A typical structural glazing system is composed of a number of essential components:

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Gervais Hutchinson
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1 DESIGN COMPONENTS FOR STRUCTURAL GLAZING A typical structural glazing system is composed of a number of essential components: 1. Structural framing or support structure 2.!Glass! 3. Structural silicone sealant! 4. Compatible spacers, setting blocks and gaskets. The specific functions and relevant concerns relating to each of these components are: 1. STRUCTURAL FRAMING! The design professional specifies the structural framing members in accordance with design parameters of the building project. The structural framing is normally fabricated from an aluminum alloy and may be either anodized or finished with an architectural coating. If an architectural coating is applied, it must be done in-shop by a licensed applicator in strict conformance with the manufacturer s specification and quality control procedures. Anodized coatings on aluminum tend to be variable, and may create problems for silicone adhesion if not strictly monitored. ALL FINISHES MUST PROVIDE A SUBSTRATE TO WHICH STRUCTURAL SEALANTS WILL ADHERE ON A LONG-TERM BASIS. 2. GLASS!The glass manufacturer specifies the glass type and thickness in accordance with the specified design wind load and size limitations made by the architect. Glass types the glass type normally used is clear vision

2 glass, which may be tinted or treated with a reflective coating. Other types of glass considered by the design professional include: annealed, heat-strengthed, tempered, laminated, or insulating glass.! Considerations if the glass is treated with a reflective coating, low E coating, or an opacifier (as with some spandrel types), structural silicone sealant adhesion to, and compatibility with, this coating must be verified. If insulating glass is used, it shall be a high quality, dual-seal unit with a silicone secondary seal in compliance with local specification. Compatibility of the structural silicone sealant with the insulating glass edge seal shall be verified If the structural silicone sealant used in conjunction with insulating glass units in an acetoxy cure type, be certain that joint details are designed in accordance with GE Momentive Silicones product use suggestions, as shown on details 1, 2 and 3. IT IS ESSENTIAL THAT THE JOINT DETAIL AVOIDS TRAPPING THE CURE BY-PRODUCTS WITHIN THE GLAZING ASSEMBLY IF 2-COMPONENT SILICONE INSULATING GLAZING SECONDARY SEALS ARE USED. 3. STRUCTURAL SILICONE SEALANTS!The sealant manufacturer recommends the structural silicone sealant to be used in the glazing project. Although sealant selection is made based on several factors, including: the type of system being used, the design parameters to be met and the requirements of the glazing contractors, ONLY HIGH STRENGTH SILICONE SEALANTS SPECIFICALLY DESIGNED AND TESTED FOR STRUCTURAL GLAZING SHALL BE USED. General or multi-purpose silicone sealants not specifically designed for structural glazing should not be used. The reasons

3 for this are that these sealants are:! Generally exhibit relatively low cohesive strength, a characteristic NOT desirable in structural glazing applications;! May allow excessive edge deflections in glass units under high wind load conditions. In the case of insulating glass units, this excessive deflection could result in spacer bar displacement, the rupture of the primary seal, and premature unit failure. 4. SPACERS, SETTING BLOCKS AND GASKETS! The glass manufacturer shall be consulted for the specific design requirements of spacers, setting blocks and gaskets. These requirements include factors like size, location and hardness (Shore A durometer). Compatibility of materials!the sealant manufacturer shall be consulted regarding the compatibility of 1) pre-formed rubber parts, and/or 2) materials for fabrication of these parts, with the recommended structural silicone sealant. GE Momentive SILICONES WILL PERFORM COMPATIBILITY TESTS WITH PRODUCTION SAMPLES OF THESE MATERIALS. The reason for this vital step is that these materials must not cause a color change to the sealant. Color change indicates a chemical reaction between the gasket material and the structural silicone sealants. This reaction may, in the long term, cause a complete loss of adhesion between the structural sealant and the glass and/or metal substrates when they are exposed to ultraviolet light. EXPERIENCE SHOWS, FOR EXAMPLE, THAT MANY ORGANIC RUBBERS SUCH AS NEOPRENE AND EPDM, WHEN EXPOSED TO UV LIGHT CAN CAUSE COLOR CHANGE RESULTING IN ADHESION LOSS OF THE SILICONE SEALANT AND THEREFORE, ARE JUDGED NOT COMPATIBLE IN A STRUCTURAL GLAZING SYSTEM.

4 Selection of Spacer, Setting Block and Gasket Types:!There are two types of pre-formed spacers, setting blocks and gaskets that can be selected, based upon the function that each part will play. Type I, where the silicone sealant must not adhere to the part, such as in: 2 and 4 sided systems, where the spacer is in direct contact with the sealant, and must act as a bond-breaker to prevent three-sided adhesion.!type I spacers, setting blocks and gaskets can be made of either compatible organic or silicone rubber. IF THE MATERIAL IS SILICONE, SPECIAL TALC IS APPLIED TO THE PART TO PREVENT ADHESION BETWEEN THE PART AND THE STRUCTURAL SEALANT. Prior to installation, excess talc shall be removed by wiping, to prevent contamination of substrates to which structural sealant is required to bond. Type II, where the silicone sealant must adhere to the part, such as in: a. FOUR-SIDED SYSTEMS, where the sealant must adhere to the setting blocks, and!b. TWO-SIDED SYSTEMS, where the sealant must adhere to the head and sil gaskets. Type II pre-formed spacers, setting blocks and gaskets may be made from silicone rubber. These products are available in a wide range of durometer values and in selected colors. Consult GE Momentive Silicones for specific recommendations. DESIGN COMPONENTS FOR STRUCTURAL GLAZING Only high strength silicone sealants, specifically designed and tested for structural glazing, shall be used in structural glazing applications. High strength sealants generally have high modulus characteristics. Before defining modulus, the following terms must be understood.

5 Elastic Limit! Stress! Strain! Hooke s Law! Break Strength or Stress! Bond Line Elastic Limit The greatest stress which can be applied to a sealant without leaving a permanent deformation upon complete release of the load. Stress (Unit Stress) Load in kg (P) acting on sealant bead divided by the area of sealant bead on which the load is acting (A). Stress (S) = Kg (P) / Square cm (A) = Kg / cm2 Strain (Expressed in %) Calculated by dividing the amount of a sealant bead is extended (cm) by the original bead dimension. Can be also called elongation. Hookes Law States that the amount of extension (displacement) of a sealant bead is proportional to the stress applied.!break Strength or Break Stress Stress (kg/cm2) developed at point of sealant failure = maximum load divided by the area on which the load acts. Bond Line = Contact Area The area of sealant is bonded to the substrate. To simplify the design calculations, the length of the bead may be considered to be 1cm. Therefore, the contact area = sealant width in cm (in contact with substrate) x 1 cm = square cm of Contact Area or Bond Line. Modulus Modulus is usually expressed in terms of stress at a specific strain or elongation. Modulus curves for typical high and medium modulus sealants can be shown in graph form as follows:

6 To the design professional, modulus is a key consideration because too much strain (elongation) may allow excessive flexure, or bending of the glass, when a building is subjected to high wind loads. With insulating glass, excessive flexure may cause edge seal failure or primary seal rupture.!a high modulus sealant reduces the amount of strain (elongation) and therefore, the amount of flexure (bending) of glass. A low modulus sealant implies high strain (elongation). SEALANT DESIGN CRITERIA Application of Wind Loads to Rectangular Areas: It has been an industry standard to accept that wind loads applied to a rectangular glass area are distributed to the structural sealant in accordance with the conservative trapezoidal loading area as shown below. Contact Width: The following formula to determine contact width is derived from the loading diagram shown above where- S = Short Dimension (m)!d = Design Wing Load (kg/m2)!p = Total Load in Kg = {S (m) x I (m) x D (kg/m2)} / 2 (Crosshatched Area of Figure)!CW = Contact Width (cm2)!a = Total Contact Area of Silicones in Crosshatched area = 100 cm x CW Maximum stress on silicone is:!p/a = ((S/2)*D)/(100 *CW) or P/A = (S * D)/(200 * CW) Solving this equation for Contact Width (CW), it now follows that: CW = (S * D)/(200 * CW) The accepted maximum sealant stress has been set by industry at 1.4 kg/cm2 (0.14 N/mm2) or 0.14 (MPa). Therefore the

7 formula for Contact Width becomes: CW (cm) = (S * D)/(200 * 1.4) Knowing any of the three values, this formula can be adjusted to solve for the third variable, i.e. Contact Width, Design Wind Load or Sealant Stress. SIGNIFICANCE OF A SAFETY FACTOR The term safety factor as applied to structural glazing is the ratio of a sealant s ultimate strength (usually in tension) to the most commonly used design stress of 1.4 kg/cm2. Safety factors of 5 : 1 and 6 : 1 have proven adequate since the inception of structural glazing and are the most commonly specified. Considering the number of variables which can be encountered in a structurally glazed assembly and the potential liability to all participants in the project, the higher the safety factor the more tolerant of errors the system becomes. Some of the variables which may affect the sealant strength after application are:! Sealant shelf age! Compatibility of sealant and substrate! Proper surface preparation (cleaning and, if required, priming)! Applicatoin of sealant to insure proper surface contact, avoiding are entrapment and air voids! Proper cure time prior to the moving of factory assembled units or removal of temporary stops in field applications! Environmental conditions during application and cure such as dust, wind, temperatures, rain, etc! Batch to batch variations in metal finishes (paint or anodizing). Any of these variables can affect the ultimate strength of the assembly. It is the position of GE Momentive Silicones that the highest safety factor possible is required for structurally glazed systems.

8 MAINTENANCE PROGRAM - Cleaning of glass surfaces should be performed on a regular basis in accordance with the recommendations of the glass manufacturer. - All structural joinery should be inspected annually by a reliable agency approved by the design professional and the building owner. During these inspections, special attention should be given to those installations involving structurally glazed insulating glass. Any units that exhibit evidence of the formation of condensation within the confined air space between the glass lights should be replaced as soon as possible. Failure to replace such defective units may eventually impact on the structural integrity of the system and possibly cause danger to building occupants and / or pedestrians. MAINTENANCE PROGRAM The subjects covered in SECTION II: DESIGN of this structural glazing guide are essential elements to be considered when designing and constructing a structurally glazed curtain wall system. The design professional should make no distinction between the two- or four-sided support when the design of the structural silicone bead is undertaken. The function of the structural seal is critical in either system. Two- sided systems require the same degree of care in design and application as four-sided systems.

SECTION METAL FRAMED SKYLIGHT. A. Engineering, preparation of fabrication drawings and structural calculations for the entire skylight system.

SECTION METAL FRAMED SKYLIGHT. A. Engineering, preparation of fabrication drawings and structural calculations for the entire skylight system. SECTION 08625 METAL FRAMED SKYLIGHT PART 1 GENERAL 1.01 SECTION INCLUDES: A. Engineering, preparation of fabrication drawings and structural calculations for the entire skylight system. B. Fabrication