In metal box framing, the window mullions are the principal members of the grid; horizontal members rarely form the support for curtain walls. Vertical members spanning from floor to floor must withstand axial stresses caused by self-weight and bending caused by wind loads, and it is in this direction that the mullion must have greatest stiffness and strength, a principle also illustrated by the 'fin' section of a patent glazing bar The depth of the mullion is thus dependent upon its
7.6 Glazed wall at Stansted Airport (architects: Foster Associates).
span and the area of glazing it is required to support (module spacing). Prouvé illustrated as early as the I 930s how the use of a range of proprietary sections could cope with a variety of spans (Fig. 7.9).
The size of the curtain wall grid is determined by the costs of the vertical members, the section size being weighed alongside the number of members required. Proprietary systems offer a module size of between 760 mm and 1200 mm, and according to Elder (1977), where mullion spacings are over 1200 mm, glass costs and mullion sizes increase considerably.
7.7 Mullion section for BA Combined Operations Centre by Nicholas Grimshaw & Partners.
It may be that, with the Introduction of designs of glazing systems for particular projects rather than for general use, the sections can be engineered for larger mullion spacings. Float and sheet glasses are now available for use with these larger grid dimensions. Information on maximum sizes normally available for glasses used in curtain walling can be obtained from the various glass manufacturers, including Pilkington Bros, St Helens. BS 6262:1982 Code of practice glazing for buildings also gives information on design considerations affecting spanning characteristics of different types of glass.
The logic underlying the frequency at which prefabricated curtain walling elements are jointed together can be summarized in terms of manageability (both in transportation and in site handling) and reduction of movement per joint, particularly thermal, in the system and between the system and the supporting framework.
The framing members of the curtain wall, which are fixed back at points to the supporting structure, must be allowed to contract and expand freely with changes of temperature. Rostron (1964) shows various methods of allowing for thermal movement (Figs 7.10 and 7.1 I) between the framing and structure using sliding joints. Reduction and friction at sliding connections is provided by slotted holes and plastic washers.
It is also essential to provide some kind of discontinuity in the frame itself. Frequent jointing within the frame will reduce the thermal movement per joint in the system and between the system and the main structure. There are essentially three types of such joints:
7.8 Cast aluminium truss for'Alucasa' System by Renzo Piano.
- slip joints;
- butt joints between solid mullions and transoms;
- spring connections between adjacent panels.
Both Rostron (1964) and Schaal (1961) illustrate examples of such joints. Slip joints in vertical framing members, using a loose spigot or a swaged offset, are shown in Fig. 7.1 2.
Figure 7.1 3 illustrates the principle of a slip joint in a split mullion, where the male and female sections permit thermal movement between the frames.
Butt splicing between solid mullions and transoms allows a joint thermal movement (Fig. 7.14). Examples of spring joints are shown in Figure 7.1 5. Flexible metal closure strips can also be used.
Joints between frame and infill panels take the form of beads, gaskets and sealants.The following requirements must be met.
- Joints should be wind- and rainproof (if the inside of the infill panel is sensitive to moisture, the edges should be protected).
- Self-weight of panels and wind loads should be transferred evenly to the frame.
- Panels and framing members should be free to expand and contract independently (if subject to movement; thermal or structural).
- Joints must allow for dimensional and alignment variations between shop and site.
7.9 Range of sections (depth) to cope with a variety of spaces.
Rostron (1964) describes two main types of joints, open and closed:
- Open joints allow water to enter the joint, control its passage and provide drainage.
- Closed joints form a completely weatherproof barrier
Open joints have several advantages: they easily accommodate movement, erection is quick, and sub-
7.1 I Allowance for thermal movement in methods of fixing (from Rostron, 1964).
sequent maintenance of joint seals is reduced. How-even the joint profile tends to be more complicated, and closed joints are more commonly used sealed with either rigid or flexible seals.
Rostron (1964) also defines two main classes of joint shape: integral and accessory.
- In integral joints, members are shaped in such a way that bringing them together forms the joint.
- Accessory joints require additional parts to form the joint.
Each of these joint shapes has three principal forms (Fig. 7.1 6). The choice of joint shape is determined by requirements for tolerance in assembly, movement, sequence of assembly, performance factors and aesthetics.
The component nature of curtain walling means that there are many joints in the envelope.These joints, at frequent intervals, are necessary to enable components to be of manageable dimensions. Frequent jointing will also reduce the problem of thermal movement per joint. Recent years have seen the development of more sophisticated joints moving away from metal and mastic joints between panels towards patent neoprene gasket designs. Neoprene also helps to accommodate thermal movement between the panel and its framing.
When glass is the infilling panel the edge cover given to the panel at the joint has to be limited in order to avoid too great a temperature differential occurring between the edge and the exposed surface, which may lead to the glass breaking. Stroud, Foster and Harington (1976) (pp. I 70-171) recommend that the edge cover to the glass should be limited to 10 mm, although a dark-coloured frame will alleviate the problem of thermal stressing. A more detailed discussion of glass fracture is contained in! A. Schwartz's paper to the Second International Conference on the Durability of Building Materials, I 981 (Schwartz, 1981), in which he advises designers to consider the strength-degrading effects of long-term loads, environment and surface defects as part of their glass selection process. At the same conference Zarghamee and Schwartz (1981) reported on a study designed to discover causes for the loss of metal edge bands from insulating glass units during service. Such studies have been carried out as a result of experience with glazing units that have
7.12 Two types of slip joint in vertical framing members (based on Rostron, 1964): (a) loose spigot or sleeve; (b) swaged offset.
7.13 Slip joints in split mullions (based on Schaal, 1961)
7.14 Butt joints between solid mullions and transoms (based on Schaal, 1961).
7.1S Spring connections (based on Schaal, 1961): (a) flexible metal closures in the joints; (b) example of a spring joint.
failed either by seal fracture or by glass fracture due to cyclic movement between the glass and the curtain wall. The most dramatic example is that of the Hancock Tower in Boston, where it is necessary to stabilize the main frame in order to reduce the amount of movement.
Edge clearance should take account of the thickness, strength and spanning capacity of the glass, its deflection, its thermal movement and the condition of the edge of the glass (edge fracture). It is also necessary to take into consideration the strength of the supporting member; its deflection, its thermal movement and the type and characteristics of the weatherseal. The possible creep under dead load and characteristics and correct placement of the setting block must also be taken into consideration.
Some glass manufacturers will offer general guidance, such as the rule of thumb that'designed edge cover should equal the thickness of the glazing', and most of their recommendations hover around this general principle.
The received opinion from persons within the curtain walling industry is that it is normal that recommendations for minimum edge cover be quoted as guidance for design. In setting the measurable minimum to be achieved on site, safety factors should also allow for the particular conditions of use and geometric complexity of the project under consideration.
Some individual manufacturers carry out tests on typical assemblies to take account of variations that occur in practice and any reduction in performance (weathering or strength) that could thus occur Schuco, for example, in their FW50 curtain walling advise 12 mm as minimum (measured) edge cover governed by the width of the rear airseal.
7.16 Integral and accessory joints (based on Rostron, 1964).
The Architectural Aluminum Manufacturers' Association Guide 1979, p. 100, Table C, recommends 9/1 6 in edge cover for 9/1 6 in glass.
The thermal performance of the assembly is largely determined by the insulating properties of the infilling panel. The nature of the frame, does, however; have some influence upon performance. If the frame is made of metal and is unprotected from the external environment, then it will set up a cold-bridging pathway. The effect can be overcome by discontinuity in the framing members using a patent plastic thermal break (Fig. 7.17). In some systems the metal frame is continuously protected from external conditions by a
7.17 Patent plastic thermal break.
neoprene gasket, and in this way cold bridging is avoided. If the frame is constructed from timber members then no problems of cold bridging arise.
Was this article helpful?