Description

• Stick glazing
• Unitised glazing
• Clamped glazing
• Bolt fixed glazing
• Glass blocks
• Steel windows
• Aluminium windows
• Timber windows

Like clamped glazing, bolt fixed glazing has a high degree of transparency provided by the bolts, which replace the need for metal framing around the edges of the glass sheet or double glazed units. One of the main advantages of bolt fixed glazing is the small size of the fixings when compared to the larger and more frequent clamped fixings. This system can also be double glazed much more easily since a spacer can be introduced into double glazed units during manufacture, away from the edge of the glass, where it also has the advantage of reducing the glass span, thus minimising glass thickness. However, the sight lines associated with double glazed units are almost the same as those for stick glazing, with the black line around the edge of two adjacent units, plus a 20mm (0.78in) average joint in silicone giving an overall 50mm (2in) thick joint width in a dark colour. Since the edges of double glazed units are made opaque to conceal the edging strip, translucent silicone is not used in the joint between two opaque edges. Instead, a dark coloured silicone is used, usually black.

Support methods

In common with some other glazed wall types, bolt fixed glazing is either hung from the top of the unit or is supported at the bottom. When hung from the top, bolt fixings are secured to stainless steel cables or rods fixed at the top and tensioned at the bottom (a). Fixing tolerances are provided in three directions: vertically, horizontally and laterally. Because the joints between the glass units or sheets are visible, and need to be well aligned, and due to the fact that the glass is drilled during its manufacture of the glass, fixing tolerances have to be provided by the bolt fixing and its connection to the supporting bracket.

The bolt fixings themselves vary in complexity from economic types where a threaded bolt is secured with nuts to a fixing bracket, to sophisticated types where all threads are concealed behind sleeves to give a very smooth appearance. All bolt types allow rotation to occur between the glass and the fixing, usually to a maximum of 12 degrees. A ball joint is housed where the fixing intersects with the glass for this purpose. Any sleeve that is set over the threaded rod behind the disc is positioned so that it does not interfere with the free movement of the ball joint. Support brackets to bolts are either cast or are machined and welded from plate. The choice will depend largely on shape and the number of brackets needed, since castings are economic only in large numbers cast from a single mould. Castings can be formed to take up complex geometries that look very cumbersome in welded plate but are more expensive and time-consuming to produce, except in large quantities. The choice of painted mild steel or stainless steel (polished or brushed) for the support bracket is very much a visual decision, particularly where a large amount of welding is required. If the welding is not performed to the highest standards then the results can be visually disappointing. However, the bolts themselves are always in stainless steel. Junctions between mild steel and stainless steel are isolated to avoid the strong electrolytic corrosion that occurs between these two metals. Construction tolerances between supporting structure and glass panels are accommodated between the glass and the bolt fixing, between the bolt fixing and the support bracket or ‘spider’ and between the support bracket and the supporting column or truss. The gap between glass bolt and fixing is to allow three of the typical four bolt fixings on a glass unit to move freely, while the fourth unit is clamped tight without damaging the glass at the edge of the hole. If adjustment is provided between bolt fixing and support bracket in order to take up dimensional differences between glass and supporting structure then this results in different joint widths between glass units, with slightly uneven corners where four glass units meet. This method does, however, allow the support spider to be set in a fixed relationship with the supporting truss or column. If this adjustment is instead formed between spider and primary structure then the glass remains in perfect alignment, but the alignment between spider and column/truss will vary. The choice of where to position the dimensional adjustment varies between designs and is largely based on visual preference.

Because all components in bolt fixed glazing are visible, both from inside and outside, the choice of fixing bolts and screws is visually very significant, as is the extent of visible thread on the bolt fixing itself. Countersunk bolts and pig-nose bolts (those with two small holes on their face to provide points for tightening) are often preferred to the more common hexagonal head types. Bolts with Allen keys (a square hole cut into a circular bolt head tightened with a socket wrench) are also preferred, as are smooth circular washers with small holes for tightening drilled into their edges.

Bottom-supported glazing

Glass supported at its base can be fixed by a variety of methods. Glass units are most commonly supported with glass fins or steel posts. When glass fins are used, the glass is stacked from its base, with the dead load passing partly to the fins, and partly through the bolt fixings to the glass below, depending on the design. The glass fins serve primarily to stiffen the glass wall and resist wind loads. This method is very much a development of the patch plate system described in the previous topic in order to provide maximum transparency with a minimum of metal supporting structure.

The glass units and fins are clamped at their base to the floor slab. Because the clamping plates are big, secured by four to six bolts which pass through the glass, they are often concealed below the finished floor level. The floor slab supporting the glazed wall is either stepped down to accommodate the base clamps or forms part of a raised floor zone. With either method, the pocket formed can also be used to accommodate convector heating used to reduce down-draughts in temperate climates. Above the base level, glass units are fixed with bolt fixings that transfer loads from the top glass to lower ones.

Structural posts, usually in mild steel, are an alternative method of supporting the glazing without introducing a visually intrusive element, though concrete columns are sometimes used where they form a necessary part of the primary structure. This method has the advantage of allowing each glass to be individually supported. In generic examples, the glass is fixed back to a steel post at each vertical joint. Lateral restraint is provided by vertical or horizontal wind trusses in either steel tube or cable (b). Cable is preferred for its visual lightness. The size (diameter) of the supporting posts or trusses can be reduced by adding outrigger brackets to posts set at wider centres. This reduces the number of support posts by two thirds, but each post will necessarily be bigger. The outriggers can also be reduced to become lateral restraints only by introducing small diameter posts, typically around 10mm (0.4in), to take the vertical load.

In all cases the bolts supporting the glass are fixed back to steel columns with brackets or to outriggers with similar brackets. The general shape of the bracket is determined both by the position of the holes in relation to the edge as well as by the method of fixing the bracket back to its supporting structure. The positions of the holes are usually so as to reduce the span of the glass in order to keep it within an economic thickness, typically 12mm (0.5in). An essential aspect of bracket design is in accommodating fixing tolerances. Since the supporting structure is erected before the glass, tolerances are introduced into the bolt and bracket fixing to take up the differences.

Top-hung glazing

A supporting structure to bolt fixed glazing can be made visually very lightweight by the use of cables or rods hung from the top and tensioned at the base (MCE_155, d). Stainless steel cables or painted mild steel rods are used. The choice is usually made for visual reasons. However, like clamped glazing this method is associated with high deflections under wind load. This can be accepted either in the design (some cable-assisted walls deflect up to 800mm/2ft 8in at their centre) or be restrained with cable trusses or outriggers, typically at floor levels. Bolt fixings are secured with brackets that are secured directly to the cable or rod. They usually fit together in two halves when secured to cable (MCE_152, b). Where rod is used, the material is usually in lengths that connect into a fixing made as a single piece.

Again, the mixture of stainless steel and mild steel requires separators to avoid bimetallic corrosion, caused by contact between different metals in the presence of rainwater. While mild steel rod can be painted to match other parts of the construction such as adjacent supporting structure, care must be taken during assembly on site to ensure that factory-coated components and assemblies are not damaged. Where stainless steel is used instead of either mild steel rod or stainless steel cable, the diameter of the rod will be bigger than either of the other two options.

Corners

Corners are made by either introducing supporting structure at the corner or alternatively by cantilevering or fixing the glass at the corner and pinning the glass together with two bolts forming a corner bracket (MCE_151, d). Where structural support is provided at a corner, a special bracket is usually required, though manufacturers are increasingly providing standard corner brackets as part of their systems. Variations of corner condition are usually as economic as one another. Where double glazed units are used, care is taken to avoid exposed edges of glass where they are vulnerable to damage, usually from cleaning equipment.

Seals and interfaces

Like clamped glazing, bolt fixed glazing has essentially a single barrier to water penetration. The workmanship of the silicone sealant on site is critical to its successful application. In common with clamped glazing, silicone extrusions are slowly being introduced to make installation easier (MCE_156, a). The depth of double glazed units (often around 30mm/1.2in) allows a small drainage channel to be introduced. This information can be found in the section on clamped glazing.

Where bolt fixed glazing interfaces with adjacent glazing systems, the edge of the glass is usually glazed-in directly to those systems. Where it meets a wall type in a non-glazed material, the bolt fixed glazing is usually terminated by a glazing channel or metal angles that are fixed to the adjacent wall. The detail is formed in a way that conceals the angles, leaving only the glass visible. Where bolt fixings are used at the interface with another material, half brackets are used.

Supplementary Material

Originally published in: Andrew Watts, Modern Construction Envelopes: Systems for architectural design and prototyping, Birkhäuser, 2022.