The Journal of the Society of Façade Engineering January 2008

Report: SFE Complex Geometries Event

Technical: Brise Soleil and Thermal Performance

The theme of this issue of Elevation highlights the ever increasing dependence we have on our software. Whether it is simulating the dynamic energy interactions of the building fabric, occu-pants, M+E systems and local climate in a particular project; predicting stress and strain in a structural system; or detecting clashes between elements in a complex building form, we are ir-revocably locked into our relationship with the digital algorithm. As the speakers at the UK autumn event of the Society of Façade Engineering elaborated about non-orthogonal building skins, they all touched on the realm of the representation of a designer’s intent, the conveyance of these intents to the fabricator, and the challenge of interpreting that representation. Some of the more complex projects inevitably employed the emerging software that is becoming an integral part of the process of designing non-rectilinear buildings. To quote from Waagner Biro’s seminar handout, “It is obvious that the succession of nontrivial tasks to be completed during construction profits immensely from a good knowledge of the vari-ous interrelations between the primitives used in construction, an from a profound understanding

of the constraints they put on this process, at a level which is perhaps even more fundamental than structural analysis”. That knowledge has to be acquired and used by all members of the procurement chain in order to move such complexity towards designed performance. If parametric software is to be used from inception to final production, questions arise as to who owns copyrights and liabilities. The im-plications are absolute that the concept designer must have knowledge of the technology and methods of fabrication, assembly and installa-tion; not to mention the physics and materials science that goes into the making of even the most mundane façade. The role of the façade engineer can be re-emphasised in bridging the gap between designing and making. But an onward implication is that the bridging de-signer/engineer must have the knowledge of the new breed of software to be able to link beginners to endings.

mesh match:

stories of seminal skins

Review of the lectures from the society's UK autumn event: Facades and Complex Geometries Following introductions by Chris Macey, Chairman of the society, five speakers pre-sented to a packed basement of the Build-ing Centre in London. There was repre-sentation from various sectors of the indus-try: two specialist contractors, an engineer, an architect and a software provider. the fabricator Johann Sischka Waagner Biro The proper place to commence a discus-sion about any form of technology is on its history, and Johann drew parallels between today’s free form building skins and the long history of the development of auto-motive surfaces carried out by the me-chanical engineering industry. The chal-lenges are similar, but the building indus-try does not have the luxury of producing a single product which leaves the factory

doors as a single finished entity. He then proceeded to give an insight into how pieces of a jigsaw can be conceived, planned, fabricated, transported and re-assembled on site, to form objects not far removed from vehicles in form.

Case study: The British Museum, London The brief required a transparent roof of 6000m2 to cover the open space of the Great Court, with the circular reading room slightly offset within the centre. The form chosen was a component of a dough-nut, which resulted in the slightly domed structure that revolves around the reading room. The first task was to find the optimum mesh and panel size. The solution was 4,878 welded hollow sections for the net and 1,566 connecting nodes. The resulting 3,312 glazing units were virtually all unique, as there was only one axis of sym-metry in the roof.

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Cover photo: Marcin Czajkowsjki. Courtesy: The Jerde Partnership

Loads had to be brought to the perimeter of the roof, but not place any undue hori-zontal forces on the existing structure. The challenge was to ‘float’ the roof on an edge beam that distributed the forces. Each node was made from a thick steel plate with 6 radiating arms perpendicular to the plate surface. The glazing comprises stepped edge sealed units toggle fixed to the glazing bars with dry gaskets and silicone top seals. Case study: Sage Music Centre, Gateshead The task was to create a freeform skin around a more rectilinear enclosure. The mesh chosen this time was rectangular instead of triangular of the British Mu-seum. The majority of the skin is non-transparent, allowing it be formed from metal panels, which can be pre-curved. The use of rectangular panels allows the efficient use materials in their commonly manufactured form and consequent sav-ings in cost. An important principle of Waagner Biro’s method of erection of complex building

Left: British Museum roof under construction

Above: Finished mesh Right: Glazing detail

Below: The Sage, Gateshead

size, rigidity etc. The ladders are usually installed with a single row of sticks be-tween them. Single sticks are then site installed between the pre-assembled lad-ders. This method allows for rapid instal-lation, but also accommodation of toler-ances. Once the intermediate sticks are installed, the mesh becomes stronger as forces can be distributed throughout the emerging skin.

skins was demonstrated at Sage: the mesh is divided into sections called ladders. These comprise rows of pre-connected sticks that are either small enough to trans-port on lorries or, if space is available on site, to be assembled locally and hoisted into position. Some temporary propping or bracing may be required if the assem-blies are not inherently rigid enough. The ladders can be formed as single, double or even triple rows of sticks, depending on

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Case study: Zlote Tarasy, Warsaw In the words of the Waagner Biro seminar technical handout: “This atrium project… took freeform surface development to a new level”. From images shown and the description of the process, this claim may be fully justified. The brief was to build a 10,200m2 trans-parent roof over a shopping centre with convex and concave bulges. Construction-ally, this consisted of sections of negative and positive spheres that interlock at the lines of geometric contraflexure (see arti-cle, ‘Complex Surfaces: The Basics’). The challenge given was to keep the num-ber of supporting columns to a minimum to ensure maximum flexibility and space in the volume below.

Top left and top right: Rectilinear panels at the

Sage Centre Below: Structural grid at the Sage

Centre

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[continued on page 7]

converging sticks all meeting correctly at the nodes, allowing the glazing to be sim-ple triangles. The node also accommo-dates the misaligning inside convergence of the sticks. The glass fixing differed from the British Museum glazing stick, where wet silicone was applied to effect the seal; at Warsaw a dry gasket system was used with a drain-

The undulating roof nestles between other structures on the site, and flows down to street level at the front, effectively unify-ing and linking the ensemble. This building skin illustrates the problem of planar offset at connections faced by the designers. This means that when adjacent planes on the surface of a complex skin are parallel offset as shells to express the zones of the structural members, it can be found that the vertices no longer meet as they did on the original plane. For exam-ple, if the centrelines of the supporting sticks meet at all the intersecting nodes of the external surface of the mesh, they will not necessarily meet at node points inter-nally offset from the external surface. The inside surface of sticks will not intersect with the other converging sticks at a given node, even though their centrelines all meet precisely at the external surface. Somehow, Waagner Biro needed to ac-commodate the offsets in every node in the final mesh. The solution was custom stainless steel nodes with individually an-gled spigots for each stick. This allows the sticks to remain at the bisecting angle of the two planes they are required to sup-port, with the outside centrelines of the

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age channel built into the gasket profiles. Capping disks were used to give additional mechanical support and anchors for safety wire lines were also integrated into the design. The design of the gaskets also allowed the

Above: Panel installation at the Sage Centre

Below: Zlote Tarasy

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Technical

There are different types of complex geometries, some more complex than others. A building may have a non-rectilinear form made up of facetted planes in a variety of juxtapositions and orienta-tions. Many of the new breed of high rise commercial and residential build-ings follow these lines: our city sky-lines are transforming as this form of expression becomes more manage-able in design and construction terms. The advantage of these forms is that they can be made from sticks that are straight in two axes of orientation. A development of this is where a flat plane is curved in one direction. An example is the surface of a cylinder.

The next level of complexity is where a simple curved surface is pinched to form a segment of a cone. These sur-faces can be joined in opposite orienta-tions to form wave like surfaces that appear more complex than they are. Cylinder and cone skins can be formed from straight sticks in one orientation, but require either facetted or curved sticks in the second orientation. A higher form of complex geometry involves the curving of the surface in two directions with either a stretch or a bulge. This can be represented in three dimensional geometry by part of a sphere or a doughnut. Added complexity can be given to a segment of a pure geometric form by combining like forms in positive and negative orientations, joining the parts

Complex Surfaces: The Basics

at the points of geometric contraflex-ure (the point at which a positive arc becomes a negative arc, such as in a sine wave) to ensure seamless inte-gration. Having chosen the type of surface, the next task is to find the appropriate mesh to represent the form in the model and then manufacture a panel, that when assembled, will reproduce that form. The resultant planes between mesh lines form geometric panels, the sim-plest being triangular, but they can be polygonal. Some polygons don’t inter-lock on either a flat or curved surface, so they require other simpler polygons to form the mesh, much like the hex-agonal and pentagonal panels on the soccer ball.

Top: polygons of a football Above: mesh at Zlote Tarasy

showing parts of negative and positive spheres

Below left: London Bridge Tower

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glazing to be positioned at slight angles to the central axis of the glazing bar, essential for the glass skin to follow the contours as designed. Additional support is given by steel tree columns which distributed load from a number of points on the delicate mesh to the slab below. The prefabricated ladder components were three glazing bays wide. This was larger the British Museum solution, but site ac-cess was better. The nature of the fabrication implied a hierarchy of three levels of tolerance: of trees to ladders, ladders to sticks and sticks to nodes. On site, the trees were hoisted into place and the ladders, with additional temporary props, were loaded onto the trees. Con-necting sticks and final glazing were then located from a temporary access deck just below the roof.

[continued from page 5]

Top: pre-assembled ladder Right: nodal offset of upper and

lower centrelines Below: Installation

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Mario Perin Permasteelisa Permasteelisa are another company that has started to harness the power of parametric modelling as a way of taking the creation of the façade smoothly from design through production and onto installation. Mario Perin highlighted the importance of the interface of software as a bridge between the designers and the CNC machinery that can produce components that make up a façade. Case study: Walt Disney Concert Hall The outer skin of this façade is made from 3 long stainless steel panels which are curved and formed to the shape of the sur-face. These panels are mounted on sticks (or studs) that are in turn mounted on the primary structure. Both the primary struc-ture and the secondary studs are straight. The primary members are highly facetted and the studs are less facetted and inclined to the general form of the surface. Only when the finished panels are applied does the surface become smooth and un-facetted as planned.

Case study: BMW World, Munich The vortex for the BMW centre stands on the corner of a major intersection next to the Olympic park in Munch. 2500 unique elements make up the composition, neces-sitating the writing of digital subroutines to handle the process of rationalising the complexity. The subroutines contained rules, for instance, that each steel frame member is located on the centre line of a

glass joint. The mesh was triangulated with common lines running horizontally in order to achieve the tight radii of the curves in the vertical plane. Computers were also highly instrumental in writing cutting and welding schedules, and with modularisation and transportability stud-ies.

the architect Christian Male Ian Simpson Architects Case study: Chester City Council Offices This project is an office for a local author-ity close to the site of the old city wall. The designers chose to make reference to the wall to inspire the form and look of the building. They wanted the facade to take cues from the existing wall, which became layered over successive historical periods from Roman times. Red sandstones are interlaced with grey stones, and the colours within the wall offer in insight into its his-tory. The designers therefore sought to create a memory of the wall in the colour of this, mainly transparent façade. The striking use of interlocking planes to create the form and massing of the building gives it a contemporary and dynamic feel that, coupled with transparency, met the aspira-tions of the client. Being an office building the facade had to conform to certain standards to ensure the right levels of daylight entered the internal spaces, but that sunlight and solar gain are controlled. Also, standard office modules and interfaces had to be addressed, ensur-ing connectivity at slab levels and parti-tions. These factors helped inform deci-sions about panel size, transparency and shape. Infill panels are either clear or col-oured with various shades of red and grey. When constructed, the façade will comprise unitised elements that span between floor slabs. Where planar cranks are required at crease lines in the wall, additional support will be given by steel beams. A discreet gutter has been designed to catch drizzle along the near horizontal crease lines. It is envisaged that higher levels of rainfall will cleanse the façade and therefore not require a larger gutter. Stack joints will be standard unitised hori-zontal connections with make-up pieces to take up the angular differences with the floor. Many studies were carried out dur-ing the design stages to ensure repetition of façade elements to reduce cost, but without compromise to the complex look of the building. Case study: One Blackfriars Road The slender tapering form of this residen-tial/hotel tower presents some challenges for the design of the façade. Although there are some areas of flat vertical wall,

much of the remainder is either curved or canted, or a combination of both, with some double curved at the very top. The façade will have two layers. The single glazed outer skin will be curved to the form of the building, which has been carefully analysed to allow for a certain amount of cold warping of the glass. It will contain louvers where required. The inner layer is the environmental wall and will be facetted to vertical at each floor, apart from at the curved nose of the building. It will contain sliding screens and approximately 30% opacity. The interstitial spaces will act as winter gardens. the engineer Duncan Richards | Arup Façades Duncan described the process of develop-ment for the Ian Simpson Chester City Council scheme from the engineer’s per-spective. Rationalisation of the form is the first step to making the design buildable, both tech-nically and economically. This was done by either identifying repetition of elements, size and shape of panels, or reduction of materials wastage. For Chester, the differ-ent angles were consolidated to simplify the design and increase repetition. Using the new data, the form was remodelled with minimal impact on the architectural vi-sion. Arup Facades used a combination of hand sketching and modelling to work out the interfaces and connections. 3D modelling tools made the process much quicker. Hand sketches which had been developed at design workshops were scanned and extruded to form the model, from which further investigation of facade interfaces and components were made. Later itera-tions also applied thermal modelling to show areas of cold bridging and possible condensation.

Top left: BMW World Bottom left: Walt Disney Concert

Hall Top: Chester City Council

Above: Blackfriars Road Tower

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Tristam Simmonds | Arup Tristam works in the advanced geometry group at Arup and has worked with a num-ber of artists on complex installations. Artists conceive spaces, colours, forms, impressions and surfaces in sometimes ethereal and always inspiring terms, offer-ing interesting engineering challenges to be solved. He described the writing of soft-ware which can build and optimise a mesh from a scanned form and turn it into 90% equilateral triangles. Case Study Anish Kapoor: Marsyas Installation Typical of many freeform constructions, this installation began life as stretch fabric maquettes and evolved into the eventual space gulping leviathan that drew crowds in the turbine hall at the Tate Modern. To construct the artist’s vision the engineers had to digitise the surfaces, and then ana-lyze the forces generated by the installation and the resilience of the exiting structure to carry the loads. The finished work had to have the very lightest touch on the turbine hall surfaces, in order to give the appear-ance of floating. The panels were opti-

mised to take the highest loads along their seams, rather than across them, and to de-form into the desired final shape, much like the making of yacht sails. Case Study Anthony Gormley: Body Sculptures This fascinating project involved the ab-straction of the form of a human body in the foetal position into a series of easily fabricated metal cubes of diminishing or-der. The project demonstrated the power and functionality of software to take a bio-morphic form and translate it into sub-forms that are more familiar to the world of construction. The body was digitally mod-elled as a complex free form, and then the software was used to explore its replication from large cubes. Over the course of sev-eral iterations, increasing smaller cubes were added to the model to add detail and explore alternative orientations. In essence, the sculpture is a fractal, with cubes formed from smaller cubes, which are in turn formed from even smaller cubes. Many parallels could be seen between these exercises in the creation of pure art and the creation of the facades of the sculptural buildings featured in the other seminar talks.

Above: Marsyas installation Below: Gormley sculpture

[continued on back page]

the Building Regulations. But this does not alleviate the issues associated with poten-tial condensation. The heat flow through a section of curtain wall can be accurately simulated through the use of 2 and 3 dimensional computer software. This software can also help to identify areas of increased heat transfer by showing heat flow lines on the detail. The distribution and increased density of these lines indicate path of least thermal resis-tance. Most readers will be familiar with flow line diagrams of curtain wall sections, in-dicating increased concentration of heat flow around the aluminium spacer bars in the double glazed units. This signifies rela-tively high rates of heat transfer across these components when compared to the heat flow across the glazing cavity and the plastic thermal break profile of the mullion bar.

When the brise soleil bracket is introduced to the nose of the mullion, a significantly smaller mass of heat flow lines around the aluminium spacer bars in the double glazed units is evident (Figure 1). This indicates that the spacer bars are no longer the area of highest heat transfer. Unmis-takably, the highest concentration is cen-tred on the bracketry elements and mullion back box. This shows the path of heat transfer from the internal environment, through the mullion box, along the bracket and out to the external environment.

The effects of this thermal bridge are not confined to the height of the bracket, though. The heat flow within the system of

by James Smith MSc. Façade Engineer at Wintech Ltd. A philosophy of the design of commercial buildings is to utilise the available natural light to its maximum potential. Large ar-eas of vision glazing allow the penetration of daylight into the building, thus reducing the demands on the artificial lighting sys-tem. But visible light is only part of the energy radiated from the sun, and as direct sunlight enters through a window, so does the solar heat energy which can lead to overheating. It is primarily for this reason that a range of external solar shading de-vices, such as brise soleil have been devel-oped. The potential energy saving benefits of external solar shading devices are widely documented. These types of system are generally acknowledged to be amongst the most efficient means of reducing excessive internal solar heat gains. The connection of these devices to curtain wall systems is commonly achieved through the use of aluminium bracketry. Although structurally aluminium is an ideal material to manufacture such a bracket from, its principle disadvantage in this application is its high thermal conduc-tance. Modern curtain wall systems are designed with thermal breaks between the outer and inner aluminium sections, with effective seals to prevent the ingress of weather and air infiltration. The brackets puncture this thermal break, creating a ‘path of least resistance’, and thus thermally bridging the system locally. Thermal bridges are shown to have the potential for a dramatic effect upon the thermal performance and life expectancy of the whole façade, not just the area di-rectly affected. Locations of increased thermal transmittance can drastically raise the overall U-value of the building enve-lope and until recently have often been ignored in simplified thermal calculation methods. Even when thermal bridges are accounted for in the thermal calculation and the overall façade U-value is amended, often a double glazed unit with a higher insulating performance is installed to compensate and ensure compliance with

the surrounding area is also increased, as indicated in Figure 2. The compromised thermal performance can also lead to lower internal surface temperatures, which in turn can result in condensation on sur-faces not intended to be wetted. Calculations conducted by the author indi-cate the average U-Value of the sample mullion (Uf) increases significantly when the brise soleil bracket is incorporated and rises further when the full brise soleil as-sembly is attached (by over 38%).

Further calculations indicate the overall U-value of a glazed curtain wall screen may be increased by up to 10%. This may be sufficient to jeopardise Part L compliance of the project if not factored into the over-all building energy consumption model early enough. Brise soleil assemblies offer many benefits in terms of reducing energy consumption and maintaining occupant comfort levels in summertime, but consideration should be given to the potential for increased heating loads during winter from a com-promised façade U-Value.

Figure 1

The Effect of Brise Soleil on the Thermal Performance of a Facade

Figure 2

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ing on from last year’s successful event which is repeated with a different route. Commences 5.00pm 26 - 30 October 2008 Interbuild at NEC Birmingham. www.interbuild.com

Feedback Please submit comments, articles, case studies, technical features, diary dates, or any other newsworthy items to the editor, Mark Taylor at : editor@facadeengineeringsociety.org www.facadeengineeringsociety.org

Diary 7 December 2007 - 25 March 2008 Exhibition: The Poetics of the Technical Object. The work of Jean Prouve, pioneer of curtain walling. Design Museum, Lon-don. 20 February 2008 SFE event. Visit to Pilkington glass float line and processing plant at St Helens. There will be seminars on topics such as vacuum glazing units and other new developments. Cost free. Meet-ing 2.00pm at Pilkington. Please reserve a place by emailing: sfe@cibse.org

26 February 2008 NCE Conference: ‘Façade Engineering’ Milleniun Copthrone, London W8 www.ncefacades.co.uk 1 April 2008 CWCT spring meeting (10.00am) and SFE annual general meet-ing (4.00pm). Bath University. 8 - 10 April 2008 Glassex and Glass Proc-essing and Technology exhibitions. NEC Birmingham www.gexhibition.com 8 - 9 April 2008 Glass Performance Days. Beijing. www.gpd.fi 18 June 2008 SFE Summer Event: City Walk 2. Follow-

the software writer Edoardo Luzzato | Gehry Technologies Gehry technologies provide software solu-tions for building information modelling (BIM), which allows designers to trans-form ideas into digital models that can be passed throughout the design, supply, manufacturing and construction web. The aspiration is that a single model can be used or added to by all parties to stream-line the process. Eduardo described the latest software as more that just parametric; and highlighted a number of benefits, including reduction of time, risk, errors, clashes, waste and cost. Alternatives can be tested by adjust-ment of contained parameters and 3D con-struction documentation can be produced. 2D drawings can be associated to the prin-ciple 3D model and auto update as the main model is amended. The models work hand in hand with spreadsheets containing the parametric information in a readable and editable fashion, which can eventually be used to programme manufacturing ma-chinery and logistics.

Above: Digitised Gormley body

Below: Gehry designed IAC headquarters building