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ELSEVIER Construction and Building Materials 20 (2006) 761 - 768 MATERIALS www.elsevier.comlIoca te/con b ui Idma t

Structural performance of laminated and unlaminated tempered glass under monotonic transverse loading

Amir Fam a,., Sami Rizkalla b a Queen's University. Kingston. Ont .. Canada K7L 3N6

I> North Carolina SW(e University. Centennial Campus. Raleigh. NC 27695-7533. USA Received II August 2004; received in revised form 3 November 2004; accepted 31 January 2005

Available online 7 April 2005

Abstract

A total of thirty six bending tests have been conducted on 1220 x 460 mm sheets of glass, 9.5-, 12.7- and t5.9-mm thick, using slow-rate monotonic loading. Twenty four specimens were laminated on one side using either one or two 0.36-mm thick polyester transparent laminates. The study showed that lamination has significantly changed the failure mode of glass from a catastrophic failure, where fragments of glass shatter in different directions, to one which is still brittle yet safer, as the fractured glass remains fully intact. The average gains in flexural strength, stiffness and strain energy, as a result of lamination, were 20%, 10% and 34%, respectively, while the maximum gains in flexural strength, stiffness and strain energy were 36%, 33% and 52%, respectively. Because of the scatter of data, no specific correlation between the gains and reinforcement ratio (expressed as the ratio of laminate-to-glass thickness) could be established. The load-deflection behaviour of both laminated and unlaminated glass was linear up to failure. No rupture or delamination of the laminates were observed. 2005 Elsevier Ltd. All rights reserved.

Keywords: Glass; Retrofit; Laminated; Shatter; Monotonic loading; Pressure; Plexure

L Introduction

Glass is increasingly being used in the construction industry. Tempered glass, in particular, is being used in roofing applications. Glass, which is quite a brittle material, is generally vulnerable to failure due to a num-ber of reasons, including excessive wind loading in hur-ricanes, accumulation of snow in overhead roofing applications or due to terrorism and vandalism acts. An additional problem with broken tempered glass in overhead applications is its tendency not to break into small parts. It may rather fall in large clumps, which could lead to serious human injuries and possible life threats [1].

Corresponding author. Tel.: + I 6135336352; fax: + I 613 533 2128. E-mail address:fam@civi1.queensu.ca (A. Fam).

0950-06 18/$ see front matter 2005 Elsevier LId. All rights reserved . doi: 10.1016Jj.conbuildmat.2005.01 .05 I

Tempered glass, sometimes referred to as toughened glass, is produced by heating ordinary annealed glass to just below its softening point, to about 650C, and then cooling it rapidly with blasts of air. This causes the surface of the glass to cool more rapidly than the in-ner core, which in turn causes the outer zones of the glass to be under compressive stresses, while the inner core is under tensile stresses. These stresses are in a state of equilibrium and are generally not less than 70 MPa [2], This 'prestressing' effect results in increasing the bending strength of glass by four to five times, compared to ordinary glass. It also changes the failure mode of the glass, from shattering into few large and sharp pieces; to small pieces (diameter less than 10 mm), without sharp edges [3]. However, clusters of the small broken pieces are often lumped, and when falling from a height, could induce severe injuries as indicated earlier.

762 A. Fam. S. Rizkalla I Construction and Building Materials 20 (2006) 761- 768

The term " laminated glass" or "sandwich glass" of-ten refer to two or more glass plies bonded together with an elastomeric interlayer such as polyvinyl butyral (PYB) to improve the post-breakage characteristics of the glass. This type of glass is usually prefabricated in this form before installation and commonly used in automotive vehicle windshields. In this paper, however, the term "laminated glass" is used within a different con-text to indicate a regular single shect of tempered glass, which is retrofitted, either under service conditions or before installation, using a special polymeric transparent lamina attached to the cxternal surface of the glass to enhance its performance and failure mode. This simple technique is quite useful and economical, compared to sandwiched glass. While the structural performance and failure modes of standard tempered glass and sand-wiched glass have been studied experimentally and numerically under transverse loading [4- 6], the behav-iour and failure mode of externally laminated tempered glass have not been studied. In this paper, tempered sheets of glass of different thicknesses have been exter-nally laminated using transparent polyester laminate and tested in flexure. The study is focused on examining the effects of lamination on failure mode, flexural strength, stiffness, and strain energy, as compared to unlaminated glass.

2. Experimental program

In this section, the transparent polymeric laminate and the installation procedure are described. The test specimens, along with the test setup and procedure are also described.

2.1. Laminate material and installation

The laminate used in this study consists of three lay-ers of polyester films, bonded together using acrylic

Table 1 Summary of test matrix and results

Specimen Number of Glass Thickness Lamination J.D. specimens product (mm)

Al A2 A3 A4 A5 BI B2 B3 B4 B5 B6 B7

3 each

3 each

A 12.7 12.7 12.7 15.9 15.9

B 9.5 9.5 9.5

12.7 12.7 12.7 15.9

No I-Laminated 2-Laminated No I-Laminated

No I-Laminated 2-Laminated No I-Laminated 2-Laminated No

Fig. 1. Typical polyester O.36-mm laminate, showing removal of protective layer.

pressure-sensitive adhesive. The cold-lamination process under high pressure provides high shock absorption per-formance and superior optics at the same time. The lam-inate has a total thickness, including adhesive, of 0.36 mm, a tensile strength of 193 MPa and a Young's modulus of 3.8 GPa, determined in accordance with ASTM (D 882-75, 1004-76 and D 1938-67) [7,8]. The laminate has a visible light transmittance capacity of 92%, a total solar energy rejection of 17% and an ultra-violet light transmittance of 0-5%. The side of the lam-inate, which is bonded to glass, has a layer of acrylic pressure-sensitive adhesive, coated with a thin protective film. This film can easily be peeled off, prior to installa-tion, in a similar fashion to wall paper, as shown in Fig. I. For retrofit application, the surface of ordinary glass is cleaned and dried, followed by installation of the lam-inate (or multiple laminates) under high pressure. For glass replacement applications or new glass installations, the laminate is pre-installed on typical standard size an-nealed or tempered glass, using the same process, and shipped to the site. The peel strength of the laminate is 3.86 MPa.

2.2. Description of test specimens and parameters

The experimental program included a total of 36 tests conducted on both unlaminated and laminated

Reinforcement Ratio C% age) Average test results at maximum load Load Deflection Stiffness Energy (kN) (mm) (kN/m) (kNm)

0 9.25 30.9 300 0.143 2.83 9.53 31.4 303 0.150 5.67 9.05 31.0 292 0.140 0 14.00 24.3 576 0.170 2.26 17.37 29.5 588 0.257

0 3.80 37.5 101 0.072 3.79 4.90 44.0 III 0. 109 7.58 4.59 46.0 100 0.106 0 6.86 30.5 225 0.105 2.83 8.95 34.6 293 0. 155 5.67 9.36 31.1 300 0.146 0 14.49 22.8 637 0.167

A. Ftt/H, S. Ri=kttlla I Construction and Building MateriaJ~' 20 (2006) 761- 768 763

.:;;:;::=3t.~~ Steel rollers

330

Potentia meter

-1< Elevation

330 -I

Fig. 2. Test setup for monotonic loading.

tempered glass specimens. Two different glass products, provided by two suppliers from the United States and Canada were used in this study and referred to, herein, as Glass A (provided by Virginia Glass) and Glass B (provided by Laurier Glass) . All specimens consisted of 1220 x 460 mm sheets of glass with polished edges. Three different thicknesses were investigated in this study, namely 9.5, 12.7, and 15.9 mm. Table I provides summary of test specimens. Specimens AI - AS are of Glass A, whereas BI- B7 are of Glass B. Three identical specimens were tested for each parameter to provide reliable average values. Specimens AI , A4, BI, B4 and B7 were unlaminated, and were used as control speci-mens. Specimens A2, AS, B2 and B5 were laminated with a single laminate applied to one face . Specimens A3, B3 and B6 were laminated using two laminates (O.72-mm thick), both applied to the same face. The laminates were applied to the entire surface area of glass. This scheme has resulted in reinforcement ratios, defined as the ratio of thickness of laminate to glass, ranging from 2.26% to 7.58%.

The behaviour of specimens (A I, B4) and (A4, B7) was used to compare the two types of glass (A and B),

20 A4

16

Z 12 e. "0 ro 0 8

...J

4

10 20 30

15.9mm AS UIlLam

40 50 10 Deflection (mm)

(a) Glass A.

20 30

for the same thickness. The behaviour of specimens (AI - A3), (A4, AS), (BI- B3), and (B4-B6) was com-pared to examine the effect of lamination, including the number of laminates, on the behaviour. Finally, specimens (AI, A4), and (BI, B4, B7) were com-pared to examine the effect of thickness of glass on the behaviour.

2.3. Test setup and procedure

The experimental program described in this paper was intended to simulate lateral pressure applied gradu-ally to glass at a slow rate, over a period of time. As such, all specimens were tested to failure under mono-tonic loading, using a four-point bending configuration as shown in Fig. 2. An MTS hydraulic actuator was used to apply the load using stroke control at a rate of 0.5- 1.0 mmlmin. This setup is a modified version of the one used to determine the strength of glass in flexure (ASTM C 158-95) [9], mainly to accommodate full scale specimens in this case. The span between supports was 990 mm, while the distance between the loads was 330 mm. Steel rollers of 50-mm diameter were used at

20 15.9mm B7 15.9mm

ILam UIILAm 16

Z12 e. "0 ro 0 8

...J

4

40 10 20 30 40 50 Deflection (mm)

(b) Glass B. Fig. 3. Load-deftection behaviour of 15.9mm glass.

764 A. Fam, S. Rizkafla I Construction and Bllilding Materials 20 (2006) 761- 768

20 A1 12.7mm A2 12.7mm A3 J2.7mm

16 Un-LAm I-lAm 2-um

Z 12 C " '" .3 8

4

10 20 30 40 50 10 20 30 40 50 10 20 30 40 50 Deflection (mm)

(a) Glass A .

20 64 12.7 mm 65 12.7mm 6 6 12.7 mm

Ull-Lam I-Lam 2-Lam 16

Z 12 C " '" 0 8

...J

4

0 0 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50

Deflection (mm) (b) Glass B.

Fig. 4. Load-defleclion behaviour of 12.7-mm glass.

20 61 9.Smm 62 9.Smtn 6 3 9.5mm

UIl-lAm I-Lam 2-Lam 16

Z 12 C " '" 0 8

...J

4

0 0 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50

Deflection (mm) Fig. S. Load-deflection behaviour of9. 5-mm Glass B.

both loading and support points. Rubber pads, 12 mm thick, were placed between the steel rollers and surfaces of glass in order to avoid stress concentrations. Deflec-tions were measured using two electric potentiometers

at mid span, at both sides, and also at the center of each support to account for the settlement resulting from the rubber pads. This configuration allows for measuring the net deflection.

A. Fam, S. Rizkalla I Construction and Building Materials 20 (2006) 761- 768 765

24

22

20

16 ( 1 layer) (2~) (ll1yw) (2I1)'1f)

16

Z 14 ~

" ..,

I (a) Maximum load capacity I ~ 10 0 -'

8

6

4

2

0 60

54

48 I (b) Maximum deflection I E

42

.s 36 0 30

.Q 1:1

~ 24 = ID Cl 18

12

6

0 800

no

640

:f 560 I (e) Stiffness I Z C 480 0 0 400 ID

"' ij5 320

240

160

80

0.3

0.25 I (d) Energy I f 0.2 Z

~ 0.15 '" e>

::1 ID 0 W 0.1

0.05 :' .,1 :,

Fig. 6. Comparison of unlaminated and lami nated specimens o f Glass A and B.

3. Test results and failure modes

The load--

766 A. F(lIIl, S. Ri:kalla I Comtrue/iull and Building Materials 20 (2006 ) 76/- 768

3.1. Effect of type of glass 3.3. Effect of lamination

Specimens A I and B4 (of Glass A and B) were both 12.7-mm thick and un laminated. The ultimate load and stiffness of Al were 35% and 33';', higher than B4, respec-tively. Specimens A4 and B7 were both 15.9-mm thick and unlaminated. The ultimate load and stiffness of A4 were about 3% and 9% lower than B7, respectively. Therefore, the strength and stiffness ratios of types A and B glass do not appear to be consistent for all thick-nesses. As will also be confirmed later, specimen B7 seems to have higher strength and stiffness than expected.

3.2. Effect of thickness of glass

Specimens Al and A4 of Glass A were 12.7- and 15.9-mm thick, respectively. The ratios of strength and stiff-ness of A4-Al were 1.51 and 1.92, respectively, which are In good agreement with thickness ratios [(15.9:12.7)' = 1.57] for strength and [(15.9:12.7)3 = 1.96] for stiffness, using basic mechanics principals. Specimens BI , B4, and B7 of Glass B were 9.5-, 12.7-and 15.9-mm thick, respectively. The ratios of strength and stiffness of B7 to B4 were 2.11 and 2.82, respec-tively, which are not in good agreement with thickness ratios [(15.9:12.7)' = 1.57] for strength and [(15.9:12.7)' = 1.96] for stiffness. The ratios of strength and stiffness of B4 to BI were 1.81 and 2.23, respec-tively, which are in good agreement with thickness ratios [(12 .7:9.5)' = 1.79] for strength and [(12.7:9.5)3 = 2.39] for stiffness. These results confirm that an inconsistency, perhaps related to quality control and tempering pro-cess, appears to be associated with specimens B7 (15.9-mm Glass B) and results in higher strength and stiffness than expected.

60 Load

50 0 Deflection

6 Stiffness .. 40 '"

Energy

'" i" "

30 E II .. co 20 '"

0

.... 0 10

0 6 ~ -10

o 2 3

0

6

Fig. 6 shows a comparison of all test specimens, both laminated and unlaminatcd, in terms of maximum load , deflection , stiffness and elastic strain energy. The stiffness is defined as the slope of the load-deflection curve in this case, whereas strain energy is defined as the area under the load- deflection curve. Fig. 7 shows a relation between the reinforcement ratio, represented as the ratio of thickness of laminate to that of glass, and the percentage increase in ultimate load , deflection, stiffness and the strain energy. While Figs. 6 and 7 show that gains in strength, stiffness, and strain energy, up to 36%, 33%, and 52%, respectively, were observed as a result of the lamination, Fig. 7 clearly shows a wide scatter of data, which makes it very difficult to establish a correlation between the reinforcement ratio and the gain in strength, stiffness, or strain energy. This could be attributed to the very brittle nature of glass and its sensitivity to any slight difference in quality control during the tempering process. Based on test re-sults, however, the average gains in strength, stiffness and strain energy were 20%, 10% and 34%, respec-tively. It is also clear from Fig. 6 that using two lami-nates does not provide additional gains, compared to one laminate. This is possibly attributed to the lack of composite action (horizontal shear transfer provided by adhesive) between the layers at high loads. Also, the structural contribution of lamination is low because of its low stiffness. The ultimate strain of glass, based on the measured loads, is less than 0.0019, which corre-sponds to very low stress in the lamina, due to its low modulus. Perhaps the most pronounced advantage of lamination is its effect on failure mode as will be dis-cussed next.

6 ~

>l !! u

4 5 6 7 8 Reinforcement ratio (% age)

Fig. 7. Effect of laminate reinforcement ralio on ultimate load. deflection, stiffness and strain energy.

A. Fam, S. Rizkalla I Construction alld Building Materials 20 (2006) 761- 768 767

3.4. Failure modes

A distinct difference in failure mode between the lam-inated and un laminated glass was observed for all range of thicknesses and for both Glass A and Glass B. Fig. 8(a) shows an un laminated IS.9-mm Glass A, specimen A4, with high deflection, moments before failure . Fig. 8(b) shows the same specimen A4 and also specimen B7 immediately after failure. The unlaminated glass shatters violently, once the modulus of rupture of glass is attained. Fig. 8(b) clearly reflects the very brittle and catastrophic nature of the failure. Fragments of glass were scattered and have travelled more than 6 m away

from the specimen. All unlaminated specimens failed in this manner. It is envisioned that serious injuries and panic would have definitely resulted, had a similar sce-nario been encountered in a real structure. Careful examination of the failure of both un laminated Glass A and B shows that, while both were extremely brittle, Glass B shatters into smaller fragments compared to Glass A as shown in Fig. 8(b). This could be attributed to slight differences in the tempering processes of both types.

Fig. 8(c) shows the laminated glass after failure . Although the glass itself was completely fractured in every direction, throughout its entire surface, all pieces

(a) Glass sped mens with large deflection just before failure.

(b) Unlaminated glass just after failure.

(c) Laminated glass just after failure. Fig. 8. Deflection and failure modes of unlaminated and laminated glass.

768 A. Fum. S. Ri:kuffa I Con;yirtlctioll and Bllilding Mmeria/s 20 (2006 ) 761 -768

were contained together as one unit due to the presence of the laminate. All laminated specimens failed in this manner. Fig. 8(c) also shows that the specimen main-tains the same deflected shape after fai lure. No signs of failure of the laminates were observed. It appears that, once the bottom fibre of glass reaches the modu-lus of rupture and cracks, the bonded lamina fails to sustain the tensi1e stresses and maintain the applied load (unlike other composite systems, such as rein-forced concrete for cxample). This is mainly due to the very low modulus of the lamina (3.8 GPa, com-pared to 66 GPa of glass), which leads to excessive deformations without rupture of the lamina. This fail-ure mode was distinctly different from that of unlami-nated glass and was certainly quite safer and less catastrophic. A failed laminated specimen was easily removed from the test setup as one unit. Similarly, in an actual structure, failed laminated panels would be easily removed and replaced.

4. Summary and conclusions

This study included a total of 36 tests conducted on both unlaminated and laminated clear float tempered glass sheets of thrcc different thicknesses (9.5, 12.7, and 15.9 mm). Monotonic loading was applied at a slow rate to simulate a rather gradual and sustained pressure over a short period of time. The lamination used in-volved either one or two 0.36-mm thick polyester lami-nates attached to one side of the glass. Based on this study, the following conclusions are drawn:

I. The most distinct advantage of lamination is that it significantly changes the failure mode from a brittle and catastrophic failure, where small fragments of glass shattcr in different directions, potentially caus-ing serious injuries, to one which is still biittle yet safer, as the fractured glass remain intact and can eas-ily be removed and replaced.

2. The average gains in flexural strength, stiffness and strain energy as a result of lamination were 20%, 10% and 34%, respectively. Although gains as high as 36%, 33% and 52% in strength, stiffness and strain energy were observed, the wide scatter of data made it difficult to establish a specific correlation between the reinforcement ratio (thickness of laminates-

to-thickness of glass) and the gains. The scatter of data could be attributed to the very brittle nature of glass and its sensitivity to slight variations in temper-ing process, especially for different thicknesses, where the cooling rate across the thickness affects the level of residual stresses.

3. Adding a second laminate may have an insignificant effect on strength, stiffness, strain energy, and failure mode.

4. The load-deflection behaviour of both laminated and unlaminated glass is generally linear up to failure.

5. No rupture or delamination of the laminates were observed.

6. The lamination process is quite suitable for both ret-rofit of glass in existing structures as well as for new structures, where pre-laminated glass can be installed in the field.

Acknowledgements

The authors acknowledge Clear Defense Inc., for supplying the test specimens, Mrs. Jerry Atkinson, Ro-berto Nunez and Jeremy Bloom for their assistance dur-ing the experimental work and the Constructed Facilities Laboratory of NC State University.

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[4] Pantel ides CP, Sallee GP, Minor JE. Edge strength of wind ow glass by mechanical test. J Eng Mech 1994;120(5):1076-90.

[5] Carre H. Daudcville L. Loadbearing capacity of tempered structuml glass. J Eng Mech 1999; 125(8):914--21.

[6] Norville HS, King KW. Swofford Jl. Behavior and strength of laminated glass. J Eng Mech 1998: 124( I ):46-53.

(7J ASTM D882. Standard test method for tensile properties of thin plastic sheeting; 2002.

[8] ASTM D1938. Standard test method for tcarpropagation resis tance (trouser tear) of plastic fi lm and thin sheeting by a singlc+tear method; 1994.

[9] ASTM CI58-95. Sta nd ard test methods for strength of glass by flexure (determination of modulus of ru pture); 1995.

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