SSG Curtain Wall: Structural Glazing Systems Explained

Published 9 October 2026 — Hanart Facade Engineering Team

What Structural Glazing Actually Means

In a conventional captured curtain wall, glass is held in the frame by aluminum pressure plates and gaskets, which visibly outline every pane. Structural silicone glazing changes that: the glass edge is bonded directly to an aluminum or steel setting block with structural silicone, and the bonding bead — not a mechanical clamp — carries the wind load from the glass into the frame. SSG curtain wall facades therefore present glass-to-glass or glass-to-insulated-panel surfaces with no aluminium on the outside face: an all-glass, frameless curtain wall — in effect a hidden frame curtain wall, because the support exists but is invisible. This is what a structural glazed curtain wall delivers: transparency without visible metal, achieved by engineering rather than by removing the structure.

SSG is not a single product; it is a family of configurations. In a two-sided SSG, the two vertical edges are structurally bonded and the horizontal edges are still captured or drained. In four-sided SSG, all four edges of the unit are bonded, giving the fully frameless reading. There is also SSG in unitised systems, where each factory-assembled panel is glazed under controlled factory conditions and bonded to the frame before dispatch — by far the most common SSG route on high-rise buildings because site glazing of four-sided bonds is weather and labour dependent.

The Silicone Joint as a Structural Element

Structural silicone is an elastomer with a design tensile strength typically taken as one-tenth to one-eighth of its ultimate strength, per the industry factor of safety of about 5–6 against ultimate. A correctly sized SSG joint resists three concurrent demands. The wind suction pulls the glass away from the frame and stresses the bond in tension; the positive wind pressure pushes the glass inward and stresses the bond in compression and shear; and thermal and drift movement between frame and glass strains the bead in shear. The design width and depth of the bead must satisfy all three together, with the tension case almost always governing on tall buildings.

Joint sizing follows a simple but unforgiving logic. The required bite (the width of the bond on the glass) is proportional to the design wind pressure times the pane area divided by the allowable strength of the silicone divided by the length of the bonded edge. Doubling the pane area therefore doubles the required bite on the same edge, which is why very large panes are either bonded on four sides or moved to point-supported systems. Four-sided SSG redistributes load better but turns every edge into a QC-critical seam; two-sided SSG keeps the structural seams to two edges but puts the full pane width on each bond. Neither is "better" in the abstract — each is a trade between sightlines, pane size and test evidence.

Design Rules: Bite, Edge Clearance & Movement

Three dimensions control an SSG design. First, the structural bite described above — typically 6–19 mm depending on pane size and wind zone. Second, the edge clearance between glass edge and frame, which gives the silicone room to deform in shear without the glass kissing the metal at the corner — usually 4–8 mm. Third, the back-up depth and the perimeter sealant that protects the structural bead from UV and water. The structural silicone itself must never be exposed to sunlight or standing water: UV degrades the surface, and water in the bond line is both a durability and an adhesion risk.

Because silicone adheres tenaciously but not indiscriminately, the substrates matter as much as the geometry. The glass edge must be clean, dry and free of any release agent; the aluminum must be degreased and, if anodised or coated, verified coat-to-silicone adhesion. Ionomer, certain laminated glass interlayers and some decorative coatings are incompatible with standard structural silicones and require compatibility testing before anything is bonded. This is why SSG design on landmark projects is always a package: glass supplier, silicone manufacturer and system manufacturer must sign off on the same bond schedule.

SSG in Unitised Systems: Factory Quality Wins

On high-rise projects, Hanart's default SSG recommendation is the unitised SSG curtain wall: complete glazed panels are assembled and bonded in the factory, where temperature, humidity and cleanliness are controlled, then transported to site and hoisted into pre-fixed brackets. The structural bond is applied in a clean room environment, cured under monitored conditions, and inspected edge-by-edge before the panel leaves the plant. Site work then reduces to bracket attachment, panel alignment and the dry gaskets between panels — the inter-panel joints are mechanical, so the weather line is continuous and reproducible.

Factory SSG also unlocks the transport and logistics constraint elegantly: because the glass is bonded to the frame as a complete panel, the frame protects the glass edges during shipping, and the unitised handling process that Hanart has refined across export projects from Shanghai to New York applies unchanged. For an introduction to how unitised panels are fabricated and tested, see in-house fabrication and our comparison of unitised versus stick-built systems.

Adhesion & Compatibility Testing: The Non-Negotiable Step

Every SSG curtain wall project must demonstrate that its specific glass, coating, frame finish and silicone combination actually bonds — and keeps bonding. The standard sequence starts with compatibility testing between the silicone and every adjacent material, followed by adhesion pull tests on production-representative samples. On project scale, the acceptance regime includes: a minimum adhesion verification on every silicone batch (Hanart's standard is 1.0 MPa minimum on pulled samples), periodic destructive sampling from the production line for the factory-glazed route, and field peel tests on site-applied beads during repair or retrofit work. These tests follow the framework of EN 13022 and the GB/T 21086 annex requirements, which together define how SSG joints are sized, applied and verified.

Neglecting the test regime is the surest way to convert an aesthetic decision into a liability. Structural silicone does not fail gradually like a bolt — it loses adhesion at the interface, invisibly, under cyclic wind and thermal loading, and a failed bond on a high-rise pane is a falling-glass event. That is why the SSG specification on any serious project is inseparable from the silicone manufacturer's warranty documents and the test certificates attached to the shop drawings. For the wider envelope performance context, see performance testing of facades; for the glass side of the system, our curtain wall glass selection guide covers IGU configuration and coatings in depth.

Frequently Asked Questions

Is SSG safe on high-rise buildings?

Yes, when engineered and verified properly. The design factor of safety, factory-controlled application, batch adhesion testing and periodic destructive sampling give SSG a track record of decades on skyscrapers worldwide. The risk profile comes from shortcuts — site glazing in poor weather, skipped compatibility tests, or unverified coatings — not from the technology itself.

What is the difference between 2-sided and 4-sided SSG?

Two-sided SSG structurally bonds two opposite edges (usually vertical) and captures or drains the other two; four-sided SSG bonds all four edges of the pane. Four-sided SSG gives the cleanest frameless look and spreads load across more bond length, but it multiplies the QC-critical seams and is almost always factory-glazed. Two-sided SSG is simpler to verify and is a common choice for moderate pane sizes.

Can structural silicone be repaired or re-applied on site?

Yes, but only by removing the damaged bead back to sound material, re-priming and re-bonding with an identical approved silicone, followed by field adhesion testing. Repair work must be sequenced away from rain and high humidity, and the repaired joint should be documented with a peel test record and warranty transfer in mind.

Does SSG cost more than a captured system?

Typically yes on materials and factory processes — structural silicone, controlled glazing and the testing regime add cost. The premium is usually justified where the frameless aesthetic is a design requirement or where a continuous glass ribbon adds marketable value. When the budget does not move, specifiers can achieve a near-SSG look with narrow-profile captured systems at lower risk.

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