Point Fixed System (Spider)
Point fixed glass systems are used most frequently in structural glass curtain walls to create transparency and depth of vision for the occupants.
The glass panes are either bolted or clamped with components providing attachment to the structural system. The most common system type is often referred to as a “spider” system. The metallic fingers that support the glass façades allow the designer to increase transparency by minimizing the structural framing.
A four- armed fitting, usually of cast stainless steel, supports four glass panes at adjacent corners on the glazing grid and ties back to the structural system. The spider fitting is designed to provide for glazing system movement under environmental loading, as well as to accommodate specified field tolerance during assembly. A variety of spider systems are available from the suppliers of cable and rod rigging systems.
Point fixed glass - Bolted:
The approach for the point-fixing of glass, since the development of the suspended glass wall, has been mechanical attachment with a fitting that accommodates a bolt through a hole drilled in the glass panel that ties it to supporting structure.
Multi-layer glass panels presented a particular problem at the start of the point-fixed systems. A method had to be found to seal around the fixing component so as not to compromise the air cavity of the panel.
Glass manufacturers and fabricators developed a ringed spacer that could be sealed around the holes to this purpose. This development has made the sourcing of this type of product become easier and more competitive worldwide.
Point fixed glass - Clamped:
Bolted method of point fixing has the disadvantage of requiring drilling and countersinking of the glass panes, and with insulated glass units the insertion of a sealing ring in the space between the glass panes around the bolt hole.
Each laminated or insulated glass unit requires the drilling of at least eight holes. Insulated-laminated panes require a minimum of 12 holes. Obviously, this adds to the cost of the glass panels.
An alternate approach that eliminates the need for drilling and instead clamps the glass at the perimeter is frequently referred to as a “pinch-plate” system. With a spider-type system, the spider component is rotated 45 degrees so that the spider arms are aligned with the glass seams.
A narrow blade of metal penetrates from the spider through the center of and parallel to the glass joint. A relatively small clamp plate on the outside surface of the glazing plane is then fixed to the blade, clamping in place the two glass panels on either side of the seam.