Glass

 

I. Types of glass

Soda (soda ash) - lime - silica glass (sand, silica dioxide)

90% of all glass is this type

Not a solid (non-crystalline) - supercooled liquid with no fixed melting

Aged glass looks like it runs

 

The only type used for construction projects

Several other types of glass are made but are for other uses such as high temperature and thermal shock resistant types such as Pyrex.

 

II. Glass Manufacturing

A. Float glass - molten glass is drawn over a bath of molten tin and then annealed (gradually cooled)

1. Single strength 3/32, double strength 1/8, commercial up to 1 thick

B. Sheet glass - less expensive and older method of glass manufacturing.

C. Molten glass is drawn through rollers and annealed. More distortion in sheet glass than float glass.

 

III. Glass Properties

 

Mechanical properties - the strength of a piece of glass is limited by the condition of the glass surface. Minute scratches in the surface will cause failure at that location.

 

Thermal properties - U-value is about 1.04 to 1.10 (R .96 - .9) for single sheet glass meaning that heat moves through it very readily. U-value can be increased by doubling the glazing with Y2" air space - reduces the U-value to .49 to .56

 

IV. Solar transmittance through glass

A. Solar energy strikes glass

1. Part is transmitted through the glass and part is absorbed.

2. The radiant heat energy is transformed into short wave lengths after it passes through the glass and very little is reradiated back out through the glass. This is the "greenhouse effect".

3. Of the part that is absorbed, a portion is convected/reradiated inward and a portion convected reradiates outward.

 

C. Wired glass - light gage wire fabric is embedded in the molten glass.

1. The wire holds the glass shards together when they break.

2. Required in fire doors and fire rated wall openings.

D. Spandrel glass - ceramic frit coated glass to cover interior frame members.

E. Glass for architectural effect can be beveled on the edges, bent or made into glass blocks.

 

 

 

2. Float glass - (90% of all glass made today)

a. Heated drawn onto a bath of molten tin and annealed - slowly cooled. Helps to avoid defects.

 

3. Definitions:

a. glazing - (v) the installation of glass (n) the glass in the opening

b. glazier - the glass installer

c. lights or lites - individual panes of glass

d. relight or relite - a pane between two interior rooms.

 

4. Thickness of glass

a. Single - strength glass 3/32

b. double - strength glass 1/811

c. made thicker for most uses. Most commercial glazing is 3/16'1 thic greater up to 3/4 or 1 I'

d. Thickness depends on use and wind load

 

5. Specifying glass:

a) wind speed and exposure

b) insulating factor

c) impact exposure

d) color

e) reflectivity

 

Types of Glass

 

1. Annealed glass - occurs in the manufacturing process. Annealing (gradual uniform cooling) increases strength. Annealed glass breaks into shards

Fire finish - clear annealed glass finish is known as a fire finish.

 

2. Tempered glass - heating the glass sheet and then exposing it to jets of cold air.

Increases strength significantly

Glass crumbles when it breaks instead of breaking into shards.

Increases the safety factor in glass exposed to breakage or human contact.

a. annealed glass is cut to size - heated to 1200 F

b. both sides glass is cooled with a blast of cold air

c. bends more, 4 times more resistant to breakage.

d. When it breaks it crumbles rather than shatters into shards.

e. Specified where safety from breaking is required

f. Glass is more expensive to produce

g. Used in locations where impact is imminent

h. Required by building code in doors, around doors, and within 1 E the floor.

i. Used for all-glass doors, floor to ceiling glass panes, sunroom roo building entrances, sliding doors, etc.

j. Tempered glass causes distortion of light passing - distortion is noticeable "

 

2. Heat-strengthened glass

a. Similar to tempered glass but less expensive to produce

b. Stronger than annealed glass, but breaks like annealed glass c. Doesn't distort as much as tempered glass.

 

3. Laminated (safety) glass

a. Made by bonding sheets of glass together under heat and pressure clear vinyl sandwiched in between the glass layers.

b. vinyl holds the pieces of glass together when it breaks

c. May be used where impact resistance is required by code

d. Resists sound transmission - used in airport terminals etc.

e. May be made in any thickness. can be thick enough to stop bullets

Immersing in molten salt bath then strengthens glass.

f. Code requirements

1. Breakage resistance like bodily impact, gymnasiums, etc.

2. Polycarbonate core for bullet resistance.

3. Laminated type with a soft sound-absorbing plastic core for recording studios, radio and TV stations.

 

4. Wired glass -

a. light patterned wire embedded in glass

b. Used before there was laminated or tempered glazing c. Required by code in fire walls and fire doors.

c. wire holds glass together longer in a fire even if broken

d. New types of glass are being made that is fire rated - without wire

 

5. Patterned or obscure glass

a. surface pattern or texture is molded into the hot glass

b. used where light is desired by vision must be obscured.

c. Many different types and patterns are available

d. Etching - chemical dipping or spraying with acid. The degree of opacity depends on the lengthof time the glass is exposed to the acid. Opaque but smooth to the touch.

e. Sandblasting - glass is exposed to sand blown by a stream of compressed air. Etching and sandblasting abrade the surface - reducing glass strength.

 

6. Tinted or colored glass

a. Colored by adding small amounts of chemicals to the raw glass

b. Used to reduce glare and light transmission - heat conductivity

c. Can reduce visible light to 14% in dark gray glass (compared to 85 clear glass)

d. Provide aesthetic appeal to the building exterior

e. Common colors are gray, bronze, blue, green, gold

f. Tinting - The most common method of coloring glazing. Color producing ingredients are added to the molten glass. Reduces glare..

 

7. Reflective coated glass

a. Reflective glazing - a thin coating of metal applied to the glass to provide a reflective surface.

1. Decreases solar gain. Reduces cooling load on the building.

2. Reduces transmitted light through the pane.

b. Metallic films or metal oxides are bonded to the glass to reflect heat andlight. Architects will specify for how it looks and for high shading coefficient. Can reflect glare into neighborhoods.

c. Shading coefficient ratio of total solar heat transmission through tinted or reflective glass to the total solar heat transmission through double-strength clear glass.

d. Silvering - spraying the glass with a mixture of silver nitrate and tin chloride. Producesmirrors. The coating is then covered with shellac, varnish, or a layer of copper electroplated to it.

e. Ceramic frit - Glass surfaces are coated with a colored powder of ceramic frit and then fired at high temperature. Used for exterior glazing that is exposed to weather to hide spandrels.

 

8. Insulating glass

a. Glass is a very poor insulator (R-1 or less)

b. Insulating glass is used in almost all exterior glazing, exterior doors, windows two or three sheets of glazing bonded on the edges with space in between the panes.

c. Must be made in the factory in controlled conditions - edges are hermetically sealed with splines with desiccant crystals in them.

d. made with tempered glass or safety glass

e. made with any type of colored or reflective glass

f. space between panes can be filled with low-conductivity gas like argon reduces heat transmission (and noise)

g. made with low emissivity coatings (low-E) on the glass which facesthe inside of the thermal pane. Low -E coatings allow heat from the to pass through but reflect the heat from inside the building back towards the inside. good for cold climates.

h. Low-E Glass - (low emissivity) a coating on interior surface of double or triple pane units, that reflects ultraviolet and infrared (heat) wave lengths. The coating creates a heat barrier that keeps heat out in the summer and inside during winter.

 

9. Low-iron glass

a. Ordinary glass has low amounts of iron

b. Low-iron glass transmits heat better for solar collection

 

10. Plastic glazing

a. very strong but scratches easily

b. expands and contracts measureably - causing glazing problems

c. can be bent using heat into shapes (skylights)

d. acrylic glazing is common (Plexi-glas or Lexan)

e. polycarbonate glazing is almost unbreakable - used in hockey rink and can be used for bullet proof glass if thick enough

f. specified when vandal resistance is necessary

g. Used for light fixture lenses on outdoor lights

 

Glazing

 

1. Goals of good glazing system

a. Support the weight of the glass evenly so that the light is not subjected to abnormal stress

b. Support the glass against wind pressure and suction

c. Isolate the glass from movement in the structure and window mulli

d. Allow for expansion and contraction of the glass and the frame e. to avoid contact between the glass and the frame

 

2. Steps in Glazing

a. Glass is supported by setting blocks to suspend in the frame.

b. Sealed around the edges with different sealants including perform solid tape sealants, gunnable sealants - caulking - synthetic rut - silicones compression gaskets of silicone or butyl rubber -

c. to lock the edges in the frames.