# Structural Glazing Systems: Glass Held by Silicone, Not by Frames

> Structural glazing bonds glass to its supporting frame with structural silicone, so the adhesive carries wind load rather than a mechanical pressure plate. The result is a facade with no visible external framing, and a system whose performance depends entirely on the quality and geometry of the silicone bond.

## What the system is doing

In a conventional curtain wall, a pressure plate clamps the glass mechanically
and a cap covers it. That cap is what you see as a grid on the outside.

Structural glazing removes it. The glass is bonded to the frame behind with
structural silicone, and wind load passes through the adhesive. From outside,
the facade reads as a flush plane of glass with only silicone joints between
panes.

The appearance is the reason clients choose it. The engineering consequence is
that **an adhesive joint has become a structural element**.

## What that changes

| | Conventional glazed system | Structural glazing |
| --- | --- | --- |
| Load path | Glass → pressure plate → frame | Glass → silicone bond → frame |
| External appearance | Visible capping grid | Flush glass, silicone joints |
| Critical quality item | Gasket and fixing | The silicone bond |
| Where it is made | Partly on site | Bonding in the factory |
| Inspection | Visual, mechanical | Bond geometry, adhesion, compatibility |
| Repair of a pane | Unclip and replace | Remove and re-bond the unit |

## Bond geometry is calculated, not chosen

The bite — the width of the silicone bond — and its thickness are derived from
the design wind pressure, the pane size and the silicone's rated strength.

Design wind pressure comes from **IS 875 (Part 3)** for the specific site,
height and terrain, and varies across a building, with corners and upper levels
highest. So the bond dimensions on one project do not transfer to another, and
within one building they may differ by elevation.

Design practice is addressed by guidance such as **ASTM C1401** and the
structural sealant glazing standard **EN 13022**, within the curtain walling
framework of **EN 13830**. The values must come from the project's facade
engineer.

## Compatibility is a real failure mode

Structural silicone touches several other materials: the glass coating, the
frame finish, setting blocks, spacers, gaskets, the insulating unit's edge seal,
and any weather seal beside it.

Some combinations degrade the bond. This is why **compatibility and adhesion
testing** between the actual silicone and the actual materials being used is
part of the design process, not a formality — and why a substitution of any
component late in procurement requires the testing to be revisited.

## Fabrication control

Because the bond is structural, factory conditions matter:

- Surfaces cleaned and primed to the silicone manufacturer's procedure
- Controlled temperature and humidity during application and cure
- Bead geometry verified, not assumed
- Cure time respected before handling or transport
- Records kept per unit, so the fabrication of any panel can be traced

A facade whose structural bonding was done on site, in ambient conditions, to
unrecorded procedures, is not a structurally glazed facade in any meaningful
engineering sense.

## What to ask for

- The design wind pressures used, and who established them
- The bond calculation, and the silicone's rated values
- Compatibility and adhesion test results for the actual material combination
- The fabrication procedure and quality records
- Whether mechanical retention is provided, and on what basis
- A performance mock-up, tested before production
- The inspection and cleaning regime for handover

## Where it suits, and where it does not

It suits buildings where a flush, uninterrupted glass plane is the architectural
intent and the budget supports the fabrication control that requires.

It suits less well where the facade is highly irregular with many one-off panes,
where the programme cannot accommodate factory bonding and curing, or where the
project cannot support the quality regime the system depends on. In those cases
a captured or partly captured system is the more honest choice.

## Standards referenced

Structural sealant glazing guidance in **ASTM C1401** and **EN 13022**; curtain
walling product standard **EN 13830**; wind loading to **IS 875 (Part 3)**.
Bond design, wind pressures, retention provisions and compatibility for a
specific building must be established by the project's facade engineer and the
silicone manufacturer.

## Frequently asked questions

### What actually holds the glass on?

A bead of structural silicone bonding the glass to an aluminium frame behind it. Wind load on the glass is transferred through that bond into the frame and then into the building structure. The silicone is doing structural work, which is why its dimensions are calculated rather than chosen, and why the bond is the element the whole system depends on.

### Why must the bonding be done in a factory?

Because the bond needs controlled conditions to achieve its designed strength — clean, correctly primed surfaces, controlled temperature and humidity, accurate bead geometry and proper curing. Site conditions cannot reliably deliver any of those. Structural silicone applied on site to a facade that will rely on it structurally is a serious quality risk, and reputable practice keeps that work in the factory.

### Is two-side or four-side structural glazing used?

Both. In two-side systems, two edges are captured mechanically and two are bonded, which gives a partly framed appearance and a mechanical back-up on two edges. In four-side systems all edges are bonded and there is no external framing at all. Four-side gives the cleanest appearance and places complete reliance on the silicone, so it demands the highest standard of design and fabrication control.

### What happens if the bond fails?

This is why the design basis matters. Systems are designed so that a bond failure does not immediately release the glass, commonly through a mechanical retention arrangement as a secondary measure. Whether such provision is required, and in what form, depends on the system, the building height and the applicable requirements, and must be determined by the project's facade engineer.

### How long does structural silicone last?

Properly specified, applied and protected, it is a long-life material — but it is a material with a service life rather than a permanent fixing. Ultraviolet exposure, incompatible materials in contact with it, and poor surface preparation all shorten it. This is why compatibility testing between the silicone and every material it touches is part of the design, not a formality.

### What maintenance does it need?

Periodic inspection of the bond lines and the weather seals, which are different things — the weather seal is usually a separate silicone bead that can be renewed, while the structural bond is not renewable in place. Cleaning regimes matter too, since some cleaning agents attack silicone. The inspection regime should be established at handover with the system supplier.

## Sources

- [ASTM C1401](https://www.astm.org/products-services/standards-and-publications.html) — Guide for structural sealant glazing
- [EN 13022](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Glass in building — structural sealant glazed systems
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings and structures
- [EN 13830](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Curtain walling — product standard

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Source: https://hagerstone.com/services/facade-glazing/structural-glazing-systems