hong kong budding scientists awards (2008-2009) · introduction on 24. th. september, 2008, some...

Topic 2: Safety Analysis of the Glass-walled Buildings

Group members:

So Wun Fat

Fung Tsun Wai

Li Ka Kit

Li Kwan Yeung

Chu Wai Lun

Cheung Sha Wan Catholic Secondary School

2008-09 Hong Kong Budding Scientists Award

(Secondary Section)1st Runner-up

1

Content

1. Introduction………………………………………P.3

2. Characteristics of tempered glass………………...P.3

3. Aim……………………………………………….P.3

4. Experiment Reports

a)Stage1…………………………………………P.4

b)Stage2…………………………………………P.5

c)Stage3…………………………………………P.8

d)Stage4…………………………………………P.9

e)Stage5…………………………………………P.12

5. Conclusion……………..…………………………P.14

6. Further information………………………………P.14

7. Reference…………………………………………P.15

2

Introduction

On 24th

September, 2008, some tempered glass windows of a commercial building in Hung Hom

broke under strong wind. In this research, we will investigate the conditions causing the breakage of

tempered glass, and suggest some workable ways to minimize the chance of tempered glass breakage.

Characteristics of tempered glass

In order to reduce the possibility of glass breakage, the strength of glass can be increased by

tempering which involves heating the glass followed by rapid cooling of outer surface. This will cause

surface compression and internal tension, resulting in a smaller yield of fractures on the surface on the

glass.

Aim of Investigation

In this investigation, we are going to study the factors on the breakage of glass. Pieces of glass with

size 3ft ×3ft×3/16 inch are used to estimate the strength of glass windows (9ft ×9ft×9/16 inch in

dimension) under the following conditions:

(1) wind pressure

(2) degree of bending

(3) being hit by a free-falling rigid body

(4) being scratched

(5) improper installation (glass is only supported by a few sharp points)

3

Experiments

Stage1 Prince Rupent’s Drop As the manufacturing process of tempered glass is difficult to achieve in the school laboratory, Prince

Rupent’s Drop, showing similar properties as tempered glass, was produced for testing its properties.

Purpose: To observe properties of tempered glass.

Materials: 1. A beaker of water 2.Glass rod 3. Blowpipe 4. Tempered glass and

ordinary glass

Procedure:

1. Heat the glass rod until it melts.

2. Place a beaker of water underneath to cool down the molten glass drops.

3. Hit the Rupent’s Drop and ordinary glass with a hammer (result 1).

4. Break the tail of the tempered glass drop (result 2).

Fig 1.1 Breaking the tail of Prince Rupent’s Drop

Results:

1. The tempered glass will not break upon hitting while the ordinary glass is broken.

2. The tempered glass drop shatters when we break its tail.

Fig 1.2 Fragments of Prince Rupent’s Drop

Conclusion:

Rupent’s Drop is much stronger than the ordinary glass. However there is a weak point, by damaging

its tail, fine fragments shot out at high speed.

4

Stage 2 Strength of tempered glass under wind pressure

As the tempered glass breaks more easily in typhoon season, it is believed that the tempered glass

breaks due to strong wind. We use the difference of air pressure between the interior and the exterior

of the wooden panel to simulate the force acting on the tempered glass by the strong wind.

Purpose: To determine if the difference of air pressure can break a piece of tempered glass.

Materials: 1.Wooden panel 2. Tempered glass and ordinary glass 3. Vacuum pump

4. Pressure sensor 5. Data logger system 6.Computer

Fig 2.1 Set up of experiment

Procedure:

1. Stick the tempered glass to the frame of wooden panel.

2. Use the vacuum pump to pump out the air inside and lower the inner air pressure.

3. Measure the air pressure inside until the glass breaks by using the data-logger system.

4. Repeat the experiment with an ordinary glass piece.

5

A =Cross sectional area of glass

v =velocity of air

V =volume of air

=density of air

F =force act by the air

P =pressure

t =time

In t seconds, the volume of air hitting on the glass: V=Avt

So, the mass of air: m =V=Avt……………………………………..(1)

Assume that the air losses its momentum when it hit the window, such that

Impulse = Change of momentum

F t = mv ………………………………………....(2)

Substitute (1) into (2), we have

F t =Avt

v

So, pressure difference, P = F/A = v2

Or v = /P

Fig 2.2 Theory: Estimation of the wind speed on the glass window

Experimental Results:

Table of experimental data

Expt Type of glass Atmospheric

pressure/kPa

Lowest

pressure

attained/kPa

Difference in

pressure/kPa

Estimated

wind speed

/ kmh-1

Result

1 Tempered glass 102.5 80.5 22 487 Not broken



4 Ordinary glass 102 85 17 428 broken

Window Air

vt

Cross Sectional area, A

6

The Pressure Time Graph of tempered glass: The Pressure Time Graph of ordinary glass:

Fig 2.3 Pressure time graph of tempered glass Fig 2.4 Pressure time graph of ordinary glass

The broken ordinary glass after the experiment:

Fig 2.5 Broken pieces of ordinary glass after the experiment

Conclusion:

The tempered glass didn’t break even under the estimated wind speed of 487kmh-1

while ordinary

glass broke at 428kmh-1

. Since the highest wind speed during typhoon ever recorded is 380km h-1

, it is

concluded that the wind cannot break both the ordinary glass and the tempered glass because it is less

likely to have wind which speed is as high as 428km h-1

.

7

Stage 3 Deformation of glass

As tempered glass can withstand high pressure, we tried to find out the reason behind.

Purpose: To investigate the effect of deformation on the glass

Materials: 1. G-clamp 2.A piece of glass 3.A piece of tempered glass

Procedure:

1. Fix the position of the tempered glass with a G-clamp.

2. Tighten the G-clamp until the glass breaks and record the bending distance.

3. Repeat the experiment with ordinary glass.

Results:

1. The ordinary glass breaks when the glass reaches a bending distance of 2.5cm.

2. The tempered glass did not break even with a maximum bending distance of 7cm.

Fig 3.2 Broken ordinary glass after the experiment Fig 3.3 Deformed tempered glass after the experiment

Conclusion:

The observation shows that tempered glass has a higher flexibility and can withstand a larger

deformation. This implies that tempered glass can also withstand a greater degree of deformation than

ordinary glass and that’s why tempered glass can withstand higher pressure difference.

8

Stage 4 Testing the tempered glass by hitting it with a rigid body (part 1)

Previous experiments prove that the breakage of tempered glass is not due to strong wind alone. It is

suggested that the tempered glass may be broken when hit by fast-moving rigid body during typhoon.

Materials: 1. Tempered glass 2. A wooden stand

3. A dry cell (93g) 4. A copper mass (200g)

5. A roll of thread 6. 2 punches

Part A

Purpose: To estimate how much force can break the tempered glass and regular glass by

dropping a rigid body to the glass at certain height.

Fig 4.1 Set up A of experiment

Procedure: (set up A)

1. Fix a piece of tempered glass on the wooden panel which is placed horizontally on the ground.

2. Release a rigid body to hit the tempered glass at different heights until it breaks.

3. Repeat the experiment with ordinary glass.

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Part B

Purpose: To simulate how the presence of small particles stuck in the window frame weaken the

strength of the glass by supporting it with two punches.

Fig 4.2 Set up B of experiment

Procedure: (set up B)

1. Fix two punches onto the lower groove of the wooden frame.

2. Insert the tempered glass into the wooden frame which is placed vertically.

3. Release the copper mass to hit the tempered glass at different heights until it breaks.


(set up A)

Type of

glass

Height (h/m) Mass (m/kg) Estimated velocity

ghv 2

(ms-1

/ kmh-1

)

Scale up

mass

(M/kg)

Scale up velocity

(ms-1

/ kmh-1

)

Result

Tempered 4.4 0.2 9.4 / 33.8 5.4 28.1 / 101 Not broken

Tempered 8.2 0.2 12.8 / 46 5.4 38.4 / 138 Broken

Ordinary 4.4 0.2 9.4 / 33.8 5.4 28.1 / 101 Not broken

Ordinary 8.24 0.2 12.8 / 46 5.4 38.4 / 138 Broken

(set up B)

Type of

glass

Height from

the floor

(h/m)

Mass (m/kg) Estimated velocity

ghv 2

(ms-1

/ kmh-1

)

Scale up

mass

(M/kg)

Scale up velocity

(ms-1

/ kmh-1

)

Result

Tempered 0.55 0.2 3.32 / 11.9 5.4 9.95 / 35.8 Not broken

Tempered 1.5 0.2 5.48 / 19.7 5.4 16.4 / 59.2 Not broken

Tempered 2.5 0.2 7.07 / 25.5 5.4 21.2 / 76.4 Broken

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Fig 4.3 Broken ordinary glass after the experiment A

Fig 4.4 Broken tempered glass after the experiment B

Conclusion:

The tempered glass and ordinary glass can both withstand a 5.4kg mass with a speed of

101 kmh-1

while the tempered glass supported by punches is already broken at a speed of 76.4 kmh-1

.

There is a possibility that the glass window may break when being hit by fast-moving rigid body

during typhoon.

However, in experiment A, we cannot distinguish the difference between the strength of tempered

glass and ordinary glass in normal installation as repeated impacts may weaken the glass.

The result also shows that tempered glass is much safer than ordinary glass. The shards of ordinary

glass are obviously large and sharp and will cause serious injury if anyone is behind. However, the

small fragments of tempered glass are round and will no doubt cause minor injuries only.

By comparing part B with A, the tempered glass is severely weakened when it is supported by

punches. Therefore, if there are particles present in the window groove, tempered glass is weakened.

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Stage 5 Testing the tempered glass by hitting it with a rigid body (part 2)

To achieve higher accuracy at a cheaper cost, we mass produce homemade tempered glass by heating

microscope slides at 650oC in a ceramics oven and then cool down rapidly by hairdryers and a

ventilation fan.

Fig 5.1 Producing homemade tempered glass

As sand in the wind can make scratches on the surface of the glass during typhoon, we try to find out

if tempered glass is weakened after it is scratched.

Purpose: To estimate how much force can break the ordinary glass, tempered glass and scratched

tempered glass.

Materials: 1. Tempered glass 2. 2 sets of clamp and stand

3. A metal sphere (8.4g) 4. 2 wooden blocks

5. An electromagnet 6. Adhesive tape

Fig 5.2 Set up of experiment

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Procedure:

1. Fix the position of tempered glass on the wooden block using adhesive tape.

2. Hold the metal sphere using the electromagnet.

3. Release the metal sphere and hit the tempered glass at different heights.

4. Repeat the experiment using ordinary glass and scratched tempered glass.


Type of

glass

Height

(h/m)

Estimated velocity

ghv 2

(ms-1

/ kmh-1

)

Scale up

mass

(M/kg)

Scale up velocity

(ms-1

/ kmh-1

)

Result

Tempered 0.7 3.74 / 13.5 18.5 48.6 / 175 Not broken

Tempered 1.1 4.69 / 16.9 18.5 61.0 / 220 Broken



Tempered 1.085 4.66 / 16.8 18.5 60.6 / 218 Broken

Tempered 1.08 4.65 / 16.7 18.5 60.4 / 218 Broken

Ordinary 0.55 3.32 / 11.9 18.5 43.1 / 155 Not broken

Ordinary 0.56 3.35 / 12.0 18.5 43.5 / 157 Not broken

Ordinary 0.57 3.38 / 12.2 18.5 43.9 / 158 Broken

Scratched 0.5 3.16 / 11.4 18.5 41.1 / 148 Not broken

Scratched 0.62 3.52 / 12.7 18.5 45.8 / 165 Broken

Scratched 0.59 3.44 / 12.4 18.5 44.7 / 161 Not broken



Conclusion:

The tempered glass slide broke when the 8.4g metal sphere hit it at about 1.08m high while the

ordinary glass slide broke when the sphere hit it at about 0.57m high.

It is concluded that tempered glass is stronger than ordinary glass. However, when it is scratched, it is

severely weakened to the strength similar to that of ordinary glass.

13

Conclusion

According to our investigation, tempered glass is stronger than ordinary glass. It is unlikely that wind

pressure alone can break the tempered glass. However, the tempered glass can also break when it is hit

by a rigid body at high speed. Yet, the tempered glass is safer because it is stronger and its fragment is

not large and sharp when it breaks.

It is proved by our experiment that a piece of glass will be seriously weakened when it is installed

improperly. Therefore, the installation workers should make sure there is no debris in the window

groove before any installation process.

Furthermore, it is proved that even a small scratch will severely weaken the tempered glass, so surface

protection such as coating should be introduced.

Further information

We found from the internet (ref. 5) that 0.3%-3% tempered glass can break due to spontaneous

shattering, because of the presence of the impurities like nickel sulphide. Consequently, laminated

glass with sheets of polyvinyl butyral (PVB) sandwiched should be used. Even if this kind of glass

breaks, the PVB sticks it onto its original position. This makes the damage visible and allows people

to replace it before any accidents occur. Yet we cannot test these properties in school laboratory.

14

Reference

1. Glass on Web

http://www.glassonweb.com/glassmanual/topics/index/tempered.htm

2. Toughened glass From Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Tempered_glass

3. Architectural glass From Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Architectural_glass

4. Tempered safety glass

http://www.prelco.ca/pdf/fiches/Prel-Gard_A.pdf

5. A review of the nickel sulphide induced fracture in tempered glass

http://www.aisglass.com/heat-soaking/AIS-13.pdf

6. Exploding Glass: Rupert's Drop - Museum of Glass

http://hk.youtube.com/watch?v=8-GOwtikSO0

7. Prince Rupert’s Drop

http://hk.youtube.com/watch?v=6V2eCFsDkK0

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http://www.glassonweb.com/glassmanual/topics/index/tempered.htm

http://en.wikipedia.org/wiki/Tempered_glass

http://en.wikipedia.org/wiki/Architectural_glass

http://www.prelco.ca/pdf/fiches/Prel-Gard_A.pdf

http://www.aisglass.com/heat-soaking/AIS-13.pdf

http://hk.youtube.com/watch?v=8-GOwtikSO0

http://hk.youtube.com/watch?v=6V2eCFsDkK0

Comments

The experiments were well designed. The students could make good use of the school laboratory apparatus and equipments to simulate the real life situation. In order to increase the reliability of the report, the students may also further investigate the effect of glass size on the strength of glass windows under different conditions.

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hong kong budding scientists awards (2008-2009) · introduction on 24. th. september, 2008, some...

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