IGMA: August 2014
By Bill Lingnell
Living on the edge
By Bill Lingnell
Architects and designers are requesting more and more the use of insulating glass units to have unsupported edges.
Architects and designers are requesting more and more the use of insulating glass units to have unsupported edges. This occurs in designs that demand more unobstructed views for vision and light transmission and give a smooth appearance using glass. It is typical to see this concept used in butt joint units and point-supported installations where both systems use spans of insulating glass with the edges unsupported.
How much deflection can the spacer and sealant be subject to without causing premature failure? The deflection of the spacer and sealant will be dependent on the type of edge seal technology that is being used. As one would expect there will be certain spacers and sealants that offer different responses. The question actually cannot be answered in a manner that will cover all conditions. For example there will be differences in metal spacers with regard to the profile, thickness of the profile, width and height, mechanical properties and unit size to name a few factors. A steel spacer has different mechanical properties than an aluminum spacer with regard to allowable stresses and modulus of elasticity. Thus one spacer cannot be compared to another on a direct one-to-one basis with regard to deflection. Sealants also respond relative to their capability to resist transverse shear stresses when the edge seal deflects. Silicones have different properties than polysulfide, polyurethane and butyl. The amount of sealant will also have a direct bearing on the resistance to the shear and tension stresses in the sealant (both adhesion and cohesion) that occur when glass deflects. As you would imagine, flexible spacer systems will also challenge the fabricator to limit or minimize edge deflection on insulating glass units.
Is the L/175 a good rule of thumb? Is ¾ inch a limit that should be taken into consideration as far as deflection of the spacer and sealant? The L/175 edge deflection limit is a reasonable method to be employed to limit the edge stress on glass and give the designer of the framing system proper guidance so that the glass is properly supported in an effort to use the appropriate glass load resistance based on testing and analytical methods for determining stresses and deflections of glass. The limit assures that there is edge support around the perimeter of the supported edges of glass. The edge deflection should not be confused with the center-of-glass deflection. The center-of-glass deflection will be analyzed based on aesthetic conditions, edge pull-out characteristics, effect on gaskets and sealants, and possible contact with other building materials, to name a few factors that may cause the designer to limit center deflection. Glass strength and resistance to breakage is the primary issue with center deflection. It is a factor that should be thoroughly reviewed and, if need be, limited by the designer. The ¾-inch limit, like the L/175 limit, is a maximum limit that could be considered for deflection of the edge of the insulating glass. It has been used by the glass and glazing industry for many years and has proven to be a safe limit when selecting insulating glass deflection limits as well as general edge-of-glass limits for most window designs that are subjected to wind and snow loads. To date there have not been any reported failures when the L/175 or ¾-inch (whichever is less) edge deflection limits are used for the spacer/sealant designs when using insulating glass.
The present ASTM E 1300 and CAN/CGSB 12.20-M89, Structural Design of Glass for Buildings standards do not address unsupported edges in insulating glass units and, due to the vast amount of variables that exist in the fabrication of insulating glass, there is not a common method that will address all cases. I would suggest that for the current standards updates commentary be made that would give guidance to users of the standard with reference to the deflection limits and working with the manufacturers on the concerns of the edge seal used and its ability to be used in this method both structurally and environmentally.
Bill Lingnell has over 46 years of experience in the technical field of glass and architectural products. He holds three Masters of Science degrees in engineering. Lingnell is the technical consultant for the Insulating Glass Manufacturers Alliance.