Why did the glass break!

Causes of edge impact in commercial glazing systems;

· Thermal unit was impacted by trapped water in system that froze

1. The curtain wall system in which the thermal unit is installed provides drainage for water entering the system.

2. The drainage provided requires several orifice to be free and clear of obstructions to function correctly.

3. The drainage orifice in this system should be in the form of a slot ( 3/8” x ¾”) and not a hole as is found in many instances.

4. The drainage route in this system requires the vertical structural joints to have a seal between glass and frame, and a weather bead of silicone between glass surfaces on the face, separated by backer rod. The backer rod separation facilitates the movement of water around and thru the system. Obstructions in this path cause water to be trapped.

5. This system was missing a section of backer rod and void was filled with structural silicione.  Water flowing through the joint was trapped at this point.  Extreme cold temperatures in Winter caused freezing, expansion, and pressure against glass edge.

Other breakage patterns would be interior impact, thermal stress, or mechanical stress.

· Interior impact (type A)

1. Someone or something falls against the glass causing impact spread over a moderate surface area – result is mechanical stress (see below)

· Interior impact (type B)

1. Something falls against the glass causing impact over a small surface area – object goes thru glass at point of impact (hole), force is not translated through entire glass to edge.

· Thermal stress

1. Temperature gradient across glass surface is extreme (either interior or exterior)

• Interior (example: black body placed against part of interior glass surface which absorbs thermal energy, air conditioning blows against opposite end of the glass causing loss of thermal energy)

• Exterior (example: a portion of the glass is in full winter sun, the balance in shade is affected by extreme wind chill.

2. Inherent edge imperfections are weakened with this manipulation and produce cracks.

3. Single line crack with non linear path to usually to completion is produced. (often whip like if not in conjunction with Mechanical stress)

· Mechanical stress

1. External forces are applied to the glass causing twisting, bending, and/or compression. (deflection due to wind load is included in this class of breakage)

2. Inherent edge imperfections are weakened with this manipulation and produce cracks.

3. Single line crack with path related to the nature of the stress is usually produced (often straight along seam of force plane), often to completion.

4. Wind load forces usually result in exterior lite breakage since the compressible nature of the air space results in approximately 70% transmission of force to interior lite.