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Construction and Building Materials 20 (2006) 761 - 768

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Structural performance of laminated and unlaminated tempered glass under monotonic transverse loading Amir Fam I>

a,.,

Sami Rizkalla

b

a Queen's University. Kingston. Ont .. Canada K7L 3N6 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 t 5.9-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 number of reasons, including excessive wind loading in hurricanes, 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 6 13 533 2128. E-mail address:[email protected] (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 650°C, and then cooling it rapidly with blasts of air. This causes the surface of the glass to cool more rapidly than the inner 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.

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

762

The term " laminated glass" or "sandwich glass" often 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 context 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 sandwiched glass have been studied experimentally and numerically under transverse loading [4- 6], the behaviour and failure mode of externally laminated tempered glass have not been studied. In this paper, tempered sheets of glass of different thicknesses have been externally 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.

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 performance and superior optics at the same time. The laminate 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 ultraviolet light transmittance of 0-5%. The side of the laminate, which is bonded to glass, has a layer of acrylic pressure-sensitive adhesive, coated with a thin protective

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.

film. This film can easily be peeled off, prior to installation, 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 laminate (or multiple laminates) under high pressure. For glass replacement applications or new glass installations, the laminate is pre-installed on typical standard size annealed or tempered glass, using the same process, and shipped to the site. The peel strength of the laminate is 3.86 MPa.

2.1. Laminate material and installation

2.2. Description of test specimens and parameters

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

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

2. Experimental program

Table 1 Summary of test matrix and results

Specimen

Number of

Glass

Thickness

J.D.

specimens

product

(mm)

Al

3 each

A

A2

A3 A4

A5 BI B2

B3 B4 B5

B6 B7

3 each

B

12.7 12.7 12.7 15.9 15.9 9.5 9.5 9.5 12.7 12.7 12.7 15.9

Lamination

No I-Laminated 2-Laminated

No I-Laminated

No I-Laminated 2-Laminated

No I-Laminated 2-Laminated

No

Reinforcement Ratio C% age)

Average test results at maximum load Load

Deflection

(kN)

(mm)

Stiffness (kN/m)

(kNm)

Energy

0 2.83 5.67 0 2.26

9.25 9.53 9.05 14.00 17.37

30.9 31.4 31.0 24.3 29.5

300 303 292 576 588

0.143 0.150 0.140 0.170 0.257

0 3.79 7.58 0 2.83 5.67 0

3.80 4.90 4.59 6.86 8.95 9.36 14.49

37.5 44.0 46.0 30.5 34.6 31.1 22.8

101 III 100 225 293 300 637

0.072 0. 109 0.106 0.105 0. 155 0.146 0.167

A. Ftt/H, S. Ri=kttlla I Construction and Building

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20 ( 2006) 761 - 768

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

rollers

Potentia· meter

-1


::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--l

u

!!

-10

o

2

3

4

5

6

7

8

Reinforcement ratio (% age) Fig. 7. Effect of laminate reinforcement ralio o n ultimate load. deflection, stiffness and strain energy.

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

3.4. Failure modes

A distinct difference in failure mode between the laminated 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

767

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 scenario 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 maintains 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 modulus of rupture and cracks, the bonded lamina fails to

3.

sustain the tensi1e stresses and maintain the applied

load (unlike other composite systems, such as reinforced concrete for cxample). This is mainly due to the very low modulus of the lamina (3.8 GPa, compared to 66 GPa of glass), which leads to excessive deformations without rupture of the lamina. This failure mode was distinctly different from that of unlaminated 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. 5. 6.

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 tempering process, especially for different thicknesses, where the cooling rate across the thickness affects the level of residual stresses. Adding a second laminate may have an insignificant effect on strength, stiffness, strain energy, and failure mode. The load-deflection behaviour of both laminated and unlaminated glass is generally linear up to failure. No rupture or delamination of the laminates were observed. The lamination process is quite suitable for both retrofit of glass in existing structures as well as for new structures, where pre-laminated glass can be installed in the field.

Acknowledgements 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 involved either one or two 0.36-mm thick polyester laminates 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 causing serious injuries, to one which is still bi·ittle yet safer, as the fractured glass remain intact and can easily 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-

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

References (IJ Wheeler I. Glass failure: the unacceptable risk. Environ Healt h J 2002: 196-9. [21 Shepard CL, Cannon BD. Khalecl MA. Measurement of in ternal stress in glass articles. J Am Ceram Soc 2003;86(8): 1353- 9. [3J 851 , BS 6206. Impact performance requirements for flat safety glass and safety plastics for use in buildings: 198 1. [4] Pantel ides CP, Sallee GP, Mino r JE. Edge st rength of wind ow glass by mechanical test. J Eng Mech 1994;120(5):1076-90. [5] Carre H. Daudcville L. Load·bearing 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 fo r tensile properties of thin plastic sheeting; 2002. [8] ASTM D1938. Standard test method fo r tcar·propagatio n resis· tance (trouser tear) of plastic fi lm and thin sheeting by a singlc+tear met hod; 1994. [9] ASTM CI58-95. Sta nd ard test methods for strength of glass by flexure (determination of modulus o f ru pture); 1995 .

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