thinking

Non-linear thinking

Software is helping deliver beautiful structures that span huge distances with minimal materials. Peter Debney, application specialist at Oasys, shares some great examples in tension and compression

Apart from the chain or cable assumes weight, there About the author under its own weight, is nothing familiar to us from lightweight Peter Debney is a struc- suspension bridges) is about lightweight struc- tural engineer with over stabilised out-of-plane tures. With traditional structures, the loads are 20 years experience in computing applications, in practice and in engiby gravity and possibly additional factors resisted by the stiffness neering software, BIM, such as a bridge deck. in the beams, columns, analysis and design. However, such strucand walls; with tension- tures are still vulneraonly and compression- ble to sway, whether only structures, the overall form of the induced by wind or pedestrians. A solustructure becomes critical. Get the form tion to this problem (though not normally right and the structure can span huge dis- for bridges) is to have cables going in multances with minimal material; get the form tiple directions, so sway in one direction is wrong and you are in trouble. resisted by cables at other angles, giving

In fact, all structures are nonlinear; it’s what is called a cable net. just that the simpler linear analysis usually If such a cable net is horizontal and gives answers that are close enough for the loaded it will deflect down with an essenmajority of engineering design challenges; tially catenary shape, giving resistance to but new lightweight structures and materi- gravity loads, but there is still a problem: als call for a more thorough analysis. what about uplift forces on such a cable

So what makes nonlinear different to lin- net when it is clad? Suction would be ear analysis? One of the most important resisted purely by the self-weight of the things to remember is that while with line- structure, but that is potentially minimal ar analysis you establish equilibrium of the with lightweight structures. The solution forces on the original geometry, with a non- is to have the cables in one direction linear analysis you get equilibrium of the curved down to resist gravity loads, and forces on the deflected shape. The problem those in the other direction curved is, you do not know what the deformation upward to resist suction loads. This douis until you have resolved the forces and ble-curved hyperbolic surface is characyou cannot resolve the forces until you teristic of many cable nets and the shape know the deformation. All nonlinear analy- naturally gives stiffness in all directions. ses requires iteration. The faster the better. So what does such a double curved surTension structures Velodrome for the London 2012 Olympics, Cables famous for its ‘Pringle’ shaped roof. While linear structures resist lateral loads face look like? An example is Expedition’s with bending stiffness, light-weight nonlin- Fabrics ear tensile structures have to deflect a long The modern science and engineering of fabway until the forces are in balance. ric structures was pioneered by Frei Otto,

A single loaded cable, describing a cate- with his roof to the Munich Olympic nary (the curve that an idealised hanging Games. Rejecting the heavy wartime archi-

Exemplifying the potential for lightweight structures is Barcelona’s emblematic Sagrada Familia: Arup engineers are designing some of the remaining structures to withstand both gravity, wind and seismic forces

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1 Westminster Abbey: any movement in compression-only structures increases the risk of buckling 2 Kurilpa: the world’s first bridge created using tensegrity 3 London’s Olympic Stadium with its iconic double curved surface 4 Frei Otto’s roof to the Munich Olympic Games

tecture of Nazi Germany, Otto aspired to make modern architecture as light as possible, in both senses of the word: the Munich roof achieved this by using both a minimum of material and maximum glazing.

Fabrics are woven and this gives rise to warp-weft interaction, making them sensitive to the balance of pre-stresses in the two principle directions. The fabric will wrinkle as a whole if the pre-stress is much higher in one direction than the other. A correctly tensioned fabric will be smooth; unbalanced tensions will wrinkle the surface.

Fabrics require an edge support, which can either be solid, such as a beam, or flexible, such as a cable. With flexible edges, the cable’s curvature is dependent on the balance in the pre-stress between the cable and fabric and calculating this has quite an impact on the aesthetics and geometry of the fabric structure.

Foils

Foils, which are isotropic plastic sheets made from materials such as ETFE, have been used on iconic structures such as The Eden Project in Cornwall and the Beijing 2008 Olympics’ Water Cube.

Foils are generally used in inflated pillows, so each cladding panel is actually two or three separate layers supported by pressurised air. Wind loads on one surface are carried through the contained air to load the opposite face, so the whole remains in tension, plus the air can be heated to prevent snow loads. Unlike fabrics, foils have a good shear strength so have to yield under load to achieve their final form.

Like fabrics, form finding and the correct cutting patterns are critical. Compression structures

Flying buttresses at Westminster Abbey

While tension-only structures deflect forces to balance the loads, compression-only structures, which can take the take the form of arches, shells, grid shells, and walls do not have this luxury as any movement increases the risk of buckling. This is a major risk for masonry structures as they have little or no bending capacity other than that provided by the compression thrust. While a masonry structure behaves linearly when the forces are low, it becomes nonlinear as soon as bending and buckling starts to open the joints.

Sagrada Familia

Exemplifying the potential for lightweight structures is Antoni Gaudi’s emblematic Sagrada Familia in Barcelona, due for completion in 2026 or 2028, more than 140 years after it was started.

While columns and walls in medieval cathedrals were braced with flying buttresses, the Sagrada Familia has angled columns so as to take the loads in direct compression. This removes the need for buttressing and the end result is something much more natural-looking. Gaudi used physical models to form find the structure; we can now use software such as Oasys GSA to achieve the same result.

Arup engineers are designing some of the remaining structures to withstand both gravity, wind, and seismic forces.

Tensegrity

One progression of lightweight structural design is tensegrity, where elements are either in pure tension or compression, and ideally no two compression elements should be in contact. Tensegrity was a term coined by Buckminster Fuller and is described variously as “islands of compression in an ocean of tension” and “God’s geometry”. Brisbane’s Kurilpa, the world’s first bridge created using tensegrity, was designed by Arup engineers and called for purpose-written software that linked into Oasys GSA for the form finding, nonlinear static, and vibration analyses.

CAD Manager’s Guide to the Galaxy

ith the introduction of W digital design tools came the mantra ‘create once and reuse many times’. Pretty soon after this, we started to hear the term ‘knowledge capture’.

Most CAD managers agree the only real constant in their field is constant change. The best managers adapt easily and wear many hats: they manage time and resources effectively, facilitate communication between and within multiple teams, stay on top of industry trends and best practices, and keep hardware and software systems updated. In today’s rapidly changing world, even the best CAD managers need help staying ahead of the curve.

Autodesk University (AU) is a great resource if you’re looking to hone your skills and understand the latest software, workflows, and best practices. Many AU sessions also offer professional development in other important soft skills. If you didn’t make it to AU Las Vegas last month, don’t worry. We’ve cherry-picked the top classes for CAD managers on everything from the latest hardware to boosting your people skills so you can become a more effective manager. You can access these classes anytime at AU online, so grab your laptop and get ready to learn.

A Hardware Wonk’s Guide to Specifying the Best 3D and BIM Workstations, 2015 Edition

Matt Stachoni, CAD expert and BIM specialist for Microsol Resources, walks you through how to build out new workstations for different classes of BIM usage in the Autodesk Building Design Suite. Learn to identify user requirements, understand the computing demands for different applications, and discover how current AEC and IT trends affect your decision. Matt covers 5 critical subsystems and components to look for in every workstation, from CPUs to GPUs. tinyurl.com/AU-hardware

Tip: Download the handout for this class, which is chock full of stats and details you can reference.

How’s BIM Doing in Your Multioffice Pond?

Do you know the difference between a WAN and a LAN? David Butts, a toprated AU speaker and BIM specialist with 3 decades of experience under his belt, knows how to scale BIM best practices across a multioffice environment. This class provides actionable tips to make sure your entire team is sharing information accurately, quickly, and completely, so you can resolve conflicts, meet standards, and deliver a well-designed project on time and on budget. It all starts with structuring a strong information system.

tinyurl.com/AU-multioffice

Tip: David shares best practices for worksharing in multiple environments, including creating a set of default project folders that are specific to BIM and/or CAD files.

Becoming an Awesome CAD / BIM / Tech Manager

Now that you’ve got the right workstation and worksharing practices in place, let’s talk people skills—you can’t be a good manager without them. Mark Kiker, editor of both caddmanager.com and bimmanager.com, covers key character traits and skills that set successful managers apart from the crowd. Learn to identify your strengths, connect with people, and motivate progress.

tinyurl.com/AU-awesome

Tip: Mark recommends keeping end users informed when you’re troubleshooting, and letting them know you have a plan B—good communication is crucial to success. CAD is still used in nearly every AEC project. This class also covers export settings and formats to make all your tools “play nice” together, and includes a downloadable template you can use to make CAD files Revit compatible.

tinyurl.com/AU-standards

Tip: David recommends starting with CAD standards, which address construction documentation, before BIM standards, which address modelling aspects and workflow.

For a complete listing of AU Las Vegas 2015 classes now available for free online, visit au.autodesk.com/au-online

Seamlessly Integrating CAD and BIM Standards for AEC Projects

David Butts leads another informationpacked class all about how to get BIM and CAD tools, work methods, and standards to work together—both ways. Even with Revit leading the way in BIM projects,

(Above) BIM is changing our building, construction, and engineering processes. Source: Autodesk.com

(Below) Autodesk University attendees hone their BIM skills with hands-on learning.

(Bottom) David Butts demonstrates how to integrate CAD and BIM standards.