Cable-supported Curtain Wall

What is a cable-supported curtain wall?

A glazed wall in which the glass is supported by pre-tensioned steel cables rather than by aluminium or steel mullions. Each pane is held at its corners by a fitting that connects to the cable. There is no frame around the glass, so from inside the wall reads as glass and a thin grid of steel.

Single-span height

8 – 30 m

Stainless spiral strand

12 – 40 mm

Laminated glass build-up

21.5 – 52 mm

Deflection under design wind

L/50 – L/100

It is known by several names, which describe the same system from different angles:

• Cable wall / cable-supported curtain wall — named for the structure.
• Spider glazing — named for the fitting, the four-armed bracket that connects glass to cable.
• Point-fixed glass façade — named for how the glass is held: at discrete points rather than continuously along its edges.
• Frameless glass façade — named for what it does not have.

Three things it is often confused with, and the differences matter:

The distinction that matters commercially: a spider fitting on a rigid frame is a detail. A spider fitting on a cable is a system. Everything on this page follows from that.

The trade: transparency is bought with movement

Movement under wind is an order of magnitude greater than a framed wall.

That is the sentence to understand before anything else, and the specification puts a number on it: deflection limit L/50 – L/100 under design wind.

Work that through. On a 10 m wall, L/50 permits 200 mm of movement at mid-height. On a 20 m wall it is 400 mm. That is not a defect or a tolerance — it is how the system works. A cable resists load by changing shape until the geometry balances the force. Restrain it and you no longer have a cable wall.

• Section through the wall The cable is pre-tensioned between the structure above and below. Under wind it bows until its shape balances the load, and the movement δ is greatest at mid-height.

• • 10 m wall at L/100 100 mm
  • 10 m wall at L/50 200 mm
  • 20 m wall at L/100 200 mm
  • 20 m wall at L/50 400 mm

  Movement at mid-height The same wall at each end of the deflection limit. Bars are to one scale, so the step from 10 m to 20 m reads as it will on site.

Everything else on this page is a consequence:

• The fixings must rotate, because the glass stays flat while the cable behind it curves.
• The glass is sized by the fixing, not only by the span, because movement concentrates stress at the fixing points.
• The perimeter seals must move by the full deflection without losing their air and water seal.
• The anchorage must be stiff, because any movement in the supporting structure adds directly to the wall's own.
• Everything the wall touches — soffits, floors, adjacent construction — needs a joint that accommodates it.

Tell the client about the movement before they see it. On a tall lobby wall, 200 mm is visible in a storm, and a building owner who has not been warned reports it as a defect. Architects who have built one know this; clients building their first do not.

The cable arrangement

Stainless spiral strand, 12 – 40 mm diameter, over a maximum height of 8 – 30 m single span.

The usual arrangement is a set of vertical cables spanning head to base, pre-tensioned between the primary structure above and below, with glass fixed across them. Single span means no intermediate support — the cable runs the full height, which is what keeps the elevation clean and also what produces the deflection.

Pre-tension is derived from form finding, as on every system in this family, and it does two jobs here:

• It keeps every cable in tension under wind from either direction. A cable that goes slack under reversal is a wall that moves suddenly rather than smoothly.
• It sets the wall's stiffness. Higher pre-tension means less deflection — and a larger permanent force into the structure above and below.

That second point is the cost trade and it is the same one as on the parent system, cable structures: pre-tension is cheap to increase on a drawing and expensive to resist in a slab edge.

Stainless is specified here rather than galvanised for a reason beyond durability: the cable is at arm's length from people, permanently visible, and often lit. It has to look right for the life of the building.

Fixings: spider, countersunk bolt, or clamp plate

Three types on this system, and the choice changes the glass, the appearance and the cost.

Spider fitting. The familiar four-armed casting, clamped to the cable and picking up the corners of four adjacent panes. Articulated, so each arm rotates as the wall moves. The most tolerant of movement and the most forgiving of setting-out variation. Visible as an object — which on most projects is the point.

Countersunk bolt. The fixing passes through a conical hole in the glass and sits flush with the outer face. The flattest, cleanest exterior available. It demands the most of the glass: the hole is a stress concentration, the countersink removes material from the outer surface, and tolerances on hole position are tight.

Clamp plate. The glass is gripped between plates at the edge rather than drilled. No hole, therefore no hole stress, and glass that is not toughened becomes possible in some configurations. The plate is visible at the joint, so the elevation reads as a grid of small plates rather than of spiders.

• Spider fitting Clamped to the cable, picking up the corners of four panes. Articulated, so each arm rotates as the wall moves. Seen from outside.

• Countersunk bolt Sits in a conical hole, flush with the outer face — the cleanest exterior, and the most demanding of the glass.

• Clamp plate The glass is gripped between plates at the joint instead of drilled. No hole, so no hole stress — and the elevation reads as a grid of small plates.

The property that matters in all three is rotation. The glass is flat and stays flat. The cable behind it curves under load. Something has to absorb the angular difference, and it is the fitting. A fixing that cannot rotate transfers bending into the glass at the one point where the glass is weakest — its drilled hole.

Glass: the fixing sizes it, not the span

Point fixings concentrate stress at the corners. Glass thickness and lamination are driven by the fixing, not only by the span.

Build-up: laminated or laminated insulating, 21.5 – 52 mm overall.

This inverts the usual logic. On a framed wall, glass is supported continuously along its edges and thickness follows from the pane size and the wind load. On a point-fixed wall the load funnels into four small areas, so the governing stress is local to the fixing, and the glass gets thicker than its span alone would suggest.

• Framed wall The pane is supported continuously along its edges. Thickness follows from the pane size and the wind load.

• Point-fixed wall The load funnels into four small areas. The governing stress is local to the fixings, so the glass is thicker than its span alone would suggest.

Three specification consequences:

Laminated, always. If a pane breaks, the interlayer retains the fragments. On a point-fixed wall there is a second reason: if a drilled pane fails, the interlayer holds it on its fixings rather than releasing it.

Drilled glass must be toughened or heat-strengthened. Annealed glass cannot carry the local stress at a hole. Toughened glass raises the nickel sulphide question — it can contain inclusions that cause spontaneous breakage years later, and heat soaking is the process that forces most of those failures to happen in the factory instead. Whether the glass is heat-soaked is a specification decision to settle, not an assumption.

Hole position tolerance is tighter than glass tolerance. The holes locate the pane. If they are out, the pane is out, and unlike a framed system there is no gasket to absorb it. This is a fabrication quality question more than a design one, and it is where cheap point-fixed glazing goes wrong.

Anchorage stiffness: the wall is only as stiff as what it is tied to

Slab edges and ring beams are designed for cable pre-tension, not just for gravity.

This is the most under-appreciated thing about cable walls, and it is where they fail commercially rather than structurally.

A cable wall pulls vertically and permanently on the structure above and below it. A conventional curtain wall hangs off the slab edge and pushes horizontally under wind; a cable wall tries to pull the head beam down and the base structure up, continuously, for the life of the building.

Two consequences:

• The supporting structure must be designed for it from the outset. A slab edge sized for a framed curtain wall will not take cable pre-tension, and finding that out after the frame is priced is the same sequencing failure that kills cable roofs.
• Any movement in the support adds to the wall's own. If the head beam deflects 20 mm, the cable loses tension and the wall's mid-height movement can increase by much more than that 20 mm. The structural engineer's deflection limit for the supporting beam is therefore part of the façade specification, not separate from it.

• Stiff support The wall pulls the head beam down and the base structure up — vertically, permanently, for the life of the building. Both are designed for it from the outset.

• Soft support If the head beam deflects 20 mm, the cables lose tension and the wall’s mid-height movement can grow by much more than 20 mm. Dashed: the same wall on a stiff beam.

Deflections exaggerated so they can be seen.

Practical guidance: issue indicative cable reactions to the structural engineer at concept stage. Approximate figures are enough for the head beam and slab edge to allow for them, and they cost little to produce. The Moghaddas Ardebili Office street elevation is a delivered cable-net wall with the frame set back behind the glass line.

Perimeter seals and interfaces

Air permeability Class AE to EN 12152. Watertightness Class RE to EN 12154.

Achieving those classes on a wall that moves 200 mm is the real detailing work, and it is entirely at the perimeter.

• Head and base take the full movement. The seal has to compress and extend through the whole range without losing contact, repeatedly, for decades. This is a moving joint that also has to be airtight.
• Jambs and abutments are where the wall meets the rest of the building. The adjacent construction does not move; the wall does. Every junction needs a designed movement allowance.
• Soffits, floors and internal finishes that touch the wall need the same consideration. A plasterboard soffit scribed tight to a cable wall will crack in the first storm.
• Mock-up testing is how the classes are demonstrated, on the actual geometry with the actual joints. Programme it before fabrication is released.

Thermal performance, honestly

Ucw 1.4 – 1.9 W/m²K. Minimal framing removes the thermal break. Insulated units and edge detailing carry the whole thermal load.

It is worth being straightforward about this, because the comparison is on this site. The framed curtain wall system achieves Ucw 1.2 – 1.8 double glazed and 0.9 – 1.3 triple. A cable-supported wall reaches 1.4 – 1.9, and triple glazing is generally not practical on it — the weight goes straight into the cables and the fittings.

• Cable-supported curtain wall (this system) 1.4 – 1.9
• Framed curtain wall, double glazed 1.2 – 1.8
• Framed curtain wall, triple glazed 0.9 – 1.3

0.81.01.21.41.61.82.0

← lower is better

Ranges as published for each system on this site — see the curtain wall system.

So a cable wall is thermally weaker than the framed alternative, at both ends of the range. That is the honest position, and it follows directly from what makes the system attractive: there is almost no frame, so there is almost no thermally broken framing to help. The whole thermal load sits on the glass and its edge seal.

What follows from that:

• On a large lobby or atrium wall, the cooling and heating consequence is real and belongs in the energy model early.
• The edge seal of the insulating unit becomes disproportionately important, because it is a much larger fraction of the assembly than in a framed wall.
• Where thermal performance governs the design, this is the wrong system, and we would say so.

What drives the cost

In order of leverage:

1. The structure behind it. If the slab edge and head beam were designed for cable reactions, this is a normal façade package. If they were not, the cost of stiffening them can exceed the wall. This is decided before the façade is tendered.
2. Glass thickness, which the fixing sets. Moving from clamp plates to drilled fixings, or from spiders to countersunk bolts, changes the glass specification, and the glass is usually the largest single material cost.
3. Height. 8 – 30 m single span. Taller means higher pre-tension, larger cables and fittings, thicker glass, and more movement to detail out at every interface — the cost rises faster than the height.
4. How many fitting types are unique. The same rationalisation argument that governs panel sets on free-form structures applies here. A wall using one spider type two hundred times costs a fraction of one using eight types across the same area.
5. The perimeter. Movement joints at head, base, jambs and every abutment are bespoke detailing and mock-up scope. A simple rectangular wall is far cheaper per square metre than one with the same area and a complicated edge.

What to decide, and when

1. 01

   Concept

   The height and whether it is a single span. What is above and below the wall structurally, and whether those elements can be designed for permanent tension. The thermal target — if it is demanding, decide now whether this system can meet it.

2. 02

   Developed design

   Fixing type, which sets the glass. Pre-tension range, which sets the reactions. Indicative reactions issued to the structural engineer before the frame is fixed. The movement budget agreed with every trade that touches the wall.

3. 03

   Technical design

   Cable schedule and fittings, glass specification including the heat-soak position, perimeter movement details, mock-up scope and programme, and the tensioning sequence.

4. 04

   Before tender

   Confirmed support stiffness and a defined fitting family. Tendering a cable wall without knowing what it is tied to produces a price with a large risk margin in it.