Windows and curtainwalls

Earthquake-induced interstorey drifts damage thin and brittle panes of glass. Glass breakage alone cost more than any other single item after the 1971 San Fernando, California, earthquake. More recently, during the 1994 Northridge, Los Angeles, earthquake up to 60 per cent of storefront glazing suffered damage in the worst affected areas. Little glazing damage was observed in high-rise buildings. From the dual perspective of injury prevention and reducing economic loss glazing is worth protecting.

Clearance Glazing n

Clearance Glazing

180 mm

Distortion of

Distortion of

▲ 11.14 A glass windowpane set inside a frame with clearance all around (a). In a distorted frame the glass has slid and rotated relative to the base of the frame (b). Further distortion will lead to glazing damage.

▲ 11.16 Damage to corner windows. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from David C. Hopkins).

▲ 11.16 Damage to corner windows. Mexico City, 1985 Mexico earthquake.

(Reproduced with permission from David C. Hopkins).

180 mm

Seismic mullion

▲ 11.15 An extruded aluminium seismic mullion that allows for ±35 mm of movement.

▲ 11.14 A glass windowpane set inside a frame with clearance all around (a). In a distorted frame the glass has slid and rotated relative to the base of the frame (b). Further distortion will lead to glazing damage.

Glazing

Seismic mullion

▲ 11.15 An extruded aluminium seismic mullion that allows for ±35 mm of movement.

Figure 11.14 shows a window frame before and after structural distortion. A small clearance around all four sides of a glass pane protects the glass under a small deformation imparted to the window frame during interstorey drift. Where drift exceeds that provided by such simple detailing, seismic mullions with their greatly enhanced provision for movement are provided (Fig. 11.15).

Current practice of glass protection from seismic movement varies from country to country and upon the size of a building. In New Zealand, for example, there is little, if any, intentional provision for movement between glass panes and wooden frames in typical houses; although domestic-scale aluminium joinery does provide a nominal clearance in the order of 10 mm.In larger buildings designed by architects and structural engineers, glazing detailing faces more stringent code requirements. During small earthquakes, glazing is to be protected and during a design-level earthquake when interstorey drifts can approach 90 mm in the most flexible building allowed by the code, glass panels must not fall out.5 A similar approach is taken in the USA, where the drift at which a glass panel falls out is to be less than the calculated design earthquake drift multiplied by both a small factor of safety and one other factor reflecting the importance of the building.6 Obviously provision for such large movements can pose practical and visual difficulties, particularly at corners where two-dimensional movements must be allowed for (Fig. 11.16).7 During the design and specification of glazing, architects need advice from a structural engineer in order to provide the necessary amount of movement.

▲ 11.17 The elements of a curtain wall system.

(a) Plan Reinforced concrete column provides face-load support

(a) Plan Reinforced concrete column provides face-load support

Steel bracing bracket

Steel bracing bracket

Roof

(b) Elevation Steel bracket

Roof

(b) Elevation Steel bracket

▲ 11.18 Out-of-plane support to parapets using regularly spaced columns or brackets. Horizontal reinforcement in some mortar courses might be required to ensure the brickwork can span between vertical supports.

A lightweight cladding system consisting of glass, plastic or metal panels constrained within a light, often aluminium frame, is referred to as a curtain wall. The supporting frame is usually designed as a fully-framed prefabricated storey-height unit. It moves horizontally with the structural frame easily, offering little resistance (Fig. 11.17). Depending on the degree of interstorey drift seismic mullions may need to be incorporated into a curtain wall system. The basic approach is to isolate glass panes or other panels from their frames by providing suitably large movement clearances. Although simple in theory it is harder in practice to achieve the required separation. A high quality of workmanship is necessary to ensure that sliding or other movements are not hindered by tight-fitting gaskets or other devices. One set of tests found that the deflection capability of the tested panels was 40 per cent less than that calculated using a conventional formula.8 Manufacturers ' full-scale mock-up tests to demonstrate adequate seismic performance should be part of the process of design, specification and installation of curtain walls.

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