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Dynamic response of tall timber Buildings under service load – the DynaTTB research program

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Wind-induced dynamic excitation is becoming a governing design action determining size and shape of modern Tall Timber Buildings (TTBs). The wind actions generate dynamic loading, causing discomfort or annoyance for occupants due to the perceived horizontal sway – i.e. vibration serviceability failure. Although some TTBs have been instrumented and measured to estimate their key dynamic properties (natural frequencies and damping), no systematic evaluation of dynamic performance pertinent to wind loading has been performed for the new and evolving construction technology used in TTBs. The DynaTTB project, funded by the Forest Value research program, mixes on site measurements on existing buildings excited by heavy shakers, for identification of the structural system, with laboratory identification of building elements mechanical features coupled with numerical modelling of timber structures. The goal is to identify and quantify the causes of vibration energy dissipation in modern TTBs and provide key elements to FE modelers.
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EURODYN 2020
XI International Conference on Structural Dynamics
M. Papadrakakis, M. Fragiadakis, C. Papadimitriou (eds.)
Athens, Greece, 2224 June 2020
DYNAMIC RESPONSE OF TALL TIMBER BUILDINGS UNDER
SERVICE LOAD THE DYNATTB RESEARCH PROGRAM
Rune Abrahamsen
1
, Magne A Bjertnæs2, Jacques Bouillot3, Bostjan Brank4, Lionel
Cabaton5, Roberto Crocetti6, Olivier Flamand7, Fabien Garains3, Igor Gavric8, Olivier
Germain9, Ludwig Hahusseau3, Stephane Hameury7, Marie Johansson10, Thomas Jo-
hansson6, Wai Kei Ao11, Blaž Kurent4, Pierre Landel10, Andreas Linderholt12, Kjell
Malo13, Manuel Manthey7, Petter Nåvik2, Alex Pavic11, Fernando Perez14, Anders
Rönnquist13, Haris Stamatopoulos13, Iztok Sustersic8, Salue Tulebekova13
Corresponding Author: Olivier Flamand
CSTB
olivier.flamand@cstb.fr
Other addresses in the footnote below.
Keywords: Timber building, wind load, discomfort, modelling, damping, full scale.
Abstract. Wind-induced dynamic excitation is becoming a governing design action determin-
ing size and shape of modern Tall Timber Buildings (TTBs). The wind actions generate dynamic
loading, causing discomfort or annoyance for occupants due to the perceived horizontal sway
i.e. vibration serviceability failure. Although some TTBs have been instrumented and meas-
ured to estimate their key dynamic properties (natural frequencies and damping), no systematic
evaluation of dynamic performance pertinent to wind loading has been performed for the new
and evolving construction technology used in TTBs. The DynaTTB project, funded by the Forest
Value research program, mixes on site measurements on existing buildings excited by heavy
shakers, for identification of the structural system, with laboratory identification of building
elements mechanical features coupled with numerical modelling of timber structures. The goal
is to identify and quantify the causes of vibration energy dissipation in modern TTBs and pro-
vide key elements to FE modelers.
The first building, from a list of 8, was modelled and tested at full scale in December 2019.
Some results are presented in this paper. Four other buildings will be modelled and tested in
spring 2020.
1
Moelven Limtre, rune.abrahamsen@moelven.no, 2 Sweco, magne.bjertnaes@sweco.no, petter.navik@sweco.no, 3 Eiffage,
jacques.bouillot@eiffage.com, ludwig.hahusseau@eiffage.com, 4 Uni Ljubjana, Bostjan.Brank@ikpir.fgg.uni-lj.si, bkurent@fgg.uni-lj.si, 5
Arbonis, lcabaton@arbonis.com, fgarains@arbonis.com, 6 Moelven SE, crocetti@kth.se, thomas.johansson@moelven.se, 7 CSTB,
olivier.flamand@cstb.fr, stephane.hameury@cstb.fr, manuel.manthey@cstb.fr, 8 InnoRenew CoE, igor.gavric@innorenew.eu,
iztok.sustersic@innorenew.eu, 9 Galeo, o.germain@galeo.fr, 10 RISE, marie.johansson@ri.se, pierre.landel@ri.se, 11 Uni Exeter, wka203@ex-
eter.ac.uk, A.Pavic@exeter.ac.uk 12 LNU, andreas.linderholt@lnu.se, 13 NTNU, kjell.malo@ntnu.no, anders.ronnquist@ntnu.no, haris.sta-
matopoulos@ntnu.no, saule.tulebekova@ntnu.no 14 Smith&Wallwork, fernando.perez@smithandwallwork.com
Corresponding author: Olivier Flamand
1 INTRODUCTION
1.1 Background
Due to population expansion and concentration in urban regions, housing shortages are an
increasing global issue, necessitating the efficient use of space in urban areas. One solution is
an increased supply of tall buildings to optimize the best use of limited space. This has sustain-
ability impacts surrounding the choice of materials and carbon footprint ramifications. Using
more wood-based products in building construction, as a renewable raw material, could assist
in the transition of society towards a circular based bio-economy, moving away from a depend-
ence on fossil fuels and extractive materials, whilst helping meet the commitments contained
within the UN Sustainable Development Goals. The use of tall buildings with timber structures
would be an opportunity to create value and impact by encouraging the increased use of forest-
based replenishable products. Tall Timber Buildings could provide cost effective, environmen-
tally sustainable developments, maximising the limited space available in urban regions.
Tall Timber Buildings (> 10 storeys) is still a very new concept, some examples are:
Murray Grove (London, UK (2009)) 9 storeys,
Forté (Melbourne, Australia (2012)) 10 storeys,
Treet (Bergen, Norway (2015)) 14 storeys,
Brook Commons (Vancouver, Canada (2017)) 18 storeys,
Origine (Quebec City, Canada (2018)) 13 storeys and
Mjøstårnet (Brumunddal, Norway (2019)) 18 storeys.
The load-bearing system of these buildings are very different giving the buildings different
properties. Building tall means several new challenges such as higher loads vertically and hor-
izontally, earthquake loads, fire safety and increased needs regarding technical systems as some
examples. One aspect that is of special interest is the dynamic behavior related to wind-loads
that leads to a need to limit acceleration levels at the top of buildings. This is a factor that has
shown to govern the design of the stabilization system of tall timber buildings from a height of
12-14 storeys and above, see [1] for example. The understanding of the wind-induced dynamic
behaviour of Tall Timber Buildings (TTBs) and their components is till poor and results in a
lack of confidence by designers in the use of timber as a construction material. A lack of reliable
data for modelling is one of the main barriers in the further development of TTBs and wider
utilisation of timber within construction.
This lack of reliable data regarding modelling wind-induced vibrations for tall timber build-
ings was the motivation for starting the ForestValue project “DynaTTB - Dynamic Response
of Tall Timber Buildings under Service Loads”. The project will be performed by partners from
academia, research institutes and companies
2
from five different countries in Europe during
2019-2022. The research hypothesis for this project is that it is possible to create computational
models for Tall Timber Buildings, based on system identification and calibration of advanced
Finite Element (FE) models. This will be underpinned by data from full-scale tests of a number
of representative mid-to-high rise timber buildings in Norway, Sweden, France, Slovenia and
UK. The project plan will utilize unique horizontal electro-dynamic sliding shakers from the
University of Exeter (UK) and CSTB (France) to perform vibrational tests and use the data to
2
RISE Research Institutes of Sweden, NTNU Norwegian University of Science and Technology, University of
Exeter, University of Ljubljana, InnoRenew Renewable Materials and Healthy Environments Research and Inno-
vation Centre of Excellence (InnoRenew CoE), CSTB Centre Scientifique et Technique du Bâtiment, Linnaeus
University, Moelven Töreboda AB, Moelven Limtre AS, SWECO Norge AS avd Lillehammer, Smith and Wall-
work Engineers Ltd, Galeo, Eiffage Immobilier Sud Ouest, Arbonis.
M. Johansson, M. Manthey and O. Flamand
estimate Frequency Response Functions (FRFs) for a number of TTBs with different building
systems.
The overall objective of the project is to identify experimentally a number of full-scale TTB
structures within Europe and, based on these results, develop representative FE-models
for predicting the vibration response of TTBs exposed to wind-induced dynamic loading.
This paper will present the structure of the project and the first preliminary results from the first
two tested buildings.
1.2 State-of-the-art
There are several all timber building systems that can be used for the stabilizing system in
TTBs. In principle these systems can be made up of one-dimensional elements in the form of
beams and columns, mostly made of glulam, or two-dimensional elements such as cross lami-
nated timber (CLT) or laminated veneer lumber (LVL). The one-dimensional elements can be
used in moment-resisting frames or as trusses while the two-dimensional elements are used as
shear walls to stabilize the buildings, see Figure 1. It is also possible to use hybrid structures
with parts of the building system in steel or concrete to help support the building against espe-
cially horizontal loads. CLT elements are typically connected using shear or tension angular
steel brackets and self-tapping screws [2]. The glulam members are typically jointed using dow-
elled connections with slotted-in steel plates - a jointing technique successfully applied in large-
span timber bridges and sport arenas.
Hybrid system
© Marie Johansson
Figure 1: Principal buildings systems for Tall Timber Buildings, a) moment-resisting frames, b) truss-systems, c)
shear wall systems and d) hybrid system combining timber with other materials.
TTBs have, in principle, sufficient strength capacity to resist lateral loads (e.g. wind, earth-
quake) for the ultimate limit state. However, instead the vibration serviceability limit state (SLS)
governs the design, leading to the need to restrict wind-induced sway vibrations to within cer-
tain limits [4, 5]. The lowest natural frequencies, dependent on mass and stiffness, of the TTB
sway motion is in same frequency range as the wind spectra. The sway is, however, also largely
dependent on the damping of the structure. Timber is a light material with only moderate stiff-
ness and hence the fulfilment of the vibration SLS criteria, due to wind, results in restrictions
to the total building height [6, 7].
The amount of sway/acceleration depends on the mass and stiffness distribution of TTB
structures and the ability to dissipate vibrational energy from the structural system. Currently,
the knowledge on structural stiffness and damping in TTBs is limited, particularly regarding
the effects of different types of connections used in the load-carrying systems. These connec-
tions are crucial generators of stiffness and damping, yet little is known as to their impact on
TTB structures. Current modal damping values used in the design of TTBs are based on guess
work, with minimal underpinning rational or scientific basis.
Corresponding author: Olivier Flamand
Two major contributors to damping are: material damping and structural damping. Material
damping arises from the internal friction within the material of a timber element, whilst struc-
tural damping is due to friction and energy dissipation in the connections. Non-structural ele-
ments and their connections are also assumed to contribute to the total damping. Timber
structures are lightly damped systems and, therefore, due to the nature of resonant response
calculations, a small change in the damping ratio can lead to significant changes in the vibration
response and overall serviceability performance.
Although some TTBs have been instrumented and measured to estimate their key dynamical
properties (natural frequencies, mode shapes and damping) [3], no systematic evaluation of
dynamic performance pertinent to wind loading, has been performed for the innovative and
evolving TTB construction technology. Knowing that the wind response calculations are highly
sensitive to the damping values and natural frequencies, indicating that small variations in these
uncertain modelling parameters may yield vastly different responses on either side of the ac-
ceptable vibration response values.
Full-scale tests on TTBs has been done using Operational Modal Analysis (OMA), also ex-
pressed as Ambient Vibration Testing (AVT), where the dynamic response is measured without
knowledge of the load, [3, 8] for example). This technique gives relatively good results for
natural frequencies and mode shapes but offers considerably less reliable values for damping.
Using Forced Vibration Testing (FVT) gives the possibility of controlling the load level and
thereby establishing Frequency Response Functions (FRFs) [9]. Using FVT gives the possibil-
ity of measuring over a range of frequencies and will result in better understanding of the dy-
namic response of TTBs as a function of excitation frequency, which is important for stochastic
wind loading containing multiple frequencies. The FRFs also give a better base for model cal-
ibration of the FE-models of TTBs. The partners at University of Exeter and CSTB have unique
equipment and knowledge of measuring dynamical properties of other types of structures such
as large civil engineering structures, bridges and floor structures [10, 11] which will be used
for TTBs in this project to provide excellent opportunities to establish unique and vital data.
FE models of full-scale TTBs have numerous uncertainties, for example, whereabouts stiff-
ness and damping occur. Improved knowledge of the dynamic behaviour of TTBs can be de-
veloped through dynamic tests on full-scale building structures (in-situ), but these are time
consuming and costly. The goal of this project is to limit the required number of tests and de-
velop simulation tools such as reliable FE models.
2 PROJECT PLAN
The main objective of the project is to identify experimentally a number of full-scale TTB
structures (existing or currently being built) and, based on these, develop representative FE-
models for predicting the vibration response of TTBs exposed to dynamic loading due to wind.
Figure 2 includes 8 TTBs (plus a timber bridge) that has been identified as potential candidates.
In most cases the companies designing and building the structures are involved in the project
as industry partners making drawings and calculation models available. Table 1 gives some
data for the buildings such as height, width and depth as well as their main building system.
M. Johansson, M. Manthey and O. Flamand
Figure 2: Map of Europe with the buildings that will be measured.
Building
name
Country
Building
height
Building
depth
Load-bearing
system
Stabilising ele-
ments
Mjøstårnet
Norway
85.4
16.3
Glulam
Trusses
Treet
Norway
46
Glulam+ strong
concrete floors
Trusses
Timber bridge
(Fjell-leet)
Norway
-
Glulam
Trusses
La Tour Hype-
rion
France
57
19.1
Glulam + CLT
Concrete Core
Treed It
France
36
18.6
Glulam + Tim-
ber Concrete
Slab
Concrete Core
Balcons en
forêt
France
FlowerValley
Slovenia
12.7
21.2
CLT
Shear walls
Kv Eken
Sweden
24.4
19
Glulam
Trusses
Yoker
UK
22
28
CLT
CLT Shear
walls
Table 1: Selected data about the building included measurements and main load-bearing system.
The buildings represent different modern timber building techniques and offer good exam-
ples of a variety of building types, providing a range in dynamic response for calibration and
reliability in the modelling work. The buildings are dispersed across Europe providing a geo-
graphic spread. A bridge is included being an excellent example as to a similar type of glulam
structure, including dowel connections, but with less non-structural elements. This will support
research into the effect of connections versus non-structural elements.
The work in the project is divided into three interrelated main work-packages (WP2-4) and
two supporting work-packages 1 and 5, see Figure 2. Work-packages 2 and 3 are experimentally
based, to improve the understanding of the real behaviour of complete buildings measured in-
situ (WP3) and parts of the structural system (components, connections and sub-assemblies),
known to influence the dynamic response, as measured in the lab (WP2).
Work-package 4 includes the modelling aspects of the project, with a starting point from
using best engineering judgement information to develop models of the buildings to predict
load levels and initial estimates of the buildings dynamic responses. The data from WP2 will
Image: Smith&Wa llwork
Image: Moelven
Image: Moelven
Image: Smith&Wa llwork
Image: Eiffage
Image: Arbonis
Treet, 14 stories,
glulam+volume elem ents Yoker, 7 storeys, C LT
Hyperion, 18 storey s, hybrid + CLT
Treed-IT, 12 storey s,
Image: Mariehus
Eken, 6 storeys, Glula m
Mjøstårnet, 18 store ys,
Glulam
Image: InnoRenew Co E
Karantanika, 4 storey s,
CLT
Corresponding author: Olivier Flamand
create detailed FE-models of the connections and sub-assemblies simulating the measured be-
haviour. These models will be simplified and used for model calibration of the FE-model for
the complete buildings, whilst also being calibrated against measurement data for the complete
buildings.
Figure 3: Project structure WP2 - Laboratory experiments and WP3 - In-situ Measurements will run in parallel
whilst WP4 - Modelling supports the measurement WPs and is calibrated with data from measurements. WP1 -
Project management and WP5 - Dissemination will run during the whole project time for support and exploita-
tion of the results.
The project is still on-going and so far, measurements using forced vibrations have been
conducted on two buildings, Treed-It in Paris and Yoker in Glasgow. Preliminary results from
the measurements and modelling of these two buildings will be presented in this paper as well
as an overview of the measurement techniques and modelling techniques used.
3 EXAMPLES OF BUILDINGS AND RESULTS
3.1 The Treed-It Building - structure
The Treed-It building is 12 story building with the first podium story in concrete and then
11 stories with a glulam structure. The building is stabilized against horizontal loads with an
elevator shaft in concrete. The building is located in Champs-sur-Marne and is constructed by
Vinci Engineering with Arbonis as the main contractor for the timber parts of the structure. The
main part of the construction work has been done during 2019 with the timber structure being
raised during July to October 2019. The building will be finished June 2020.
M. Johansson, M. Manthey and O. Flamand
Figure 4: The structure of the Treed-It building with a concrete core and a glulam structure around it and mix
timber-concrete floor slabs.
3.2 Modelling of Treed-It
A FE-model of the Treed-It building has been built using Ansys by CSTB.The structure is
modelled using beams for the timber part and shells for the concrete and mix parts. As the
building was not totally completed at the time of the dynamic measurements, the modelling
takes into account the missing dead loads corresponding to inner partition that were not in place
when natural frequencies have been measured. Oppositely, the stiffness of the structure was
already the same as the final one. Cladding, that was already finished, is considered as bearing
only load and no rigidity because façade is not contributing to the horizontal strength.
Figure 5: Ansys model of the Treed-It building by CSTB, using beam elements for the columns and beams and
shells for the slabs and the concrete core.
Corresponding author: Olivier Flamand
Figure 6: Dynamic analysis of the Treed-It Building performed by CSTB before the real testing.
M. Johansson, M. Manthey and O. Flamand
3.3 Testing of Treed-It
The dynamic properties of the Treed-It Building was measured by CSTB using a horizontal
mass shaker with a moving load of 500kg.The shaker was lifted in one piece to the top floor
by a tower crane. With a stroke of 240mm the sinusoidal force applied to the building was up
to 8000N. This applied force increases with the square of the frequency, as it is an inertial ex-
citer.
Figure 7: Horizontal force applied by the mass shaker for a given stroke (left) and shaker in
operation on the Treed It Building (right)
Three locations of the shaker have been tested. Accelerometers were placed at three levels
(11th, 7th and 2nd floor) to measure the amplitude of floor displacements, some displacement
sensors (LVDTs) put between timber beam at the 7th level to measure the deformation of the
structure. The preliminary analysis of the time domain signal show damping ratio close to 2%
of critical, calculated from the decay of the acceleration signals.
Figure 7: An example of decay of an accelerogram, amplitude in m/s²
Corresponding author: Olivier Flamand
4 CONCLUSIONS
The DynaTTB research program will provide valuable information concerning the FE
modelling of high rise timber buildings, to be used by designers for assessing the comfort
of final users. The modeling of structural damping of such structures is one of the main
input of this European research.
5 FURTHER WORK
A FE-model of the Yoker building, which is a 7 storey building in Glasgow designed by
Smith and Wallwork Ltd, made using a CLT structure, has been modelled using Ansys by Uni-
versity of Ljubljana. The dynamic properties of the Yoker building was measured Feb 2020 by
University of Exeter using horizontal shaker. Data processing is under way.
An extensive measurement campaign in Scandinavia is planned late summer 2020.
6 ACKNOWLEDGEMENT
The research leading to these results has received funding from the ForestValue Research
Programme which is a transnational research, development and innovation programme jointly
funded by national funding organisations within the framework of the ERA-NET Cofund
‘ForestValue – Innovating forest-based bioeconomy.
REFERENCES
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of the 14-storey timber framed building “Treet” in Norway. European Journal of Wood
and Wood Products, 74(3), pp.407424.
[2] Brandner, R., Flatscher, G., Ringhofer, A., Schickhofer, G., Thiel, A. 2015. Cross lami-
nated timber (CLT): overview and development. Holz als Roh- und Werkstoff 74(3).
[3] Feldmann, A., Huang, H., Chang, W., Harris, R., Dietsch, P., Gräfe, M., Hein, C. 2016.
Dynamic properties of tall timber structures under wind-induced vibration. In WCTE
2016 - Word Conference on Timber Engineering. Vienna, Austria.
[4] Anon 2005. SS EN 1991-1-4:2005 Eurocode 1: Actions on structures Part 1-4: General
Actions Wind actions, SIS (2002).
[5] Anon 2008. SS-ISO 10137:2008, Basis for Design of StructuresServiceability of Build-
ings and Walkways against Vibration, ISO (2008).
[6] Johansson, M. et al., 2016. Tall timber buildings a preliminary study of wind- induced
vibrations of a 22-storey building. In WCTE 2016 - Word Conference on Timber Engi-
neering. Vienna, Austria.
[7] Reynolds, T., Casagrande, D. & Tomasi, R., 2016. Comparison of multi-storey cross-
laminated timber and timber frame buildings by in situ modal analysis. Construction and
Building Materials, 102, pp.10091017.
[8] Fjeld Olsen, M., Hansen, O. 2016. Measuring vibrations and assessing dynamic proper-
ties of tall timber buildings, Master thesis NTNU, Trondheim Norway.
M. Johansson, M. Manthey and O. Flamand
[9] Ewins, D. J. (2000). Modal Testing: Theory, Practice and Application. Research Studies
Press.
[10] Magalhães F., Caetano E., Cunha A., Flamand O., Grillaud G., Ambient and free vibra-
tion tests of the Millau Viaduct: Evaluation of alternative processing strategies, Engineer-
ing Structures, Volume 45, December 2012, Pages 372-384.
[11] Zivanovic S, Pavic A, Reynolds P. (2007) Finite Element Modelling and Updating of a
Lively Footbridge: The Complete Process, Journal of Sound and Vibration, volume 301,
pages 126-145
[12] On site testing of the Treed-It building CSTB report.
... Therefore, structural engineers need guidelines and models to predict the modal properties of a complete building. Investigations combining experimental and numerical methods aim at quantifying the three structural governing parameters Abrahamsen et al. (2020), Kurent et al. (2021), Manthey et al. (2021) and Tulebekova et al. (2022). In the following three parts, simplified estimations and guidelines from several design codes based on traditional building types are presented as well as some main outcomes from the DynaTTB project and highlights from other papers. ...
... 1.0 -3.5% Shut down FVT, Abrahamsen et al. (2020) 14-story all-GLT (Treet, Norway) ...
Thesis
Full-text available
Climate change and densification of cities are two major global challenges. In the building and construction industry, there are great expectations that tall timber buildings will constitute one of the most sustainable solutions. First, vertical urban growth is energy and resource-efficient. Second, forest-based products store carbon and have one of the highest mechanical strength to den-sity ratios. If the structural substitution of concrete and steel with wood in high-rise buildings awakens fears of fire safety issues, engineers and research-ers are particularly worried about the dynamic response of the trendy tall tim-ber buildings. Indeed, due to the low density of wood, they are lighter, and for the same height, they might be more sensitive to wind-induced vibrations than traditional buildings. To satisfy people’s comfort on the top floors, the ser-viceability design of tall timber buildings must consider wind-induced vibra-tions carefully. Architects and structural engineers need accurate and verified calculation methods, useful numerical models and good knowledge of the dynamical properties of tall timber buildings. Firstly, the research work presented hereby attempts to increase the under-standing of the dynamical phenomena of wind-induced vibration in tall build-ings and evaluate the accuracy of the semi-empirical models available to esti-mate along-wind accelerations in buildings. Secondly, it aims at, experimen-tally and numerically, studying the impact of structural parameters – masses, stiffnesses and damping – on the dynamics of timber structures. Finally, it suggests how tall timber buildings can be modeled to correctly predict modal properties and wind-induced responses. This research thesis confirms the concerns that timber buildings above 15-20 stories are more sensitive to wind excitation than traditional buildings with concrete and steel structures, and solutions are proposed to mitigate this vibra-tion issue. Regarding the comparison of models from different standards to estimate wind-induced accelerations, the spread of the results is found to be very large. From vibration tests on a large glulam truss, the connection stiff-nesses are found to be valuable for predicting modal properties, and numerical reductions with simple spring models yield fair results. Concerning the struc-tural models of conceptual and real tall timber buildings, numerical case stud-ies emphasize the importance of accurately distributed masses and stiffnesses of structural elements, connections and non-structural building parts, and the need for accurate damping values.
... An alternative to ambient vibration testing (which is highly dependent on the wind speeds at the time of recording) is forced vibration testing (FVT), whereby horizontal electro-dynamic sliding shakers induce vibrations in a building and record its response (see Figure 2.18). The DynaTTB project, a collaboration between numerous European companies, academia and research organisations, plans to test seven timber buildings and one bridge between 2019-2022 using FVT (Abrahamsen et al. (2020)). The first building to be tested was the 36 m "Treed It" in France. ...
... 18 The 500kg mass shaker used for forced vibration testing, in operation on Treed It in France(Abrahamsen et al. (2020)). ...
Thesis
Despite all being less than 100 metres tall, the world's tallest timber buildings all utilise concrete to increase their mass such that they do not vibrate excessively under wind loading. Wind-induced vibrations must be minimised to ensure that the building's occupants remain comfortable and do not regularly experience motion sickness during high winds. Despite the difficulties with wind dynamics for the current generation of timber towers, numerous concept designs have been announced that propose to build much taller with timber. However, at present, there has been little consideration of how the architecture of timber towers can be suitably designed to help combat the problem. This thesis investigates the effects of different structural typologies on the dynamic performance of timber buildings by studying four iconic skyscrapers; the Gherkin, the Shard, the John Hancock Center and 432 Park Avenue and examining how they would perform if built from timber. First, they are assessed at their existing heights and across a range of shorter heights, with their steel or concrete frames but examining the effect of replacing their concrete floors with CLT. Secondly (and again across a range of heights), the buildings are redesigned with a timber frame to test how their dynamics would change if their steel or concrete beams, columns and walls were replaced with glulam sections and CLT panels. The Shard and 432 Park Avenue, which have concrete cores, have also been examined to see how they perform if they kept their concrete cores, but if the remainder of their structures were built from timber. In total, 144 combinations of building, floor material, height, and frame material are assessed. Retaining their existing steel or concrete frames but replacing their concrete floors with CLT resulted in the buildings' natural frequencies increasing by an average of 30\% and the peak accelerations by 47\%. These changes are due to the CLT floors being considerably lighter than the original concrete floors. By comparison, the change from a steel or concrete-framed structure to a timber-framed structure (with no change in floor type) made little difference to the peak accelerations, but caused natural frequencies to increase by 11\%. If their existing structures were retained, but CLT panels with a thin layer of concrete screed were used for their floors (instead of deep concrete slabs), then the Gherkin at 182 m, the Shard at 200 m, the John Hancock Center at 196 m and 432 Park Avenue at 137 m would have acceptable vibrations (for residential occupancy) if located in a low wind speed environment like London. Across the four buildings, this change in floor type would save an average of 24 $kgCO_2$ per $m^2$ of floorspace if sequestered carbon is excluded, and 170 $kgCO_2/m^2$ if sequestered carbon is included. When sequestered carbon is included in the calculation, the net carbon stored in CLT is enough to offset the embodied carbon of the steel and concrete of the Shard (at 200 m) and 432 Park Avenue (at 137 m). When sizing the columns and diagonals of the Gherkin and the John Hancock Center, the strength criteria was the limiting factor (rather than stiffness). This is because both towers have well-braced tubular designs that are inherently stiff, thanks to the majority of their columns and diagonals being located on their perimeters. With strength as the governing criterion, the size of the structural members could be reduced when lightweight CLT floors were used instead of concrete. For example, the columns of the Gherkin would have required 32\% less steel if CLT floors had been used instead of concrete decks. Such savings would not be possible for the Shard or 432 Park Avenue, where the stiffness criterion limits the sizes of the sections. If the four skyscraper designs were built with a timber frame, the Gherkin would comfortably be the best performing structure thanks to its inherently stiff diagrid shell and its circular cross-section. It could easily satisfy the ISO 10137 human comfort criterion for residential occupancy in most locations at its full height of 182 metres. Taller versions of the structure are also likely to be viable. If built in London, a fully-timber Shard at 134 m (or 200 m with a concrete core and glulam frame), a timber John Hancock Center at 196 m, and a fully timber 432 Park Avenue at 80 m (or a hybrid at 137 m) could also satisfy the same criterion (all with CLT and screed floors). Across a set of the 135 m versions of the four skyscrapers, the change from a steel or concrete frame to a glulam and CLT structure would result in a saving of 130 $kgCO_2/m^2$ (including sequestered carbon) or a saving of 92 $kgCO_2/m^2$ for a hybrid (timber beams and columns, but retaining a concrete core). Overall, when different typologies were compared on a like-for-like basis, braced tubular forms like the Gherkin and the John Hancock Center worked the best in timber, producing lower wind-induced vibrations than 432 Park Avenue and the Shard. Furthermore, their tubular structures required smaller column sizes (which occupy a lower percentage of their floor space), have lower material costs per $m^2$ of floor space and would result in less embodied carbon per $m^2$ (if sequestered carbon is ignored) than those which rely on an internal core for lateral stability. The next generation of tall timber buildings looks unlikely to reach some of the super tall heights proposed without significant additional damping, added mass or suitable aerodynamic cross-sections that can minimise wind-induced vibrations. However, this thesis has shown that timber does have the potential to be used in suitably designed tall buildings up to at least 200 m tall, without additional damping or mass, and as the primary structural material in the frame or as an alternative to concrete floors.
... A parametric study by Cao et al. (2021) [29] found the DAF to be approximately 2.0 for timber frames, mainly influenced by the structures' damping ratio and connection stiffness. Damping in tall timber buildings is still not fully understood, which has attracted a lot of new research [30]. ...
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... Indeed, vibration and cyclic tests have been performed to explore stiffness and energy dissipation in engineered timber products [14][15][16][17][18], in mechanical connections with steel plates and dowels [19,20], and for different lateral-resisting truss or frame prototypes [21][22][23][24]. During the last decade, many ambient vibration campaigns, on real buildings, that focus on the lowest eigenfrequencies and damping ratio have been performed [25][26][27][28], and recently, taller timber buildings have been excited with mass inertia shakers to improve estimates of modal properties [29][30][31][32]. ...
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... A sensitivity study with mesh fineness from half to 1/12 th of the bay size is shown in Figure 49, justifying the choice of the model. Damping of the structure is a large, complex topic that is actively occupying timber engineers, as the dynamic response of tall timber buildings is still largely unknown (Abrahamsen et al., 2020). There are many methods to introduce damping, of which Rayleigh damping is the most popular and one easily applied with Abaqus. ...
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https://doi.org/10.3929/ethz-b-000526211
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Based on the experimental estimation of the key dynamic properties of a seven-storey building made entirely of cross-laminated timber (CLT) panels, the finite element (FE) model updating was performed. The dynamic properties were obtained from an input-output full-scale modal testing of the building in operation. The chosen parameters for the FE model updating allowed the consideration of the timber connections in a smeared sense. This approach led to an excellent match between the first six experimental and numerical modes of vibrations, despite spatial aliasing. Moreover, it allowed, together with the sensitivity analysis, to estimate the stiffness (affected by the connections) of the building structural elements. Thus, the study provides an insight into the as-built stiffness and modal properties of tall CLT building. This is valuable because of the currently limited knowledge about the dynamics of tall timber buildings under service loadings, especially because their design is predominantly governed by the wind-generated vibrations.
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The dynamic response of semi-rigid timber frames subjected to wind loads is investigated numerically in this paper. The dynamic response of more than one million unique frames with different parameters was assessed with the frequency-domain gust factor approach, which is currently adopted by Eurocode 1, and the time-domain generalized wind load method. In the generalized wind load method, the frames were simulated for three different wind velocities with five simulations per unique combination of parameters, resulting in more than twelve million simulations in total. Qualitative and quantitative observations of the dataset were made. Empirical expressions for the accelerations, displacements, and fundamental eigenfrequency were proposed by the use of nonlinear regression applied to the obtained numerical results and a frequency reduction factor was developed. The wind-induced accelerations obtained by the two methods were compared to the corresponding serviceability criteria according to ISO10137, providing insight about the feasibility of moment-resisting frames as a lateral load-carrying system for mid-rise timber buildings. Comparison between the theoretical gust factor approach and the generalized wind load method showed that the gust factor approach was nonconservative in most cases. Finally, the effect of uniform and non-uniform mass distributions was investigated, with a theoretical reduction in top-floor accelerations of 50% and 25% respectively.
... The mass m at the head of the panel is assumed equal to 8 KN, ie a fraction of the vertical load applied in quasistatic tests. The viscosity coefficient of timber structures is approximately 2%, as found by [1,46]. However, identifying the viscosity coefficient in operational conditions generally underestimates the actual viscosity coefficient at more significant displacements. ...
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The paper presents an application of the Extended Energy-dependent Generalized Bouc–Wen model (EEGBW) to simulate the experimental cyclic response of Cross-Laminated Timber (CLT) panels. The main objectives of the paper are assessing the sensitivity of the quadratic error between experimental and numerical data to the EEGBW parameters, showing the fitting performance of the EEGBW model in matching the experimental cyclic response of CLT panels, highlighting the stability of the model in nonlinear dynamic analysis with seismic excitation. The research proves that the considered Bouc–Wen class hysteresis model can reproduce the hysteretic response of structural arrangements characterized by pinching and degradation phenomena. The model exhibits significant stability in nonlinear dynamic analysis with seismic excitation. The model’s stability and versatility endorse its application to simulate structural systems’ dynamic response when Finite Element modelling might be an impractical choice.
... Unfortunately, some dynamical properties of large timber structures are not well known [2]. The damping is the least known dynamical property and recently started research projects aim to close the knowledge gap through ambient vibration tests [3,4,5] and forced vibration tests of tall timber buildings [6]. ...
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A framework for the probabilistic finite element model updating based on measured modal data is presented. The described framework is applied to a seven-storey building made of cross-laminated timber panels. The experimental estimates based on the forced vibration test are used in the process of model updating. First, a generalized Polynomial Chaos surrogate model is derived representing the map from the model parameters to the eigenfrequencies and the eigenvectors. To overcome the difficulties caused by mode switching, we propose a novel approach to mode tracking based on partitioning an extended and low-rank representation of the k mode shapes resulting from different setups of the finite element model into k clusters by the k-means clustering algorithm. Second, the surrogate model derived with the help of mode pairing is used to efficiently perform sensitivity analysis and uncertainty quantification of the first five frequencies and the corresponding mode shapes. Finally, the surrogate-based Bayesian update of the model parameters is efficiently performed, providing engineers not only with a finite element model that gives a good fit to the experimental modal data, but also a stochastic model that represents the uncertainties originating from the initial model and the uncertainties of measuring modal properties.
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This paper describes a project that aims at evaluating the dynamic properties of tall timber structures under wind-induced vibration in serviceability level from a series of onsite measurements on existing multi-storey timber buildings and timber towers. Ambient vibration measurement was carried out on nine timber towers and three tall timber buildings to derive dynamic parameters including natural frequencies, damping ratios, and mode shapes. The results were analysed by time and frequency domain approach. The factors that influence the dynamic parameters, such as construction type, height, and vibration amplitudes, are discussed. The outcomes of this paper are useful for engineers to evaluate the dynamic parameters in the design of tall timber structures.
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“Treet” is a 14-storey timber apartment building in Norway currently under construction. Ground works started in April 2014, and the residents can move in autumn of 2015. The building will be one of the tallest timber buildings in the world. The building consists of load-carrying glulam trusses and two intermediate strengthened levels. Prefabricated building modules are stacked on top of the concrete garage and on top of the strengthened levels. There is CLT in the elevator shaft, internal walls and balconies. But, CLT is not a part of the main load bearing system. Glass and metal sheeting protect the structural timber from rain and sun. The paper presents the design of the building as well as many of the investigations, considerations and discussions which took place during the design process. Finally some of the design verifications are presented.
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Cross laminated timber (CLT) has become a well-known engineered timber product of global interest. The orthogonal, laminar structure allows its application as a full-size wall and floor element as well as a linear timber member, able to bear loads in- and out-of-plane. This article provides a state-of-the-art report on some selected topics related to CLT, in particular production and technology, characteristic material properties, design and connections. Making use of general information concerning the product’s development and global market, the state of knowledge is briefly outlined, including the newest findings and related references for background information. In view of ongoing global activities, a significant rise in production volume within the next decade is expected. Prerequisites for the establishment of a solid timber construction system using CLT are (1) standards comprising the product, testing and design, (2) harmonized load-bearing models for calculating CLT properties based on the properties of the base material board, enabling relatively fast use of local timber species and qualities, and (3) the development of CLT adequate connection systems for economic assembling and an increasing degree of utilization regarding the load-bearing potential of CLT elements in the joints. The establishment of a worldwide harmonized package of standards is recommended as this would broaden the fields of application for timber engineering and strengthen CLT in competition with solid-mineral based building materials.
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The dynamic properties of two common structural systems for multi-storey timber buildings are compared through in situ testing of completed buildings. The two five-storey buildings examined are identical except for their structural system, which in one is sheathed stud-and-rail timber construction, and in the other a cross-laminated timber panel system. Both also have a reinforced-concrete core located at the centre of one edge of the rectangular plan of each building. An output-only modal analysis method was used to identify the modal properties of the buildings: the random decrement technique was applied to the stochastic measured response, and then the time-domain random decrement signature was used for modal analysis by the Ibrahim Time Domain method. The natural frequencies, damping ratios and mode shapes of the first three vibration modes of the buildings were identified, and compared with those modelled based on the properties of the core and timber walls. The variations in properties between the two buildings are discussed. The two structures show very similar natural frequencies and damping ratios, suggesting that they could perhaps be considered as the same class of building in design for lateral movement.
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The finite element (FE) model updating technology was originally developed in the aerospace and mechanical engineering disciplines to automatically update numerical models of structures to match their experimentally measured counterparts. The process of updating identifies the drawbacks in the FE modelling and the updated FE model could be used to produce more reliable results in further dynamic analysis. In the last decade, the updating technology has been introduced into civil structural engineering. It can serve as an advanced tool for getting reliable modal properties of large structures. The updating process has four key phases: initial FE modelling, modal testing, manual model tuning and automatic updating (conducted using specialist software). However, the published literature does not connect well these phases, although this is crucial when implementing the updating technology. This paper therefore aims to clarify the importance of this linking and to describe the complete model updating process as applicable in civil structural engineering. The complete process consisting the four phases is outlined and brief theory is presented as appropriate. Then, the procedure is implemented on a lively steel box girder footbridge. It was found that even a very detailed initial FE model underestimated the natural frequencies of all seven experimentally identified modes of vibration, with the maximum error being almost 30%. Manual FE model tuning by trial and error found that flexible supports in the longitudinal direction should be introduced at the girder ends to improve correlation between the measured and FE-calculated modes. This significantly reduced the maximum frequency error to only 4%. It was demonstrated that only then could the FE model be automatically updated in a meaningful way. The automatic updating was successfully conducted by updating 22 uncertain structural parameters. Finally, a physical interpretation of all parameter changes is discussed. This interpretation is often missing in the published literature. It was found that the composite slabs were less stiff than originally assumed and that the asphalt layer contributed considerably to the deck stiffness.
Measuring vibrations and assessing dynamic properties of tall timber buildings
  • M Fjeld Olsen
  • O Hansen
Fjeld Olsen, M., Hansen, O. 2016. Measuring vibrations and assessing dynamic properties of tall timber buildings, Master thesis NTNU, Trondheim Norway.