

Applied Failure load:

weight of sand and bucket

(mF/kg)

Average specimen strength*

(Fs/Newton)

T1 4 mm-wide bar 24.5 cm 3

1.1.312.1.213.1.45

38.906

T2 6 mm-wide bar 24.5 cm 3

1.1.652.1.683.1.93

51.548

T3 8 mm-wide bar 24.5 cm 3

1.2.102.1.673.1.82

54.782

C1 10 mm x 10 mm tube 5 cm 3

1.3.132.3.203.3.02

91.630

C2 10 mm x 10 mm tube 10 cm 3

1.2.802.2.763.2.85

82.418

C3 10 mm x 10 mm tube 16 cm 3

1.2.212.2.053.1.90

60.368

C4

8 mm x 8 mm x 8mm

triangular tube

5 cm 3

1.3.422.3.393.3.43 100.352

C5 8 mm x 8 mm x 8mm 7 cm 3

1.3.362.3.32 98.098

triangular tube

3.3.33

C6

8 mm x 8 mm x 8mm

triangular tube

9 cm 3

1.3.232.3.043.3.17 92.512

Because of the set of the equipment, according to the lever law, the actual stress taken by the sample was:

Fs=(3/2L)/(L/2)×mF×9.83.2.4 Discussion

The graphs of the results were made in order to discover the relationship between the different conditions of the samples and their ability of taking stress correspondingly:

4 mm 6 mm 8 mm0

10

20

30

40

50

60

Bar sample

Figure 3

5 cm 10 cm 16 cm0

10

20

30

40

50

60

70

80

90

100

Square Tube

Figure 4

5 cm 7 cm 9 cm88

90

92

94

96

98

100

102

Triangular Tube

Figure 5

According to the Figure 3, the width and the strength of bars follow the linear relationship although the third data might have some error. This shows that the strength of bars is proportional to the width of bars. However, as the slope of the line is quite small, it can be deduced that the growth of the strength cannot be considerable even if the width is enlarged. In addition, the general strength of bars is quite weak.In terms of tubes, according to Figure 4 and 5, the length and the strength of tubes follow inverse relation. And the relationship is not linear, which means

the longer tube will be much weaker. As the barycenter of the long tube is high, the shear force followed by the vertical stress will break the balance of the tube easily. That is why short tubes can be stronger. Moreover, generally, triangular tubes are much stronger than square tubes. This result has been mentioned in Experiment 1. The best part is that the decreasing amplitude of the strength of triangular tubes is much less than that of square tubes. Therefore, to this bridge, the triangular tube is really an appropriate material.

3.2.5 Conclusions

Experiment 2 gave the strength of each component, which could be significant to calculate the factor of safety. Besides, triangular tubes were proved to be the reliable material of building the bridge according to Experiment 2.