Structural Silicone Bite on a Four-Sided Curtain Wall Unit
A glazing crew is installing four-sided structural silicone glazed (SSG) curtain wall units on a 12-storey commercial building. Each vision unit is 1.5 m wide by 3.0 m high. The structural engineer's specified design wind pressure is 3.0 kPa. The shop drawing notes a 12 mm structural silicone bite on all four sides and specifies a structural silicone with a design tensile strength of 0.14 MPa. The units arrive with the silicone already cured in the shop. On site, the installer notices that the bottom edge of each glass unit is bearing directly on the aluminum sill member, with no setting blocks visible. The installer proposes adding a 6 mm bead of structural silicone along the bottom edge after installation to “make up the bite” and to support the glass. The general contractor wants to proceed because the schedule is tight.
1. Calculate the required structural silicone bite for the 1.5 m × 3.0 m unit at 3.0 kPa and determine whether the 12 mm shop bite is adequate. Show your formula and reasoning.
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Use the common four-sided SSG bite relationship:
b = (0.5 × w × a) / σ
where:
- b = required bite width
- w = design wind pressure = 3.0 kPa = 3.0 kN/m²
- a = short span of the panel = 1.5 m
- σ = design tensile strength of the structural silicone = 0.14 MPa = 140 kN/m²
Substitute:
b = (0.5 × 3.0 × 1.5) / 140 = 2.25 / 140 = 0.0161 m ≈ 16.1 mm
The 12 mm shop bite is therefore not adequate for the specified wind load, even though it exceeds the typical 6 mm minimum. The controlling dimension is the short span because the 0.5 factor accounts for load sharing around the perimeter; the short sides carry a larger tributary width and govern the uniform bite. The calculated 16.1 mm is a minimum design value, so the shop should provide a bite greater than 16 mm after allowing for tolerance and tooling. A 12 mm nominal bite may also measure less in practice. The unit should not be accepted without an engineered revision or re-glazing.
2. Evaluate the installer's proposal to add a 6 mm field bead at the bottom edge and to rely on it for dead-load support. What is wrong, and what should be done?
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The proposal is not acceptable for two separate reasons.
First, structural silicone bite is not simply additive. A field-applied bead is only structural if the joint is properly designed, the surfaces are prepared and primed as required by the silicone manufacturer, the bead dimensions and cure are controlled, and the engineer accepts the repair. Adding 6 mm to a 12 mm bite does not automatically create an 18 mm structural bite, because the new bead may not develop uniform adhesion, may be applied over contaminated or aged silicone, and may not have the correct contact width or cure. A field bead may also create a hard spot that changes load distribution.
Second, structural silicone is not a dead-load support. The glass weight must be carried by setting blocks in the frame, typically two blocks located at approximately the quarter points of the bottom edge, sized and hardened to suit the glass and frame. If the glass is bearing directly on the aluminum sill, point loading and edge damage can occur. The correct action is to remove and re-glaze the unit with the required bite, or obtain a written engineer-approved repair procedure, and to install proper setting blocks so the glass does not hang from or bear on the silicone.
3. List three quality-control checks that should be completed before this SSG unit is accepted.
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Acceptance should not be based on visual appearance alone. Three checks are:
Bite measurement and joint geometry: Measure the actual cured bite width and thickness at several locations, including the short sides, and compare with the engineer's required minimum. Check that the silicone contact is continuous and that there are no voids, unbonded areas, or excessive squeeze-out.
Adhesion and cure verification: Confirm the silicone is the specified product and that it has cured according to the manufacturer's data. Review shop adhesion tests or perform field peel tests on representative samples if required. Verify surface preparation and primer use.
Dead-load support and edge clearances: Confirm setting blocks are present at the correct locations, that the glass does not bear on the sill or fasteners, and that edge clearances and joint gaps meet the specification. Also verify the frame is not deflecting excessively and that the unit is labeled and traceable to the shop drawings. These checks close the gap between a drawing that says “12 mm” and a unit that actually performs safely.