Elsevier

Structures

Volume 13, February 2018, Pages 8-25
Structures

Optimizing the architectural layouts and technical specifications of curtain walls to minimize use of aluminium

https://doi.org/10.1016/j.istruc.2017.10.004Get rights and content

Highlights

  • Efficient design reduces a curtain wall's embodied energy and construction cost.

  • Changes to extrusion shapes and facade layout geometry result in substantial savings.

  • A genetic algorithm has been used to optimize thousands of curtain wall designs.

  • Findings are summarized in simple heuristics for practising design professionals.

Abstract

During the recent decades it has become common to enclose large buildings with lightweight, weathertight walls that hang, like curtains, from the floor edges. The frames of these curtain walls are, usually, extruded aluminium – a material whose production is highly energy-intensive. Although means of enhancing the thermal performance of building envelopes have been scrutinized, comparatively little attention has been given to the cost and embodied energy savings that can be achieved through efficient structural design. No guidelines for efficient use of aluminium in a curtain wall have been published, and architects therefore have not known the impact that their decisions have upon the facade's material content.

In this study more than 1000 unique curtain wall systems have been optimized numerically, each one to a different set of design criteria, and the results show the extent to which aluminium content is influenced by floor height, locations of supports, number of horizontal members per panel, width of the extrusions, spacing between mullions, design wind pressure, and the minimum allowable thickness of aluminium. The conditions in which the amount of metal required to construct a window wall (glazing spanning between two floors) might be less than that required for a curtain wall (an uninterrupted, multi-floor shroud), also have been explored. The results show that substantial metal savings – reductions of 40% or more – can be realized by making modest changes to the layout geometries and specifications that are in common use. The value of the corresponding construction cost reductions is significant: in the worldwide construction market, the potential savings are in billions of dollars per year.

The practical steps that an architect and specifier should take in order to reduce metal content in a curtain wall are set out in a list. These savings are separate from, and in addition to, any that might be attained by optimizing the cross-sectional shapes of extrusion profiles.

Unlike improvements in a facade's thermal performance, which usually require capital investment in insulating materials for returns that accrue over decades, material-efficient design methods are free to apply, and the benefits can be enjoyed immediately.

Introduction

At the start of the last century, when the world's tallest skyscraper was not much more than 100 m high [1], it was still common to design tower buildings with thick masonry walls that served not only to protect occupants from the weather, but also to support the weight of the floors and to resist lateral forces [2]. There is however a practical limit [3] to the height of these load-bearing walls. To create taller towers, another construction technique evolved in two cities – New York and Chicago – which were already the largest in America, and which were still growing rapidly [4, p. 492,504]. There, it became the norm to construct a freestanding structural frame made up of beams and columns, and then use that frame to carry the floors and walls. By moving away from masonry enclosures, it was possible to build to much greater heights and, partly for this reason, by the mid-1920s New York had become the world's most populous city [4, p. 505]. In the process, a market had been established for lightweight exterior walls that could be suspended, like curtains, from the edges of a tower's floors [5].

In the earliest of these curtain walls, the main structural component – the vertical member, or mullion, spanning from floor to floor – was a simple steel section. At those locations where windows were needed, the glass was carried by a separate metal frame fixed mechanically to the mullion [6, p. 108]. For decades this was the dominant design approach and, in the late 1950s, it was the method used to create the facades of the first fully-glazed towers. It was however in these early “glass box” buildings that the limitations of a curtain wall made up of window frames supported by steel verticals, particularly difficulties in achieving an effective weather barrier, were revealed [7, p. 17]. Higher performance standards were attained as facade engineers exploited the freedom afforded by the aluminium extrusion process to create mullions with more complex cross-sectional shapes. Conventional structural forms – I-sections, T-sections and boxes – were combined with features such as gasket keyways, so that a separate frame for glass was no longer required [e.g. 6, p. 111; 8].

During the ensuing period of innovation there emerged a new type or variety of curtain wall, the unitized systems, the first of which was patented in America in 1962 [9]. Facades of this type are made up of discrete panels, each one being, typically, one floor in height, prefabricated and preglazed away from the building site. The anatomy of such a panel is shown in Fig. 1. Because of the advantages conferred by factory fabrication [10 p. 4-5; 11 p. 86], today the majority of the world's new curtain wall is unitized [12, p. 82].

When two unitized panels are brought together, side by side at the exterior of a building [13, in photos, p. 69], their extruded aluminium frames engage to create a two-piece mullion – the split-mullion – within which the joints are weatherproofed by rubber gaskets. Each of the two profiles in a modern split mullion is, usually, shaped like the letter E, and many extrusions of this sort may be found in the industry's technical literature [e.g. 14, p. 6–51; 15, p. 90; 16, p. 52; 17 pp. 6–11; 18]. In the particular example shown in Fig. 2, the base shape of both the male and female profile is E-shaped, but an additional web has been added to create a box in the exterior part of the female side.

In this paper, curtain wall has been introduced in its historical context in order to emphasize that, by the standards of the construction industry, the technology is still young. It was only in the 1980s that unitized building techniques entered the mainstream [14, p. 2–4]. The first structurally-glazed tower facade – using sealant to secure the glass to the aluminium frame, as shown in Fig. 2 and discussed in Section 3.9 – was completed as recently as 1986 [13, p. 53]. Design know-how has had to propagate rapidly between contractors, especially during the period between 2005 and 2012, when the global market for unitized curtain wall doubled in value, to around US $ 12 billion per year [12, p. 82]. It would therefore be unsurprising to find that opportunities for further technical refinement exist within this relatively new field.

The authors of this paper have previously examined the efficiency with which aluminium is used in bespoke curtain walls conceived, by respected specialists, for real facades [19]. The mass of aluminium in twenty-four existing unitized wall systems, each one custom-designed for a specific building, was compared with the mass of metal in a numerically optimized design complying with the same performance criteria. The solutions obtained numerically were found to be consistently superior to those conceived by experienced facade designers. It proved to be easy to identify cases in which metal savings of 20 % or more could have been achieved through better optimization of the extrusion shapes. This finding is of interest for at least two reasons. One, most obviously, is that material savings bring cost savings. The other is that, of all the materials used in significant quantity in construction, aluminium has the highest embodied energy per unit mass (approximately 80 times that of reinforced concrete [20]), so there is an environmental incentive to use this metal sparingly.

This past investigation demonstrated that the task usually undertaken by a curtain wall contractor's designers – finding the most efficient cross-sectional shapes for extruded framing members capable of satisfying a given set of performance requirements – can be handled effectively, or more effectively, by computational algorithms. The research described in this present paper goes further: it investigates the effects that decisions made by architects and their consultants – regarding the facades' layout, and its performance criteria – have upon the mass of metal in a building's curtain wall.

The method of investigation has been to consider, initially, the geometric layout and specifications for an archetypal curtain wall – a wall typical of the sort used to enclose large numbers of modern buildings – and then, by varying one design constraint at a time, it has been possible to quantify the extent to which each of the variables influences the mass of metal in the wall system.

In this paper, the specifications for a total of more than 1000 unique curtain walls have been considered. In each case, the wall system's extrusion shapes have been optimized using numerical algorithms implemented in the software whose workings are outlined, briefly, in 1.1 , 1.2 Structural design of glass, 1.3 Material cost and embodied energy. Results are set out in Section 2: these show the extent to which the mass of aluminium is affected by changes in floor-to-floor height, mullion bracket location, number of transoms, mullion width, mullion spacing, and also by the magnitude of the design wind pressure. The implications of these results, which are presented in Section 3, are formulated as a set of simple guidelines. By following these recommended design strategies, practising architects and facade engineers, who will not have access to the sort of numerical optimization tools that have been used in this research, will be able to make more efficient use of aluminium in their buildings' curtain walls.

For each unique combination of facade layout and performance specification, the shapes of the extrusions in an optimized curtain wall system have been found numerically. The optimization software, named Acweds, was written for this purpose. The program's features and complexities – it is made up of 5000 lines of C++ code – are not detailed here, but a description of its workings has been published separately [19]. Its four main operative parts are:

  • (a)

    A parameterized geometric model of a unitized curtain wall system's extrusions.

  • (b)

    A set of procedures by which to evaluate whether proposed extrusions are structurally viable, and whether they can in practice be manufactured. One of the verifications made during these analyses ensures that the magnitude of the mullion's deflection is not greater than the specified allowable. Also, stresses are computed in each inter-transom span, for each of the panel's mullion profiles, for each specified wind load condition: these values are checked, using the algebraic rules given in the Aluminum Design Manual (ADM) [21], to ensure that they do not exceed the allowable proportion of the extrusion's yield strength or local buckling limit or lateral torsional buckling limit.

  • (c)

    A numerical search function, a genetic algorithm (GA), programmed to look for that set of dimensions that, when applied to the parametric model, produces a curtain wall design satisfying the constraints using the minimum possible quantity of aluminium.

  • (d)

    Computer code capable of converting the program's data into human-readable format. Output includes structural calculation reports, drawings of optimized extrusions, and statistics with which to track the search algorithm's progress.

The software's efficacy has been assessed by comparing its extrusion designs with those developed by experienced practitioners. Also, the algorithm's robustness – its ability to reject local optima – has been investigated by executing it repeatedly, each time starting at different points within the search space. The results of these tests suggest that the mass of metal in the machine-generated solutions is consistently within a few percent of the global minimum, and that this level aluminium usage efficiency is considerably better than that achieved by human designers [21].

In order to admit light to a building, and in order to allow the occupants to see outside, it is usual that the sheet material used to cover a large proportion of a curtain wall's surface area – sometimes the entire surface area – will be glass. Although the central goal of this research is to find effective means by which to minimize the mass of aluminium in a curtain wall, it is desirable to understand also the way in which those strategies influence the thickness of the glass. This information is of interest because the amount of energy required to create architectural glass, by melting silica sand and subsequently heat treating the cut panes, is energy intensive. The finished material's embodied energy, and hence its cost, is significant, although in a typical curtain wall glass contributes less than aluminium to the total embodied energy and total cost. So that these contributions may be assessed, Acweds has been programmed to select – from amongst the six standard thicknesses of architectural glass between 6 mm and 19 mm – the minimum allowable thickness for each pane in the curtain wall panels that it analyzes.

The spandrel glass and vision glass shown in Fig. 1 are separate rectangular panes, each of which is simply supported along its four sides. The load resistance of these panes is determined using a closed-form algebraic expression deduced from the British Standard for glazing in buildings [22]. Glass deflections, on the other hand, are computed by the algebraic method set out in ASTM E1300 [23, Appendix X1]. The reason for mixing the design rules published in two different countries is simply that the British standard does not provide a method for finding deflections, and the ASTM's procedure for estimating load resistance relies on graphs whose data are not readily incorporated within a computer program. The glass thicknesses determined using Acweds therefore might not comply precisely with either the British or the American design conventions.

For the purpose of estimating the combined cost of glass and aluminium in a given curtain wall design, the price of extruded and painted aluminium has been taken to be US$ 3 per kg. The cost of tinted, heat strengthened, monolithic glass with a single coating of metal oxide, has been assumed to vary linearly with the thickness of the pane. Based on a review of current “factory gate” prices – that is to say, without transportation fees, taxes or duties – for high-volume glass purchases [24], [25], the authors have developed the following algebraic expression to describe glass costs: - cgl=4+3100tglwhere,cglis the cost of glass in US $ per m2,and,tglis the thickness of the pane, in m.

In reality, glass cutting wastage, and hence cost, will be influenced by the sizes of the panes in a production batch. Also, individual glass fabricators and aluminium extruders frequently adjust their pricing policies, and so the material costs presented in this study should be considered indicative, not exact.

The embodied energy in extruded aluminium and tempered glass is taken to be 154 MJ/kg [20, p. 74] and 36 MJ/kg [mean value, 20, p. 16] respectively.

Section snippets

Numerical optimization studies

Throughout the history of the “glass box” architectural style, critics have complained that the curtain wall facades of many of the world's large buildings are similar to one another in appearance. There is some truth in this allegation. Because of practical constraints, different architects arrive at similar design solutions: floor-to-floor heights vary only within a narrow range; a rectilinear grid is the most practical arrangement for the facade's skeletal frame; transportation logistics

Research-based heuristics for efficient curtain wall design

Metal-saving strategies that might be used by the different members of a building's facade design team – the architect, the specialist consultant and the facade contractor – are presented below. These recommendations follow from the numerical studies described in this paper.

Further metal-saving strategies

The rules of thumb set out above, in Section 3, follow directly from the numerical studies described in Section 2. The list below contains further suggestions that are not based upon the research results, but may nonetheless be of help to those building designers and code committees who have an interest in minimizing the mass of aluminium in curtain wall facades.

Quantifying material optimization's benefits

This study has shown that, just by moving the mullion's support point away from the top of the panel 2.2 Stack height, 3.2 Location of mullion brackets and relaxing the allowable minimum metal thickness 2.4 Minimum extruded thickness, 3.4 Minimum metal thickness, the mass of metal in a unitized curtain wall may be reduced by more than 40%. Other small deviations from current common practice, such as adjustment of the spacings between members 2.3 Unbraced length & mullion width, 2.5 Mullion

Conclusions

In the course of this study, the designs of more than 1000 curtain wall facades have been optimized numerically, using a cluster of high-performance computers. Analysis of the results has revealed that the criteria defined by an architectural team – for example, the distances between framing members, the positioning of attachment brackets, and any requirement for a minimum thickness of metal in extrusions – have a marked influence upon the quantity of aluminium in a unitized curtain wall. It

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