This is the final version of the article. It first appeared from ICE Publishing at http://dx.doi.org/10.1680/jcoma.16.00034.This briefing presents some of the early findings of the super tall timber project, an innovative research and design project exploring the potential of engineered timber as a structural material in the construction of tall buildings. The project brings researchers together with practising engineers and architects in a process that combines design-led research with research-led design in a constructive feedback loop, providing a new model for addressing the fundamental challenges of designing with a new structural material. The first result of this project is a concept design for a 300 m tall timber building sited in London.The authors also gratefully acknowledge the financial
support of the Engineering and Physical Sciences Research
Council under grant EP/M01679X/1

Foster, Robert M.

This briefing presents some of the early findings of the super tall timber project, an innovative research and design project exploring the potential of engineered timber as a structural material in the construction of tall buildings. The project brings researchers together with practising engineers and architects in a process that combines design-led research with research-led design in a constructive feedback loop, providing a new model for addressing the fundamental challenges of designing with a new structural material. The first result of this project is a concept design for a 300 m tall timber building sited in London

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Briefing: Super tall timber –Oakwood TowerRobert M. Foster MEng, PhDResearch Associate, Department of Architecture, University of Cambridge,Cambridge, UK (corresponding author: rfm41@cam.ac.uk)Michael H. Ramage MArch, PhD, CEng, MIStructESenior Lecturer in Architectural Engineering, Director Centre for NaturalMaterial Innovation, Department of Architecture, University of Cambridge,Cambridge, UKThis briefing presents some of the early findings of the super tall timber project, an innovative research and designproject exploring the potential of engineered timber as a structural material in the construction of tall buildings. Theproject brings researchers together with practising engineers and architects in a process that combines design-ledresearch with research-led design in a constructive feedback loop, providing a new model for addressing thefundamental challenges of designing with a new structural material. The first result of this project is a conceptdesign for a 300 m tall timber building sited in London.1. IntroductionWhile not exactly a new material, engineered timber is notwidely used as a construction material for the design of tallbuildings. At present, the tallest modern building using timberas a primary structural material is the 14-storey Treet buildingin Bergen, Norway. While it is expected that timber will becomeincreasingly common as a structural material for the construc-tion of taller buildings, there is uncertainty as to how thematerial may be best used (Foster et al., 2016).The first of a number of results from the super tall timberproject is a concept design developed with Smith andWallwork engineers and PLP Architecture, sited provocativelyin London’s Barbican complex. The design was presented tothe then Mayor of London as an exemplar of the potential ofengineered timber as a construction material and was promptlydubbed ‘the toothpick’ by the popular press. The building willbe referred to here as Oakwood Tower (Figure 1) and will beused in this briefing to highlight some of the challenges andopportunities presented by engineered timber as a constructionmaterial.2. Self-weightOakwood Tower is designed to be over 300 m tall; this meansthat it meets the definition of super tall as set out by theCouncil on Tall Buildings and Urban Habitats (CTBUH,2016). Tall and super tall building design is characterised bythe increasing influence of lateral and often dynamic loads onthe structural design. Efficient design of such structures thusrequires the designer to direct the permanent vertical loadsinto the lateral load resisting structure. This is expected to beparticularly important for the design of super tall timberbuildings due to the lightness of typical timber construction.Concrete-framed buildings have a typical bulk density of300 kg/m3 (Yang et al., 2004). Steel buildings, including theconcrete decking that often makes up the greater part of theirmass, have a typical bulk density of 160 kg/m3 (Huang et al.,2007). The Treet building has an estimated bulk density of140 kg/m3, including 30 kg/m3 of non-structural concreteballast. This indicates that a timber structure without addi-tional concrete, or a steel structure with a timber floor system,may have considerably less permanent vertical load to resistoverturning and load reversal than conventional tall buildings.While the design approach adopted for this project did notprohibit the use of concrete, the designers preferred to avoidthis initially so as not to side-step what was seen as a funda-mental challenge presented by the use of engineered timber asa construction material. The intention to avoid supplementarymass also follows from the fact that the low self-weight oftimber buildings is likely to provide an important part of theirvalue proposition with regard to reduced foundation costsor even the reuse of existing foundations. As a result, theOakwood Tower design incorporates no concrete in the super-structure and has a bulk density of 125 kg/m3.The importance of directing loads to the lateral load resistingsystem is compounded in a timber structural system due to thereduction in stiffness in typical timber connections when sub-jected to load reversal (Reynolds et al., 2014). This providesa strong incentive to minimise interior vertical load paths bymaximising clear spans and directing vertical load into alateral load resisting system at the perimeter. This has thearchitectural benefit of encouraging a ‘soft’ core that need notbe central or even internal, allowing flexibility in space plan-ning and the consideration of innovative vertical transportation1Construction MaterialsBriefing: Super tall timber – OakwoodTowerFoster and RamageProceedings of the Institution of Civil Engineershttp://dx.doi.org/10.1680/jcoma.16.00034Paper 1600034Received 21/06/2016 Accepted 23/07/2016Keywords: buildings, structures & design/materials technology/timber structuresPublished with permission by the ICE under the CC-BY licencehttp://creativecommons.org/licenses/by/4.0/Downloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/08/16]. Published with permission by the ICE under the CC-BY license solutions. A further architectural desire to lighten the presenceof the tower, while achieving a variable wind profile to resistdynamic excitation, generated the spiral stepping of buildingheight that also allows for buttressing of the central towerby the bundled sub-towers (Figure 2). Although this initiallyappears to lead to inefficient cladding (i.e. poor floor-plate per-imeter to area ratio), this is offset by the improved naturallight penetration and the markedly increased provision ofhigh-value dual-aspect office or residential spaces.3. Mechanical propertiesEngineered timbers such as glued-laminated timber (glulam),cross-laminated timber (CLT) and laminated veneer lumber(LVL) obtain enhanced characteristic values of strength and stiff-ness compared with raw constituent timber material by virtueof homogenisation. Homogenisation refers to the reduction invariability associated with the lamination of n>1 constituentelements such as sawn boards or veneers in a sub-parallelload-sharing arrangement. Layers can also be laid-upcross-wise, as is the case in CLT, providing for enhanceddimensional stability and reduced anisotropy. The thickness ofthe CLT element allows out-of-plane loads to be resisted andreduces the propensity for buckling that has limited the appli-cation of other dimensionally stable panelised wood productssuch as plywood or oriented strand board (Brandner et al.,2015). Figure 3 shows the specific mean stiffness, as mightreflect the stiffness for serviceability design, plotted againstthe specific design compressive strength, as might reflect ulti-mate strength design, for a selection of construction materials.Softwood timbers in particular exhibit specific compressivestrength and stiffness comparable to those of steel, and signifi-cantly higher than of concrete, in the parallel-to-graindirection. The beneficial effect of homogenisation means thatthe strengths of engineered timbers such as glulam and LVLcompare even more favourably. However, like un-homogenisedtimber, glulam and LVL as typically laid-up are highly aniso-tropic and their perpendicular-to-grain strength and stiffnessare very much less than those parallel-to-grain. There is thus atrade-off to be considered in design between efficient use ofFigure 1. Architect’s visualisation of Oakwood Tower.Reproduced with permission, image copyright PLP Architecture2Construction Materials Briefing: Super tall timber – OakwoodTowerFoster and RamageDownloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/08/16]. Published with permission by the ICE under the CC-BY license parallel-to-grain capacity and mitigation of anisotropy throughoff-parallel use of material. This is a trade-off familiar todesigners of fibre-reinforced composites and the transfer ofunderstanding from fibre composite design is likely to be animportant feature of future advanced engineered timber design.Notwithstanding the potential for cross-lamination to mitigatesome anisotropy, linear rather than planar elements were deter-mined to be most efficient for withstanding the very highforces associated with the design of a super tall building. Thisdrove the design towards a perimeter mega-truss system usinghomogeneous glulam or LVL timber elements with steel con-nections. Malo et al. (2016) suggest that such systems mayrequire a similar mass of structural material and exhibit similardynamic behaviour to equivalent steel mega-truss systems.Initial in situ output-only analysis of the dynamic behaviour ofthe Treet building using the random decrement methodsuggested that this may be the case (Reynolds et al., 2016).The use of steel in the connections envisaged for OakwoodTower is a response to the anisotropy of the timber. There isa fundamental requirement at these junctions for a more iso-tropic material, such as steel, to transfer forces betweennon-concentric, non-parallel elements. A consideration thatinformed the design approach adopted here was the recog-nition that concrete buildings typically contain something ofthe order of 100 kg of steel reinforcement per cubic metre ofstructural concrete, meaning that Oakwood could be up toabout 10% steel by mass, without using any more steel than anequivalent concrete building. In reality, tall timber buildingscan be expected to use much less steel than this. An analysis ofSkidmore Owings and Merrill’s (SOM, 2013) design for an ‘alltimber’ redesign of the Dewitt-Chestnut building in Chicagosuggests that a timber design would use only approximatelyone-fifth of the steel of the existing concrete design.4. Fire resistanceLarge solid-timber elements have a high degree of inherent fireresistance against structural failure provided by the propensityof the surface to form a protective charred layer. Charringoccurs at a well-established rate of 0·7–0·8 mm/min in solid-timber materials. While there is no question that further devel-opment of the structural fire design of Oakwood Tower isrequired, the current conceptual design does positively addressa number of key considerations. The vertical and bracingFigure 2. Architect’s birds-eye visualisation of Oakwood Tower.Reproduced with permission, image copyright PLP Architecture3Construction Materials Briefing: Super tall timber – OakwoodTowerFoster and RamageDownloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/08/16]. Published with permission by the ICE under the CC-BY license elements that form the mega-truss and primary structuralsystem are necessarily very large and are preliminarily sized fora combination of serviceability and ultimate considerations. Inthe accidental design case that corresponds to fire, the designloadings and material safety factors are substantially reduced.There is thus a considerable unused capacity in the structuralsection providing a very great thickness of potential sacrificialmaterial for charring. Similarly, the widely spaced, large ele-ments reduce the potential for rekindling.5. RobustnessAn area for further design review of the Oakwood Towerconcept is in relation to robustness against disproportionatecollapse. The current design may not permit sufficient redistri-bution of load by way of alternative load paths in the event ofa corner column removal to permit a demonstration of safetythrough redundancy. These elements would therefore requirefurther special design to demonstrate that they are sufficientlyresistant to accidental actions, in order to justify their use asa key load path in a building of this size. The considerationof alternative load paths is likely to pose a challenge for build-ings using structural timber due to the anisotropy of theelements and the resulting dependence of connection strengthon the direction of loading. However, the relative lightness ofbuildings constructed using timber does mean that the loadsto be redirected are considerably smaller than those for a con-ventional building. The relatively high material safety factorsapplicable to timber also prove beneficial when reduced in anaccidental design situation.6. ConclusionsThe use of timber as a construction material at the scale con-sidered for Oakwood Tower has raised a number of areasrequiring further investigation in relation to the performanceof the engineered material and its use in structural systems. Atthe materials level, these include the effects of size, hygrother-mal variations, time dependency and weathering on materialperformance. At the systems level, they include methods forcreating tight-fit or preloaded connections in order to increasethe stiffness of joints under unidirectional load and under loadreversal, the potential of ‘mega-carpentry’ for the formation ofprincipally compressive element-to-element connections, anddesign for fire and robustness. The favourable specific strengthand stiffness of engineered timber combined with its low densityallows potential for very efficient long-span floor solutions.However, strategies for mitigating the resulting dynamic effects,such as economical tuned mass damper systems, requirefurther development. A further consideration that has recurredin various forms during this preliminary phase of research hasbeen the relationship between material variability and theBasalt aggregate concreteQuartzite aggregate concreteLimestone aggregate concreteSandstone aggregate concreteStructural steelReinforcing steelHardwood EC5Oak EC5American oakAmerican ashAmerican tulipLVL beechSoftwood EC5CLT softwood 1CLT softwood 2Glulam-h EC5Glulam-c EC5LVL softwood35302520Mean Young’s modulus/mean density: MPa/(kg/m3 )1510500 0·01 0·02 0·03 0·04 0·05 0·06 0·07Design compressive strength/mean density: MPa/(kg/m3)Figure 3. Specific stiffness and compressive strength of selectedconstruction materials4Construction Materials Briefing: Super tall timber – OakwoodTowerFoster and RamageDownloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/08/16]. Published with permission by the ICE under the CC-BY license structural system within which that material is used. Eurocode5 (BSI, 2004) does allow limited consideration of redundancyand redistributive capacity of a structural arrangement througha ‘k’ factor. However, there may be potential to develop moreprecise relationships between the sensitivity of a system to theperformance of individual elements and the required variabilityof the properties of the materials constituting those elements.7. Future directionOakwood Tower and the other concept designs developed aspart of the super tall timber project represent only the firstphase of the research. The second phase will involve a series ofexperimental investigations into the actual performance ofengineered timber as a construction material used at the scaleconsidered in these designs. The experimental results will thenbe fed back into the design process so that the design can befurther developed and improved. The authors’ expectation isthat this combination of research-led design with design-ledresearch will lead to a rapid development of understanding ofthe use of engineered timber as a construction material.AcknowledgementsThe authors thank their collaborators in the design ofOakwood Tower: Simon Smith of Smith and Wallwork engi-neers, and Ron Bakker and Kevin Flanagan of PLPArchitecture, for their generous contribution of time and crea-tivity. The authors also gratefully acknowledge the financialsupport of the Engineering and Physical Sciences ResearchCouncil under grant EP/M01679X/1. Data supporting thispaper can be obtained at the University of Cambridge’s insti-tutional repository (http://dx.doi.org/10.17863/CAM.515).REFERENCESBrandner R, Flatscher G, Ringhofer A, Schickhofer G and Thiel A(2015) Cross laminated timber (CLT): overview anddevelopment. European Journal of Wood Products 74(3):331–351.BSI (2004) EN 1995-1-1: Eurocode 5: Design of timberstructures – part 1-1: general – common rules and rulesfor buildings. BSI, London, UK.CTBUH (Council on Tall Buildings and Urban Habitats) (2016)Criteria for the Defining and Measuring of Tall Buildings.CTBUH, Chicago, IL, USA. See http://www.ctbuh.org/LinkClick.aspx?fileticket=zvoB1S4nMug%3d&tabid=446&language=en-US (accessed 29/04/2016).Foster RM, Reynolds TPS and Ramage MH (2016) Proposal fordefining a tall, timber building. Journal of StructuralEngineering, http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0001615.Huang S, Li QS and Xu S (2007) Numerical evaluation ofwind effects on a tall steel building by CFD. Journal ofConstructional Steel Research 63(5): 612–627.Malo KA, Abrahamsen RB and Bjertnaes MA (2016) Somestructural design issues of the 14-storey timber framedbuilding ‘Treet’ in Norway. European Journal of Wood andWood Products 74(3): 407–424.Reynolds T, Harris R and Chang WS (2014) Stiffness ofdowel-type timber connections under pre-yield oscillatingloads. Engineering Structures 65(21): 21–29.Reynolds TPS, Feldmann A, Harris R, Chang WS and Dietsch P(2016) Design parameters for lateral vibration ofmulti-storey timber buildings. In InternationalNetwork of Timber Engineering Research, Graz, Austria(Rainer Gorlacher (ed.)). Timber Scientific Publishing,Karlesruhe, Germany.SOM (Skidmore Owings and Merrill) (2013) Timber TowerResearch Project: Final Report. SOM, Chicago, IL, USA.Yang J, Agrawal A, Samali B and Wu J (2004) Benchmarkproblem for response control of wind-excited tallbuildings. ASCE Journal of Engineering Mechanics 130(4):437–446.HOW CAN YOU CONTRIBUTE?To discuss this briefing, please email up to 500 words tothe editor at journals@ice.org.uk. Your contribution willbe forwarded to the author(s) for a reply and, if con-sidered appropriate by the editorial board, it will be pub-lished as discussion in a future issue of the journal.Proceedings journals rely entirely on contributions fromthe civil engineering profession (and allied disciplines).Information about how to submit your paper onlineis available at www.icevirtuallibrary.com/page/authors,where you will also find detailed author guidelines.5Construction Materials Briefing: Super tall timber – OakwoodTowerFoster and RamageDownloaded by [ UNIVERSITY OF CAMBRIDGE] on [23/08/16]. Published with permission by the ICE under the CC-BY license 

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