Chapter 17 Glass and Glazing

Glass

Benefits of Using GlassAllows entry of natural lightProvide “views” of exterior environmentEntry of sunlight provides warmth

Disadvantages and/or Design ConsiderationsLimits occupant’s privacy Lower resistance to thermal transmission

heat in the summer & Cold in the winter

Initial & operating costs

Glass HistoryMaterial 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 Heated glass blown into sphere 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

Glass History (cont.)

Plate GlassIntroduced in the 17th CenturyProcess

Molten glass cast into frames Spread into sheets by rollers Cooled Each side ground / polished

Larger sheets of High optical qualityCostly (until process was mechanized)

Glass History (cont.)

Drawn GlassReplaced cylinder glass, early 20th centuryFlat 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

Glass History (cont.)

Float GlassProcess 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)

Ribbon of Float Glass

Process Benefits•Surfaces parallel•High Optical Quality

(Comparable to Plate)•Brilliant Surface Finish•Economical •Virtually all flat glass produced

TerminologyGlazing - “...installation of a transparent material (usually glass) into an opening”I.E. “Glass & Glazing”

Glazier A glass installer

Lites (lights)Individual pieces of glass

Glass as a Material

Major ingredient - Sand (silicon dioxide)StrengthIndividual fibers stronger than steel, but less stiffIn 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

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)

Wind TestingCommon on tall Buildings - Purpose: Establish expected loads

Mockup for a 24 Story Condo

Tempered Glass

Ordinary Glass - Annealed glass cooled slowly under controlled conditions to avoid

internal stresses

Tempered GlassAnnealed 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

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

DisadvantagesMore CostlyProcess may cause noticeable distortionsCutting & Drilling must be prior to tempering

Uses of Tempered Glass

Heat-Strengthened Glass

Substitute for Tempered GlassLower Cost, but Less of the desirable qualities of tempered

Lower strength Less desirable breakage characteristics

Process Similar, howeverLower induced stressesLess strength (only twice annealed)Breakage characteristics more similar to annealed

Laminated Glass

Sandwiching 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

PVBLayer

Skylight @ the Bellagio Hotel

Hurricane Resistant GlassLarge Missile Impact Test

Laminated and Tempered

Fire Rated Glass

Required for:Fire rated doorsRated Window and wall assemblies

Glass TypesSpecially 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)

Wire Glass

Spandrel Glass

Interior faceCeramic based paints w/

pigmented glass particles (frits) applied

Heated / Tempered to form a ceramic coating

Opaque LiteMatch or contrast other glassOften tempered - resist thermal

stresses behind lightPurposeConceal structure behind glass /

curtainwall

Spandrel Glass(view from the inside)

Spandrel Glass(view from the outside)

Spandrel Glass

Tinted & Reflective Glass

Why tint or apply a reflective coating to glass?

Reduce glare from sunlight

Reduce solar heat gain

Architectural look - Aesthetics

Clear Float Glass

Sunlight

Outside InsideReflected

Sunlight

Absorbed & Reradiated as Heat

85% +/-sunlightenters

Tinted Glass

Sunlight

Reflected

14% to 75%

ReradiatedReradiated

Outside Inside

Result: •Lower Cooling Costs•Less “sunlight”

Glare for peopleFading FF&E

Tinted Glass

ProcessChemical elements added to the molten glassColors available

Grays, bronzes, blues, greens, golds, etc.

Clear (untinted) Glass

Lightly tinted glass

Lightly tinted glass

Tinted glass

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

ReflectiveFilm

Reflective Glass

Reflective Glass

Reflective Glass

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

Visible Transmittance

“Measures the transparency of glass to visible light (rather than solar heat gain)

Ranges:Clear Glass .9Tinted & Reflective < .9

Glazing Luminous Efficacy (Ke)

Ke = Visible Transmittance Shading Coefficient

High KeHigh amount of solar heat blocked whileConsiderable amount of sunlight allowed to enterGreen & blue glass

Low KeSimilar amounts of solar heat & sunlight blocked Darker interior (less light)Bronze, gold, & grays

Glass & Thermal Transmission

Single PaneGlass

1” Polystyrene

“Well”Insulated

Wall

1/5 of Glass

Thermal Transmission

1/20 of Glass

Disadvantage of Glass: Higher Initial & Operating Costs, Reduced Comfort

Insulating Glass

Two or more sheets of glass separated by an air spaceDouble Glazing: Two (2) sheetsTriple Glazing: Three (3) sheets (somewhat uncommon)

Primary purpose of additional sheets of glassImprove insulating value - reduce thermal transmissionTwo (2) sheets - cuts heat loss in half (1/3 for 3 sheets)

Increases initial cost but:Reduces operating costsIncreases comfortProvides additional architectural options

Insulating Glass

Glass

AirSpace

Spacer

Sealant

Spacer (Spline)•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

Insulated Glass (tinted)

Insulated Glass (tinted)

Metal Spline

Low-Emissivity GlassLow-E Glass

Improves thermal performanceUltra-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

4321

Insulated Glass

Thermal Performance Data Obtained from PPG Glass

Clear, insulated, alum. spacer, air filled

Add Low-E Glass

Add “special” spacer

AddArgon

33% Improvement

6% Improvement

13% Improvement

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)

Self-Cleaning GlassProprietary product w/ coating of titanium

oxideCatalyst allowing sunlight to turn organic dirty

into carbon dioxide and water

Plastic Glazing SheetMaterials – acrylic & polycarbonateMore expensive, higher coefficients of thermal

expansion

Glazing - Small Lights

Design ConsiderationsLow stresses from wind loadingLow stresses from thermal expansion / contraction

Glazier’s points &Putty

WoodStop

Snap-in Glazing Beads &Synthetic Rubber Gaskets

Glazing - Large Lites

Design ConsiderationsGreater spans, Larger wind loadsGreater stresses from thermal expansion / contractionMinor Frame irregularities can induce stresses

Design ObjectivesEffectively support glass weight (w/o inducing abnormal stresses)Support glass against wind pressure (both positive & negative) Isolate glass from from the supporting frame & building structureAllow for independent expansion/contraction (glass & supports)Separate glass from support materials that could induce stress or cause

abrasion

Glass Support &Isolation from Frame

Setting Blocks•Synthetic Rubber•Set @ the bottom edge•Often two/lite @ quarter pts.

Centering Shims•Synthetic Rubber•Center Lite•Isolate Lit from the Frame

Bite

Support against Wind Pressure•Bite

•Too little - pop out•Too much - stress w/ glass deflection

•Supporting Mullion•Support Glass•Transmit loads to structure

Mullion

Gaskets•“Seals” the Glass (1st line of defense)•“Isolates” glass (from abrasion)•Allows for Thermal Expansion/Contraction•Accommodates Structure/Support deflection

Anchorage of Glass to Mullion(s)

‘Dry’ Glazed Lite (using compression)

Mullions being anchored to the structure

Glass being installed

Insulated Glass

Gaskets

Retainer (compression) strips installed

Finished Installation(Snap on Covers applied over the retainer strips)

Lock-Strip Gasket

Structural Silicone Flush Glazing

Mullions onthe “inside” of the

Glass

Glass adheredby SiliconeSealant orRetainer

Structural Silicone Flush Glazing

Structural Silicone Flush Glazing

Butt-Joint Glazing

•Head & Sill with conventional frames•Vertical Mullions eliminated•Vertical joints - caulked

Butt-Joint Glazing

Truss-like Mullions with ‘architectural qualities’ to

support wind loading on a tall entrance

Weeps / “Drainage” System

Sampling

of

Mullion ColorsAluminum & Glass L.C.

Suspended Glazing System

Tempered & Laminated Glass

Glass support & anchorage

Structural Glazing System(exterior)

Structural Glazing System(interior)

Glass Mullion Systemtempered glass

Glass & Design

Methods to compensate for its poor thermal propertiesDouble & triple glazingLow E coatingsLow conductivity gas fills Tinting, reflective coatingsCurtains, shuttersWindow sizing & orientation on the buildingShading or overhangs

Glass & the Building CodesCodes 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