Structural. Structural Performance of Laminated Architectural Glass A SAFLEX DESIGN GUIDE FOR ARCHITECTURAL GLAZING SYSTEMS

Transcription

1 Pub. No A (Supercedes No ) Structural Structural Performance of Laminated Architectural Glass A SAFLEX DESIGN GUIDE FOR ARCHITECTURAL GLAZING SYSTEMS

2 The Guide provides a new, technically sound basis for the structural design of architectural glazing systems under a wide range of environmental conditions. ACKNOW LEDGEMENTS This design guide was prepared by Solutia Inc. using results from extensive research conducted at the Glass Research and Testing Laboratory at Texas Tech University and the Building Envelope Research Laboratory at the University of Missouri-Rolla. Contributions by individuals at these two research institutions are gratefully acknowledged: Dr. Joseph E. Minor, Dr. H. Scott Norville, Dr. C.V.G. Vallabhan, Dr. Richard A. Behr, Mr. Magnus P. Linden, Mr. Sesha R. Nagalla, and Mr. Paul Kremer. Matching contributions by Texas Tech University, the University of Missouri-Rolla and the States of Texas and Missouri through programs that assist university/industry cooperative efforts are also acknowledged. FOREWORD A Design Guide to the Structural Performance of Laminated Architectural Glass (the Guide) was developed to provide the designer with the latest information and data on the performance of this glazing product. The Guide provides a new, technically sound basis for the structural design of architectural glazing systems under a wide range of environmental conditions. Since wind and snow govern the design of most architectural glazing systems, special attention is given to these loads. In addition, windborne debris that accompanies hurricanes, typhoons and other extreme windstorms has become recognized as an important factor in the performance of the building envelope. Hence, new attention is given to designs for windborne debris a condition in which laminated architectural glass is especially effective. Testing in the 1980s at Texas Tech University revealed that laminated architectural glass fabricated with Saflex polyvinyl butyral (PVB) plastic interlayer is as strong as monolithic glass under wind and snow loads. In the 1990s, building codes and standards began to emphasize protection of the building envelope from windborne debris in extreme wind prone regions. Testing at the University of Missouri-Rolla defined the attributes of laminated glass that enable it to perform effectively in this new application. These two developments have projected laminated architectural glass as the product of choice for a strong, passive window system under wind, snow and debris impact. The newly reported research and new provisions in building codes and standards noted above are incorporated into the Guide. The new procedures make it possible to design architectural glazing for wind and snow loads using a simple process, to address designs for windborne debris, and to employ the strengths and attributes of laminated architectural glass in protecting the building envelope from a wide range of environmental and man-made hazards.

3 Structural Performance of Laminated Architectural Glass contents Introduction Listing of Figures and Tables 2 The Changing Design Environment 3 Attributes of Laminated Glass 3 Research Results 3 New Design Methods 3 Purpose of the Guide 4 Organization of the Guide 4 section 1 Structural Design Methods Introduction 5 Simple Design Procedure: Wind and Snow 6 Comprehensive Design Procedure 10 Designs for Windborne Debris 17 Additional Design Requirements: 22 Earthquakes 22 Human Impact 22 Human Loads 23 Hail Impacts 23 Overhead Glazing 24 section 2 Basic Factors in Glass Strength Annealed Glass Strength 25 Glass Type Factors 25 Load Duration 26 ASTM E Impact Strength 27 section 3 Laminated Glass Behavior Research Summary 28 Lateral Pressure 28 Verification of Computer Programs 29 Laminated Glass Behavior 30 Time Duration of Load Effects 30 Interlayer Thickness 31 Failure Strengths 31 Impact Strength 32 section 4 Windborne Debris Windstorm Experiences 33 Hurricanes and Typhoons 34 The Nature of Windborne Debris 35 The Building Envelope 35 Post-Breakage Behavior 36 Test Protocols 36 Research and Development 37 Laminated Architectural Glass 37 Closure Summary 38 The Future 38 Appendix Charts A.1-A.12 Glass Thickness Selection Charts 39 Charts A.13a-A.13i Deflections in Glass Plates 41 Table A.14 Load Sharing in Asymmetrical IG Units 43 Table A.15 Thickness Designations for Laminated Glass 43 Table A.16 Equivalent Monolithic Glass Thickness for Laminated Glass under Long-Term Load at Room Temperature 43 Abbreviations 44 Symbols 44 Glossary 45 References 46 Specification 48

4 Listing of Figures, Charts and Tables Figure Title Page 1 Window Glass Design Chart (Symmetrical Products 4-Side Support) 7 2 Window Glass Design Chart (Symmetrical Products 2-Side Support) 9 3 Butt Glazed System 9 4 Deflection Calculations Chart (6 mm) 13 5 Symmetrical Laminated Glass 14 6 Laminated Insulating Glass Unit 14 7a Laminate Acting as Monolithic 16 7b Laminate Responding as Layers 16 8a Strength Room Temperature 16 8b Strength Elevated Temperature 16 9 Areas Requiring Consideration of Debris Impact Glazing by Construction Glass Industries Inc Glazing Detail Passing Small Missile Impact Test Glazing Detail for Small Missile Impact 21 13a Tempered Glass 25 13b Stress Distribution in Fully Tempered Glass Effect of Load Duration on Glass Strength Glass Plate Systems 28 16a Monolithic Maximum Stress 29 16b Monolithic Deflection 29 16c Layered Maximum Stress 29 16d Layered Deflection 29 17a Maximum Stress 30 17b Deflection Effects of Sustained Load: Maximum Deflection 30 19a Maximum Stress 31 19b Maximum Deflection Houston, Texas Hurricane Alicia Miami, Florida Hurricane Andrew Hurricane/Typhoon Wind Field Breaching of the Building Envelope Doubles Forces Silicone Anchored LAG Sacrificial Ply LAG 37 Table Title Page 1 Table of Factors 7 2 Requirements for Windborne Debris 18 3 Establish Debris Impact Criteria 19 4 Select Glazing Concept or Product 20 5 LAG Constructions That Meet Missile Impact Standards and Codes 20 6 Qualify Concept or Product for Use 21 7 Safety Glazing Requirements Consumer Products Safety Commission 23 8 c 2 Factors for Glass Floors 23 9 Strength Factors Average Minimum Impact Velocity Causing Fracture Inner Ply Breakage Rates in LAG Under Impacts Impact Resistant LAG Constructions Failure Strengths of AN Monolithic and LAG at Room Temperature Failure Strengths of AN Monolithic and LAG as a Function of Temperature Failure Strengths of AN, HS and FT LAG LAG Breakage Rates as a Function of Interlayer Thickness and Heat Treatment Typical Standard for Windborne Debris Impact Tests 36 A.14 Load Sharing in Asymmetrical IG Units 43 A.15 Glass Thickness Designations for Laminated Glass 43 A.16 Equivalent Monolithic Glass Thickness for Laminated Glass under Long-Term Load at Room Temperature 43 Chart Title Page A.1-A.12 Glass Thickness Selection Charts 39 A.13a-A.13i Deflections in Glass Plates 41 2

5 Introduction THE CHANGING DESIGN ENVIRONMENT Recent windstorm disasters have focused attention on the building envelope (windows, doors, wall coverings and roof cladding) as an important part of any enclosed structure. Failures of the building envelope during windstorms have produced unacceptably large insured losses and have sharpened awareness toward hazards presented by falling glass. This new attention to the building envelope has produced building code changes and forced a reexamination of design methods for architectural glazing. The fluctuating nature of wind pressures and the presence of windborne debris in some extreme wind events are now being addressed as part of the design process. Perhaps most significantly, the postbreakage behavior of architectural glazing has become a crucial element to successful construction in the new design environment. ATTRIBUTES OF LAMINATED GLASS Two attributes of laminated architectural glass make this product attractive in the new design environment. First, recently completed research has shown that the strength of laminated architectural glass under wind and snow loads is equivalent to that of monolithic glass of the same nominal thickness. Second, the ability of laminated architectural glass to remain in its supporting frame following breakage by windborne debris, or by unexplained events, is important to the preservation of the integrity of the building envelope and to the limitation of hazards to people who may occupy space below. RESEARCH RESULTS Research conducted at the Glass Research and Testing Laboratory at Texas Tech University provides a basis for the strength of laminated architectural glass defined in this Guide. Theory has been verified by experiment, and experiments were both non-destructive and destructive (tests to failure). Full-scale experiments performed at the Building Envelope Research Laboratory at the University of Missouri-Rolla established the ability of laminated architectural glass to accept debris impacts and to remain in the opening during the application of pressure cycles representing wind gusts that follow. In addition, full-scale tests established the superior performance of laminated architectural glass in earthquake environments. NEW DESIGN METHODS The new design environment and the attributes of laminated architectural glass defined by recent research results have altered conventional approaches to design for architectural glazing. A simple approach to designing for wind and snow is outlined first. This procedure is based on observations that laminated architectural glass acts like monolithic glass under these design conditions, and that most insulating glass units are thin and symmetrical. A comprehensive design procedure is offered for the small percentage of design situations that do not meet these conditions. A new design method for windborne debris recognizes new requirements in the South Florida Building Code, ASCE 7-98, The BOCA National Building Code, the Texas Department of Insurance Building Code for Wind Resistant Construction, and an increasing number of municipal codes. Finally, information is presented for earthquakes, human impact, human loads, hail, and overhead glazing to assist the designer in addressing these topics. 3

6 Introduction PURPOSE OF THE GUIDE This publication has been prepared for the building design professional by Solutia Inc., manufacturer of Saflex polyvinyl butyral (PVB) plastic interlayer for laminated architectural glass. The principal purpose of this publication is to present easy-to-follow guidelines for designing laminated glass systems with Saflex for wind, snow, impact, earthquake and other loads. These methodologies are devised to enable the architect and engineer to develop designs of glazing systems using laminated architectural glass in a logical and a rational manner. The need for the Guide is enhanced by the more extensive use of laminated architectural glass in glass curtain walls, storefront facades, atriums, skylights, canopies and other glazing systems. ORGANIZATION OF THE GUIDE To provide a user friendly design tool, the Guide has been organized as follows: Section 1 contains (1) a simple design method for wind and snow, (2) a comprehensive design procedure for lateral pressures, and (3) a design method for windborne debris. Information is also presented for earthquakes, human impact, human loads, hail, and overhead glazing. The design methods are presented in condensed formats. Supporting data are contained in subsequent sections and the Appendix. Section 2 contains discussions of factors which are important to the definition of strength for annealed glass, laminated architectural glass, heat treated glass and insulating glass. Effects of the time duration of loading on glass strength are included in these discussions. Section 3 summarizes extensive research on laminated architectural glass that was used as a basis for the design guidelines contained herein. Section 4 contains information on windborne debris that is currently being addressed in national standards and building codes (ASCE 7-98, the BOCA National Building Code, SBCCI SSTD 12-97, ASTM E and TDI 1-98), as well as in the South Florida Building Code and an increasing number of municipal building codes in hurricane-prone regions. The Appendix contains design charts, laminated glass thickness designations, abbreviations, symbols, a glossary, lists of references, a model specification and other supporting information. 4

7 section Structural Design Methods 1 INTRODUCTION Architectural glazing products are employed in a myriad of situations that require structural design. Windows, spandrel units, skylights, doors, storefronts, atriums, greenhouses, passageways, side lites, and many other uses of glazing each has its own requirements for design. These requirements may involve combinations of loads (wind, snow, dead, live), as well as additional conditions involving earthquakes, human impact, human loads, and hail. Since most glazing designs are governed by wind loads or snow loads, this section of the Guide begins with a simple approach to the design of common architectural glazing products for wind and snow. This simple design method is followed by a comprehensive design procedure for all products that experience lateral pressure, including wind and snow. A new design procedure a method of design for impacts from windborne debris is presented next. Finally, many products must also be designed for earthquakes, human impact, human loads, and hail. Design methods for these additional conditions are addressed in the final part of Section 1. The Simple Design Procedure addresses the most commonly occurring design loads: wind and snow. These loads dominate and control many designs; hence, the simple procedure presented in this method will be sufficient for most glazing design situations. The Comprehensive Design Procedure addresses design cases with lateral pressures that cannot be handled by the simple procedures. For example, this procedure can be used to design overhead glazing for snow loads using an asymmetrical insulating glass unit. Designs for Windborne Debris is a new procedure that will assist designers with requirements that are appearing in national standards and building codes. The procedure helps the designer determine when windborne debris must be addressed, the impact criteria that may apply, design concepts and glazing products that can resist debris impact, and test protocols that can be used to qualify products for use in situations requiring consideration of windborne debris. Additional Design Requirements address conditions that may influence products that have already been designed for wind and/or snow loads. Earthquakes, human impact, human loads (e.g., people walking on glass floors), hail and overhead glazing are addressed in this part of Section 1. 5

8 Structural Design Methods SIMPLE DESIGN PROCEDURE: WIND AND SNOW The design of an architectural glazing product that employs monolithic and/or laminated architectural glass (LAG) can be very complex. However, if the glazing product meets the following conditions, designing it for wind and snow loads is very simple: 1 its aspect ratio is 2:1 or less (length width 2) 2 it is comprised of monolithic glass or LAG with symmetrical* plies, and 3 if the product is an insulating glass (IG) unit, it is symmetrical* with thin lites. Note: A LAG lite has two glass plies and an IG unit has two glass lites. One or two LAG lites can be used in an IG unit. See Glossary. Simplicity is achieved because: the 12 annealed (AN) glass strength charts from ASTM E Standard Practice for Determining the Load Resistance of Glass in Buildings (Reference 1.1) have been reduced to a single chart for aspect ratios of 2:1 or less (see Window Glass Design Chart 4-Side Support); lites in IG units share loads equally when the lites are both symmetrical and thin (see Section 2); LAG behaves like monolithic glass under wind and snow loads (see Section 3); and dead loads do not exist (vertical glazing) or can be neglected (sloped glazing)** and glass must not be subjected to live loads. *Plies (LAG) or lites (IG) of same glass type with identical designated thickness are symmetrical. ** For example, a 1/4-inch glass plate weighs less than 3 psf; typical design wind and snow loads exceed 30 psf. PROCEDURE (Symmetrical*, 4-Side Support) See Design Chart and Table of Factors on Page 7 Step 1 Obtain design load from building code: wind load or snow load (lbs/ft 2 or kpa). Step 2 Calculate opening area (L x W) in ft 2 or m 2 and aspect ratio (L W). Step 3 Check for Simple Design Procedure (L W must be 2). Step 4 For AN, HS or FT glass, divide design load by appropriate strength factor: 1 (AN), 2 (HS) or 4 (FT), respectively (see Table page 7). Step 5 If product is an IG unit, also divide by IG strength factor: 2*** (see Table page 7). Step 6 Divide load from Step 4 or 5 by appropriate time factor*** (see Table page 7 for time factor); the result is the modified design load*** in lbs/ft 2 or kpa. Step 7 Use opening area (from Step 2) and modified design load (from Step 6) to define a point in the Design Chart on page 7. Step 8 Required overall lite thickness is the thickness (t) associated with a line corresponding to the aspect ratio (from Step 2) that is above-right of the point defined. If the aspect ratio (from Step 2) is other than 1:1 or 2:1, draw a line (between the 1:1 and 2:1 lines) for the specific aspect ratio calculated. Linear interpolation between 1:1 and 2:1 lines for a given thickness provides acceptable accuracy. Step 9 If IG unit, verify that lites are thin (W t)>150 where W = width (short dimension) and t = thickness of one lite. *** If ASTM E must be followed: Step 4: If LAG is used as a single lite, also divide by 0.9 (short duration load) or 0.75 (long duration load). Step 5: Divide by 1.8 instead of 2 (for IG unit). Step 6: If design load is a wind load, do not divide by time factor. 6

9 1 FIGURE 1 Window Glass Design Chart (Symmetrical Products 4-Side Support) Opening Area Square Meters Second Strength PSF M Denotes Monolithic Nominal Thickness Designation L Denotes Laminated Nominal Thickness Designation /8 in. (3 mm M) /4 in. (6 mm M & L) 3/16 in. (5 mm M & L) 3/8 in. (10 mm M & L) 5/16 in. (8 mm M & L) 2-1 1/2 in. (12 mm M & L) Second Strength kpa /4 in. (19 mm M & L) 5/8 in. (16 mm M & L) 7/8 in. (22 mm M & L) Opening Area Square Feet NOTES TO FIGURE 1 DESIGN CHART: 1 Plies (LAG) or lites (IG) of same glass type with identical designated thickness are symmetrical. 2 Glass strengths in the chart are 60-second duration glass strengths obtained from ASTM E for aspect ratios (length width) of 2:1 or smaller. 3 Linear interpolation between 2:1 and 1:1 lines for intermediate aspect ratios provides acceptable accuracy. 4 Glass strengths may be adjusted by time factors to account for changes in glass strength from 60-second to 3-second (wind) and to two-week (snow) load durations. (If ASTM E is followed, product strengths for wind loads must be determined by using 60-second strengths.) 5 The Simple Design Procedure uses 1.0 as a strength factor for LAG and 2.0 as a strength factor for IG. If ASTM E is followed, strength factors for LAG of 0.9 (short duration load) and 0.75 (long duration load) and a strength factor for IG of 1.8 must be used; see Footnotes, page 6. 6 If deflections must be calculated, see Comprehensive Design Procedure, Procedure 3. 7 If the glazing product is an IG unit placed over an occupied space that is or may be heated, the interlayer temperature in the lower (LAG) lite may be >32 F when under snow load. In this case, the Comprehensive Design Procedure, Procedure 2, must be followed. TABLE 1 Table of Factors Glazing Strength Time Factors Product Factor Wind Snow AN HS FT IG 2.0 LAG 1.0 7

10 Structural Design Methods EXAMPLES (follow procedure on page 6) Find required lite thickness for a symmetrical 1 IG unit comprised of HS LAG. Opening is 5 ft. x 8 ft. and 3-second design wind load is 100 psf. Step 1 3-second wind load = 100 psf. Step 2 Opening area: 5 ft. x 8 ft. = 40 ft. 2 Step 3 Aspect ratio: 8 5 = 1.6:1 (1.6 < 2; OK for Simple Design Procedure) Step = 50 psf (HS strength factor = 2). Use (IG) 1.8 (HS LAG, short duration load) = 31 psf if E must be followed. Step = 25 psf (IG strength factor = 2). Step = 17 psf (HS time factor = 1.6). Do not use wind time factor for HS glass (1.6) if E must be followed. Step 7 40 ft 2 and 17 psf define a point within the 3/16-inch thickness lines but above a 1.6:1 line interpolated between the 1:1 and 2:1 lines. For E , 40 ft. 2 and 31 psf define a point within the 5/16-inch thickness lines but above a 1.6:1 line interpolated between the 1:1 and 2:1 lines. Step 8 Required thickness of each HS LAG lite in the IG unit is 1/4-inch (overall designated thickness ) For E , required thickness of each HS LAG lite in the IG unit is 3/8-inch (overall designated thickness ). 2 Find required lite thickness for a HS LAG lite over an unheated occupied space. Opening is 54 inches x 81 inches and design snow load is 62 psf. Step 1 Snow load = 62 psf. Step 2 Opening area: (54 x 81) 144 = 30.4 ft. 2 Step 3 Aspect ratio: = 1.5:1 (1.5<2; OK for Simple Design Procedure) Step = 31 psf (HS strength factor = 2). Use (HS LAG, long duration load) = 52 psf if E must be followed. Step 5 (Not an IG unit.) Step = 39 psf (HS time factor = 0.8). Step ft. 2 and 39 psf define a point within the 5/16-inch lines, near the 1:1 line. For E , 30.4 ft. 2 and 52 psf define a point above the 3/8 in. lines. Step 8 Interpolated 1.5:1 line passes below defined point; use 3/8-inch HS LAG (overall designated thickness ). For E , required thickness of HS LAG is 1/2-inch (overall designated thickness ). Step 9 (Not an IG unit.) Step = 240 (240 > 150); thin lite = 160 (b/t=160 > 150) 8

11 1 FIGURE 2 Window Glass Design Chart (Symmetrical Products 2-Side Support) FIGURE 3 Butt Glazed System /8" 3/4" Head Design Load lbs./sq. ft /16" 1/4" Example: 60" x 30" (60" Unsupported) p = 30 psf Use 5/8" 5/16" 3/8" 1/2" U Sill Butt Joints U = Unsupported Span Length of Unsupported Span Inches PROCEDURE (2-Side Support) Step 1 Obtain design load from building code: wind load or snow load. Step 2 Determine length of unsupported span (U) in inches. Step 3 For AN, HS or FT glass, divide design load by 1 (AN), 2 (HS) or 4 (FT), respectively (see Table page 7). Step 4 If product is a symmetrical IG unit, also divide by 2 (see Table page 7). Note: It is not common practice in North America to use IG units in 2-side support configurations. Step 5 Divide load from Step 3 or 4 by appropriate time factor (from Table page 7); this is the modified design load. Step 6 Use length U (from Step 2) and modified design load (from Step 5) to define a point on the design chart (Figure 2 above). Step 7 Read minimum allowable overall lite thickness. If point falls between two thicknesses, choose the thicker lite thickness. Note: ASTM E does not address 2-side support conditions. 9

12 Structural Design Methods COMPREHENSIVE DESIGN PROCEDURE Glazing products that do not meet the conditions for using the simple procedure for wind and snow loads must use a more detailed analysis. The comprehensive design procedure presented herein uses the ASTM E annealed glass strength charts, but differs from the other data in the ASTM process in two important respects. First, the strength of laminated architectural glass (LAG) is defined by recent research that recognizes LAG strengths that are equivalent to monolithic glass strengths under wind and snow loads (see Section 3, Laminated Glass Behavior). Second, the increased strength of glass under wind loads (which are shorter in duration than the 60-second duration employed in ASTM E strength charts) is recognized. Because of these adjustments, strengths of glass products calculated using procedures outlined in the comprehensive design procedure will not be the same as strengths calculated using ASTM Standard E The comprehensive design procedure is divided into four parts. Procedure 1 addresses monolithic glass, LAG, and IG units with lites that share load equally; Procedure 2 handles IG units with unequal load sharing; Procedure 3 treats the calculation of deflections. Special cases using LAG are treated in Procedure 4. Procedure 1 addresses all monolithic glass, LAG made with plies of the same glass type, and IG units that are symmetrical (lites identical in glass type and thickness, including LAG lites equal in designated thickness to their monolithic companions). Procedure 2 handles asymmetrical IG units, IG units with thick lites, and IG units with LAG that must be treated as layered (e.g., a LAG lower lite of an IG unit under snow load that is over an occupied space that may be above 32 F). Procedure 3 treats deflections in monolithic glass, LAG and IG units of all types. Procedure 4 treats special cases of LAG design that use different ply thicknesses and glass types. The comprehensive design procedure addresses wind and snow loads only, as dead loads do not exist (vertical glazing) or can be neglected (sloped glazing), and glass must not be subjected to live loads. Combinations of wind and snow loads, when required, are treated by converting each load to an equivalent 60-second duration load and adding effects according to the combination formulas contained in standards and codes. If ASTM E or building codes which do not recognize the science employed in the comprehensive design procedure must be followed, notes at appropriate points in the presentation indicate adjustments that must be made. 10

13 1 PROCEDURE 1: Monolithic and Laminated Glass, and Insulating Glass Units With Symmetrical Lites Includes LAG containing plies with same heat treatment, and IG with lites of same glass type and designated thickness (including IG units with one LAG lite). Approach: Modify the design load to account for glass type and time duration of load, select a trial thickness, enter the corresponding chart using opening dimensions, and compare the chart defined strength with the modified design load; proceed to a thicker or thinner thickness chart, as necessary, to close on an acceptable thickness. Step 1 Obtain design load from building code: wind load and/or snow load. (Note: if wind and snow design load combinations must be considered, see note in Step 4.) Step 2 Determine opening dimensions (L = long dimension and W = short dimension). Step 3 For monolithic glass, LAG and IG units with AN, HS, or FT glass, divide the design load by 1 (AN), 2 (HS) or 4 (FT), respectively. If product is an IG unit, also divide by 2*. Step 4 Divide modified design load from Step 3 by appropriate time factor: 1.2 (AN), 1.6 (HS), or 1.8 (FT) for wind load, or 0.6 (AN), 0.8 (HS), or 0.9 (FT) for snow load.* (Note: if load combinations must be considered, perform the conversions as described for both wind and snow, and add according to the load combination formula.) Step 6 If the strength of the trial thickness is larger (smaller) than the modified design load, move to a thinner (thicker) glass thickness chart and repeat Step 5. Repeat this process until a glass thickness is found that exhibits a strength that is the same as or slightly larger than the modified design load. Step 7 If the glazing design is an IG unit, divide the short dimension (W) by the designated thickness of one lite. If this number is >150, the IG unit design is acceptable; if not, the IG unit design is too thick to behave as a symmetrical unit and Procedure 2 must be used. * ASTM E combines strength factors and time factors into four tables of glass type (GT) factors : single lites (monolithic and LAG) and IG, for short and long duration loads. To adjust the comprehensive design procedure (Procedure 1 only) to ASTM E , strength factors and time factors must be separated. ASTM E uses the following strength factors: 1 (AN), 2 (HS), 4 (FT), 1.8 (IG; monolithic, symmetrical lites), 0.9 (LAG, AR 2 and b/t > 150; short duration load), 0.75 (LAG, AR > 2 or b/t 150; short duration load), 0.75 (LAG, AR 2.5; long duration load), 0.5 (LAG, AR > 2.5; long duration load). ASTM E modifies strength factors by time factors for long duration loads: 0.6 (AN), 0.8 (HS), 0.9 (FT). Note: ASTM E does not specifically address IG with two LAG lites and does not include a time factor for a 3-second wind load. In ASTM E : aspect ratio (AR) = long dimension short dimension, b = short dimension, short duration load lasts 60 seconds or less, and long duration load lasts approximately 30 days. Step 5 Select a trial thickness and find the corresponding chart in the Appendix, Charts A.1-A.12. (The trial thickness is a designated monolithic or LAG thickness, and is the thickness of one lite if the glazing is an IG unit; ASTM C1036 designated thicknesses are contained in the Appendix, Table A.2.) Draw a vertical line from L and a horizontal line from W ; the intersection of these lines represents the strength of the trial thickness (in kpa) for the glazing being designed. (Interpolation between load lines along a radial line from the chart origin may be necessary.) 11

14 Structural Design Methods PROCEDURE 2: Asymmetrical Insulating Glass Units Includes IG units with lites of different thickness and/or heat treatment, thick IG units, and IG units with lite(s) of LAG that must be considered layered. Approach: Establish a trial design by defining glass types and thicknesses; apportion design load between lites and modify the apportioned loads by strength and time factors, as appropriate. Enter charts to determine if strengths of lites in trial design exceed modified apportioned design loads. Step 1 Obtain design load from building code: wind load and/or snow load. (Note: if wind and snow design load combinations must be considered, see note in Step 5.) Step 2 Determine opening dimensions (L and W); W is the short dimension. Step 3 Establish a trial design by defining the asymmetrical IG unit as follows: select a trial thickness t L for the loaded (outer) lite; select a trial thickness t U for the other (inner) lite; select a trial glass type (AN, HS, or FT) for each lite. Step 4 Calculate t U t L and W t L. (Note: if lower lite is LAG that will be > 32 F under snow load (lower lite is over an occupied space that may be heated), substitute an equivalent monolithic glass thickness for t U from Table A.3.) Obtain percent of design load carried by loaded lite % t L from Table A.1.* Calculate percent of design load carried by inner lite % t U (% t U = percent of design load carried by loaded lite % t L ).** Step 6 Find appropriate glass strength charts for t L and t U in the Appendix (Charts A.1-A.12). For each thickness, draw a vertical line from L and a horizontal line from W ; the intersection of these lines represents the strengths of the thicknesses t L and t U (in kpa) for the trial design defined in Step 3. (Interpolation between load lines along a radial line from the chart origin may be necessary.) Step 7 If the strengths of the trial thicknesses t L and t U are larger than the modified design loads L tl and L tu, the design is acceptable. If either of the individual glass strengths is smaller than the corresponding modified design load, or if one or both of the glass strengths are much larger than the corresponding modified design loads, an alternate design is indicated. Modify glass thicknesses and/or glass types in the trial design and repeat the process beginning with Step 3 (if thicknesses are changed) or with Step 5 if glass types (only) are changed. Deflections may be found using Procedure 3. * Load share factors in Table A.1 were obtained from Reference 1.2. ASTM E contains an alternate method for calculating load sharing. If the ASTM E procedure must be followed when laminated glass is employed in an asymmetrical IG unit, use the process detailed therein. ** ASTM E does not recognize the equivalency between monolithic and laminated glass under wind and snow loads. Step 5 Calculate portions of design load carried by each lite (percents from Step 4 x design load). Divide each of these loads by 1 (AN), 2 (HS), or 4 (FT) and by time factors 1.2 (AN), 1.6 (HS), 1.8 (FT) for wind, or 0.6 (AN), 0.8 (HS), or 0.9 (FT) for snow, as appropriate. The results are modified design loads for each lite: L tl and L tu. (Note: if load combinations must be considered, perform Step 5 on both wind load and snow load, and add the modified design loads for each lite according to the load combination formula.) 12

15 1 PROCEDURE 3: Deflection Calculations The model building codes contain no requirements that limit deflections in architectural glazing. ASTM Standard E offers no recommendations regarding acceptable deflections. Dashed lines in the glass thickness selection charts (Charts A.1-A.12 in Appendix) indicate loads and plate geometries for which the maximum lateral deflection of the glass exceeds 3/4-inch (19 mm). Industry practice varies, but limits on deflection often relate to the perceived deflection of glass lites under load. Deflections on the order of the thickness of the lite are perceptible, especially for windows under wind load. Since there are no specific building code limitations for deflections and guidelines for allowable deflections in glass lites are stated only in general terms, there is little need to obtain deflections with great accuracy. Charts A.13a-A.13i (Appendix) contain approximations, for each thickness designation, of the deflections of 4- side, simply supported glass lites subjected to uniform lateral loads (wind and/or snow). Enter a graph for the appropriate glass thickness from the top (in.) or bottom (mm) with the width (smallest dimension) of the lite. Move vertically to a load line (psf or kpa) for the appropriate aspect ratio (L W). Load lines may be drawn for aspect ratios between 1:1 and 2:1 by interpolating using the loads shown. Load lines for other loads may be drawn by interpolating between load lines for a specific aspect ratio. Use of 2:1 load lines for aspect ratios above 2:1 provides acceptable accuracy. Deflections in IG units are found by using the apportioned load acting on one lite (see Procedures 1 and 2). LAG unit defections are the same as deflections of monolithic plates with the same thickness designations. Deflections of warm LAG lites in IG units under long-term (snow) load are found by using the equivalent monolithic thicknesses specified in Table A.3 (Appendix). The chart for deflections in 1/4-inch (6 mm) glass plates is presented below. Illustrated examples are included. Center Deflection (mm) FIGURE 4 Deflection Calculations Chart (6 mm) psf (1 kpa) 40 psf (2 kpa) psf (4 kpa) Example Example 3 Example Width (in.) 1,000 Width (mm) 1, psf (1 kpa) 40 psf (2 kpa) 80 psf (4 kpa) Aspect Ratio 1 Aspect Ratio 2 Example 1: 34 x 34 in. (864 x 864 mm) lite, 40 psf (1.9 kpa) Center Deflection = 0.15 in. (3.8 mm) Example 2: 34 x 68 in. (864 x 1,728 mm) lite, 40 psf (1.9 kpa) Center Deflection = 0.32 in. (8.1 mm) Example 3: 34 x 51 in. (864 x 1,295 mm) lite, 40 psf (1.9 kpa) Center Deflection = 0.22 in. (5.6 mm) ,400 1,600 1,800 2,000 Center Deflection (in.) 13

16 Structural Design Methods EXAMPLES Find the required lite thickness for a symmetrical LAG unit 30 x 90 inches in size that must 1 carry a wind load of 100 psf. (Aspect ratio = 3; Comprehensive Design Procedure 1, page 13, must be used.) Step 1 3-second wind load = 100 psf. FIGURE 5 Symmetrical Laminated Glass 100 psf t Step 2 L = 90 inches, W = 30 inches; aspect ratio L W = 3. (Simple Design Procedure, page 6 is limited to aspect ratios 2.) Step 3 For AN LAG: = 100 psf (4.8 kpa) For HS LAG: = 50 psf (2.4 kpa) For FT LAG: = 25 psf (1.2 kpa) [strength factors] Step 4 For AN LAG: = 83 psf (4.0 kpa) For HS LAG: = 31 psf (1.5 kpa) For FT LAG: = 14 psf (0.8 kpa) [time factors] Step 5 From Charts A.1-A.12: AN LAG thickness = 3/8 inch (10 mm) HS LAG thickness = 1/4 inch (6 mm) FT LAG thickness = 3/16 inch (5 mm) FIGURE 6 Laminated Insulating Glass Unit t L (Note: 5/32-inch FT monolithic is also OK, but is not available in LAG.) Using ASTM E : (Table 1) AN LAG: = 133 psf (6.4 kpa); t = 1/2 inch (12 mm) HS LAG: = 67 psf (3.2 kpa); t = 3/8 inch (10 mm) FT LAG: = 33 psf (1.6 kpa); t = 1/4 inch (6 mm) 75 psf t U 14

17 1 2 Design an IG unit with an outer monolithic fully tempered glass lite and an inner laminated glass lite for a 60 x 80 inch opening. Snow load (acting normal to surface of glass) is 75 psf. The IG unit is supported on four sides and is located over a heated occupied space. (Asymmetrical IG design and heated occupied space; hence, Comprehensive Design Procedure 2 for asymmetrical IG units, page 12, must be used.) Step 1 Design load = 75 psf (snow) acting normal to surface of glass. Step 2 Opening dimensions: L = 80 inches, W = 60 inches. Step 3 Trial design: t L (loaded lite) = 1/4 inch FT; t U (other lite) = 5/16 inch AN LAG. Step 4 From Table A.3: equivalent monolithic glass thickness for 5/16 inch LAG lite at room temperature t U = 1/4 inch From Table A.1: W/t L =60/0.25=240 t U /t L =0.25/0.25=1.0 % of design load carried by loaded lite = 50 % of design load carried by unloaded lite (100-50) = 50 Step 5 Outer lite t L carries 75 x 0.50 = 38 psf. Inner lite t U carries 75 x 0.50 = 38 psf. Modified design load for t L (outer lite) = = 11 psf. Modified design load for t U (inner lite) = = 63 psf. Step 6 From Chart A.6 (1/4 inch): strength t L = 26 psf (>11 psf, OK). From Chart A.6 (1/4 inch): strength t U = 26 psf (<63 psf, not acceptable). Step 7 Change glass type (only) for t U (inner lite) to HS; return to Step 5 (Note: since thicknesses have not changed, load share factors are unchanged.) Modified design load for t U (inner lite) = = 24 psf. From Chart A.6 (1/4 inch): strength t U = 26 psf (> 24 psf, OK). Notes: A HS 1/4-inch outer lite (modified design load =24 psf) is also acceptable. A thinner FT outer lite may be acceptable; select alternate t L and repeat from Step 3. If occupied space may not be heated, check design with lower LAG lite acting as monolithic. PROCEDURE 4 SPECIAL CASES Special Case 1: LAG with Different Ply Thicknesses (same glass type) LAG fabricated with plies that are the same glass type but with different ply thicknesses will act as a monolithic plate with a total thickness equal to the sum of the ply thicknesses, when subjected to wind or snow loads. This conclusion may be inferred from the extensive research on the behavior of symmetrical LAG reported in Section 3. Should LAG with the same glass type but different ply thicknesses have to be treated as layered (a design condition that will not occur often), the analysis becomes mathematically complex. For simplicity, bending only behavior is assumed (membrane behavior is ignored) and each ply will assume a share of applied load in proportion to the cube of its thickness (see Example 1, next page). The load assumed by the thicker ply will always produce larger stresses in the thicker ply than the load assumed by the thinner ply will produce in the thinner ply. Hence, if the thicker ply can carry its share of the proportioned load, the design is satisfactory. Special Case 2: LAG with Different Glass Types This special design condition may occur when a combination of two plies of AN, HS, or FT glass is employed. The procedures outlined below apply to LAG with a thicker ply that is less than two times the thickness of the thinner ply. If the stronger ply is placed in tension and the unit is not elevated in temperature, the unit will behave as a monolithic plate with a total thickness equal to the sum of the ply thickness. Unit strength is determined by considering the monolithic plate to be made of the stronger type of glass (see Example 2, next page). If the stronger ply is placed in tension and the unit is at an elevated temperature, the strength of the unit is equal to the strength of the stronger ply, acting alone (see Example 2, next page). Independent of temperature, if the stronger ply is placed in compression, the strength of the unit is equal to the strength of the ply in compression, as if it were acting alone (see Example 3, next page). 15

18 Structural Design Methods EXAMPLES (Special Cases) Determine the strength of a 4 foot x 5 foot AN 1 LAG unit (4-side support) composed of 1/8-inch and 1/4-inch plies (a) at room temperature and (b) at elevated temperature: (a) At room temperature (<100 F) the LAG unit will behave under uniform load as a 3/8-inch monolithic glass plate. From Window Glass Design Chart (page 7), 3/8-inch, 20-ft 2, AN plate with AR = 1.25 (interpolate linearly between 1:1 and 2:1 lines): 60-second strength is 72 psf. 2 Determine the strength of a 4 foot x 5 foot AN LAG unit (4-side support) with an AN 1/8-inch ply and a HS 1/4-inch ply, with the HS 1/4 ply in tension (a) at room temperature and (b) at elevated temperature: (a) At room temperature and below (<100 F), the 60-second strength is equal to the strength of an HS 3/8-inch monolithic plate: 72 psf x 2 = 144 psf (see Example 1a, above). FIGURE 8a Strength Room Temperature FIGURE 7a Laminate Acting as Monolithic Compression Tension (b) The LAG unit at elevated temperature (>100 F) will behave under uniform load as a layered system with 1/8-inch and 1/4-inch plies. The plies will share load as follows: 1/8-inch ply /( ) = 11% 1/4-inch ply /( ) = 89% FIGURE 7b Laminate Responding as Layers (b) At elevated temperature (>100 F) the 60-second strength is the same as an HS 1/4-inch plate: 38 psf x 2 = 76 psf (see Example 1b, above). 3 Determine the strength of a 4 foot x 5 foot LAG unit (4-side support) with an AN 1/8-inch ply and an HS 1/4-inch ply, with the HS 1/4-inch ply in compression (a) at room temperature and (b) at elevated temperature. The strength of the LAG unit at all temperatures is equal to the strength of the HS 1/4-inch ply, acting alone: 76 psf (see Example 2b, above). Note: the 1/8-inch AN ply that is in tension may fail under a load equal to the strength of the 1/4-inch HS ply in compression. FIGURE 8b Strength Elevated Temperature The strength of a 1/4-inch AN monolithic glass plate (20 ft 2, AR 1.25, 4-side support) is 38 psf (Window Glass Design Chart, page 7). The 60-second strength of the layered system is = 42 psf (thicker ply controls). Compression Tension 16

19 1 DESIGNS FOR WINDBORNE DEBRIS Glazed openings in buildings located at sites exposed to windborne debris during extreme windstorms should be designed for possible debris impacts. In some areas, designs for windborne debris are mandatory, while in other areas designs for windborne debris are required if the building is not designed for full internal pressure. In many other situations (e.g., urban areas where windborne debris may be generated from adjacent buildings and the urban environment), designing for windborne debris is voluntary, but prudent. Debris impact requirements in the South Florida Building Code (SFBC, Reference 1.3) apply in Dade and Broward Counties of Florida. Provisions of SBCCI Test Standard for Determining Impact Resistance From Windborne Debris, SSTD (Reference 1.4), apply in Palm Beach County, and some municipalities within the county, in Florida. Texas Department of Insurance Standard TDI 1-98, Test for Impact and Cyclic Wind Pressure Resistance of Impact Protective Systems and Exterior Opening Systems, applies in coastal counties of Texas, seaward of the Intracoastal Waterway (Reference 1.5). The origins of these code requirements, test standards and test methods are discussed in Section 4. The procedure for designing for windborne debris is as follows: 1 Determine if consideration of windborne debris is mandatory, an alternative to designing for internal pressure, voluntary (prudent), or not needed (see Table 2 for guidance). 2 Establish appropriate debris impact criteria (see Table 3 for guidance). 3 Select glazing product or design concept that meets debris impact criteria (see Table 4 for guidance). 4 Qualify concept, design or product for use (see Table 6 for guidance). ASTM E Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors and Storm Shutters Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials (Reference 1.6) and its companion specification (ASTM E ) can be specified by architects and building owners. Consideration of windborne debris as an alternative to designing for full internal pressures is required in hurricane-prone regions by ASCE Standard 7-98 Minimum Design Loads for Buildings and Other Structures (Reference 1.7) and the 1996 BOCA National Building Code (Reference 1.8). 17

20 Structural Design Methods TABLE 2 Requirements for Windborne Debris Requirement Geographic Locations Comment/Reference Mandatory Dade and Broward Counties of Florida; Palm Beach County, Dade and Broward Counties: South Florida Florida (outside of municipalities); some Palm Beach County, Building Code; Palm Beach County: SSTD 12- Florida municipalities; residential construction in coastal 97; other Florida cities: contact local counties of Texas, seaward of Intracoastal Waterway (to building official; coastal counties of Texas, obtain insurance through Texas Windstorm Insurance seaward of Intracoastal Waterway: TDI Association); Areas defined in ASCE 7-98 and 1996 BOCA National Building Code. Alternative to Areas defined in ASCE 7-98 and 1996 BOCA National See Figure 9 for areas requiring designs for internal pressure Building Code. debris impact or for full internal pressure. Voluntary Urban and suburban areas with potentials for windborne Owner/designer can cite ASTM E , debris in the form of roof gravel, roof tile, shingles, fascia, SSTD 12-97, South Florida Building Code, mechanical equipment and other debris from adjacent roof TDI 1-98, or can specify site specific criteria. tops, buildings, and the general environment. Not needed Open suburban and rural sites with no adjacent buildings Consider future development adjacent to site. or other debris sources. FIGURE 9 Areas Requiring Consideration of Debris Impact (ASCE 7-98 and 1996 BOCA Basic Building Code) Areas with windspeed 110 mph (49 kph) or greater 18

21 1 TABLE 3 Establish Debris Impact Criteria Requirement Criteria Comment South Florida Building Below 30 feet: 2 x 4 timber weighing 9 lbs. impacting end-on See Reference 1.3 for specific Code, Section 2315 (Impact at 50 ft./sec. (two per specimen). test requirements contained Tests for Windborne Debris); Above 30 feet: 2 gm rocks impacting at 80 ft./sec. (30 per in South Florida Building Code; Dade County Protocols specimen). contact Dade County Office of PA (Impact Test Pressure cycles: each of above impacts followed by 9000 cycles Code Compliance or a Dade County Procedures) and PA of pressure representing hurricane wind gusts. certified lab for test protocols. (Cyclic Wind Pressure Loading) SSTD 12-97: SBCCI Test Large missile impact test: 2 x 4 timbers impacting end on See Reference 1.4 for specific test Standard for Determining (Chapter 4): protocols contained in SSTD Impact Resistance from windspeed* lbs. at 50 fps Windborne Debris 100 < windspeed < lbs. at 40 fps 90 < windspeed lbs. at 40 fps Impact each of three specimens twice (center and corner) or each of six specimens once (three center, three corner). Small missile impact test: 2 gm steel balls impacting at 130 ft./sec. (Chapter 5): Each of three specimens receives 30 impacts in three groups of 10 (center, corner and center of long dimension). Pressure cycles: 9000 cycles. Acceptance: Three specimens from each group of three shall pass the test. * windspeeds are fastest mile design windspeeds in mph TDI 1-98: Test for Impact Large missile impact test: 2 x 4 weighing 9 lbs. impacting end See Reference 1.5 for specific test and Cyclic Wind Pressure on at 50 ft./sec.: protocols contained in TDI Resistance of Impact Impact each of three specimens twice (center and corner) or Protective Systems and each of six specimens once (three center, three corner). Exterior Opening Systems Small missile impact test: 2 gm steel ball impacting at 130 ft./sec.: Each of three specimens receives 30 impacts in three groups of 10 (center corner and center of long dimension). Pressure cycles: 9000 cycles. Acceptance: three specimens from each group of three shall pass the test. ASTM E : Large Missile Impact Test: 2 x 4 weighing 4.5 to 15 lbs. impacting See Reference 1.6 for specific Performance of Exterior between 0.10 and 0.55 of basic wind speed (number, size and test protocols contained in Windows, Curtainwalls, impact speed specified by user). ASTM E Doors and Storm Shutters Small Missile Impact Test: solid steel ball having a mass of 2 gm Impacted by Missiles(s) and impacting between 0.40 and 0.75 of basic wind speed (number Exposed to Cyclic Pressure and impact speed specified by user). (Note: Companion Differentials specification to ASTM E-1886 is available as ASTM E ) ASCE 7-98, Table 6-4: In hurricane-prone regions (V 110 mph; see Fig. 9) glazed See Section 4 for discussion of Internal Pressure Coefficients openings in lower 60 ft not specifically designed to resist the effects of internal pressure for Buildings, GC pi windborne debris or are not specifically protected from windborne and advantages of debris impact debris impact must use internal pressure coefficient GC pi = 0.8 protection. for partially enclosed buildings. (from Reference 1.7) 1996 BOCA National Openings which are likely to be breached by windborne See Section 4 for discussion of the Building Code, projectiles where the basic wind speed is 110 mph or greater effects of internal pressure and Table (6): Internal (see Fig. 9) must use internal pressure coefficient for partially advantages of debris impact Pressure Coefficients enclosed buildings. (from Reference 1.8) protection. for Buildings, Note e Voluntary Site specific conditions may warrant large missile and/or small See Section 4 for conditions that missile impact tests: urban areas with gravel and debris on may warrant consideration of adjacent roof tops, urban and suburban areas with glass above debris impact, design examples, walkways, and spaces occupied by people. and references. 19

22 Structural Design Methods TABLE 4 Select Glazing Concept or Product Impact Requirement Glazing Concepts References Large missile test in South Laminated glass with inch or thicker interlayer and silicone See Figure 10 and Sections 2 and 3 Florida Building Code, anchor detail (Impact Strength) Section 2315 and in SSTD 12-97, Chapter 4 Small missile test in South Sacrificial ply concept using laminated glass with in. See Figure 11 and Sections 2 and 3 Florida Building Code, interlayer (steel ball) or in. interlayer (rock) in dry glazed (Impact Strength) Section 2315 and in SSTD system - Laminated glass and silicone anchor detail (single lite) 12-97, Chapter 5 IG unit with laminated glass outer lite Large debris (general); site Single lites: silicone anchored laminated glass units with See Table 5 and Sections 2 and 3 specific impact criteria in. (minimum) interlayer (Impact Strength) developed by architect or IG Units: one or more laminated glass units with in. glazing consultant (minimum) interlayer Small debris (general); site Single lites: laminated glass using sacrificial ply concept or See Figure 12, Sections 2 and 3 specific impact criteria anchored lite concept with in. (minimum) interlayer (Impact Strength), and Section 4 developed by architect or IG units: both lites monolithic or outer lite laminated glass glazing consultant FIGURE 10 Glazing by Construction Glass Industries Inc. (approved by Dade County for Large Missile Impact) TABLE 5 LAG Constructions That Meet Missile Impact Standards and Codes Standard or Code SFBC Small Missile* SSTD12-94,97 Small Missile ASTM E Small Missile TDI 1-98 Small Missile SFBC Large Missile SSTD12-94,97 Large Missile TDI 1-98 Large Missile ASTM E Large Missile LAG Construction Glass/0.030 in. Saflex/Glass Glass/0.060 in. Saflex/Glass Glass/0.060 in. Saflex/Glass Glass/0.060 in. Saflex/Glass Glass/0.090 in. Saflex/Glass Glass/0.090 in. Saflex/Glass Glass/0.090 in. Saflex/Glass Glass/0.090 in. Saflex/Glass * All missile impact standards and codes include pressure cycles (see Table 17). Notes: 1. Glass shall be designed to meet ASCE-7 wind load requirements. 2. Use plies with different glass types to obtain differential break patterns. 3. Glass bite minimum 1/2 inch. 4. Use structural wet seal or high adhesion glazing tape. 20

23 1 FIGURE 11 Glazing Detail Passing Small Missile Impact Test (Sacrificial Ply Concept) FIGURE 12 Glazing Detail for Small Missile Impact (Non-hurricane Region; Reference 1.9) 3/16" (5 mm) 3/8" (10 mm) Gasket Sacrificial Outer Ply (AN, HS, FT, CT) Laminated Glass With Saflex Interlayer Saflex Interlayer Heat Strengthened or Fully Tempered Inner Ply Saflex Interlayer Structural Anchor Bead 3.0" (75 mm) " (19 mm) TABLE 6 Qualify Concept or Product for Use Impact Requirement Mandatory Selected as alternative to internal pressure Voluntary Qualification Procedure Test at certified Dade County (Florida) laboratory, listed SBCCI laboratory, or listed TDI laboratory in accordance with prescribed test protocols (SFBC, SSTD 12-97, TDI 1-98), or use approved product listed by Dade County Office of Code Compliance ( Use test procedures or products approved by a jurisdiction with mandatory impact requirements (e.g., SFBC); test according to SSTD 12-97, TDI 1-98 or ASTM E ; or use designs accepted as standard of practice. Use products approved by a jurisdiction with mandatory impact requirements, test using protocols appropriate for specific design condition, or use designs accepted as standard of practice. 21

24 Structural Design Methods ADDITIONAL DESIGN REQUIREMENTS Once a glazing product has been designed for wind, snow or some combination of these common design loads, it may be necessary to consider additional design requirements. In some design situations earthquakes, human impact, human loads, hail and special requirements for overhead glazing must be addressed. The information presented below will assist the designer in meeting these additional design requirements. EARTHQUAKES Design The response of buildings to earthquakes has been studied extensively. Earthquake engineers can define, in very precise terms, how the structural frame of a building moves during an earthquake. Interstory drift (displacement of the top of a story relative to the bottom of the story) can be defined for a specific building in a design earthquake. It is common practice to simply provide enough clearance between glass edges and the supporting frame to accommodate the interstory drift. Design objectives for earthquakes are (1) to prevent breakage and (2) if glass breaks, prevent it from falling out. LAG exhibits superior performance in meeting the second design objective. Tests There is only very limited guidance available to the designer who wishes to test the behavior of a glazing system under earthquake motions. An informal test procedure imposes, through the application of static forces, a prescribed interstory drift on a full-scale mockup of an architectural glazing system. This test evaluates the ability of the glazing system to accommodate the interstory drift without engaging the glass in a way that will produce breakage. While reasonably effective, this static test does not replicate the multi-cycle, dynamic motion induced by an earthquake. Further, this informal static test, while commonly prescribed as a component of mock-up test regimes, is not defined in a formal standard or in a building code. Research Solutia has joined the U.S. National Science Foundation in developing methods to evaluate architectural glazing systems under earthquake motions. The Building Envelope Research Laboratory (BERL) at the University of Missouri-Rolla has conducted extensive tests to evaluate glazing system performance in earthquakes. Full-scale architectural glazing systems have been subjected to dynamic racking that simulates motions that can be experienced by buildings in earthquakes. Results of this research have produced a proposed standard method of test (Reference 1.10) and comparisons of seismic performance of architectural glazing systems (Reference 1.11). Proposed Standard Test Method A format for a standard test method is offered by Behr, et al. (Reference 1.11). A crescendo test imposes a steady increase of cyclic drift amplitudes. Interstory drift magnitudes are related to serviceability limit states (glass contact with frame) and ultimate limit states (glass fallout). The crescendo test consists of a continuous series of alternating ramp-up and constant-amplitude intervals, each comprised of four sinusoidal cycles at a frequency of 0.8 Hz. Each drift amplitude step is ± 0.25 inch. The number of cycles at each step and the test frequency were selected to be reasonable representations of drift-time histories that could occur in building envelope wall systems under seismic loadings. Preliminary Test Results In a series of tests using the proposed test method, LAG exhibited consistently larger ultimate limit state (glass fallout) drift amplitudes than monolithic glass and PET film coated monolithic glass (AN, HS, and FT). In other tests, heat treatment (HS and FT) only marginally increased drift amplitudes associated with breakage. Structurally glazed systems performed well in earthquake tests. HUMAN IMPACT Industry standard ANSI Z97.1, Safety Performance Specifications and Methods of Test for Safety Glazing Material Used in Buildings (Reference 1.12) and federal standard 16 CFR 1201, Safety Standard for Architectural Glazing Materials (Reference 1.13), contain provisions for architectural glazing materials that can be broken by human contact. Where architectural glazing can be subjected to human contact, these additional provisions may apply. Generally, these glazing products are doors, windows adjacent to sidewalks or passageways (with sills near floor level), side lights, and openings that may be mistaken as passageways. Note that these safety glazing standards do not apply to sloped glazing and skylights (overhead glazing) unless these components can be broken by human contact. Also note that glazing products that meet the safety glazing standards do not necessarily qualify for use in sloped glazing and skylights. (See Overhead Glazing, next page, for model code provisions for Sloped Glazing and Skylights.) 22

25 1 TABLE 7 Safety Glazing Requirements Consumer Products Safety Commission Category I Category II Definition 9 sq. ft. or less, except patio doors, shower Greater than 9 sq. ft. and patio doors, and tub enclosures shower and tub enclosures of any size Test Requirement* Break safely at 150 ft.-lb. impact Break safely at 400 ft.-lb. impact Test Standard CPSC 16 CFR 1201 Category I or equivalent CPSC 16 CFR 1201 Category II or equivalent model code standard model code standard Complying Laminated Glass Two-ply with in. plastic interlayer or greater Two-ply with in. plastic interlayer or greater With Saflex Interlayer * Category I certification requires the glazing withstand one 150 foot-pound impact, produced by impacting a 100-pound shot bag from a vertical height of 18 inches. Category II certification requires the glazing withstand one 400 foot-pound impact, produced by impacting a 100-pound shot bag from a vertical height of 48 inches. HUMAN LOADS Glass can be designed to minimize the risk of breakage or fallout under human loads. Overhead Glazing Overhead glazing should not be exposed to the weight of a person and should be designed to discourage people from walking on glass surfaces. If necessary, the ability of a lite in an overhead glazing unit to withstand human loads can be checked using the equation for a concentrated load, as shown below. Glass Floors The equations below may be used to design glass floors (walking surface of floors, landings, stairwells, and similar locations) for human and other loads. The design should be based on the load that produces the largest stresses from the following equations. Uniformly distributed load: 2F u + D F fa x c 2 x 0.67 Concentrated load: (8F c /A) + D F fa x c 2 x 0.67 Actual load: F a + D F fa x c 2 x 0.67 where: F a = actual intended use load (psf); double for dynamic applications F u = uniformly distributed load (psf), from building code D = glass dead load (psf) = 13 t g t g = total glass thickness (inches) F c = concentrated load (lbs.), from building code c 2 = glass type factor (see Table 8) F fa = maximum allowable load on glass (from Charts A.1-A.12) A = area of rectangular glass (sq. ft.) TABLE 8 c 2 Factors for Glass Floors (Single Glass) * Use lower value for L/W 2 or W/t 150; use higher value for all other cases (L = long dimension, W = short dimension, t = lite thickness); factors apply to two ply laminates only. LAG should have a minimum of three plies and should be capable of supporting the total design load with any one ply broken. Surface damage caused by people or by objects placed on glass can significantly reduce the strength of glass, subjecting it to breakage under subsequent loads. HAIL IMPACTS AN 0.6 HS 1.6 FT 3.6 LAG AN 0.3/0.45* LAG HS 0.8/1.2* LAG FT 1.8/2.7* Sloped glazing, skylights and some vertical window systems may be subjected to impact from hail. Fully tempered (FT) glass is more resistant to hail impact than annealed glass. Tests at Texas Tech University have shown that most hailstones will not break 6 mm (1/4 inch) FT glass. At terminal velocities (maximum speeds attained by a falling hailstone) and higher, ice balls representing hailstones up to 75 mm (3 inches) in diameter shattered themselves and did not break 1/4-inch FT glass. Hail may break FT glass in thinner thicknesses and all thickness of other types of glass (AN and HS). Field experience and laboratory tests have shown that when laminated architectural glass (LAG) is broken by hail impacts, only the outer ply breaks. Hence, AN, HS 23

26 Structural Design Methods 1 and FT LAG are good choices for protection from hail impact. Should breakage occur, it is important to have an unbroken inner ply (single thickness lite) or an unbroken inner lite (LAG inner lite in an IG unit) to help resist subsequent loads, to preserve the building envelope and to prevent glass fragments from falling from the opening. OVERHEAD GLAZING The three model building codes (Standard, Uniform and Basic) define overhead glazing as glass that is positioned over space that may be occupied by humans. These model codes, in effect, prescribe PVB laminated glass for overhead glazing that is either a single lite or the lower lite in an insulating glass unit. There are exceptions and refinements to this general requirement, as noted below (language in each model building code is similar). Allowable Glazing Materials Sloped Glazing shall be of any of the following materials, subject to the limitations specified below. For single-layer glazing systems, the glazing material of the single light or layer shall be laminated glass with a minimum 30-mil polyvinyl butyral (or equivalent) interlayer, wired glass, approved plastic materials (meeting special requirements), heat strengthened glass, or fully tempered glass. For multiple-layer glazing systems, each light or layer shall consist of any of the glazing materials specified above. Annealed glass may be used as specified within Exceptions 2 and 3 (below). Screening Heat-strengthened glass and fully tempered glass, when used in single-layer glazing systems, shall have screens installed below glazing. The screens shall be capable of supporting the weight of the glass and shall be substantially supported below and installed within 4 inches of the glass. They shall be constructed of a noncombustible material not thinner than 0.08 inch with a mesh not larger than 1 inch by 1 inch. In a corrosive atmosphere, structurally equivalent noncorrosive screen materials shall be used. Heat-strengthened glass, fully tempered glass and wired glass, when used in multiple-layer glazing systems as the bottom glass layer over the walking surface, shall be equipped with screening which complies with the requirements for monolithic glazing systems. Exceptions: 1 Fully tempered glass may be installed without required protective screens when located between intervening floors at a slope of 30 degrees or less from the vertical plane if the highest point of the glass is 10 feet or less above the walking surface. 2 Allowable glazing material, including annealed glass, may be installed without required screens if the walking surface or any other accessible area below the glazing material is permanently protected from falling glass for a minimum horizontal distance equal to twice the height. 3 Allowable glazing material, including annealed glass, may be installed without screens in the sloped glazing systems of commercial or detached greenhouses used exclusively for growing plants and not intended for use by the public, provided the height of the greenhouse at the ridge does not exceed 20 feet above grade. 4 Screens need not be provided within individual dwelling units when fully tempered glass is used as single glazing or in both panes of an insulating glass unit when all the following conditions are met: (a) The area of each pane (single glass) or unit (insulating glass) shall not exceed 16 square feet. (b) The highest point of the glass shall not be more than 12 feet above any walking surface or other accessible area. (c) The nominal thickness of each pane shall not exceed 3/16 inch. 5 Screens shall not be required for laminated glass having a minimum inch polyvinyl butyral interlayer within dwelling units. Such laminated glass shall not exceed 16 square feet in area nor shall the highest point of the glass exceed 12 feet above any walking surface. 24

27 section Basic Factors in Glass Strength 2 ANNEALED GLASS STRENGTH The fundamental strength of annealed (AN) glass is presented in 12 charts, one for each standard glass thickness, in ASTM E (Reference 1.1). These strengths are based upon a theoretical glass breakage model which relates the strength of AN glass to its surface condition. A conservative estimate of the weathered surface condition of in-service AN glass was used in producing the charts. Since the strength of AN glass is dependent on both the time duration of loading and the probability of glass breakage, the strengths presented in the charts are referenced to a 60-second constant load for an 8 per 1,000 probability of breakage. FIGURE 13a Tempered Glass Surfaces in Compression Center in Tension GLASS TYPE FACTORS Strengths obtained from the charts are multiplied by glass type factors to account for differences in strength (strength factors) and for time duration of loading (time factors). Glass type factors are applied to AN, heat strengthened (HS), and fully tempered (FT) glass, as well as to laminated architectural glass (LAG) and to insulating glass (IG). FIGURE 13b Stress Distribution in Fully Tempered Glass Heat treatment produces large compressive stresses on the surfaces of glass plates (see Figure 13a). For breakage to occur, stresses from loads such as wind and snow must first overcome the pre-stressed surface condition and then induce tensile stresses that are sufficiently large to produce fracture. As defined in ASTM C1048 (Reference 2.1), initial surface compressive stresses of at least 3,500 and 10,000 psi are produced in HS and FT glass, respectively. Strength factors of 2 for HS glass and 4 for FT glass are commonly applied to AN glass strengths as strength factors for these types of glass. 10,000 psi (min) 0.2 t Compression Zone 0.2 t Neutral Zone 0.2 t Tension Zone 0.2 t Neutral Zone 0.2 t Compression Zone 25

28 Basic Factors in Glass Strength Laminated architectural glass (LAG) behaves like monolithic glass with the same nominal thickness under short-term (wind) loads at room temperature (< 100 F), and under long-term (snow) loads of 32 F and lower. (This behavior was established through extensive research reported in Section 3.) Hence, under wind and snow load conditions with temperatures as noted above,* the strength of LAG is the same as monolithic glass with the same nominal thickness (strength factor = 1). Should a LAG lite experience long duration loads while the interlayer temperature is above 32 F (e.g., the lower lite in an IG unit over a heated occupied space under snow load), it is appropriate to treat the LAG lite as a layered system. Insulating glass (IG) contains a sealed airspace that results in load sharing between lites. If the lites in an IG unit are the same type (AN, HS, FT), equal in thickness and relatively thin, the lites will share load equally. In these cases, therefore, a strength factor of 2 is appropriate (i.e., the strength of an IG unit is twice the strength of one lite). If the IG unit is not symmetrical (i.e., lites are different in thickness and/or glass type) or is thick (i.e., short dimension thickness of one lite 150), load sharing will not be equal. In these cases, determination of the strength of an IG unit becomes very complex and involves the calculation of load share factors. Fortunately, most IG units employed in buildings are symmetrical and thin; hence, the Simple Design Procedure (Section 1) uses a strength factor of 2. Design situations in which these conditions are not met are addressed in the Comprehensive Design Procedure (Section 1). * Design wind loads usually occur during windstorms (e.g. hurricanes and thunderstorms) that are accompanied by clouds and rain or cold fronts. Hence, the ambient temperature and the temperature of LAG interlayers is usually below 100 F when design wind loads occur. LOAD DURATION Glass strength varies with the length of time that the load is applied (Figure 14). Breaking Stress (psi) FIGURE 14 Effect of Load Duration on Glass Strength 15,000 10,000 5,000 0 In recognition of this phenomenon, 60-second glass strengths may be adjusted by time factors to account for short-term (wind) loads and long-term (snow) loads. Recommended time factors that adjust 60-second strengths for time duration of load are listed below. TABLE 9 Strength Factors Short-term Long-term Glass Type (wind, 3 sec.) (snow, 2 wk.) ASTM E second Annealed Glass 1 hour 1 day 1 week 1 month Duration of Stress AN HS FT ASTM E does not recognize the equivalency of monolithic glass and LAG under the temperature and time duration of loading conditions described above. This standard assigns strength factors for LAG that range from 0.5 to 0.9, and assigns a strength factor of 1.8 to symmetrical, thin IG units. Further, this standard overlooks the increase in glass strength under wind (3-second duration) loads. Hence, glass strength factors and adjustments for load duration (time factors) that are contained in ASTM E as glass type (GT) factors differ from those presented above. The Simple and Comprehensive Design Procedures presented in this Guide (Section 1) include provisions for utilizing alternate strength factors and load duration factors (combined into GT factors) for users who must use ASTM E

29 2 IMPACT STRENGTH Architectural glazing may be required to reject impacts from large objects such as 2x4 timbers and from small objects such as roof gravel. The South Florida Building Code (SFBC), SBCCI SSTD12-97 and TDI 1-98 (References 1.3, 1.4 and 1.5) define impact criteria using these missiles for use in design under conditions involving extreme windstorms (see Section 1 and Section 4). A 2 x 4 timber represents large objects and steel balls or rocks represent small objects that may occur in windstorms. Glazing products respond to impacts from these objects in three ways. First, single plates of monolithic glass of various thicknesses and heat treatments have the capacity to resist breakage from a small missile as shown below. Note that steel balls traveling at 80 feet/second (as prescribed in the SFBC) and at 130 feet/second (as prescribed in SSTD12-97) can be expected to break all thicknesses and types of monolithic glass. It has been found in other tests that, while hard rocks (commonly river run gravel) are equivalent to steel balls, certain types of rocks traveling at 80 feet/second may not break some thicknesses of fully tempered glass. Testing with rocks of unspecified hardness is not recommended. TABLE 10 Average Minimum Impact Velocity Causing Fracture (2 gm steel ball) t (in.) AN FT 3/16 30 fps 65 fps 5/16 30 fps 65 fps 3/8 35 fps 60 fps 1/2 40 fps 50 fps 3/4 55 fps 55 fps Second, LAG has the ability to accept impacts from small missiles through breakage of the impacted (outer) ply while preserving the integrity of the inner ply. LAG allows designs for wind load using the strength of the inner ply (only) with the assumption that the outer ply will break but protect the inner ply from the initial and subsequent impacts. Strengths of LAG acting in this capacity are shown in Table 11 (Reference 2.2). These data suggest that LAG constructed with interlayers inch and thicker, and with heat treated glass lites 3/16 inch and thicker, can be expected to perform under the 80 feet/second small missile impact specified in the SFBC without breaking the inner ply. Inner ply breakage can also be avoided by using thinner plies and/or thicker interlayers. For example, LAG construction with 1/8 inch AN/0.060 inch interlayer/1/8 inch AN has performed well in this application. TABLE 11 Inner Ply Breakage Rates in LAG Under Impacts Impacts from 2 gm Steel Balls at 117 ft/sec (80 mph) (outer ply/saflex interlayer/inner ply) Glazing Construction Inner Ply (interlayer thickness, in.) Breakage Rate 3/16HS/0.060/3/16HS /16FT/0.060/3/16FT /16HS/0.090/1/4HS /16HS/0.120/1/4HS Finally, LAG can stop impacts from large objects represented by the 2 x 4 timber. In this application, breakage of both plies is allowed, but penetration by the impacting timber is prevented. A relatively thick PVB interlayer (0.090 inch) may prevent penetration of the unit by the impacting object and holds broken glass particles together. The specific LAG constructions listed in Table 12 are some of the successful configurations used to stop two impacts of the 9 lb 2 x 4 at 50 feet/second, one at the center of the lite and one within 6 inches of a corner, specified as the impacting missile in the SFBC, in SBCCI SSTD 12-97, in ASTM E and in TDI TABLE 12 Impact Resistant LAG Constructions LAG Constructions That Can Arrest Without Penetration the 9 lb. 2 x 4 at 50 ft./sec. Specified in the SFBC, SSTD12-97, ASTM E and TDI 1-98 (outer ply/saflex interlayer/inner ply) Opening Size Glazing Construction (glass and (in.) interlayer thickness in inches) 44 5/8 x 92 1/4HS/.090/1/4HS 38 x 72 3/16HS/.090/3/16HS 37 3/8 x 50 1/4 3/16HS/.090/3/16FT 48 x 72 1/8AN/.090/1/8HS 37 x 63 1/8AN/.090/1/8HS 26 1/2 x 50 5/8 1/8AN/.090/1/8HS 48 x 60 1/8AN/.090/1/8AN 60 x 100 1/8AN/.090/1/8AN 27

30 section Laminated Glass Behavior 3 RESEARCH SUMMARY A comprehensive series of research studies on laminated architectural glass (LAG) has provided the technical basis for the design recommendations contained in this Guide (References 1.10, 3.1, 4.9). LAG behavior was studied under lateral pressures representing wind and snow loads, under impacts from several sizes of windborne debris, and under conditions simulating dynamic earthquake motions. The attributes of LAG were established in each study. Under lateral pressure, LAG acts like monolithic glass of the same nominal thickness under short-term (wind) loads when the temperature is below 100 F and under long-term (snow) loads when the temperature is 32 F and below. Under impact from windborne debris (large and small missiles), broken LAG (both plies) tends to remain in the opening following breakage. Further, small missile impacts may break only the outer ply, allowing the inner ply to remain integral and carry subsequent wind pressures. Under dynamic earthquake motions, broken LAG resists fallout at larger drift amplitudes than monolithic glass and monolithic glass with PET film. LATERAL PRESSURE Layered and monolithic glass lites (See Figure 15) were analyzed theoretically using experimentally verified computer programs. In addition, tests to failure (glass fracture) of approximately 400 LAG lites were employed to confirm predicted performances. Test samples were constructed with annealed (AN), heat strengthened (HS) and fully tempered (FT) glass laminated with Saflex interlayer by Solutia. FIGURE 15 Glass Plate Systems Monolithic Laminated Layered 28

31 Laminated Glass Behavior 3 VERIFICATION OF COMPUTER PROGRAMS The first research step involved theoretical development and experimental verification of computer programs that describe monolithic and layered thin plate behavior. Verification was achieved for both monolithic and layered systems. The extraordinarily good correlations for both stress and deflection (Reference 3.2) are summarized in Figures 16a-d. (Specimens were 60 x 90 x 1/4 inch; T - Theory; E - Experiment.) FIGURE 16a Monolithic Maximum Stress FIGURE 16c Layered Maximum Stress E T Maximum Stress (ksi) E T Corner E T Center y σ 2 σ 2 Maximum Stress (ksi) σ 1 x 2.0 σ Center Corner E T Pressure (psi) Pressure (psi) 0.75 FIGURE 16b Monolithic Deflection FIGURE 16d Layered Deflection T 1.5 E 1.5 T E Deflection (in.) Deflection (in.) Pressure (psi) Pressure (psi)

32 Laminated Glass Behavior LAMINATED GLASS BEHAVIOR The second research step compared the behavior of LAG to that of monolithic lites and layered lites. Nondestructive tests under uniform lateral pressures applied linearly with time to achieve 0.4 psi in 15 seconds were accomplished at temperatures ranging from 32 F to 170 F. Results of these tests (Reference 3.2) produced the conclusion that LAG behaves like monolithic glass under short-term loads at room temperature and below (<100 F). The temperature above which LAG begins to behave like a layered system is not clear, but is about 120 F. (Specimens were 60 x 96 x 1/4 inch; see Figures 17a and 17b). TIME DURATION OF LOAD EFFECTS In a third research step, the effects of time duration of load were studied. Sustained loads were applied in 5 seconds to a 60 x 90 x 1/4 inch LAG specimen at several temperatures and held constant for one hour (see Figure 18). Results of these tests provide additional support for a conclusion that LAG behaves like monolithic glass under wind gusts (less than 3 seconds duration) at temperatures below 100 F (Figures 17a and 17b). FIGURE 18 Effects of Sustained Load: Maximum Deflection FIGURE 17a Maximum Stress (ksi) Maximum Stress Corner Layered Monolithic 170 F 120 F 100 F 72 F 32 F Center Layered Monolithic 72 F 32 F 100 F 120 F 170 F Maximum Deflection (in.) Layered Unit Monolithic Laminated Lateral Load 0.2 psi Constant After t = 5 seconds Elapsed Time (sec.) 1,000 3, F 120 F 72 F Pressure (psi) 0.50 FIGURE 17b Deflection Deflection (in.) Layered 170 F 120 F Monolithic 100 F 72 F Pressure (psi) 30

33 3 INTERLAYER THICKNESS Further research addressed the effects of interlayer thickness on the behavior of LAG under lateral pressure. Identical lites of LAG with inch and inch Saflex interlayer thicknesses were tested at room temperature. The results (see Figures 19a and 19b) indicate that only small (less than 5%) changes in stresses and deflections occur as the interlayer thickness is changed from inch to inch (Reference 3.3). FIGURE 19a Maximum Principal Stress Near Corner (ksi) 10 FIGURE 19b Maximum Stress Layered 2.0 Lateral Pressure (psi) Maximum Deflection Laminated (0.060") Laminated (0.030") Monolithic FAILURE STRENGTHS Several series of tests to failure (glass fracture) have been conducted using LAG specimens of various sizes and thicknesses, and containing different interlayer thicknesses and glass heat treatments (Reference 3.4). Results of these tests confirm behaviors observed in the theoretical analyses and non-destructive tests described above. Annealed (AN) LAG lites with an inch interlayer exhibit failure strengths similar to monolithic AN glass lites of the same nominal thickness at room temperature, and AN LAG lites with thicker (0.090 inch) interlayers exhibit much larger failure strengths (Reference 3.5) (see Table 13). At elevated temperature (170 F) AN LAG strengths drop to about 75% of comparably sized monolithic glass (see Table 14). Heat strengthened (HS) and fully tempered (FT) LAG lites were 3.2 and 5.0 times as strong, respectively, as AN LAG lites. These tests suggest that HS and FT LAG lites have strength factors similar to strength factors for HS and FT monolithic lites at room temperature (see Table 15). TABLE 13 Failure Strengths of AN Monolithic and LAG at Room Temperature* AN LAG Interlayer Thickness Unit (inches) AN Monolithic in in. 60 x 96 x 1/4 63 psf 76 psf 144 psf 38 x 76 x 1/4 137 psf 135 psf 191 psf 66 x 66 x 1/4 107 psf 111 psf 141 psf * LAG specimens were loaded to failure at the same loading rate as comparable monolithic glass specimens. Test temperatures 75 F. Maximum Deflection (in.) Layered (theoretical) Laminated (0.060") Laminated (0.030") Monolithic (theoretical) Total Glass Thickness Is Equal for All Cases TABLE 14 Failure Strengths of AN Monolithic and LAG as a Function of Temperature* AN LAG Degrees F Unit (inches)** AN Monolithic x 76 x 1/4 137 psf 135 psf 120 psf 101 psf 60 x 96 x 1/4 63 psf 76 psf 46 psf Lateral Pressure (psi) ** LAG specimens were loaded to failure at the same loading rate as comparable monolithic glass specimens ** LAG interlayer thickness in. 31

34 Laminated Glass Behavior 3 TABLE 15 Failure Strengths of AN, HS and FT LAG* Unit (inches)** AN LAG HS LAG FT LAG 38 x 76 x 1/4 135 psf 426 psf psf ++ * LAG specimens were loaded to failure at room temperature and at the same loading rate. ** LAG interlayer thickness in. + Average surface compression was 7,700 lbs./sq. in. ++ Average surface compression 13,000 lbs./sq. in. IMPACT STRENGTH Field evaluations of windborne debris in hurricanes, typhoons and other extreme windstorms have defined the nature of objects that may impact architectural glazing. These definitions of windborne debris have been codified in standards and building codes. Impacting objects have been placed into two categories: large missiles and small missiles. The class of large missiles is represented by 2x4 timbers of several weights impacting at several velocities. The class of small missiles is represented by roof gravel and is portrayed in tests by steel balls weighing 2 gm or hard rocks of comparable weight. Background information on the evolution of missile impact criteria is contained in Section 4. Large missile impacts will break both plies of LAG. The attributes of LAG that make it valuable under this type of impact are its ability to resist penetration and its tendency to remain in the opening following breakage. LAG with inch and thicker interlayers that is secured in the frame with a silicone anchor seal or high adhesion glazing tape can preserve the integrity of the building envelope under these impact conditions. All designs for large missile impact should be tested to impact criteria similar to those outlined in ASTM E , TDI 1-98 or SSTD12-97 to ensure suitable performance. Small missile impacts may break both plies or only the outer ply of LAG. If both plies are broken, a silicone anchor seal or high adhesion glazing tape will assist in retaining the broken unit within its frame during subsequent wind gusts. In many small missile impact environments, however, the small missile will break only the outer ply, leaving the inner ply intact with an ability to resist subsequent wind gusts. If the interlayer is relatively thick and the glass plies are heat treated, the probability of breakage of the inner ply under specified impact conditions can be very low (Table 16, Reference 3.6). This concept provides a basis for designs in which the outer ply is sacrificed to small missile impacts and the inner ply is able to carry design wind pressures. In these designs, the silicone anchor seal or high adhesion tape is not required, although it may be prudent to include one or the other for structural redundancy. Standard glazing practices are sufficient for LAG performance for small missile impact if the sacrificial ply concept is employed. TABLE 16 LAG Breakage Rates as a Function of Interlayer Thickness and Heat Treatment Inner Ply Breakage Rates for 2 gm Missile Impacting at 80 MPH (117 fps) Laminated Glass Interlayer Breakage Construction* Thickness Rate (%) 3/8 HS in /16 AN in /16 HS in /16 FT in. 0.7 * All units have 2 plies of 3/16 in. glass. 32

35 section Windborne Debris 4 The sustained, turbulent winds in hurricanes, typhoons and other extreme windstorms carry large amounts of debris onto building facades, breaking windows and subjecting the building interior to internal pressures, wind and rain. Concerns with occupant safety and with large insured losses to buildings have prompted changes to building codes in the U.S. The South Florida Building Code mandates protection of windows from windborne debris. Provisions in ASCE 7-98 and the 1996 BOCA National Building Code require special designs for glazed openings in order to protect the building envelope from being breached during hurricanes or the building must be designed for the effects of full internal pressure. FIGURE 20 Houston, Texas Hurricane Alicia This section of the Guide summarizes experiences that fostered the new design requirements, describes the hurricane (typhoon) environment, outlines the new impact criteria and describes new products that have been developed to protect buildings from windborne debris (Reference 4.1). WINDSTORM EXPERIENCES Architectural glazing in several tall buildings in Houston, Texas, was broken by windborne debris generated by Hurricane Alicia (see Figure 20). Hurricane Andrew damaged facades in several major buildings in south Florida (see Figure 21). Failures of cladding systems during hurricanes result in damage to buildings, loss of building contents, interruption of business, hazards to people and marred images for buildings that have sustained damage. Events over a 25-year period during which architectural glazing in buildings was broken during hurricanes are summarized in Reference 4.2. Experiences with architectural glazing in Hurricane Andrew are summarized in Reference 4.3. FIGURE 21 Miami, Florida Hurricane Andrew 33