Window wall, structural & point-fixed glazing
Strip away the visible frame and the glass starts to do structural work - held by silicone, gripped by bolts, or spanning between floors as its own wall. Three families, three very different risk profiles.
In a true structural-glazing facade, the only thing holding a sheet of glass to a forty-storey building is a bead of silicone - and that is engineering, not recklessness, if you size it right.
The flush, frameless glass tower is one of architecture's signature looks, and it is built by quietly handing structural work to materials we usually think of as passive. In **structural silicone glazing**, the glass is bonded to its frame by a sealant that must carry wind load. In **point-fixed** glazing, the glass hangs from stainless bolts and dramatic spider arms with no frame at all. And the humble **window wall** - which looks like a curtain wall but sits between the slabs, not in front of them - changes who carries the load and where the water goes. Get the family wrong and you mis-place the seals, the loads and the risk.
Window wall, silicone, and bolts - three ways to hold glass
Window wall sits between the slabs; curtain wall hangs in front of them
From the street a window wall and a curtain wall can look identical - both are aluminium-and-glass skins. The difference is structural and it matters. A curtain wall passes continuously in front of the floor slabs, hung off them, so the slab edge is behind the skin and the joints are between panels. A window wall is installed between the slabs, floor by floor: each unit sits on the slab below and tucks under the slab above, with the slab edge exposed or separately clad.
The consequences are real. Window wall is often cheaper and simpler to install (each unit is handled within one floor, no craning past slab edges, easier for a general contractor), which is why it is common on residential towers. But it puts a horizontal joint at every slab that must seal against water and accommodate the slab deflecting and the building moving - a joint with a poor reputation for leaks if detailed badly. Curtain wall's continuous skin bridges the slab and isolates the structure from the weather line, but needs the craning and brackets to hang past the slab edges. Same look, different load path, different water risk.
Curtain wall passes IN FRONT of the slab. Window wall sits BETWEEN slabs. The slab-edge joint is window wall's blessing (cheap) and its curse (leaks).
Silicone carries the wind: 2-side or 4-side, captured or frameless
In structural silicone glazing (SSG) the glass is structurally bonded to an aluminium frame with a high-strength silicone sealant, so the sealant - not a metal pressure plate - transfers wind load from the glass to the frame. This is what lets the outside read as a flush sheet of glass with no projecting caps.
The variants:
Two-side SSG - two opposite edges of the glass are silicone-bonded (carrying wind) and the other two are mechanically captured under caps; a half-flush look, lower risk. Four-side SSG - all four edges are bonded and there is no mechanical capture on the face, giving the fully flush, frameless-looking facade; higher performance demanded of the silicone. Captured systems still hold the glass with a visible or toggle cap as a mechanical back-up; frameless four-side systems rely on the bond alone (often with a setting/safety retention as back-up at height).
The engineering point: the structural silicone bite (the width of the bonded joint) is sized so the sealant stress under design wind stays within an allowable - a long-standing rule of thumb keeps tensile stress around 0.14 MPa (about 20 psi) for standard structural silicone, which is why a bigger pane or higher wind needs a wider bite. Because a bonded joint can fail invisibly and progressively, four-side SSG above a certain height usually demands a mechanical safety retention so a debonded pane cannot fall - the lesson learned the expensive way.
No frame at all: bolts, spiders and glass fins
Point-fixed (or point-supported) glazing removes the frame entirely. The glass is toughened (it must be - holes concentrate stress), drilled at its corners, and held by stainless-steel bolts that pass through and clamp it, usually via an articulated spider fitting fixed back to a support structure. The spider's swivel head lets the bolt rotate slightly so the glass is held but not bent at the hole - critical, because a rigidly clamped hole is where point-fixed glass cracks.
What carries the wind back to the building varies: a steel structure, tension cables, or glass fins - vertical blades of laminated glass acting as the structural ribs of an all-glass wall. Point fixing gives the most transparent, frameless facade possible (think atria, entrances, flagship retail), but it is demanding: every hole is a stress raiser, every pane must be toughened and ideally heat-soaked to manage the nickel-sulphide spontaneous-breakage risk, and the joints between panes are open silicone-sealed butt joints, so the water strategy and the post-breakage retention (laminated glass so a broken pane stays in place) need real care. It is the showpiece end of the spectrum, used where transparency justifies the cost and the engineering.
Pick the family for the look you actually want and price the consequence. Want a flush, jointless glass plane? That is four-side SSG or point fixing - both beautiful, both expensive and detail-hungry, both needing back-up retention at height. Want a clean residential tower on a sensible budget? Window wall may be the honest answer, but draw the slab-edge joint deliberately because that is where it leaks. Remember that a frame you can see is doing visible work; a facade with no visible frame has handed that work to silicone or bolts, and that is a performance and maintenance commitment, not just a style.
Own the load path of whatever holds the glass. For SSG, size the **structural bite** from the design wind pressure and the allowable silicone stress (~0.14 MPa tension), check the bite against movement and the manufacturer's project-specific approval, and insist on factory bonding and a mechanical **safety retention** for four-side work at height - a field-applied structural bond is a red flag. For point fixing, the holes govern: use toughened, heat-soaked, laminated glass, allow articulation at every fitting so the hole sees no bending, and design the back-up structure (cables/fins/steel) and the open-joint water path explicitly. For window wall, the slab-edge stack joint is your critical detail - drain it, pressure-equalise it, and let it move.
These systems punish site improvisation. **Structural silicone must be factory-applied and cured under controlled conditions** - if anyone proposes structurally bonding glass on a swing stage, stop them; site silicone is for weather-sealing the open joints, not for structure. On point-fixed glass, never force a bolt or over-torque a fitting - the articulation is there so the hole does not crack, and a stressed hole fails later, not now. Handle toughened and point-fixed panes as fragile and expensive (edge and hole damage is invisible until it shatters). Across all three, the open or stack joints you weather-seal on site are the water line - your gasket and silicone work is the building's raincoat.
ASTM C1401
Structural sealant (silicone) glazing
The guide for structural silicone glazing - bite sizing, the ~0.14 MPa stress basis, factory bonding and adhesion testing. It is guidance, not a fall-safety guarantee, so retention back-up is still an engineering judgement.
ASTM E1300 / IS 2553 (Part 1)
Glass thickness & strength
ASTM E1300 sizes glass for wind; IS 2553 covers safety glazing in India. Point-fixed glass needs toughened (and usually heat-soaked, laminated) glass - these set the strength basis but holes and edge effects need separate analysis.
CWCT Standard (UK)
Facade performance & SSG testing
Sets the air/water/wind/movement performance and the structural-glazing test and retention expectations widely specified on Indian premium projects - a benchmark spec, not Indian law.
IS 875 (Part 3): 2015
Design wind loads
Fixes the wind pressure that sizes the silicone bite and the bolt/spider fixings; cladding pressure coefficients near edges and corners must be applied, or the bite is undersized where wind is worst.
“Structural silicone glazing means the glass is just glued on with sealant - that can't be safe on a tall building.”
Structural silicone is an engineered structural adhesive, not ordinary 'glue'. The bonded joint - the structural bite - is sized so the sealant stress under design wind stays within a conservative allowable (around 0.14 MPa in tension for standard structural silicone), the bonding is done in a controlled factory and project-approved by the silicone manufacturer, and four-side frameless systems at height carry a mechanical safety retention so a debonded pane cannot fall. Properly engineered, it is a proven, code-recognised system; the danger is field-applied or undersized bonds, not the principle.
Worked example - size the structural silicone bite
The defining calculation of structural silicone glazing is the structural bite: how wide the bonded joint must be so the silicone never overstresses under design wind. Let's size it for a single pane.
The design wind pressure (from IS 875-3), the pane size, and the silicone allowable stress below.
GIVEN - a four-side SSG pane on a tower:
Pane size = 1.5 m (w) x 3.0 m (h)
Design wind pressure q = 2.0 kPa = 2,000 N/m2
Silicone allowable = 0.14 MPa = 140,000 N/m2 (tension)
Bite formula (short span governs):
bite b = (q x a) / (2 x allowable)
where a = the SHORTER pane dimension (the span the bite supports)
FIND: the minimum structural bite b.- 1Identify the governing span: the structural bite resists wind over the glass tributary to each bonded edge. The short side governs the standard bite check, so a = 1.5 m.
- 2Set up the balance: wind force on a 1 m strip = q x a = 2,000 x 1.5 = 3,000 N/m. The two long silicone joints share it, so each carries 3,000 / 2 = 1,500 N/m.
- 3Apply the allowable: the bite must develop 1,500 N/m at no more than 140,000 N/m2 of stress, so b = 1,500 / 140,000 = 0.0107 m = ~10.7 mm.
- 4Round up to a buildable, robust bite: call it 12 mm minimum - and note manufacturers typically set a floor (often >= 6 mm and never less than the glass-edge cover), so check the product approval too.
- 5Sanity-check the joint depth (glueline thickness): SSG also needs a minimum glueline thickness (commonly ~6 mm) so the joint can absorb movement without overstraining - bite (width) and glueline (depth) are two different dimensions, both governed.
- 6State the limits: raise q to 3.5 kPa near a corner (higher cladding coefficient) and the bite climbs to ~19 mm - which is exactly why the worst wind zone on the elevation, not the average, sizes the bite, and why factory bonding and a safety retention back this up.
You’ll walk away with
A sized structural silicone bite (~11 mm, build 12+ mm) from the design wind and the 0.14 MPa allowable, with the corner-pressure and glueline checks - the core SSG sizing skill, and the reason flush-glass facades are engineered, not glued.
Two quick observations.
- 01Look at a flush glass facade and a capped one side by side. On the capped one you see thin aluminium lines (the pressure plates); on the flush one those are gone - the wind load went into silicone instead. You are now reading the structural system from the pavement.
- 02Find a glass entrance or atrium with visible spider fittings. Notice the bolts are at the corners through drilled holes, and the arms swivel - sketch why a rigid clamp at the hole would crack the glass.
Window wall sits between slabs (cheaper, but a leak-prone slab-edge joint); curtain wall hangs in front (continuous skin, more craning). Structural silicone glazing hands wind load to a sized silicone bite for a flush look - two-side and captured being lower-risk than four-side frameless. Point fixing removes the frame entirely, carrying glass on articulated bolts and spiders, demanding toughened, heat-soaked, laminated glass. Less visible frame means more work handed to silicone or bolts.
Window wall = between slabs, cheaper, slab-edge joint leaks if poorly detailed; curtain wall = in front of slabs, continuous, more craning. SSG bonds glass with load-carrying silicone (bite sized to ~0.14 MPa allowable); two-side/captured is lower-risk than four-side/frameless, which needs safety retention at height. Point fixing uses drilled toughened glass on articulated spider bolts - maximally transparent, maximally demanding. Silicone bonding is a factory job.
What is the difference between a window wall and a curtain wall?
A curtain wall is a continuous skin hung in front of the floor slabs, so the slab edges sit behind it. A window wall is installed between the slabs floor by floor, each unit resting on the slab below and tucking under the slab above, with the slab edge exposed or separately clad. Window wall is usually cheaper and simpler to install (handled within one floor) but creates a horizontal slab-edge joint that must seal against water and movement - a common leak point if poorly detailed.
Is structural silicone glazing safe on tall buildings?
Yes, when properly engineered. The structural silicone bite is sized so sealant stress under design wind stays within a conservative allowable (around 0.14 MPa in tension for standard structural silicone), the glass is bonded in a controlled factory and project-approved by the silicone maker, and four-side frameless systems at height carry a mechanical safety retention so a debonded pane cannot fall. The risk is in field-applied bonds, undersized bites or missing retention - not in the principle, which is code-recognised and widely used.
What is point-fixed or spider glazing?
Point-fixed (point-supported) glazing holds glass with no frame, using stainless-steel bolts through drilled holes at the glass corners, usually via an articulated spider fitting back to a steel, cable or glass-fin support structure. The glass must be toughened (and ideally heat-soaked and laminated) because holes concentrate stress, and the spider's swivel head lets the bolt rotate so the hole sees no bending. It gives the most transparent, frameless facade - used in atria, entrances and flagship retail - but is the most demanding to engineer.
Peer-reviewed journals & authoritative standards
- 01Bedon, C. et al. Performance of structural glass facades under extreme loads - design methods, existing research, current issues and trends. Construction and Building Materials, 163. — Construction and Building Materials (Elsevier), 2018.
- 02Review on Glass Curtain Walls under Different Dynamic Mechanical Loads: Regulations, Experimental Methods and Numerical Tools. IntechOpen. — IntechOpen (peer-reviewed chapter), 2023.
- 03Li, X. & Wu, Y. A review of complex window-glazing systems for building energy saving and daylight comfort. Journal of Building Physics. — Journal of Building Physics (SAGE), 2025.
- 04Material Selection and Characterization for a Novel Frame-Integrated Curtain Wall. (PMC8069006). — Materials / NCBI-PMC, 2021.
_Framed, bonded or bolted, all these are essentially single skins. But a facade can also be two layers with a ventilated gap between - a rainscreen, a ventilated facade, or a full double skin. That cavity is the next idea._