Curtain Wall, Storefront and Structural Silicone Glazing (SSG) Framing
This domain tests curtain wall and storefront system differences, movement accommodation, drainage and anchorage, plus the structural silicone bite calculation used in SSG. Questions often ask you to choose a system, set a mullion deflection limit, or compute a bite from wind pressure and glass size.
System types and movement
- Curtain wall carries its own weight to the structure and spans floor-to-floor. It must accommodate slab deflection, interstorey drift and thermal movement; it is not a loadbearing element.
- Storefront framing fits within an opening and typically bears on a sill or header. It is less able to accommodate structural movement than curtain wall.
- Stick-built systems are assembled on site; unitized systems are factory-assembled and set as complete units, improving quality control and speed.
- Pressure-equalized rainscreen design uses an outer seal, an air cavity and an inner seal with weeps, so water drains instead of being pushed inward.
- Anchor with slotted holes and shims; isolate aluminum from steel to prevent galvanic corrosion.
Storefront framing
Mullion depth is selected from wind load and span; a common project limit for storefront mullion deflection is L/175, while glass deflection is often limited to L/60 or 25 mm, whichever is less. Provide expansion joints at intervals, often 6–7.5 m, and at building movement joints. Glazing stops and gaskets must not restrict glass expansion; use setting blocks at quarter points.
Structural silicone glazing (SSG)
SSG transfers wind load from glass to frame through a structural silicone bite. The design bite is B = (P × S) / (2 × σ), where P is design wind pressure in Pa, S is the short span in metres, and σ is the allowable silicone stress, commonly 140,000 Pa. Enforce the specified minimum bite, often 6 mm.
Worked example
A 1.5 m × 3.0 m SSG lite sees 2.5 kPa wind. B = (2500 × 1.5) / (2 × 140,000) = 3750 / 280,000 = 0.0134 m, or 13.4 mm. Specify a 15 mm bite and confirm sealant compatibility with the glass coating and frame finish. The long span is not used because the short span governs load transfer.
Traps that cost marks
- Using the long span in the bite formula.
- Ignoring the minimum bite or the safety factor.
- Assuming any silicone is structural silicone; weatherseal silicone is not a substitute.
- Rigidly anchoring a curtain wall that must move.
- Sealing the pressure-equalization cavity or weep paths.