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Structural Silicone Glazing (SSG) Bite Calculator Façade Engineering
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Structural Silicone Glazing (SSG) Bite Calculator

Size structural silicone adhesive joints adhering vision glass to aluminum curtain walls per ASTM C1401 engineering design standards.

Smaller of width vs height
Larger dimension
Peak inward or suction wind
ASTM C1401 standard = 20.0 PSI
Summer high vs winter low for shear
Required Structural Silicone Bite (B)
0.417 Inches
Specify: 7/16" (11 mm) Minimum Contact Bite
Min Glueline Thickness
0.250 Inches
6.4 mm Minimum Gap
Total Wind Load
2,000 lbs
5.56 lbs / inch perimeter
ASTM C1401 Design Formula: B = (P × w) / (2 × σdes)
Differential Thermal Expansion: 0.089 Inches Shear Movement
Allowable Silicone Shear Strain: ≤ 50% Strain Capacity
Joint Aspect Ratio (Bite / Thickness): 1.67 : 1 (Ideal: 1:1 to 3:1)
Structural Silicone Type: DOWSIL 983 / Momentive IGS
Structural silicone joint dimensions conform to ASTM C1401 Standard Guide for SSG.

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Structural Silicone Glazing (SSG) Joint Sizing & ASTM C1401

In 4-sided structural silicone glazing (SSG), there are no exterior mechanical pressure plates or retaining caps holding the glass to the building. The glass panel is held in place against hurricane-force wind suction entirely by high-modulus, elastomeric silicone sealant. Sizing the Bite (B) and Glueline Thickness (G) is a safety-critical structural engineering calculation governed by ASTM C1401.

Key ASTM C1401 Engineering Formulations

  1. Structural Bite Dimension (B):
    For trapezoidal wind tributary loading on a rectangular lite:
    B (inches) = [ Wind Pressure (PSF) × Short Span (ft) ] / [ 2 × σ_des × 12 ]
    Where σ_des = 20.0 PSI (138 kPa) is the industry-standard allowable dynamic working stress for structural silicones (providing an ultimate safety factor of 5:1 to 6:1 over tensile rupture strength ≥ 100 psi).
    Absolute Rule: Structural bite must never be less than 0.25 inches (6.4 mm) regardless of calculation.
  2. Glueline Thickness (G):
    The thickness of the silicone joint between the glass and metal must accommodate differential thermal expansion without shearing the sealant:
    G (inches) ≥ ΔL / (2 × γ_allow)
    Where ΔL is differential thermal growth between aluminum (α = 12.8 × 10⁻⁶/°F) and glass (α = 4.8 × 10⁻⁶/°F), and γ_allow = 0.50 (50% shear strain).
  3. Aspect Ratio:
    The ratio of Bite to Thickness (B / G) should be maintained between 1:1 and 3:1 to allow proper moisture cure and joint flexibility.

Frequently Asked Questions

What is the difference between silicone "Bite" and "Thickness"?

"Bite" is the contact surface width of the silicone bonded to the glass and aluminum frame that resists wind suction. "Thickness" (glueline depth) is the spacer gap between the glass lite and aluminum that flexes to absorb differential thermal movement and building seismic sway.

Why is structural silicone design stress capped at 20 psi?

Although structural silicones (such as DOWSIL 983 or GE UltraGlaze) exhibit ultimate tensile strengths over 100 to 120 psi in laboratory tests, ASTM C1401 mandates a conservative 20 psi design stress to provide a 5:1 safety factor against long-term fatigue, UV aging, and wind gusts.

Can 1-part moisture-cure silicone be used for shop-glazed SSG?

2-part structural silicones (chemical cure) are universally preferred in factory fabrication shops because they cure completely through deep joints within 2 to 4 hours. 1-part silicones rely on atmospheric humidity diffusing into the joint, which can take weeks to cure in joints with bites exceeding 1/2".

What carries the permanent downward dead weight of the glass in SSG?

Structural silicone is strictly engineered to resist lateral wind and seismic loads. Structural silicone should never carry permanent continuous downward dead load due to polymer creep. Mechanical setting blocks placed directly beneath the glass lite carry 100% of panel gravity dead load.