Automation in Construction 22 (2012) 36–50

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Automation in Construction

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Performative skins for passive climatic comfortA parametric design process

Michela Turrin a,⁎, Peter von Buelow b, Axel Kilian c, Rudi Stouffs a,d

a Delft University of Technology, Netherlandsb University of Michigan, USAc Princeton University, USAd National University of Singapore, Singapore

⁎ Corresponding author.E-mail addresses: M.Turrin@tudelft.nl (M. Turrin), pv

(P. von Buelow), akilian@princeton.edu (A. Kilian), R.M.F.URLs: http://www.wiki.bk.tudelft.nl/designInformatic

http://www.umich.edu/~pvbuelow (P. von Buelow), http(A. Kilian), http://www.tudelft.nl/rmfstouffs (R. Stouffs).

0926-5805/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.autcon.2011.08.001

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 16 August 2011Available online 28 September 2011

Keywords:Performance-oriented designParametric modelingGenetic algorithmsPassive solar strategiesLarge roofs

The performance-oriented design of large roof structures for semi-outdoor spaces is investigated with theaim to integrate performance evaluations in the early stages of the design process. In particular, daylightand thermal comfort are improved under large structures by exploring passive solar strategies that reducethe need for imported energy. The performance oriented, parametric design approach we developed is struc-tured in two parts. One part uses parametric geometry to generate design alternatives, and the other partuses performance based exploration and evaluation of alternatives. We discuss how the performance basedexploration is accomplished using a tool called ParaGen. The potential of the method is shown in a casestudy of the SolSt roof. The design process of SolSt is based on parametric variations of its curvature, the den-sity of its modules and the geometry of its cladding, and is explored based on daylight and solar exposure ofthe covered space.

buelow@umich.eduStouffs@tudelft.nl (R. Stouffs).s/Michela_Turrin (M. Turrin),://www.designexplorer.net

rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In this paper we approach the design of large roofs in urbanspaces, with a specific focus on halls, atria, shopping galleries, publicsquares, and other covered spaces that are integrated into the urbanfabric. We specifically focus on the design of the envelopes coveringthe areas, emphasizing the importance of the early integration of per-formance evaluations in the design process, with the goal of develop-ing performative skins. The role of performative skins is paramountwith reference to their mediation between the conditions desiredby the users of the spaces and those present in the surrounding envi-ronment. Through the research presented here we emphasize theircapacity to control and filter environmental factors with respect to abroad range of architectural performances.

When investigating the climatic performance of large roofs, thefirst set of considerations which arises concerns the level of enclosureof the covered spaces and the use of energy resources to achieve theenvisioned degree of comfort. Considering the importance of energyperformance today, we have concentrated on passive strategies forclimatic comfort. Climatic comfort in large indoor and semi-outdoor

spaces is briefly introduced in Section 3, while Section 4 provides anoverview of the factors that affect climatic comfort, and how discom-fort can be mitigated using passive strategies. Dealing with the com-plex interdependencies between these factors is beyond the scope ofthis paper. Instead, the subsequent sections show an example of thepartial contribution that solar exposure and proportion of incomingdaylight contribute to the overall picture. With respect to all factorsaffecting climatic comfort, and solar exposure and incoming daylightespecially, the role of geometry is emphasized and discussed basedon a case study in Section 4. In particular we focus on how theroof geometry is affecting climatic comfort, and how it implies theconvergence of disciplines. Section 5.2 presents a selection of rele-vant geometric aspects with respect to climatic comfort as well asother interdisciplinary aspects. A performance oriented parametricdesign method is presented in the following sections, which aimsat supporting performance evaluations of design alternatives of per-formative skins. Section 6 introduces the parameterization processand the part of the method that refers to parametric modeling;while in Section 7 the performance evaluation process is discussed,together with the exploration of the solution space of the paramet-ric models. Section 8 presents an example where the method isapplied.

2. Large roofs in urban spaces

Contemporary cities increasingly integrate large roofs into theirurban fabric. Various rationales and motivations for the creation of

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large roofs exist. One is the increased demand for representativestructures that mark the built environment, such as in the case ofrepresentative halls and atria. Another is the need for outdoor spacesthat can be used independently of the weather conditions, such as inthe case of covered squares, stations and shopping centers. It there-fore has become more common for architects and engineers to en-gage in the design of large roof structures, both for indoor and semioutdoor spaces. The case of semi-outdoor areas is considered in moredetail below, while the case of indoor areas can be exemplified by theGolden Terraces complex in Warsaw [1], the Lac Mirabel in Montreal[2], amongmany others. Besides the well established structural investi-gations, the theme of climate control of such large indoor spaces hasreceived attention recently, with regard to installations [3] and rela-tions with environmental factors [4,2]. The design of the Eden Projectdomes in Cornwall [5] and of the Esplanade Theaters in Singapore [6]exemplify design processes where great attention was given to cli-matic comfort and energy related aspects, such as solar gain and ven-tilation for cooling. Among the many possible references, some areillustrated in Fig. 1.

2.1. Semi-outdoor areas in urban settlements

In addition to the well known distinction between outdoor andindoor spaces, the urban tissue integrates a number of spaces com-monly recognized as semi-outdoor (or semi-indoor) spaces. Withconditions between indoor and outdoor, Spagnolo and de Dear[7] define semi-outdoor spaces as locations that, “while still beingexposed to the outdoor environment in most respects, includeman-made structures that moderate the effects of the outdoor con-ditions.” Examples include roofs acting as radiation shields or wallsacting as vertical windbreaks. Both historical and modern citiesbear testimony to the traditional importance of semi outdoor spacesin urban areas. These include examples from vernacular architecturein the great majority of cultures, such as partially enclosed courtyardsor shaded streets and squares; from historical and traditional construc-tions, such as the public commercial galleries in Italy (among the mostfamous, the Vittorio Emanuele II in Milan) or the Mediterranean porti-cos and courts (like the Court of the Lions in the Alhambra).

A specific group of semi-outdoor spaces can be identified ascovered by large roofs or partially open envelopes, leaving a rele-vant direct connection with the outdoor environment. Museums,cultural centers, university campuses, shopping and leisure areas,hotels and resorts, are just a few examples of built environmentswhere covered semi-outdoor spaces are commonly integrated. Well

Fig. 1. Examples of large roofs in urban areas: roof of the Palazzo Lombardia (Milan) [imagesCour Visconti, Louvre, Paris [images published with permission of Mario Bellini Architects,

known examples can be recalled, such as the recently built roof forthe Fair inMilan in Italy [8] and the Kurayoshi Park Square in Kurayoshiin Japan. Both for large indoor and semi-outdoor areas, great atten-tion is currently given to energy related aspects. The Cabot Circusin Bristol is an example of a system designed to use natural ventila-tion, daylight and rainwater collection. Jungfen et al. [9] and Wanget al. [10] draw attention to the relevance and effects of waterspray systems for adiabatic cooling in semi-outdoor spaces, such asthe semi-outdoor areas of the Shanghai Expo.

Considering the importance of such spaces, we discuss a perfor-mance oriented design method in this paper focusing on the designof large roofs. Performance criteria related to climatic comfort aretaken as the main focus, with specific reference to the passive use ofon-site energy resources. The approach aims at an integrated designacross different disciplines and considers structural performance asa key criterion.

3. Climatic comfort of semi-outdoor areas

We consider climatic comfort as referring to both thermal anddaylight conditions. While great attention has been given to climaticcomfort of indoor spaces, less scientific attention as well as designconsideration has been given to climatic comfort of outdoor andsemi-outdoor areas in urban environments. Nevertheless, this is acrucial factor that must be taken into account in the urban designprocess and at the various scales of the design. This regards bothnew developments and the chance to improve the conditions ofexisting urban settlements. In this regard, large roofs can be inte-grated in the urban fabric with significant effects on the microclimate.

The importance of climatic comfort in outdoor and semi-outdoorurban spaces can be argued according to different criteria. Amongthe many possible criteria, it should be noticed that climatic comfortplays an important role in the appeal the spaces have for the users.This happens independently from the level of enclosure of the spaces.Focusing for example on thermal comfort, specific studies show thesignificant influence of comfort in the utilization rate of outdoor envi-ronments and in the behavior of the users [11]. The influence of ther-mal comfort on the number of people using an outdoor public areaemphasizes the need to confront thermal comfort during the designof such spaces.

Even though it does not diminish the importance of climatic com-fort, the level of enclosure affects the criteria for the performanceevaluations. More specifically, the expected level of comfort is differ-ent when comparing indoor, semi-outdoor and outdoor areas. By

published with permission of Pei Cobb Freed & Partners, copyright holder]; roof of thecopyright holder].

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focusing again on thermal comfort, a number of studies show, forexample, that occupants of semi-outdoor and outdoor environ-ments can tolerate a wider temperature range than the one expectedfor indoor thermal comfort [7,12,13].

A last important consideration concerns the way climatic comfortcan be achieved. This recalls the current increased emphasis onenergy-related aspects, which confronts the designer with the addi-tional challenge of reducing energy consumption and even investigat-ing the potential for energy production. The use of renewable energyresources, including both active and passive systems, needs to be con-fronted in the design. When focusing on active technologies, largeroofs offer large surfaces which can be used for the collection ofsolar energy. The solar energy thereby collected can be converted toelectricity and used in spaces covered by the roofs or in adjacentbuildings and indoor areas. When focusing on passive systems, largeroofs are expected to mitigate the climate factors for the spacesthey cover, both for thermal comfort and daylighting, allowing a re-duction in the requirement for the artificial energy needed in achiev-ing a given state of comfort.

4. Passive strategies for climatic comfort

The common reliance on lighting and heating based on supple-mental energy and air conditioning is responsible for the disconnec-tion of the indoor comfort from the outdoor climate. In contrast tothis, passive strategies for climatic comfort are directly based on therelationship between the outdoor climate and spaces where thermaland lighting comfort are required. They aim at passive heating orcooling based on thermal comfort requirements by making use ofthe local climate conditions, and they use natural daylight to satisfythe lighting requirements.

4.1. Passive strategies for thermal comfort

As emphasized by Nicol and Roaf [14], in spaces designed usingpassive strategies, the temperature is not decided by the buildingengineer, but by the physics of the form, by the amount and locationof its mass, the size and orientation of the openings, the shading, thematerial properties of the envelope, and the effect of any passivetechnology. Focusing on indoor spaces, a well designed system inte-grating such aspects would lead either to a zero-energy space, wherethere is no need of extra heating or cooling to achieve a state of thermalcomfort, or to a relevant “moderation both of the diurnal swings andseasonal changes in the indoor temperature” [14]. An analogous princi-ple can be applied to semi-outdoor spaces, where an enlarged comfortzone can also be considered.

With reference to thermal comfort, the following two sectionsemphasize some of the key factors affecting the comfort level,which can be controlled by the physics of the buildings, and morespecifically, of large roofs.

4.1.1. Thermal comfort, climatic factors, built environmentThe most commonly known definition of thermal comfort is the

one given by ASHRAE [15], which defines thermal comfort as the“state of mind that expresses satisfaction with the surrounding envi-ronment”. The possible formulations of thermal comfort are howevernumerous, and vary according to the taken approach. Peter Hoppe[16] describes three different approaches: psychological, thermophy-siological and the one based on the heat balance of the human body.The ASHRAE definition refers to psychological aspects, and includessubjective factors. These play a key role in the thermal comfort ofsemi-outdoor and outdoor spaces, where psychological adaptationof users has a strong influence [13]. Considering thermophysiologicalaspects, thermal comfort is “based on the firing of the thermal recep-tors in the skin and in the hypothalamus. Comfort in this sense isdefined as the minimum rate of nervous signals from these receptors”

[16,17]. Focusing on the heat balance of the human body a definitionof comfort can be given based on energy exchanges. In this sense,comfort “is reached when heat flows to and from the human bodyare balanced and skin temperature and sweat rate are within a com-fort range, which depends only on metabolism” [16,18]. Variousheat-balance models of the human body allow taking into accountthe large variety of interrelated factors; among them the physiolog-ical equivalent temperature, PET, [19] and the predicted mean vote,PMV [18].

The discussion of the concept of thermal comfort and its complexdefinition and measurement is beyond the scope of this paper. Thekey aspect that is emphasized here is that passive strategies aim atachieving or contributing to thermal comfort by influencing theheat balance equation of the human body based on the interrelation-ships between the meteorological parameters and the built environ-ment. Due to the faceted ensemble of environmental factors affectingthe heat balance, this means that air temperature, air humidity, air ve-locity and mean radiant temperature are expected to be controlled bymeans of interaction with the elements of the design. In general,this can occur by using thermal mass, air circulation (exchange andspeed), seasonal and daily patterns of wind and solar radiation,and, in case of cooling, also adiabatic cooling (direct evaporativecooling and indirect evaporation cooling). The use of thermal massis meant to reduce the fluctuations of temperatures by means ofthermal inertia. This can consist, for example, in cooling the massat night to absorb the daytime heat gain and reduce the coolingload, or in storing the solar gain to reduce heating load. The use ofseasonal and daily patterns includes the control of solar heat gainby maximizing it or minimizing it according to need, and the controlof air speed based on wind driven or on buoyancy driven air flow.Finally, adiabatic cooling consists of water evaporation with conse-quent reduction of the air temperature due to the absorption ofenergy in the evaporation process [20–22].

4.1.2. Performative skins for passive thermal comfortWhen focusing on large roofs, the air temperature of the spaces

underneath is largely affected by the solar exposure of the spaces,which directly depends on the geometry of the roof and its compo-nents as well as on the properties of its materials. The radiant temper-ature is affected by the temperature of the inner surface of the roof,being higher or lower than the air temperature of the spaces under-neath; this depends largely on the absorption factor of the roof. Theair velocity in the spaces underneath depends ultimately on the in-coming wind as well as on other air flows, such as convective circu-lation. Both air circulation patterns are directly affected by the shapeand openings of the roof, whereas the second one is also affected bytemperature differences between zones. Special attention needs tobe given to the balance between ventilation for cooling and therisk of uncomfortable drafts. The air humidity is directly related tothe moisture levels in the air and requires careful consideration inbalancing adiabatic cooling and uncomfortable levels of moisture,which lowers the highest acceptable temperatures. Adiabatic cool-ing can be achieved by flowing water over or spraying water onthe roof, to reduce the temperature of the roof and, as a conse-quence, the long wave radiation from it. Alternatively, cooling canbe attained by placing water, in the form of ponds or fountainsunder the roofs. Moreover, the use of vegetation is also an option.(Fig. 2).

4.2. Passive strategies for daylighting control

Similar to the approaches described for passive thermal comfort,passive strategies related to daylight adapt the building form toreflect natural light inside and reduce the need for artificial light-ing. Principles for the control of daylight need also to be integrated

Fig. 2. The control of the solar exposure of the covered spaces depends from the solar transmittance of the roof and form its shading shape; the effect of the radiant temperature onthe covered spaces depends on the absorption factor of the roof and its distance from the spaces; the air velocity underneath the roof depends on the way the roof influences theairflow (including wind) and the gradient temperatures; the reduction of fluctuations in temperatures can also be controlled by integrating thermal mass and adiabatic cooling inthe system.

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with thermal comfort strategies, due to the connection betweendaylight and solar gain.

4.2.1. Daylighting comfortIn daylight comfort, the daylight requirements are related to the

illumination levels needed according to the visual tasks taking placewithin the different spaces. As described by Baker and Steemers[23], daylight can be considered to have two components: the diffuselight from blue sky and clouds, and direct light from the sun. More-over, when focusing on the diffuse light falling on a point, this hasthree components: the sky component, consisting of light comingdirectly from the sky; the externally reflected component, consist-ing of light being reflected from external surfaces; and the internallyreflected component, consisting of light reflected by internal sur-faces. The amount of total light reaching a specific point in thespaces is a first fundamental factor affecting daylight comfort, andcan be quantified according to various models. A commonly usedsimplified approach is based on the Daylight Factor (DF) [24]. TheDF provides a measure of relative illumination within a space com-pared to that of a standardized overcast sky condition [25]. Eventhough the discussion of daylight comfort and its related measure-ment is beyond the scope of this paper, it needs to be pointed outthat the daylight requirements cannot be defined purely in termsof a needed amount of light. In this regard, the DF is a quantitativeparameter that does not express the light quality. The goal of a com-plete assessment of daylight comfort should be to provide a healthyand pleasant environment for the occupants [25]. The orientation ofopenings, their location and their distribution are important aspectsin achieving this goal.

4.2.2. Performative skins for daylightingWhen focusing on large roofs, three aspects are highlighted here.

On the one hand, areas underneath large roofs often offer the advan-tage of possible overhead natural lighting, which is not usually possi-ble in multi-story spaces. On the other hand, the configurations ofthe openings can include inclinations and consequent orientationsthat allow balancing the income of direct and indirect light, relatednot only to daylight requirements but also to influences on thermalcomfort through the solar gain [23]. Finally, it must also be notedthat courtyards and atria are often integrated in buildings in orderto increase the amount of daylight for deep buildings and buildingblocks. This implies that daylight requirements and consequentstrategies for meeting them do not come only from the coveredspaces directly (where the comfort zone might be enlarged), butalso from the adjacent spaces (often indoor rooms with well definedcomfort requirements). Examples of related studies can be found inCalcagni and Paroncini [26].

5. The relevance of geometry

Large roofs are envelopes that enclose or semi-enclose spaces,filtering and controlling the environmental factors with respect tothermal and daylight comfort in the spaces underneath. This actionis based on a combination of geometry and material properties,which are both equally important. The following example shows theimportance of geometry.

5.1. An example showing the impact of geometry

In order to introduce the important role of geometry with respectto thermal and daylight comfort, we present a case study, the Velaroof in Bologna, Italy. This case study was developed as a collabora-tion between the architects and engineers responsible for the overallbuilding project which includes the roof, together with an interdisci-plinary team at Delft University of Technology. More detailed descrip-tions can be found in previous publications [27,28]. The case studyfocused on passive strategies for reducing the summer overheatingof the spaces underneath the roof. In its entirety, this case study canbe summarized as investigations for increasing the airflow under-neath the Vela, reducing the direct solar exposure of the coveredspaces, reducing long wave radiation from the roof, and reducingthe maximum temperatures using adiabatic cooling. In keeping withthe focus of this paper, the description presented here relates moreto the aspects of direct solar exposure and the daylight factor.

From the large amount of data related to each of these aspects, afew key examples are taken, which refer to the three levels of thecovered spaces: (a) the ground floor with a semi-outdoor publicsquare, (b) the first floor with a terrace covered by the roof and(c) the second floor with terraced roofs for public use. By analyzinga reference geometry, a daily maximum PET (physiological equiva-lent temperature) of 35.1 °C for (a), 37.3 °C for (b) and 39.7 °C for(c) were expected under the roof in July for a cladding with 50%transparency. Decreasing the transparency by about 25% using semi-opaque colors, solar exposure was reduced without increasing longwave radiation, and allowed a significant increase in comfort,while reducing both the time and area with maximum daily PETsbetween 38 and 42 °C for level (c). However, a 70% opaque claddingaffects the daylight, which was shown not to be sufficient in meetingthe requirements. Introducing a cladding system based on a three-dimensional geometry with a north–south oriented printed shadingpattern (Fig. 3) was evaluated to be of great help in limiting the di-rect solar exposure underneath the roof while allowing the trans-mission of indirect daylight. This allowed avoiding maximum PETsover 34–38 °C also in the most critical level (c). Fountains, ponds,sprayed water mist for adiabatic cooling and slightly increased air

Fig. 3. The ETFE cladding design, viewed from the north and from the south sides.

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flow were combined as well, allowing maximum PETs between 30and 34 °C, which were considered acceptable when compared tothe outside.

The geometry of the roof plays a key role in this heating and cool-ing strategy as reflected in the provided data. Specifically, over a totalPET reduction of about 25%, about 12% was due to the combination ofgeometrical and material properties of the cladding. In addition to thediscussed geometry of the cladding, geometrical evaluations at alarger scale can be added as a key factor in driving the airflow.Furthermore, geometry becomes a key factor in bridging differentperformances, such as, in the case of large roofs, structural mor-phology and cladding systems.

5.2. Key geometric aspects

The above case study exemplifies the importance of geometry inaffecting the climatic comfort, and specifically in controlling the di-rect exposure and daylight factor of the spaces underneath the roof.When focusing on the geometry of large roofs and enclosure enve-lopes, various aspects need to be considered during the design pro-cess. These include the overall shape of the roof, its structuraltessellation and cladding modularization, the size of its structuralmembers and components, the fabrication process of its elementsand its construction process, just to mention some of the most rele-vant. For this paper, a set of design variables affecting climatic com-fort has been selected based on a simplified approach, which supportsa very early stage of the design process. Specifically, the consideredgeometric factors define the overall shape of the roof and the pattern,the density and the configuration of its modular cladding. However,each of these aspects involves a variety of disciplines beyond justthe climatic comfort alone. This refers to the structural geometry inparticular, which on the one hand is affected by the overall shapeof the roof, and on the other hand acts as support by connectingthe modules of the cladding. The structural geometry needs there-fore to be designed and evaluated together with the envelope, basedon an integrated design method. To simplify the approach, for theexamples discussed below, a separation in functional layers (i.e.main structure not components and cladding) has been assumed.However, a similar approach can be applied in case of structuralskins.

6. A parametric design approach

In order to investigate geometric alternatives that impact aspectsof climatic and structural performance, a parametric approach hasbeen developed. The approach is structured in two main phases.The first phase concerns the parameterization of the geometry inorder to generate an array of parametric designs, which describemeaningful alternative solutions to the desired performance criteria.This parameterization is based on overall geometric variation as

well as the interchangeability of structural patterns and claddingsystems, each of which is also parametric. Specifically, the designstrategy develops various levels of parameterization, which weschematize in three main groups. The first group corresponds tothe primary parameterization, and refers to the general geometricproperties of the roof. The other two groups refer respectively tothe cladding and the structure, and contain parameters describingspecific properties of their modules. These assume the geometricoutput of the primary parameterization as reference geometry. Theoverall approach to the structural morphology has been presentedby the authors in previous publications [29] and will only be recalledhere in its key aspects (Sections 6.1 and 6.2). In this paper we willfocus, instead, on the parametric exploration of the cladding systems.The second phase concerns the search for satisfactory design solutionsbased on the evaluation of the performance, and is based on the combi-nation of parametric modeling with performance evaluation softwareand computational search techniques.

6.1. Primary parameterization: the reference geometry

The parameterization of the reference geometry is structured toallow the automatic generation of geometric alternatives of polygonalpatterns. These are meant to be used as a basis for modeling both thestructural and the cladding systems. Moreover, the reference geome-try is expected to allow geometric variability of the overall shape ofthe roof as well as of its tessellations, according to the key aspectsillustrated in Section 5.2. A set of points is used as a key elementto parametrically describe the reference geometry. More specifically,an array of points, variable in distribution and density, is used todescribe the position of the vertexes of the polygons. Referring tothe same points, a variety of polygonal patterns can be generated.The overall shape of the roof can result from, or itself determine,the distribution of the points.

6.1.1. Parametric point gridsThe distribution of the points can be based on three possible

approaches: the first two position the points by distributing themalong a NURBS surface; the third one uses mathematical formula-tions to describe their positions. Any of these methods can providea point grid as output whose distribution in space is parametricallyvariable.

When working with NURBS, in order to parametrically vary thepositions of the points, two separate levels of parameterization aredefined. The first parameterization regards the NURBS surface andaims at controlling the shape of the roof. This is based on the param-eterization of the entities through which the NURBS surface is defined(such as boundary curves, curves to loft, a list of the points that thesurface will pass through, control points, or other). By varying thepositions or configurations of such entities, the entire surface changesits shape. The second parameterization regards the distribution of

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points on the surface, and aims at controlling the density of the grid andits proportional distribution along the directions of the NURBS.

One method that can be used to distribute a grid of points onto aNURBS surface is based on the UV coordinates. In this case, the den-sity of the grid and its proportional distribution can be regulated byparameters, which are independent from each other or reciprocallyrelated, and which are included in the equations, which express theU and V values. A set of scripts can contain these equations, andcan be reused whenever a NURBS surface is used as input to generatean array of points. Examples of these scripts are given in Turrin et al.[29].

To distribute the point grid once a NURBS surface is defined, asecond method can be used based on parallel curves (sections) ofthe surface, obtained, for example, by intersecting the surface with aset of parallel planes, which have a parametric distribution andreciprocal distances. Points can be distributed along the curves and/orat the intersections of the crossing sections.

As an alternative to distributing the points onto NURBS surfaces, athird method is based on mathematical formulations which describethe positions of the points. In this case, both the overall shape of theroof and the distribution (density and proportions) of the point gridare determined directly by a mathematical function, which uses acommon level of parameterization.

On the one hand using NURBS surfaces allows for freedom inmodeling the surface that is to be transferred to the point distribu-tion; this guarantees a free representation of large geometric surfacesthat include complex configurations, and offers a key advantage espe-cially during the exploratory phase of conceptual design. On the otherhand, even if the third method offers less flexibility and freedom inmodeling, it allows for the control of the shape through the precisionof an explicit mathematical formulation. An example of this thirdmethod will be given in Section 6.2.

Fig. 4. From top left to bottom right: a) example of quadrilateral tessellation; b) variation ie) example of diamond tessellation; and f) example of hexagonal tessellation.

6.1.2. Parametric tessellationsOnce the point grid is defined, a variety of parametric patterns can

be generated based on the previously defined point grid. Examplesare illustrated in Fig. 4. Software such as Generative Components al-ready provides a set of precompiled functions by which basic casesof polygonal grids (such as quadrilateral, triangular and diamondgrids) can be generated. In order to generate the lines or polygonscorresponding to a larger variety of the geometric configurations(such as hexagonal patterns, combinations of polygons, or others) aset of scripts has been developed that uses a grid of points as input.Examples of these scripts are given in Turrin et al. [29]. These scriptscan also be reapplied for different point grids, and can easily be cus-tomized to meet new conditions.

The output of the process described above consists of a set of dif-ferent tessellations. These are used for modeling both structural andcladding systems, and outline the geometric interface between thetwo systems.

6.2. Secondary parameterization: the structural systems

Concerning the structure and focusing on discrete systems, awide range of modular structures can be modeled based on thedifferent tessellations. These can be used to propagate the struc-tural elements or can be used directly as structural geometry. Amongthe many, the example of space frames is illustrative, since singlelayer grids can be generated directly based on different geometric enti-ties (usually lines) connecting the points. When considering a doublelayer space frame, a second layer of points can be added at either afixed or parametric distance from the first layer and used for generatingthe second layer as well as the diagonals between the layers. The resultis a parametric space frame composed of modular cells which vary in

n proportions; c) variation of the overall shape; d) example of triangular tessellation;

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density and overall shape (Fig. 5). Amore detailed explanation concern-ing the example of space frames can be found in Turrin et al. [29].

6.3. Secondary parameterization: the cladding systems

Regarding the cladding, various considerations need to be pointedout, especially concerning, on one hand the geometry of the modules,and on theother hand theway the geometry ismodeled andpropagated.

The geometry is discussed in relation to the performance criteria.Specifically, focusing on the control of the solar exposure, the configura-tion of the cladding modules might aim at various requirements. Com-mon examples are the shading of the spaces beneath the roof (such as inthe case of summer overheating) or the capturing of solar radiation(such as in the case of winter need for solar gain). Depending on theaim, various geometric configurations can be investigated based ondifferent principles. Some examples are presented in the next section.

Once the geometric principles are established, the cladding mod-ules can be predefined, modeled, and then propagated across the tes-sellated surface. For this procedure, polygons are usually preferred asgeometric input. Specifically, the modules can be modeled startingwith the polygonal tessellation, which is the interface between thecladding and the structure. Each module can be structured for differ-ent base polygons to match various tessellations or as single optionsspecifically built for a given polygon. The modeling can be based ona repetitive process of geometrical dependencies. Thus, when model-ing one module a routine of operations can be written to express thegeometric relations of the module, both among its components (in-ternal associations) and with external references (recalled as inputs).This routine can integrate independent parameters by constituting aparametric model itself. The module can then be saved as a replicablefeature. Hence, it can be propagated onto the tessellation by guarantee-ing the relationships with the structural geometry as well as with theother references assumed in determining its geometry.

The advantage of creating a module as a component, offers notonly the possibility of replicating the feature within the model, butalso in other models. This means that, as in the case of the scripts de-scribed above, the creation of libraries of predefined modules is alsopossible for the cladding systems. This possibility is discussed inmore detail in Section 6.4.

6.3.1. Examples of parametric cladding systemsExamples of parametric cladding systems are presented here in

order to show the flexibility of the method on a variety of designcases for semi-outdoor spaces, dealing with the control of direct

Fig. 5. Example of double layer space frame with hexagonal and tria

solar radiation and daylight. Three examples are illustrated, concern-ing respectively: complex geometry and the option of adaptivity.

The first example refers to the already mentioned Vela roof, inwhich the cladding gives protection from rain and mitigates the sum-mer thermal discomfort while providing daylight and view to the out-side. The energy absorption of the roof was reduced mainly based onmaterial properties. In addition, the parametric geometry of ETFEpneumatic cushions was considered in the investigation of the directsolar radiation transmittance of the roof, the indirect light transmit-tance and the view to the north. Meaningful geometric parameterswere identified in the opening angle between north–south orientedshading patterns printed on the layers of the cushion (Fig. 6). Abso-lute references were introduced in the model in order to save themodule as a replicable feature and propagate it onto the complex con-figuration of the structural geometry while still guaranteeing a properorientation of the shading pattern and its opening angle.

The second example deals with the option of adaptivity. The clad-ding is meant to provide shading, with no requirement of rain protec-tion. The primary focus is on the solar exposure and the daylightlevels of the spaces underneath, controlled through geometry. Theshading system is based on hexagonal modules with six flat panelsfixed to a bar passing through the center of the hexagonal polygon.It is explored in two different options. One option combines the shad-ing panels with a structural, single layer space frame based on hexag-onal tessellation. The bar is fixed to the frame by a set of cables,forming a triangulated structure. The other option aims at a closer in-tegration of the cladding into the structural system, without the needof a secondary supporting space frame. In this option, a set of cablesconnects the top and bottom ends of each bar, again forming a trian-gulated structure (Fig. 7). For both, two main parameters are identi-fied. The first one controls the inclination of the bar (and consequentlythe orientation of the panels). In this respect, the exploration leavesopen the option of adjustability of the structure, by regulating the inclina-tion of the bars according to different environmental conditions (such asthe position of the sun and the need to increase or decrease the solargain). The second parameter governs the shape of the panels, by control-ling the curvature of their edges, and, consequently, affecting the filteringof the solar radiation. Also in this case, once created, the module for eachof the two options has been saved as a replicable feature, and can be pro-rogated onto a tessellation having parametric variability in density, over-all shape of the whole structure, and/or other meaningful aspects.

The last example centers on the comparison of different strategiesfor solar control, given certain cladding principles. The cladding ismeant to regulate the solar exposure and the daylight levels basedon the geometric alternation of transparent and opaque panels, with

ngular tessellations, parametrically varied in shape and density.

Fig. 6. Parametric modeling of the cladding of the Vela; left) control of direct radiation and indirect light based on printed pattern, parametrically explored for different openingangles; right) shadow simulation emphasizing the reduced transmission of direct radiation.

43M. Turrin et al. / Automation in Construction 22 (2012) 36–50

different orientations. Two options following different strategies arecompared based on their performances. Specifically, in both optionseach component is based on a hexagonal pattern, defined by a combi-nation of six pyramids with a triangular base, together forming ahexagonal element. Based on an algorithmic pattern, each face ofthe pyramids can be either an opaque or a transparent panel. Inone of the two versions, opaque panels are south-oriented in orderto achieve a shading effect and reduce the direct solar radiation(Fig. 8a). Independent parameters regulate the inclination of thepanels. In the second version, opaque panels are north-orientedand elongated over the transparent panels in order to shade them(Fig. 8b). In this case, independent parameters regulate the inclina-tion of the panels, but also the length of the shading elongation.Also in this case, the routines through which the geometry was cre-ated were saved as replicable features which can also be stored in alibrary for future applications.

6.4. Hierarchy of the parametric model

The process described above allows for the generation of specificparametric models and reusable libraries of scripts and features,which can be reapplied and customized. Each parametric model canbe structured in order to allow geometric variations of a given typo-logical solution. This is the case, for example, in the variations in den-sities and proportions within a chosen tessellation. Moreover, theparametric model can be structured in order to allow parametricallythe switching from one typological solution to another, and not onlyfrom one to another geometric configuration of each single typologi-cal solution. This depends on the parameterization process, which caninclude a set of top parameters regulating the option to be applied.

Fig. 7. Parametric modeling of the cladding without space frame; left) control of direct radiatcurvatures of the edges; right) shadow simulation for a configuration with horizontal pane

Different options can refer to different tessellations (and in this casevariations in parameters imply switching from one to another patternby recalling for example different scripts); or to different claddingmodules (and in this case different parameters would recall for exam-ple different precompiled features from the cladding library); and soon. When properly articulated, such a parametric model would stillallow the parametric variation of the geometry of each option (suchas density, proportion, and so on). In this way the parametric modelscan contain a large series of alternative geometric solutions, whichare widely differentiated also in topology (Fig. 9).

The advantages offered by this structure mainly consist in an en-larged solution space of the model. Its benefits need to be assessedin relation to the design exploration in balance with the higher com-plexity of the model and computation required to generate designalternatives.

7. Performance evaluations

Establishing the parametric models allows for automatically gen-erating large sets of design alternatives. Based on a properly set pa-rameterization process, the alternative solutions can be generatedwith meaningful reference to the performance to be analyzed; andthe solution space of the parametric model can be explored basedon the various performance criteria (structural performance, thermalbehavior, daylight levels, and so forth). An increasing number of dig-ital tools for early performance evaluations are being developed whileparametric modelers allow for the integration of scripts for numericevaluations. However, performing analytical evaluations still usuallyrequires the use of specialized software in which the geometric alter-natives are imported and their behavior is simulated. The complexity

ion based on inclined panels, parametrically explored based on various inclinations andls and medium curvature.

Fig. 8. Top: Parametric modeling of the cladding with south facing opaque components; left) control of direct radiation based on inclined panels, parametrically explored based onvarious inclinations; right) shadow simulation for one of the configurations. Bottom: Parametric modeling of the cladding with north facing opaque components; left) control ofdirect radiation based on inclined panels, parametrically explored based on various inclinations and elongations; right) shadow simulation for one of the configurations.

44 M. Turrin et al. / Automation in Construction 22 (2012) 36–50

of such a design exploration and performance assessment process isevident when considering the depth of the solution space, whichcan already consist of many thousands of solutions for a very limitednumber of independent parameters. This exploration and evaluationprocess can be carried out manually, based on selection, import andevaluation of the design solutions. In contrast, automatic processescan make use of a cyclic software approach, such as the one basedon ParaGen, briefly discussed below.

Fig. 9. Scheme of par

7.1. ParaGen

ParaGen is a parametric design tool using genetic algorithms for theexploration of form based on performance criteria. It has been pre-sented in detail by the authors in previous publications [30] and ishere only briefly described in its key aspects. It is being developed atthe University of Michigan, Taubman College, where it is used for struc-tural form optimization. Based on collaborationwith Delft University of

ametric process.

45M. Turrin et al. / Automation in Construction 22 (2012) 36–50

Technology the tool is being extended toward an interdisciplinary ap-proach, which involves solar performance evaluations. The version ofthe tool discussed in this paper makes use of a parallel network of PCsrunningWindows 7 and a Linux web server, to run a series of both cus-tom written and commercial software packages. The procedure is con-stituted by a cycle based on three main components:

(1) The selection of variables: a genetic algorithm (GA) providesthe values of the independent parameters using techniques ofselection, recombination and mutation.

(2) The generation of forms: a parametric modeler generates thegeometry using the variables provided by the GA. Currentlythis step uses Generative Components (Bentley Systems), butthe system is open to different parametric modeling software(such as Grasshopper or Digital Project).

(3) The analysis of the generated forms: the geometry provided bythe parametric modeling software is evaluated in terms of per-formance. Originally, this step used STAAD-Pro, FEA softwarefor structural evaluations, but is currently extended also tothe use Ecotect, simulation software for thermal and daylightperformance.

When the program runs, the GA component initially generates aset of random solutions using randomly generated values for eachof the variables. This set needs to be relatively wide ranging inorder to include a large variability of the design alternatives. Each ofthese initial combinations of variable values is downloaded from theserver to local PCs as Excel files for processing by the parametric mod-eler. This requires importing the variable values and using them toproduce the startup population of geometrical solutions. As each geo-metric instance of the parametric model is established, the solution isprocessed through the rest of the ParaGen cycle. In this specific casethey can be processed through STAAD-Pro or Ecotect or both, accord-ing to the desired performance information. This determines thestructural, thermal, or daylight characteristics associated with the so-lution. At the conclusion of the local part of the cycle, the original setof variable values, the performance results, along with data files use-ful in a more detailed assessment of a particular solution are uploadedto the web server. In this way, JPG, DXF, or VRML files are made avail-able for visualizing the geometry of the solutions. Also STAAD and/orEcotect data files are saved for a more detailed inspection by the de-signer of the performance characteristics. Both the variable valuesand performance results are maintained in a SQL database, and linkedto the data files so that the designer can retrieve any or all of themthrough the web interface. After the initial random population isestablished, new parents are selected based on the better performingsolutions. The selected parents are passed to the GA program wherethey are bred to yield a new child data set based on half uniformcrossover of the variable values. The data for the child solution isdownloaded to the local PC running the associative parametric

Fig. 10. Scheme of the

modeler and the cycle starts again. Depending on the complexity ofthe problem, the process may continue to explore several 100 or sev-eral 1000 solutions. This results in the identification of good perform-ing solutions toward which the generated populations converge.

8. SolSt

The potential of ParaGen in supporting performance based explo-rations of a high number of parametric design variations, includingcomplex configurations, is illustrated with another case study. Theproject consists of a free-form roof, called SolSt, covering an area ap-proximately 50 m×50 m. The project is located in Milan (Italy) andcurrently under development. In this study we discuss summer con-ditions, and how to avoid overheating. Specifically, in order to facili-tate the air flow for cooling, the overall shape is designed based onroof peaks where heat extraction can occur through top openingsdue to the stack effect; and the cladding is investigated in order tolimit the solar heat gain while allowing indirect light. Fig. 10 showsthe overall concept of the roof.

8.1. Geometric constraints

After a preliminary investigation based on a parametric NURBSsurface, the overall shape of the roof and the distribution of thepoint grid was modeled based on a mathematical function. This op-tion was preferred in order to integrate into the design explorationa set of geometric constraints, meant to facilitate the design of theopenable parts. It was considered in fact that integrating openable el-ements based on planar geometry allows a relevant reduction of com-plexity, when compared to curved geometries. This was consideredboth in the case of traditional operable systems, and in the case offoldable or deployable geometries for operable modules. With this in-tent, the peak surfaces were constrained geometrically. More specifi-cally, a horizontal plane intersecting the top part of the peaks wasexpected to generate an approximately circular section. Dependingon the tessellation, different planar polygons would have beeninscribed in the circles.

8.2. Mathematically defined point grid

In order to achieve the geometric conditions described above, theoverall shape of the roof has been specified by positioning a set ofpoints based on a mathematical function, using Cartesian coordinates.The x and y values defined the density of the grid as well as its overalldimensions. The z value is described based on a sine function whoseamplitude defines the height of the peaks. In order to generate para-metric variations of the output points, a set of independent parame-ters has been introduced in the functions by targeting the geometricaspects to be investigated.

concept for SolSt.

Table 2Standard function and two examples of composed functions.

Function

Point [][] function (xdim1,ydim1,density,amp1) {Point pt={};for (int i=0; ib(density +1)/Sqrt(3); ++i) {pt[i]={};for (int j=0; jbdensity +1; ++j) {double xstep=xdim/density *i*Sqrt(3);double ystep=ydim/density *j;pt[i][j]=new Point();if ((xstepb1)&&(ystepb1)) {pt[i][j].ByCartesianCoordinates(baseCS, xstep*5,ystep*5,((Sin(720/xdim1*xstep-90)*amp1+ Sin(720/ydim1*ystep-90)*amp1)+(2*amp1))*(−1*((xstep-1)*(xstep-1))+1)* (−1*((ystep-1)*(ystep-1))+1)); }else if ((xstepb1)&&(ystepN=1)&&(ystepb=9)) {pt[i][j].ByCartesianCoordinates(baseCS, xstep*5,ystep*5,((Sin(720/xdim1*xstep-90)*amp1+ Sin(720/ydim1*ystep-90)*amp1)+(2*amp1))*(−1*((xstep-1)*(xstep-1))+1)); }……………

else {pt[i][j].ByCartesianCoordinates(baseCS, xstep**5,ystep*5,((Sin(720/xdim1*xstep-90)*amp1+ Sin(720/ydim1*ystep-90)*amp1)+(2*amp1))*1);}}}

46 M. Turrin et al. / Automation in Construction 22 (2012) 36–50

In particular, the points were generated by describing the Carte-sian coordinates generated with a for-next loop. The x and y valueswere defined as doubles, by subdividing two reference lengths ofthe sides of the roof. The resulting equations integrated independentparameters to regulate the density and the proportions of the grid, asshown in Table 1. Specifically, the density was regulated by the inde-pendent parameter “density” and the proportions by “factor”. The ref-erence lengths were both kept at 10 meters for both sides. The zvalues were defined based on a sine function, which is doubled by fol-lowing both the x and y directions in order to achieve the desired cur-vatures in both. Table 1 illustrates the standard case, by including theamplitude of the sine function as an independent parameter.

By applying the described function, a distribution of points isobtained which follows the desired peaks. The edges of the roof alsofollow the curvature given by the sine function. Since in the testcase the edges of the roof were expected to be on a planar square,the z values were smoothly driven towards 0 when close to theedges by multiplying the sine function with an additional function.Specifically, in the boundary areas a zero crossing function was mul-tiplied with the sine function to pull the boundaries down to a zerocrossing along the roof edge. This is achieved by subdividing the over-all grid in nine zones, each of which corresponds to the part close toeach of the four corners, of the four middle parts of the edges, andto the middle area of the grid. In this latter the standard functionwas applied. Differently, a set of “if conditions” identified each ofthe remaining 8 zones, where the whole function was composed bysingle couples of multiplied functions describing each zone. An exam-ple of the resulting total function is given in Table 2, by illustrating thestandard function and two examples of composed functions. The ex-ample is provided for a 50×50 meter roof, and the factor is given avalue of Sqrt(3), which allows for triangular and hexagonal tessella-tions with regular projections. Fig. 11 illustrates the parameterizedgeometry.

Based on the grid of points created, the parametric model was inthis case structured to analyze the alternative tessellations and clad-dings separately from each other. Among the possible tessellationsand claddings, the exploration of a cladding based on a hexagonalpattern is discussed here.

8.3. Cladding

Among the many possible claddings that have been investigatedfor the design, the process here is demonstrated with one of the ex-amples considered in Section 6. Specifically, performance based ex-plorations will be shown for the hexagonal pattern with northfacing transparent panels. The analyzed option refers to the designconcept using transparent glazed panels and semi-opaque glazedpanels with 90% light color serigraphy. It should be noticed that nomaterial aspects were considered so far when describing this claddingoption. When looking at the described geometric principle, the rangeof possible materials to be considered is relatively large. This concernsboth the opaque and transparent panels, thanks also to the dearth ofcomplexity in their configuration. More complex configurationsmight have required greater attention in order to integrate early on

Table 1Equations describing the Cartesian coordinates and integrating independentparameters.

Independentparameters

Functions

density double xstep=xdim/density *i*factor;factor double ystep=ydim/density *j;ampl z=((Sin(720/xdim1*xstep-90)*amp1+ Sin(720/ydim1*ystep-90)

*amp1)+(2*amp1))*1)

material considerations concerning various aspects of the geometricdescription (e.g., constraints from the fabrication process).

8.4. The ParaGen cycle

Parametric alternatives were explored by varying the overallshape of the roof, the density of the tessellation, and the local inclina-tions of the cladding panels. The roof was evaluated based on the day-light factor and the incident solar radiation of the spaces underneath.Several iterations were run for summer conditions and, for compari-son, through the year, to get convergence of the design solutions byminimizing the solar incident radiation and by maximizing the day-light factor underneath the roof. Within a single-objective optimiza-tion process, the fitness function targets the ratio between the two.Fig. 12 illustrates the cycle.

Examples from the analysis through the whole year are presentedbelow not only with respect to the optimal solutions, but also the useof sub-optima for knowledge extraction and design exploration. Overa population size of 370, the lowest ratio between the yearly incidentsolar radiation (W) and the daylight factor (%) was achieved in solu-tion 278 (67,399/23.4) and was about 3.5 times smaller than thehighest ratio (solution 3, ratio 303,172/29.6). Similarly good resultswere achieved in solution 355 (ratio 71,037/23.7) and 308 (ratio70,175/23.4). Fig. 13a illustrates the evolution with respect to the de-sign objective. In relation to the design variables, two main directionsemerged, evident already in the two best solutions, conventionallynamed A and B. Solution 278 shows high amplitude (2.1 on a scalefrom 0 to 2.5), maximum density, and relatively high inclination ofpanels evenly distributed (averagely 2.5 on a scale from 1 to 3). Solu-tion 355 shows very low amplitude (0.7), the maximum density, anda maximum inclination of panels in contrast to areas with minimuminclination. Fig. 13b shows the trend of amplitude and density sortedby fitness of the solutions; and the distribution of A and B for a groupof best solutions. Meaningfully, when lower panel inclination appearsin solutions with high amplitude, this is distributed on the north fac-ing areas of the curved roof. Fig. 13c illustrates the trend of DF and Isorted by fitness of the solutions. The best and worst solutions corre-sponded almost entirely to both the lowest and highest incident radi-ation, but also to the minimum and averagely maximum daylightfactor (22.7 in member 312; 32.8 in member 230) and minimumand maximum incident radiation (67,399 in member 278; 303,172

Fig. 11. Point grid described based on a number of arithmetically combined functions.

Fig. 12. The ParaGen cycle.

47M. Turrin et al. / Automation in Construction 22 (2012) 36–50

48 M. Turrin et al. / Automation in Construction 22 (2012) 36–50

in member 3). As for the DF, five solutions resulted in a values above40%, in all of which the roof is in its flat configuration (amplitude=0)with the largest size of cladding components (density=40); thesehave been the subject of a deeper investigation, in comparison withother individuals having null amplitude and the lowest density(40), they have a lower DF (lower than 30%).

The proportion between I and DF shows a limitation of the chosencladding, due to an almost proportional variation between the inci-dent solar radiation and the daylight factor of the spaces underneath.In fact, even though DF has high values, these might be problematicfor a semi-outdoor urban area faced by indoor spaces; and achievinglow incident radiation without relevantly affecting the DF would bebeneficial. In this sense, comparing the performance of this claddingoption with other claddings is meant to further support the designprocess. This can be seen when compared to the similar hexagonaloption with south facing transparent panels, kept with the same ma-terial properties for better comparison. Over a population size of 160,the lowest ratio was achieved for this option at 45,727/33.8 and thehighest at 132,284/15.03, showing a slight potential to decrease theproportionality of the variations between incident solar radiationand the daylight factor.

Fig. 13. Examples of results

8.5. Tessellations for integrating reconfigurable modules

Finally, we wish to mention the integration of further strategies inthe process byway of an example. A separate strategywas in fact devel-oped for exploring the tessellation patterns with the top four hexagonsconstrained at the peaks. Centering the polygons on the peaks allowsfour regular and flat polygons to be obtained at the tops of the peaks.This issue was initially approached in GC, by sliding the tessellationaccording to pre-calculated values in order to stretch and squeeze thepolygons in the lower parts of the roof. However, the method hasshown some relevant limitations, such as the effort to predefine the de-formed tessellations. As an alternative, an application was developed inProcessing, based on the use of particle springs.While guaranteeing thetop hexagons to be regular and flat, the application simulates the hexa-gon distribution with a particle spring system with the particle nodessliding on the mathematical surface function (Fig. 14).

9. Conclusions

By recognizing on one hand the importance of large roofs in urbansettlements, and on the other hand the relevance of the passive

from the ParaGen cycle.

Fig. 14. Examples of particle spring based tessellation for various heights of peaks.

49M. Turrin et al. / Automation in Construction 22 (2012) 36–50

climate control of the covered spaces, the topic of performative skinshas been discussed and a design approach has been presented. Thistargets the early integration of performance evaluations of design al-ternatives, during the conceptual phase of the design of the roofs.Addressing an integral approach, climate performance has been dis-cussed as the main focus, and structural performance as an exampleof interdisciplinary references. We have used parametric modelingto explore performance oriented design alternatives. Moreover, wehave discussed linking the exploration with performance simulations.We have shown how the tool ParaGen drives the parametric modelthrough genetic algorithms to search for suitable parameter settings.A successful example of the potentials offered by this combinationhas been shown based on a case study.

One of the advantages of the parametric design approach is its in-tuitive applicability to complex geometries. The method offers both ahighly visual interface, as well as a customizable library of scripts. Thelatter specifically increases the versatility of the parametric process,which relies on further implementations and variations of the geo-metric models. In this respect, the investment on the script library be-comes worthwhile, particularly in light of further applications. Incommon practice in fact, developing a library of script can be chal-lenging in time and skills. Structuring the library by considering andabstracting design problems that overcome the pure specificity ofone case makes the effort worthwhile. The case of tessellation forstructural geometry and cladding offers an example of a recurrent de-sign problem. Once the scripts have been finalized within a genericlogic, they can be intuitively reused or even called up through agraphic interface.

Even though this aspect would require a separate discussion, theinteroperability of the outputs can also be considered an advantage,favoring an integral design approach. The interoperability concernsespecially the possibility to import the geometric outputs into perfor-mance evaluation software for exploring the solution space of theparametric model.

During this phase of the process, the guidance given by the searchmethod also presents a clear advantage. Even though a balance be-tween required computation and benefits of the process is required,the use of ParaGen allows for further generalizations of the paramet-ric model by enlarging the solution space being explored. The inter-changeable cladding systems are an example of this potential, butother applications can be included as well. Moreover, the same solu-tion space can be easily explored for different performance require-ments, since the solutions are retained in a database in such a waythat further exploration is possible using traditional SQL searchmethods. Referring to the case study presented, the current explora-tions for summer conditions are being integrated with the winter sce-nario. While the optimization regarding the summer conditions

converged toward solutions with minimum incident solar radiationand maximum daylight factor, in winter both are maximized. In com-paring the geometries optimized for the two different seasons, we ex-pect to gain insight into the eventual benefits provided by adaptabledesign solutions (such as adjustable cladding).

Finally, a last benefit is inferred as the process makes explicit the in-trinsic relationships between geometry and performance. Specifically,the possibility to freely explore the generated solutions by sortingthem according to different criteria provides assistance in understand-ing the relationships between geometric variations and performances.This highly relies on the use of GAs not only for optimizing geometry,but also for more extensively exploring the solution space. In thisrespect, the process benefits also from information extracted fromsub-optima as well as badly performing design solutions. Also theknowledge extracted from such exploration becomes re-applicablein further design cases. Further developments of ParaGen mightconsider multi-objective optimization and clustering techniques forfurther investigating the relationships between variables and satisfac-tion of the fitness function; beside its extension to other parametricmodelers as well as performance simulation software.

In addition to the beneficial aspects, also the challenges of themethod should be considered. From a technical point of view, it mustbe noticed that exploring large parametric design solution spaces basedon performance simulations is a computationally consuming process,which benefits from parallel processing. In this regard, application incommon practice might seem limited, at the moment. As a solution,currently, the web-based visibility of the results and related data-base is meant to un-couple the physical locations of the pc-networkfrom large part of its users. Future developments on both machinesand simulation tools open promising perspective for larger applicationsin loco. From a knowledge-based point of view, such design explora-tions require high interdisciplinary collaboration. This is currently lack-ing in the conceptual design phase and is meant to be favored by theproposed method. However, a step should be taken also in the designteam composition, in favor of a higher integration of experts fromdifferent fields, in line with the tendency currently emerging inlarge firms.

Acknowledgments

The work is part of PhD research currently under way at DelftUniversity of Technology, and is performed in collaboration withthe University of Michigan where ParaGen is being developed. Theauthors would like to acknowledge Eric van den Hamwho developedthe PET analysis on the design alternatives of the Vela Roof and theHydra Lab at the University of Michigan, Taubman College wherethe servers used for ParaGen are hosted.

50 M. Turrin et al. / Automation in Construction 22 (2012) 36–50

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