Forms of cladding

There is a range of cladding forms suitable for steel framed buildings.

The classification of cladding systems is at best unclear. There are many forms of cladding system depending on types of material used and means of installation. Different authors have presented these in different ways. In relation to steel framed buildings the following list identifies the most common types:

  • Stick frame curtain wall
  • include rainscreen panels and cassettes
  • Structural silicone glazing
  •  Pre-cast concrete
  • Stone veneer
  • Brick cladding
  • Structural glazing
  • Overcladding

Stick frame systems: Stick frame curtain walls comprise glazing and cladding attached to a stick frame of aluminium extrusions, usually forming a rectangular grid of frame and panel on the façade.

Stick frame curtain wall

An example of stick frame curtain wall

There is a number of proprietary stick frame curtain wall cladding systems, which use extruded aluminium mullions and transoms. Glazed and opaque panels are clamped into the frames using pressure plates fixed with screws or bolts or wedges using compressed gaskets. The joints are sealed using extruded EPDM gaskets. Although on first inspection many of these systems many appear similar there are quite significant differences between systems, particularly in the manner of assembly and approach for excluding the weather. Some operate by being face sealed, others drained and ventilated or pressure equalised. The mullions span from floor to floor, spliced just above the floor level. The mullions are connected to each other using a sliding sleeve connection that permits differential thermal expansion between mullions.

In drained and ventilated systems the connection between transom and mullion is critical particularly the junction between gaskets. Continuous gaskets are available to form a complete seal around each infill panel.

Glazed units installed in the framing are supported off the transom using setting blocks. The seal to double glazed units should be protected from becoming wet.

Mullions and transoms can be supplied in a variety of shapes to deal with specific interfaces such as corners and eaves, for example PPG Building.

PPG Building

PPG Building

PPG is a major US manufacturer of curtain wall components, aluminium, extrusions and glazing units. It is perhaps not surprising that the architects for their corporate headquarters exploited this almost as far as one could possibly imagine in a gothic looking glass cathedral. The main tower of the building is 40 storeys high. One hundred and seventy five separate extrusion dies were commissioned to deal with all the corners and junctions in the facade. Reflective glazing glass and clear anodised aluminium mullions were used.

Glass fins can be attached to the glazing.

Glass fins can be attached to the glazing

Some of the earliest systems used structural glass fins, acting as beams. Glass panels are bolted to the fins. The fins provide horizontal support against both negative and positive wind pressures whilst the self-weight is transferred from glass panel to glass panel usually to the main structure at the top of the building. The glass carries its own weight.

Some applications use specially formed mullions, designed to resist the wind pressures in both directions.

These are quite different from the mullions used in conventional stick framed curtain walls. The glass panels are offset from the mullion using bolted connections. The self-weight of the panel is not carried by the mullion but transferred by steel rods to a beam spanning across the top of the opening. In the Festival Theatre vertical spanning mullions are used. Again the self-weight is transferred to the top of the building.

Toughened glass is necessary to accommodate the high stresses at connections.

The loads on the glass are normally transferred at the corners of the glass panels, which are therefore highly stressed. Toughened glass is necessary. Heating glass to the point where it starts to soften followed by rapid cooling produces toughened glass. The outer surface of the glass cools faster than the inner layer as the inner layer cools it contracts and pre-compresses the outer layers, increasing the flexural strength by three to four times.

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