Fabric and foil membranes offer architects unique design opportunities.

By Nicholas Goldsmith, FAIA LEED AP

Looking at a historical perspective on the evolution of architecture  — from the massive pyramids of Egypt to the framed structures of Greek and Roman construction, to the lighter Gothic vaulting and eventually modern architecture of the 20th century — there is a continuous, almost linear progression from solid mass to diaphanous skins of glass and steel.

A record of earlier brick and wood structures in Egypt gave way to the presence of the earliest stone architecture some 5,000 years ago, and because of the Egyptian obsession with the afterworld and permanence, a culture of stone was created.

As culture shifted from Egypt to Greece and Rome, stone continued to be used, but the Greek and Roman temples used larger spans for their columns and less material overall. The classical three orders started with the Doric, to Ionic to Corinthian — each becoming lighter in profile. With the advent of Byzantine and Romanesque churches, the weight to span decreased; and when the Gothic style was developed in northern Europe in the 12th century, this decrease in weight to span accelerated with the introduction of flying buttresses that allowed glass curtain walls reflecting its metaphysical attraction to light appear seemingly self-supporting in heights of up to 150 feet.

In the 20th century, a movement from mass to membrane increased in speed with the introduction of steel framing, lattice trusses and new developments in glass technology. By the beginning of this century, glass had become the material of preference. Curtain walls using truss framing, diagrids, glass mullions and cable systems all became common architectural practice. Corporations used glass towers as a symbol of prestige and transparency. Today, lighter and lighter building technologies include structural membranes composed of fabric and foils. It is interesting to note that two of the leading high-tech companies of world, Google and Apple, presently are developing glass and ethylene tetrafluoroethylene (ETFE) foil skins as elements of their new corporate headquarters.
In short, looking back over the last 4,000 years of architecture, one can see a linear evolution from the mass of the pyramids to the membranes of tomorrow. Mass to membrane is a human architectural journey — can we use less material, be more sustainable, and help reduce the carbon footprint on the planet called earth?

Fabric, foils and other membranes are materials well suited for creating building skins. They are curved for strength, provide waterproofing and insulation, can be multi-layered and can create complex surfaces with minimal seaming. Fabric skins have been around since the beginning of time as seen in yurts and leather tents of ancient civilizations. But with today’s new composite materials, computational analysis and digital patterning, membranes have developed into a new option as permanent building skins.

Building Skins

To shed light on the new building skins of tomorrow and new approaches to building skins, step back and look at human skin. There are three layers of skin: the epidermis, the dermis and the hypodermis. The outer epidermis layer contains the sweat pores and hair shafts; the middle dermis layer contains connective tissues, hair follicles, sweat glands and some muscles; and the inner hypodermis layer contains veins and arteries as well as connective tissues that tie the layers together.

This approach of multiple integrated layers all performing different functions creates a skin that gives the body a waterproof cover, keeps out disease and protects against sunlight. When the approach of multiple integrated layers is applied to architectural membrane structures, the performance requirements of the building skin first need to be understood.

Not only is human skin sensitive, sending the brain information about pressure and temperature, but it heals efficiently to maintain a protective barrier. Combining these two features into one man-made material has been the focus of Stanford University chemical engineering professor Zhenan Bao and her team. The team has succeed in making a man-made skin that is not only sensitive to touch, but also is capable of healing itself quickly and repeatedly at room temperature, which has significant implications for biomedical textile markets. The researchers succeeded by combining two materials — a plastic polymer with self-healing ability and the conductive metal.

Today, enormous growth is observed in the wearables or smart garments market. These technologies use intelligence pathways in the fibers of the materials to create e-textiles. Socks, sports bras, T-shirts with heart sensors, and casts with currents for quickly knitting bones together are now all available products. As this market develops, spin-off technologies likely will have an impact building skin technologies.

A few years ago, New York City-based FTL Design Engineering Studio assisted Klaudia Kruse and the innovation team at BMW to develop a stretch fabric body for a concept car named the Gina after the famous Italian actress Gina Lollobrigida. The stretch fabric approach was based on a lightweight aluminum frame with a fabric skin that can open and close as a complete shape (See Figure 1). This is a fabric skin approach to industrial design that is in its early stages of development, but again asks the question of how to get more when using less material.

Structure

There are three basic types of building skin structures — frame supported membranes, cable and strut supported membranes, and either air-supported or air-inflated pneumatic membranes. Each one of these systems requires the skin to be prestressed to develop strength, and requires form-finding to develop relaxed 3-D — either synclastic or anticlastic — surfaces that are in equilibrium. Ideally, they have equal tension and are considered minimal surfaces, but often for spatial reasons they have different tensions in both opposing primary directions. Whether computer programs that utilize a force density approach or a dynamic relaxation approach are used for form finding and analysis, finite element analysis is required to create balanced structural skins that can resist downward and uplift pressures.

Form

Prestressed membranes take either saddle surfaces — hyperbolic paraboloids — toroid or doughnut shapes; and surfaces of revolution — either hyperboloid or toroid surfaces. In complex building skins, these basic building block elements are developed into multiple panels where the different surfaces are bounded by ridges and valleys to create complex geometries. The intersection of the ridge lines and valley lines become 3-D linear elements that are described by physical modeling and form-finding geometries.

Acoustics

There are many different approaches to generating form including considering acoustics, site constraints, sun angles and wind. Using acoustics, one starts with a ray diagram to generate reflective surfaces that both blend and reflect sound waves. Depending on whether the structure is used for pop music or classical music determines the type of surface required for performances. Conoid shapes and synclastic shapes tend to trap sound, so most performance pavilions use a series of hyperbolic surfaces to create a rich interior experience.

A project in Sun Valley Idaho completed by FTL Design Engineering Studio in 2008 uses acoustic reflection and the site’s constraints as the form generator for the music pavilion. The site was a landscape in the Rocky Mountains at a famous ski resort frequented by Hollywood celebrities. FTL’s approach to the landscape was to develop a series of retaining walls that rose out of the land, came together to create a musical space and then faded back into the ground. The music season lasted only through the summer months, but the winter snow load criteria was more than 100 pounds per square foot (lb/ft2) with drifts of up to 200 lb/ft2. FTL understood the symphony wanted a structure that combined the seasonal and ephemeral quality of a summer tent with a more permanent facility that could accommodate the theatrical and acoustic requirements. FTL realized the local snow load requirements would not allow for a permanent fabric structure. The eventual design features a steel cable net and uses a rigid wood material that can handle high vertical loads. When developing the idea, FTL felt that the wood was very much in keeping with the materials near the site and tied into the surrounding buildings. A copper-top roofing maintained the differential in slope and added another earthy touch to the ephemeral cable structure. The audience area remains covered with a tensile fabric skin that is demounted each fall prior to the winter.

Musically, the wood roof allows for a rich sound that is augmented by a series of acoustical reflector panels suspended below the cable net, which can be fine-tuned for different types of music. In this case, a hybrid solution of the cable net and tensioned fabric solved the diverse set of design challenges.

Shading And Insulation

One of the quintessential qualities of tensile structures allows their use as shading elements. Since ancient times, membranes have been used for this purpose — whether it is the “vela” shading Roman coliseums or the Bedouin black goat hair tents of Arabia or the “toldos” covering the streets in southern Spain. With fabrics that allow anywhere from zero to 40-percent translucencies, the level of shade is controllable based on different climates and regions of the world. Based on a classification of different climates such as the 19th century professor Koeppen’s climate categorization based on plant life around the planet, one can apply both shading and insulation to lightweight structures. The strategies for membranes in cold moist climates such as northern Europe are considerably different than hot arid climates such as the American southwest or the hot and moist climates of Central America.

Technologies such as foil pillow structures make sense in the cold, moist climates; while in hot and arid climates, mesh fabrics often are used; and in hot and moist climates, solid fabrics that provide for air movement and passive cooling are used.

FTL had to consider a northern climate when designing an entryway for the Empire City Casino in Yonkers, N.Y. The design firm developed an ETFE foil pillow system supported by a steel grid shell structure that acts as an entry to the casino and also provides rain and sun protection for up to 30 cars. The design also acts as a lighting element and an icon for the casino. FTL developed a toroid shape for the grid shell, but also looked at funicular shapes to minimize steel weights.  The ETFE pillows allow for transparencies up to 98 percent, and are a soft alternative to glass roofs and facades.

Energy Generation

By their nature, membrane structures use large unsupported spans that can be modified with the application of solar thin film technologies to generate power to the space inside. In addition, tensioned membranes can serve as shrouds for wind collection turbines where minimizing turbulence allows the turbines to work efficiently at lower wind speeds. Since the first application of solar panels that FTL pioneered in 1998 at the National Design Museum in New York, the company has worked with amorphous silicone and copper indium gallium selenide (CIGS) technologies to develop alternative methods of power generation using building skins. Generating only approximately 5 watts per square foot, the method is still relatively inefficient. But because the roof area of the building skin is generally large, the inefficiency of the thin film technology is offset by its cost effective size.

Lighting

Several basic forms of illumination exist in nature— illuminant, illumination, film mode, surface mode and volume mode. Membranes can make use of surface and volume natural lighting modes. Using artificial lighting through the volume light mode creates glowing volumes of form and space. FTL’s recent project for Phoenix-based Arizona State University modeled computer lighting to create glowing volumetric light using high-intensity discharge fixtures for the main surfaces and programmable light emitting diode lights to highlight the steelwork.

Water Collection

Water collection always has been an important function of roofing in dry climates since Roman times. But with today’s added interest in sustainable development and storm water collection, interest in collecting storm water in cisterns for reuse has increased. Membrane structures are ideal collectors of rainwater, and the shape of the roof can be manipulated to become a water feature of the building.

In 2009, FTL completed the Rosa Parks Transit Center in downtown Detroit for the city (See Figure 2). The goal was to unify six bus waiting areas and connect them to the “People Mover” — an elevated rail system that serves the City. The design firm developed a 6,000-square-meter roof under which 13 busses can load and unload passengers. Because of the unique urban site and the elevated tracks of the rail system, foundation placement was very limited. However, the design was required to cover the street area up to the elevated tracks. FTL’s solution to this dilemma was to create a series of seven A-frame masts that use a tensegrity cable system to suspend eight curved trusses. By cantilevering the trusses out over the road, the bus area was covered. The section of trusses with A-frames creates peaks and low points, which provides curvature to the fabric, but also creates the opportunity to collect water from the roof, which is stored in a cistern below grade. The edge of the low point has a large gutter and feeder pipes that allow the water to pour into a landscaped element with a catch basin at grade. The water collection areas are surrounded by built-in seating for waiting passengers. The stored water is used for irrigation for plants and landscaping on site.

Future Outlook

There are many different elements required for building skins depending on the location they will be used. The task ahead focuses on integrating these different elements into multi-skin solutions, much like human skin, to offer numerous functions. The next steps in the evolution of membrane structures will be to layer integrated systems that, when connected, will transform the notion of building facades into a porous multifunctional membrane reflecting the natural world.

Editor’s Note: Nicholas Goldsmith, FAIA LEED AP, is senior principal at New York City-based FTL Design Engineering Studio; and a fellow of the American Institute of Architects, International Association for shell and spatial structures, as well as the former chair of the Lightweight Structures Association. This article is based on Goldsmith’s presentation given at the 2015 Textile World Innovation Forum.

September/October 2016

Stäubli offers cutting-edge machinery for the most sophisticated technical textiles

TW Special Report

Technological development in the production of textile surfaces has enabled the manufacture of a growing number of automobile components based on technical textiles. These components include body parts as well as interior trim parts such as floor mats, seat covers, ceiling panels and dashboards. In aircraft construction, it is now standard practice that fuselages — or nearly entire aircraft bodies — are built with the aid of industrial textiles. Highly versatile fabrics for such applications have been created through the application of significant know-how by weavers and their machinery suppliers — both of whom invest heavily in technical advancements.

Research and development continues to be intensified in the textile manufacturing industry throughout the entire production chain and supported by manufacturers and specific training programs offered by textile universities. Today, the list of potential applications for textile products can be almost infinitely extended.

Regarding composite materials, a significant step toward serial production was shown by multiple exhibitors at this year’s JEC trade shows in Europe and the United States. But in terms of more complex composites such as 3-D multilayer weaves and spacer fabrics, product engineers in all industries — including automotive, building construction, leisure, nautical and aeronautic — are striving to identify precise areas of application in order to merge the advanced weaving possibilities with their needs in terms of technical requirements and woven components. Thus, product designers must understand the possibilities and special advantages offered by latest weaving machines, such as those in the product range of Switzerland-based Stäubli International AG. These machines offer a wide range of capabilities, such as freedom in design and format, fail-safe operation, drapability, precise definition of the elasticity of the fabric, weaves with reinforcement in certain areas over the weaving width, supporting the integration of the fabric in any component, and precise and gentle yarn treatment — the basis for perfect quality in the final product. Furthermore, advanced weaving technologies can shorten workflows, making for more economical production of the fabric and the final product.

3-D Textiles In A Wide Range Of Formats

In recent years, a new type of technical multilayer fabric — so-called 3D textiles — has appeared on the market. Finished products incorporating these textiles are used in applications with increasingly stringent mechanical requirements. One specific example is the aerospace industry, where traceability and reproducibility are central aspects of product specifications. Critical in the production of such industrial textiles is the ability to precisely control the arrangement of the warp threads within the shed of the various layers. This precise control is perfectly supported by Stäubli’s UNIVAL 100 — a unique servo-driven single-end control jacquard machine. The warp-thread control supported by servo-driven individual actuators, allows the production of a great variety of technical textiles and 3-D multilayer fabrics. Depending on the configuration desired, the unit can be fitted with anywhere from 512 to 15,360 actuators in various designs.

Modern Airbag Manufacturing Methods

The latest jacquard weaving technology has made it possible to manufacture complex airbag structures with woven seams. These one-piece woven (OPW) airbags give designers great flexibility in creating patterns and designs. They also reduce the number of production steps, thereby reducing production time. Side-curtain airbags, activated by a lateral collision, are shaped according to the interior contour of the particular car they are fitted in. Their shape and structure are created at the weaving stage. As a result, no subsequent sewing operation is required. To effectively protect passengers in a rollover, OPW side-curtain airbags must remain inflated for several seconds. This is best accomplished by using a sealed cushion with woven seams. OPW airbags are woven on modern high-speed jacquard weaving machines. The warp material, the variety of fabric patterns, and the importance of precisely shaped airbags require the use of a robust and reliable jacquard machine. For maximum flexibility in the creation and design of airbags, weavers require a high number of hooks, which makes control of each individual warp end possible. The extra-reinforced drive elements and the rigid structure of Stäubli’s LXL jacquard machine — available in formats from 6,144 hooks to 18,432 hooks — are ideal for weaving very high-load fabrics such as airbags. The machine’s lifting mechanism ensures accurate shed geometry and vibration-free operation even at high speeds.

At the heart of the LXL are MX modules, serving as the link between the lifting mechanism and the harness. For each weft insertion, each hook of the modules can be positioned either up or down, corresponding to the individual pattern required for each type of airbag and adapting to the complexity of the design. Each hook is driven independently, thanks to the operator-friendly JC7 controller in which all weaving data are stored and then transmitted to the jacquard modules, pick by pick. All data can easily be transferred to and from the jacquard controller via USB stick or network.

Delicate Automated Weaving Preparation

Filtration fabrics are among the technical textiles upon which users place the highest demands. Filter fabrics are used in process filtration, silk screening, as filter components, in medical applications and also in architecture. Ultrafine fabrics increasingly are used in the high-tech field of smart textiles.

One thing is common to all these application areas: Precision fabrics are required — and this calls for customer-specific solutions. Highly specialized weaving mills possess the necessary know-how. These mills are not just weavers, but can respond to the individual needs of the application technology and can ensure on-time delivery of a specified end product made using the finest yarns.

But before the yarn can be woven, the warp must be drawn into the weaving harness. Warps can measure up to 400 centimeters in width, comprising tens of thousands of threads — sometimes in numbers approaching 100,000 threads.

To streamline the drawing-in process, Stäubli offers automatic drawing-in machines that can handle up to 200 ends per minute. Here, the extreme yarn count of sometimes less than 7 denier, or 32 microns, places extremely high demands on the drop wires, heddle, reed, and every other element that comes into contact with the threads.

State-of-the-art image processing makes it possible to flawlessly separate individual threads from a dense sheet of warp threads featuring up to 200 threads per centimeter. Programmable repeat control ensures correct reed dent and drawing in to harness frames, thereby ensuring the specified fabric properties. Stäubli’s automatic drawing-in machines not only handle the finest threads very carefully, but also can handle different types of threads in the same warp layer. Even abrasive yarns such as Kevlar® or fiberglass can be processed, thanks to specially coated thread-handling elements. The SAFIR S80 can even work from two stacked warp beams — up to four different thread layers and materials including S- and Z-twisted yarns.

As the technical and quality requirements for technical textiles and composites grow, machinery manufacturers will continue to support the industry with ever-increasingly sophisticated machinery and technologies.

September/October 2016