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Chapter 17 Glass and Glazing
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Glass Benefits of Using Glass
Allows entry of natural light Provide “views” of exterior environment Entry of sunlight provides warmth Disadvantages and/or Design Considerations Limits occupant’s privacy Lower resistance to thermal transmission heat in the summer & Cold in the winter Initial & operating costs
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Glass History Material used for Centuries
Early Processes (10th Century) Crown Glass Heated glass blown into sphere Reheated & spun on “punty”(rod) Sphere becomes a “disk” Cooled & cut into pieces Cylinder glass Swung like a pendulum Elongated into a cylinder Ends cut off, split lengthwise Reheated, opened, flattened into rectangular sheet Cut into pieces Neither had high “optical” quality
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Glass History (cont.) Plate Glass Introduced in the 17th Century
Process Molten glass cast into frames Spread into sheets by rollers Cooled Each side ground / polished Larger sheets of High optical quality Costly (until process was mechanized)
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Glass History (cont.) Drawn Glass
Replaced cylinder glass, early 20th century Flat sheets of glass drawn directly from a molten glass container Production Process Continuous production line - highly mechanized Drawn glass Ground & Polished (plate) To finished sheets of glass
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Glass History (cont.) Float Glass
Process invented in 1959 in England (produced in US, 1963) Has become a worldwide standard Largely replaced drawn & plate glass Production Process (Glass “floated” across a bath of molten tin)
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Process Benefits Ribbon of Float Glass Surfaces parallel
High Optical Quality (Comparable to Plate) Brilliant Surface Finish Economical Virtually all flat glass produced
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Terminology Glazing - “...installation of a transparent material (usually glass) into an opening” I.E. “Glass & Glazing” Glazier A glass installer Lites (lights) Individual pieces of glass
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Glass as a Material Major ingredient - Sand (silicon dioxide) Strength
Individual fibers stronger than steel, but less stiff In larger sheets - microscopic imperfections inherent with manufacturing process significantly reduce its strength Cracks propagate from these imperfections near the point of maximum tension Types of Breakage Thermal Stress Breaks Mechanical Stress Breaks
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Glass Thicknesses Range of Thicknesses
3/32” Single strength 1/8” Double strength Up to 1”+ Thickness Required is Determined by: Size of Glass Lites (span) Maximum Design (Wind) Loading Acceptable Breakage Rate (most always some breakage)
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Wind Testing Common on tall Buildings - Purpose: Establish expected loads
Mockup for a 24 Story Condo
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Tempered Glass Ordinary Glass - Annealed Tempered Glass
glass cooled slowly under controlled conditions to avoid internal stresses Tempered Glass Annealed glass that is: Reheated Surfaces cooled rapidly, core cooled more slowly Induces permanent compressive stresses in edges & faces and tensile stresses in the core Result: 4 times as strong in bending More resistant to thermal stress & impact
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Tempered Glass When Tempered Glass Breaks:
The sudden release of the internal stresses: Produces small square edged particles (as opposed to sharp, jagged pieces) Strength & breakage characteristics make it well suited for: Exterior Doors Floor to Ceiling Sheets of Glass All-Glass Doors, Glass walls (ex; handball courts), basketball backboards Disadvantages More Costly Process may cause noticeable distortions Cutting & Drilling must be prior to tempering
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Uses of Tempered Glass
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Heat-Strengthened Glass
Substitute for Tempered Glass Lower Cost, but Less of the desirable qualities of tempered Lower strength Less desirable breakage characteristics Process Similar, however Lower induced stresses Less strength (only twice annealed) Breakage characteristics more similar to annealed
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Laminated Glass Sandwiching PVB - Polyvinyl Butyral Uses? Glass PVB
Transparent interlayer (PVB) Between layers of glass (can be multiple layers) Bonded under heat & pressure PVB - Polyvinyl Butyral Soft interlayer Can be clear, colored, and/or patterned Improves resistance to sound transmission Upon breakage - PVB holds pieces of glass together Uses? Skylights (overhead glazing) Reduce noise (hospitals, classrooms, etc.) Security glass (typically has multiple layers) Glass PVB Layer
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Skylight @ the Bellagio Hotel
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Hurricane Resistant Glass
Large Missile Impact Test Laminated and Tempered
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Fire Rated Glass Required for: Glass Types Fire rated doors
Rated Window and wall assemblies Glass Types Specially Tempered Glass (rated for 20 minutes) Wired Glass (mesh of wire in glass, rated for 45min.) most common, but changes the appearance of the opening Optical Quality Ceramics (20min. to 3hr)
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Wire Glass
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Spandrel Glass Interior face Purpose Opaque Lite
Ceramic based paints w/ pigmented glass particles (frits) applied Heated / Tempered to form a ceramic coating Opaque Lite Match or contrast other glass Often tempered - resist thermal stresses behind light Purpose Conceal structure behind glass / curtainwall
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Spandrel Glass (view from the inside)
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(view from the outside)
Spandrel Glass (view from the outside)
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Spandrel Glass
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Tinted & Reflective Glass
Why tint or apply a reflective coating to glass? Reduce glare from sunlight Reduce solar heat gain Architectural look - Aesthetics
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Sunlight Clear Float Glass 85% +/- sunlight enters Outside Inside
Reflected Sunlight Absorbed & Reradiated as Heat Outside Inside
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Tinted Glass Sunlight 14% to 75% Reflected Outside Inside Result:
Lower Cooling Costs Less “sunlight” Glare for people Fading FF&E Reradiated Reradiated Outside Inside
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Tinted Glass Process Chemical elements added to the molten glass
Colors available Grays, bronzes, blues, greens, golds, etc.
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Clear (untinted) Glass
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Lightly tinted glass
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Lightly tinted glass
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Tinted glass
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Reflective Glass Thin films of metal or metal oxide placed on the surface of the glass Film purpose: Reflect sunlight Reduce solar heat gain Changes Appearance Colored Mirror effect Can be placed on either face, However, often on the inside face Glass Reflective Film
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Reflective Glass
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Reflective Glass
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Reflective Glass
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Shading Coefficient “Ration of total solar heat gain through a particular glass compared to heat gain through double-strength clear glass.” Shading Coefficient = Heat gain of a Glass type Heat gain thru Clear (double-strength) Tinted glass range: .5 to .8 Reflective glass range .3 to .7
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Visible Transmittance
“Measures the transparency of glass to visible light (rather than solar heat gain) Ranges: Clear Glass Tinted & Reflective < .9
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Glazing Luminous Efficacy (Ke)
Ke = Visible Transmittance Shading Coefficient High Ke High amount of solar heat blocked while Considerable amount of sunlight allowed to enter Green & blue glass Low Ke Similar amounts of solar heat & sunlight blocked Darker interior (less light) Bronze, gold, & grays
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Glass & Thermal Transmission
“Well” Insulated Wall Single Pane Glass 1” Polystyrene Thermal Transmission 1/5 of Glass 1/20 of Glass Disadvantage of Glass: Higher Initial & Operating Costs, Reduced Comfort
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Insulating Glass Two or more sheets of glass separated by an air space
Double Glazing: Two (2) sheets Triple Glazing: Three (3) sheets (somewhat uncommon) Primary purpose of additional sheets of glass Improve insulating value - reduce thermal transmission Two (2) sheets - cuts heat loss in half (1/3 for 3 sheets) Increases initial cost but: Reduces operating costs Increases comfort Provides additional architectural options
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Insulating Glass Spacer (Spline) Air Space Sealant Glass Glass Spacer
Separates the glass Often Metallic Air Space Dry Air or Inert Gas (such as Argon) Sealant “Seals” Unit Prevent air escape & moisture penetration Glass Clear, reflective and/or tinted Glass Spacer Sealant
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Insulated Glass (tinted)
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Insulated Glass (tinted)
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Metal Spline
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Low-Emissivity Glass Low-E Glass
Insulated Glass Improves thermal performance Ultra-thin, transparent, metallic coating Generally placed on: The #2 or #3 position in insulating glass or The #4 position in laminated glass Reflects selected wavelengths of light & heat radiation Allows entry of most short-wave (sunlight) Reflects most longer-wave infrared radiation from objects and humans inside the building Result: Reduced heating & cooling load, increased comfort inside 1 2 3 4
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Thermal Performance Data Obtained from PPG Glass
13% Improvement Add Argon 6% Improvement Add “special” spacer Add Low-E Glass 33% Improvement Clear, insulated, alum. spacer, air filled
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Glass with Changing Properties
Thermochromic glass (darker when warmed by the sun) Photochromic (darker when exposed to bright light) Electrochromic (changes transparency with electricity) Photovoltaic (generates electricity from sunlight)
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Self-Cleaning Glass Plastic Glazing Sheet
Proprietary product w/ coating of titanium oxide Catalyst allowing sunlight to turn organic dirty into carbon dioxide and water Plastic Glazing Sheet Materials – acrylic & polycarbonate More expensive, higher coefficients of thermal expansion
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Glazing - Small Lights Design Considerations
Low stresses from wind loading Low stresses from thermal expansion / contraction Glazier’s points & Putty Wood Stop Snap-in Glazing Beads & Synthetic Rubber Gaskets
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Glazing - Large Lites Design Considerations Design Objectives
Greater spans, Larger wind loads Greater stresses from thermal expansion / contraction Minor Frame irregularities can induce stresses Design Objectives Effectively support glass weight (w/o inducing abnormal stresses) Support glass against wind pressure (both positive & negative) Isolate glass from from the supporting frame & building structure Allow for independent expansion/contraction (glass & supports) Separate glass from support materials that could induce stress or cause abrasion
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Glass Support & Isolation from Frame
Setting Blocks Synthetic Rubber the bottom edge Often quarter pts. Centering Shims Center Lite Isolate Lit from the Frame
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Bite Mullion Support against Wind Pressure Bite Too little - pop out
Too much - stress w/ glass deflection Supporting Mullion Support Glass Transmit loads to structure
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Gaskets “Seals” the Glass (1st line of defense)
“Isolates” glass (from abrasion) Allows for Thermal Expansion/Contraction Accommodates Structure/Support deflection
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Anchorage of Glass to Mullion(s)
‘Dry’ Glazed Lite (using compression)
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Mullions being anchored to the structure
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Insulated Glass Gaskets Glass being installed
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Retainer (compression) strips installed
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Finished Installation
(Snap on Covers applied over the retainer strips)
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Lock-Strip Gasket
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Structural Silicone Flush Glazing
Mullions on the “inside” of the Glass Glass adhered by Silicone Sealant or Retainer
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Structural Silicone Flush Glazing
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Structural Silicone Flush Glazing
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Butt-Joint Glazing Head & Sill with conventional frames
Vertical Mullions eliminated Vertical joints - caulked
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Butt-Joint Glazing
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Truss-like Mullions with ‘architectural qualities’ to
support wind loading on a tall entrance
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Weeps / “Drainage” System
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Sampling of Mullion Colors Aluminum & Glass L.C.
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Suspended Glazing System
Tempered & Laminated Glass Glass support & anchorage Suspended Glazing System
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Structural Glazing System
(exterior)
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Structural Glazing System
(interior)
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Glass Mullion System tempered glass
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Glass & Design Methods to compensate for its poor thermal properties
Double & triple glazing Low E coatings Low conductivity gas fills Tinting, reflective coatings Curtains, shutters Window sizing & orientation on the building Shading or overhangs
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Glass & the Building Codes
Codes concerned with: Structural Adequacy wind & impact loads Providing natural light in habitable rooms may require a certain glass area as a % of floor area Safety concerns with breakage skylights, overhead glazing, in or near doors, “clear” sheets of glass that could be mistaken for an opening Use of laminated, tempered, etc. Prevention of fire maximum glazed area, wire glass Energy consumption may require double glazing, storm windows, limit the maximum % of glazed area
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