Snap Caps

Snap caps or dress caps are firstly a decorative cover over the pressure plate and exposed fasteners, and secondly, more functionally important as a baffle to the drainage openings in the horizontal pressure plate. Snap caps are typically made from extruded aluminum of the same alloy as the main framing member. They may also be made from formed aluminum, formed stainless steel or in some cases, formed copper. Use of formed material requires either a special pressure plate profile or modifications to a standard pressure plate. A variety of standard profiles are available.

Setting block Pressure plate

Vent/drain

SECTION

Figure 3.11: Vent/drain openings

Drainage Openings

Drainage openings must be sized to allow clear drainage and adequate venting of the glazing cavity. The recommended opening shape is an elongated horizontal slot.

As the drainage opening in the pressure plate is baffled by the snap cap, the openings in the cap must be at least as large as those in the plate if the concern is simply the provision of drainage, or substantially larger if the concern is pressure equalization as well as drainage.

Custom profiles can be made or produced to suit most architectural design intents. Caution should be exercised with especially deep or non-rectangular caps. The deeper a cap profile the less clamping engagement on the pressure plate. As engagement reduces the potential for cap disengagement in-service increases. Custom designs can be developed to reduce this problem.

Nonetheless, deep caps inherently pose a far greater risk for disengagement, and falling from the wall. Deep snap caps also influence thermal

performance by connecting more mass outboard of the thermal break. Deep caps can also pose an ice/snow collection hazard and become a roosting location for birds.

Snap cap joints, particularly the mullion caps, must be sized to accommodate actual anticipated thermal movement. The mullion caps will tend to move more than the mullions due to greater thermal cycling. Therefore, folded metal splices must be used at snap cap joints to hide the pressure plate behind and prevent the flow of surface water directly behind the cap. The splice should match the profile of the snap cap and be fixed to the upper end of the lower cap.

Snap caps

Standard Custom 50 (2 inches) or 65 mm (2 5/8 inches)

Figure 3.12: Snap caps

Fixing of Snap Caps

High profile snap caps or caps of dissimilar materials should be mechanically fastened to the pressure plate as well as snap connected. Fastening should consist of a minimum of one screw installed at the centre of the cap length. Longer caps should be fixed with two screws at third points installed in slotted holes. Screws are positioned in the side wall of the cap so as to engage extruded legs in the pressure plate. There is a tendency for some snap caps profiles to creep along the pressure plate over time. This is not usually an issue with shorter horizontal caps but can be significant with longer vertical caps. If a cap creeps sufficiently it may start to disengage. As such fixing of one end of the cap is recommended.

3-14

3.4 FASTENERS

M

echanical fasteners such as screws and bolts are used in the assembly of the curtain wall framing, the fixing of the pressure plate and the anchorage to the building. A wide variety of fastener types, thread types, coatings and materials are available. The details of fastener selection for the most part concern the structural engineer; as such, this section provides only a broad overview of some issues to be aware of regarding fastener selection.

3.4.1 Materials

Metals used for curtain wall fasteners can be either carbon steel or stainless steel, depending on where the anchor is used in the wall.

The use of carbon steel fasteners should be limited to the wall anchors, and to frame assembly if they will be located to the interior side of the air barrier. All carbon steel fasteners must be plated or polymer coated to provide protection against corrosion.

All fasteners located to the exterior side of the air barrier should be stainless steel. Stainless steel should also be used for pressure plate fastening.

Stainless steel fasteners are available in a variety of alloys. The different alloys have differing resistance to corrosion.

There are also other cautions regarding stainless steel fasteners:

Aluminum fasteners, with the exception of some riveting applications, are not appropriate for use in curtain walls.

Stainless Steel Corrosion Resistance

All stainless steel alloys have good corrosion resistance; however, they are not all equal. For example, Type 316 has better corrosion resistance than Type 304.

Seized Stainless Steel

Stainless steel has a high coefficient of friction and low thermal conductivity; both factors tend to locally heat screw threads and cause seizure. This can lead to shearing off of self-tapping fasteners during power installation.

Painting Screw Heads

Paint finishes do not adhere well to stainless steel fasteners and are often chipped during installation. Attempting to colour match screws should be avoided.

In document Curtain (Page 56-59)