Feature Performance-based design thestructuralengineer.org

Machines for living – true performance-based design

Every engineer knows that we need to of ‘doing better than last time’ on every Pete Winslow design safe and usable structures. First, project; not just the privileged few that are MEng, MA, PhD, CEng, this means using suffi cient material for publicised at the Structural Awards and in MIStructE strength and stability, i.e. satisfying the The Structural Engineer. Associate, Expedition ultimate limit state (ULS) – be it in steel, One doesn’t have to look far to see Engineering, London, UK concrete, timber or some other material. some obvious situations where we should The second fundamental principle, and challenge the approach to serviceability one which appears in all modern design movements, and instead look at the true Synopsis codes, is to design structures that are stiff performance requirements. Consider the Are codifi ed serviceability and enough, i.e. that satisfy the serviceability football stadium structure which only really stiff ness criteria appropriate in limit state (SLS), so that they don’t move or needs to meet stiff ness requirements a world increasingly focused on deform ‘too much’ under some statistically for 90 minutes once per fortnight, or an calculated characteristic loads. offi ce fl oor which, in reality, only sees reducing material use and embodied But in many cases, this traditional a fraction of the characteristic design carbon? Could new approaches to approach to serviceability and stiff ness load. These are the types of applications loading and movement off er ways needs far more material to be added than where, in my opinion, thinking based on to meet these goals? Does modern is required just for safety. Is this overkill for fi rst principles and a research approach to technology create the potential for real-world situations? Are there better ways problem solving would complement existing adaptive designs which actively of achieving the required performance? Is eff orts to bring about a new paradigm for L/250 really a good proxy for usability? performance-based design and material control defl ections? This paper With a trend for higher-strength (but use. explores ways in which the structural little stiff er) materials, longer spans, more Over recent years at Expedition we engineering profession could slender structures (Figure 1) and a drive have started to think about a number of move towards a truly performance- for reducing embodied carbon wherever the underlying principles and how to tackle based design approach, drawing possible, it is becoming increasingly them diff erently. These fall into three broad important that we, as a profession, tackle areas in which we look to: on examples from the work of these questions. In fact, I believe that the  challenge statistics on loading Expedition Engineering and simple fi rst step is to ask these questions  control movement in new ways other engineers. on every project, and to develop an ethos  relax serviceability criteria; or ‘let it move’.

Figure 1 Dune Grass by Atelier One1; high-deformation structures that still meet ULS requirements CALFLIER001 / WIKIMEDIA COMMONS / CC BY-SA 2.0 BY-SA CC / COMMONS WIKIMEDIA / CALFLIER001

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Challenge statistics climate-change/sustainable on loading energy debate has for some time The example of challenging – to reduce our resource use offi ce fl oor loading is interesting while still achieving appropriate to explore. This has been the performance. repeated focus of many diff erent practitioners and academics over Figure 2 Control movement in 6m long adaptive the years – starting with Dunham2 truss structure, IASS new ways Amsterdam EXPO 3

in 1947, Stanhope in 1992 and ENGINEERING EXPEDITION Other fi elds – be it automotive, 2004, Alexander4 in 2002, not aerospace, or the natural world forgetting Chris Wise’s IABSE Milne Medal – do not simply throw more ‘passive’ material Lecture5 in 2010 and, no doubt, others since – "THERE IS AN OBVIOUS at a problem to control movement. With 21st seemingly with little impact on the majority of century technologies in sensors, control and built projects. However, this work has tended ADVANTAGE AS REGARDS actuation, active control has real potential. to be principled truth-seeking, i.e. determining FLEXIBILITY OF USE TO Expedition has been collaborating with ‘real’ loads. DESIGN THE BUILDING SO researchers Dr Gennaro Senatore and Dr Challenging and changing the accepted THAT THESE MAXIMUM Phillippe Duff our at University College London norm for applied loading is easier if we focus LOADS CAN BE PERMITTED to investigate adaptive building structures7. on optimising the design for serviceability ANYWHERE. THERE IS JUST In addition to technical papers – which have only, rather than for all conditions. For the AS OBVIOUSLY A LACK OF their place but, I have found, rarely initiate real moment, let’s park the idea of changing change in practice – a signifi cant outcome is loading for ULS (since a large body of ECONOMY IN PROVIDING a large interactive prototype adaptive truss precedent indicates that current approaches STRENGTH THROUGH THE (Figure 2). are, in general, very safe). This pragmatic STRUCTURE THAT WILL NOT This 6m long cantilever with an incredibly focus on SLS loads is where we should take a BE USED IN 99% OF THE slender 40:1 length-to-depth ratio is sized for path of least resistance. ULS using high-strength steel. For controlling BUILDING" J.W. Dunham, 1947 In a sense, there are two aspects to SLS movements it then uses a network challenge: of strain gauges, a sophisticated control  an accepted norm where many clients value into code approaches? In the short-to- system and 10 fail-safe electric actuators. fl oor capacities in excess of the Eurocode- medium term this might only really be The actuators, embedded within the diagonal specifi ed 2.5kN/m2 + partitions appropriate for non-safety critical aspects of tension elements of the truss, shorten or  a failure to implement the research which the design. It would be achieved in practice lengthen to ensure overall defl ections remain shows that even 2.5kN/m2 is excessive for by using a shorter design life for SLS and less than 2mm as a person walks along the real offi ces. accepting that there is a chance of higher catwalk-style cantilever structure. defl ections, on rare occasions. This technological performance-based It is worth noting that, although in some This refocus on the SLS loading statistics, approach can actually go further still: the isolated cases the use of higher fl oor rather than both ULS and SLS, would make it passive steelwork is carefully designed so capacities arises from careful thought (e.g. if easier to get buy-in from project stakeholders. as not to defl ect excessively under small a change from offi ce to retail use is assessed In many cases, it is serviceability which (frequent) loads. The actuators only need as likely in the future), anecdotally this is governs the design anyway – be it defl ections to move occasionally, when higher loads mostly not the case. Even if a future change or vibrations. The Institution’s Research occur, thereby expending minimal electrical of use is deemed a signifi cant possibility, Panel now has an ‘industry focused’ research energy over the whole life of the structure. given the likelihood of incorrect predictions, a challenge in this broad topic area6, and we’d In practical terms for, say, a canopy roof more refi ned approach should be developed. love to hear from people interested (whether structure, the actuators would kick in for the This would involve strategically reinforcing or not they are applying for a seed grant). biggest windstorm of the year, but remain components which are easy to modify now However, it is likely that even the most switched off for typical winds seen on a daily but will be hard to modify in future (e.g. creative technical research on loading basis. The team has shown that, compared foundations, columns, key connections), statistics will still only go some way towards to a conventional I-beam cantilever, this can and designing other elements (e.g. beams, actually delivering value to the client and to reduce mass by 80% and whole-life energy slabs) just for loads required today while society in general. Technical studies need to (embodied plus operational) by 60–70%. checking there would be space available for go hand in hand with socioeconomic studies. The prototype received acclaim at the strengthening should it be required in many Perhaps engineering institutions and funding International Association of Shell and Spatial years’ time. bodies should embark on a programme Structures EXPO in 2015, and also at the Clearly, other types of loading can be of understanding and enlightening clients, Building Centre, London, in 2016. We hope challenged in a similar manner, be it snow, developers and letting agents (often cited that you will take forward the ideas and wind, thermal, etc. For example, it is very as a signifi cant barrier to reducing loading principles in your engineering thinking, R&D often the case that site-specifi c wind tunnel allowances for offi ces); a Class A ceoffi and construction projects – more details can testing gives design wind pressures lower can still be achieved even when the load be found online8. than the code. Since it is not practical to use allowance is substantially less than 4 + One might reasonably ask whether sophisticated physical testing methods on 1kN/m2! We should focus on demand-side there are simpler, cheaper or easier ways every project, could this not be fed through changes – in much the same way that the to achieve the same physical benefi ts. In

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the short term, adaptive designs could be evidence base that supports use of defl ection thermal, seismic and settlement-induced applied directly to a small number of hi-tech criteria such as L/250, L/360, L/500 (which are loads. pavilions and boutique structures where such essentially a proxy measurement of outcome Of course, it needed care regarding questions are of secondary importance. In – cracking of fi nishes, comfort, etc.)? If we detailing of the glass-panel photovoltaics, and the longer term, we as engineers may well made a medical analogy, the National Institute invention of a special services riser to protect be reducing the environmental impact of our for Health and Care Excellence would tell us the high-voltage cable from large relative structures in any way possible – something that we must conduct trials to determine what movements between independent structural examined in more detail by Giesekam et outcome is actually required, how it can be zones. But the benefi t was a signifi cant al.9. However, from my experience bringing properly measured and what cost-eff ective reduction in material in combination with together R&D and design on construction ‘dose’ of material is needed to produce a realising the architect’s vision. At present, projects, it is likely that our built environment particular as-built outcome. a system of pressure and displacement will benefi t from this adaptive technology in a In addition to looking at serviceability sensors is logging to a web server to help the manner very diff erent to that fi rst anticipated. criteria, understanding the fi nancial costs project team to evaluate and learn from real All projects should take benefi t from the of diff erent levels of SLS loading would performance. philosophy of decoupling ULS and SLS be another good starting point. Building It is encouraging to see that there is design and going back to fi rst principles. to a lower SLS design load and accepting growing interest in this topic area in the Some of those projects might benefi t from a slightly higher probability of, say, brittle academic research community – e.g. it a limited adaptive system; be it a single fi nishes cracking would probably make is a major theme of the GW4 university range fi nder instead of 200 strain gauges, fi nancial sense, and could be optimised with collaboration programme between Bath, or a single actuator to adequately control relatively simple ‘reliability analysis’ style Bristol, Exeter and Cardiff university defl ection rather than 10 controlling to modelling; encompassing material costs, engineering departments. laboratory precision. maintenance costs and load statistics. We, as an industry, should now move A project where the designer can fi nd A recent project Expedition has been beyond doing this kind of thing for just synergies would likely realise the largest involved in – the Stavros Niarchos Foundation (a) landmark one-off projects or (b) to benefi ts of adaptive design philosophies. Cultural Centre, Athens10 (Figure 3) – gives a enable a new class of tall buildings (sexy, For example, since high-strength steel has glimpse of what can be achieved in this area. publicity-grabbing subjects which make the same elastic modulus as regular steel, The long-span photovoltaic canopy structure the extra eff ort more easily justifi able). If combining it with adaptive systems for has a network of novel viscous-polymer the engineering model says that there is a defl ection/vibration control will enable it to springs forming part of its support and possibility of plaster in my house cracking in be used to its full potential. In areas where stability system, with resulting movements a 10-year load event rather than a 50-year settlements are a concern, foundations are three to four times greater than might load event, does it really matter? Do we likely to be subject to surveying/monitoring conventionally be allowed. This reduces know that well-being and productivity in a anyway, so combining conventional offi ce space with a low-tech movement will actually suff er if the compensation system Figure 3 vibration levels are, say Canopy with integrated would be a relatively spring-movement R=16 rather than R=8? system, Stavros Niarchos small step. Separating Foundation Cultural The answer is probably SLS from ULS in this Centre, Athens ‘not sure’, because the way and designing the construction industry is foundation itself for only organised in such a way ULS conditions could, that far too little post- for example, result in occupancy evaluation and signifi cantly shorter learning takes place (from piles, or even small a structural engineering pads instead of piles. perspective at least). To realise this potential Yes, this can be benefi t, we need research about cool tech, sensor into ‘productising’ networks, dynamic and mass-producing movement, research adjustable black box-style and adaptive systems. devices, for permanent But it’s also more than foundations or temporary that. It’s about all of works (not necessarily us as a profession a new idea, but typically challenging traditional done only on bespoke engineering philosophies. one-off projects). It’s learning how to do things better, towards a Let it move truly performance-based Is movement such a bad design approach which thing? Do we actually uses fi nite resources

have a good, current DENANC MICHEL intelligently.

12 March 2017 | TheStructuralEngineer

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Acknowledgements Pete Winslow leads Expedition London: Stanhope Properties thinking/a-new-design-philosophy- Engineering’s R&D initiatives and is adaptive-structures/ (Accessed: September a member of the Research Panel of E4) Alexander S.J. (2002) ‘Imposed fl oor 2016) The Institution of Structural Engineers. loading for offi ces: a re-appraisal’, The This article arose out of a request by Structural Engineer, 80 (23/24), pp. 35–45 E9) Giesekam J., Barrett J. and Taylor the Institution’s Structural Futures P. (In press) ‘Scenario analysis of Committee for a contribution on new E5) Wise C. (2010) IABSE Milne Medal embodied greenhouse gas emissions in structural paradigms. Lecture: Enough is enough [Online] Available UK construction’, Proc. ICE – Engineering at: http://istructe.adobeconnect.com/ Sustainability REFERENCES p87368980/ (Accessed: February 2017) E10) Frearson A. (2016) Renzo Piano E1) Calfl ier001 (2012) Blackpool E6) Institution of Structural Engineers completes Stavros Niarchos Foundation Seafront July 2012 [Online] Available at: (s.d.) Research [Online] Available at: Cultural Center on a huge artifi cial hill https://commons.wikimedia.org/wiki/ www.istructe.org/resources-centre/ [Online] Available at: www.dezeen. File:BLACKPOOL_SEAFRONT_JULY_2012_ technical-topic-areas/research (Accessed: com/2016/06/28/renzo-piano-stavros- (7581633936).jpg (Accessed: February September 2016) niarchos-foundation-cultural-center- 2017) athens-huge-artifi cial-hill/ (Accessed: E7) Senatore G., Duff our P., Hanna S., September 2016) E2) Dunham J.W. (1947) ‘Design live loads in Labbé F. and Winslow P. (2011) ‘Adaptive buildings’, Trans. ASCE, 112, pp. 725–745 structures for whole life energy savings’, J. IASS, 52 (170), pp. 233–240 E3) Fitzpatrick A., Mathys J., Taylor A. HAVE YOUR SAY and Johnson R. (1992) An assessment E8) Winslow P. (2016) A new design To comment on this article: of the imposed loading needs for current philosophy – adaptive structures [Online] Eemail Verulam at [email protected] commercial offi ce buildings in Great Britain, Available at: http://expedition.uk.com/ Etweet @IStructE #TheStructuralEngineer

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