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TALL TIMBER BUILDINGS – A PRELIMINARY STUDY OF WIND-INDUCED VIBRATIONS OF A 22-STOREY BUILDING

Authors:
  • RISE Research institute of Sweden
  • RISE Research Institute of Sweden

Abstract and Figures

During the last years the interest in multi-storey timber buildings has increased and several medium-to-high-rise buildings with light-weight timber structures have been designed and built. Examples of such are the 8-storey building “Limnologen” in Växjö, Sweden, the 9-storey “Stadthouse” in London, UK and the 14-storey building “Treet” in Bergen, Norway. The structures are all light-weight and flexible timber structures which raise questions regarding wind induced vibrations.
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TALL TIMBER BUILDINGS A PRELIMINARY STUDY OF WIND-
INDUCED VIBRATIONS OF A 22-STOREY BUILDING
Marie Johansson1, Andreas Linderholt2, Kirsi Jarnerö3, Pierre Landel4,
ABSTRACT: During the last years the interest in multi-storey timber buildings has increased and several medium-to-
high-rise buildings with light-weight timber structures have been designed and built. Examples of such are the 8-storey
building Limnologenin Växjö, Sweden, the 9-storey Stadthousein London, UK and the 14-storey building Treet
in Bergen, Norway. The structures are all light-weight and flexible timber structures which raise questions regarding
wind induced vibrations.
This paper will present a finite element-model of a 22 storey building with a glulam-CLT structure. The model will be
used to study the effect of different structural properties such as damping, mass and stiffness on the peak acceleration
and will be compared to the ISO 10137 vibration criteria for human comfort. The results show that it is crucial to take
wind-induced vibrations into account in the design of tall timber buildings.
KEYWORDS: Deflection, dynamic properties, stabilisation, sway, wind loads
1 INTRODUCTION 12 3
1.1 Background
During the last decades the interest in multi-storey
timber buildings has increased. This is due to reasons
such as development of performance based codes but
also increased interest in sustainability. The CO2
emissions could be reduced by 40% if building with
timber instead of concrete [1]. The increasing population
and ongoing urbanisation are going to increase the need
for creating cities with higher population densities. This
will lead to an increased need for tall buildings that make
the best use of limited space. The environmental
potential of high-rise buildings lies in a more efficient
use of resources.
By adding these elements together the use of tall
buildings with timber structures would be an opportunity
and provide for ecological, sustainable high and dense
developments in urban regions with housing shortage.
Timber has the benefit of having a high strength-to-
weight ratio compared to other building materials which
in many cases is beneficial. However, for the case with
1 SP Sustainable Built Environment and Linnaeus University,
Sweden, marie.johansson@sp.se
2 Linnaeus UniversityMechanical engineering, Sweden,
andreas.linderholt@lnu.se
3 SP Sustainable Built Environment, Sweden,
kirsi.janero@sp.se
4 SP Sustainable Built Environment, Sweden,
pierre.landel@sp.se
medium-to-high-rise buildings this might pose a
challenge since the dynamic properties of the building
will be quite different from a high-rise building with a
load-bearing structure in steel or concrete [2].
High-rise buildings consisting of a timber structure are
therefore the subject of a research project named “Tall
Timber Buildings Concept Studies” lead by SP
Technical Research institute of Sweden together with
Linnaeus University during the period 2015-2018.
1.2 Research project “Tall Timber Buildings -
Concept Studies”
The aim of the “Tall Timber Buildings Concept
Studies” is to develop feasible concepts for planning and
designing 22 storey high timber buildings according to
present regulations and during the process identify issues
that need more research. The specific objectives of the
project besides the issues in serviceability limit state
related to wind actions as presented in the present paper
are also to study stability, static vertical deformations,
load transfer through the building to the ground and
connection detailing regarding strength, stiffness and
continuity with respect to both dynamic and static
loading. The objective is also to develop calculation
tools and models for tall timber buildings suitable for
structural engineers and tools for analytical fire design.
The difference between high rise and low rise timber
buildings with regard to life cycle assessment will also
be studied. The aim is to increase the number of
practicing architects and designers with experience of
designing tall timber buildings and to increase the
knowledge of designing tall timber buildings also among
actors not participating in the project. Another important
aim is to develop existing timber building systems and
find new ideas and solutions that are applicable also on
lower buildings. The timber building systems involved
are KLH Sweden with CLT plates, Moelven Töreboda
with glulam columns and beams in combination with
floor elements built up from LVL and Masonite Beams
with a system consisting of planar elements built up
from I-joists. The architectural design of the concept
buildings are made by White architects and by Berg|CF
Möller architects in co-operation with the property
developers/owners VKAB and HSB and the structural
designers Bjerking and BTB that will design one of the
concept buildings each. The two fire design consultants
Briab and Brandskyddslaget as well as acoustic
engineers from WSP on the other hand will follow both
projects in parallel. Researchers from Linnaeus
University and from SP Technical Research Institute will
support the project teams with in-depth studies, detailed
calculations and testing.
1.3 Aim and objectives
This paper will present the first results from the project
“Tall Timber Buildings Concept studies”.
The paper will give a review of the requirements on
vibrations in buildings and address which parameters are
important for the design of tall buildings. The paper will
also present a preliminary finite element (FE)-model of a
22 storey building with a structure consisting of a core of
CLT elements and an outer beam-column structure of
glulam. The model will be used to estimate resonance
frequencies, stiffness and mode shapes. These data will
be used to calculate acceleration levels at the top floor
when the structure is subjected to wind loads according
to Eurocode 1 [3].
The model will be used in the Tall Timber Building
project as a base for studying the effect of changing
stiffness, mass and damping on the acceleration levels of
the structure.
The FE-model will naturally also be used to study the
performance of the building in the ultimate limit state.
2 REQUIREMENTS FOR TALL
TIMBER BUILDINGS
2.1 General requirements
All building systems are designed to withstand both
vertical gravity loads and horizontal loads due to wind.
The main focus when designing a building has always
been the safety aspect calculated based on the maximum
loads expected to occur once every 50 years. The interest
in the serviceability limit state has been attracting more
focus in the last decades. In the horizontal directions
there are limitations both for the static deformation but
also for vibrations/accelerations. For the horizontal
deformation a maximum value of h/500 is set in the
German DIN standard. Even more important for
especially tall buildings is the sway due to wind loads.
The comfort performance of a building during wind
loading is an important building design issue. The
occupants’ perception and tolerance of wind-induced
vibrations is a subjective assessment and presently there
is no single internationally accepted occupant comfort
criteria to set levels for satisfactory vibrations in tall
buildings subjected to wind loading. The requirements
that are set in the international standards are normally
based on acceleration levels where people start to notice
and comment on the motion.
There are three different international standards that deal
with horizontal vibrations in buildings and the human
perception of vibrations. There are two older ISO
standards, ISO 6897 [4] that cover the range from 0.063
Hz to 1Hz and ISO 2631-2 [5] that cover the range 1 Hz
to 80 Hz. These two are in agreement with each other
and use the Root Mean Square (RMS) value for the
acceleration due to a maximum wind velocity with a
return period of five years. ISO 10137 [6] covers the
range from 0.063 Hz to 5 Hz and uses the peak
acceleration calculated for a wind velocity with a return
period of one year. This is the standard referred to in
Eurocode 1 [3]. These two sets of standards will yield
slightly different acceptance levels for the same building.
The Swedish National Annex [7] to the Eurocode 1
states that ISO 6897 is recommended for calculation of
the effect of wind loads. In this paper, calculations
according to ISO10137 and the general requirements in
Eurocode 1 [7] are used.
In this paper the main issue for the studied building is
the sway and vibrations, therefore the horizontal effects
from the wind load are the most important phenomenon
to study.
2.2 Structural dynamics of tall buildings
A tall building is in most cases considered as a line-like
vibrating object, and its structure may be represented as
a vertical cantilever beam, fixed at the foundation and
free at the top. The global dynamics of a multi-story
building can sometimes, in a simplified manner, be
analyzed by approximating the building to be a uniform
cantilever beam, with a neglected axial load, and having
the height h. According to the well-known Euler-
Bernoulli beam theory, the homogeneous part of the
governing equation of motion is

+
= 0
(1)
in which , and  constitute the density, cross
sectional area and bending stiffness respectively. The
general solution to this equation is
()=cosh()+
sinh()+
cos()+sin()
(2)
in which =
and ,, and are
constants. When the boundary conditions for a cantilever
beam
(0)=(0)=()=()= 0
(3)
are applied, nontrivial solutions require that
(4)
which has to be solved numerically. The first solution is
= 1.875. The associated, first bending, mode then
has the mode shape
()=(sin sinh )(sin sinh )
+(cos cosh )(cos 
cosh )
(5)
which is valid for 0. Assuming that only the
first bending mode contributes to the displacement and
making the substitute
(6)
the modal mass (), modal stiffness (
) and modal load
can be calculated as:
= ()
()
(7)
= ()
()
(8)
= ()
()
(9)
In which () is the distributed wind load. The
continuous problem has then been re-placed with a
discrete single-degree-of-freedom system, depicted in
Fig. 1.
()+
()=()
(10)
Figure 1. A single-degree-of-freedom system representing the
building when one assumed mode is used.
When damping is added the governing equation of
motion becomes
()+() +
()=()
(11)
in which  is the modal damping. In the codes used for
calculation of responses due to wind loads, simplified
expressions for the mode shapes are often used.
The expression shows that the important parameters that
have an effect on the relationship between deflection,
velocity, acceleration and the wind load are the mass,
damping and stiffness.
2.3 Wind load
Wind loads vary greatly in speed, force and direction
over time and its effect on buildings are complex and
heterogeneous. To simplify this in codes, the wind is
seen as a quasi-static load for buildings with high
stiffness and damping. For high slender structures also
the gust effect is included as a turbulence factor added to
the quasi-static wind load. The effect of the wind on a
single building will be affected by the terrain around it as
well as the shape and height of the building.
In Europe, wind loads are defined in Eurocode 1, Part 1-
4 [3]. There, the fundamental basic wind velocity is
defined by the 10-minute mean wind speed, at a height
of 10 meter above ground, that is exceeded once every
50 year. The wind velocity for shorter times  (in this
case T years) can then be calculated,
 =0.75 10.2  11

(12)
The wind velocity in the vicinity of a structure is
dependent on the local terrain around the structure and is
varying with the height z above ground. According to
Eurocode 1-4 [3] the mean wind velocity () at the
height z next to a structure can be expressed as,
() = ()

(13)
The first term () is an orography factor (set to 1.0 in
normal cases) and the second term is a terrain
roughness factor.
= 0.19
,.
(14)
The term is the roughness length that varies between
terrains; the term , is a reference terrain roughness
length. The mean wind pressure () can then be
calculated using,
() = 1
2()
(15)
where ρ is the air density (normally set to 1.25 kg/m3).
For high and flexible buildings it is, however, necessary
to take also the dynamic effects of the wind load into
account. This is done by including a turbulence intensity
factor Iv(z), defined as the standard deviation of the
turbulence divided by the mean velocity as,
() =
()
(16)
where is the standard deviation of the turbulence and
() is the mean wind velocity. The standard
deviation of the turbulence is calculated as the basic
wind velocity times the terrain roughness factor .
The response of the structure due to the turbulence
intensity can be divided into two parts; the background
response () and the resonance response (). The
background response is due to the quasi-static part of the
wind load while the resonance response is due to the
dynamic properties of the building and the dynamic part
of the wind load. Eurocode 1-4, annex B gives the
following expression for calculating the Root Mean
Square (RMS) value for the acceleration, ():
() = ()()
()
(17)
Where is the shape factor for force, b the width of the
building, is the equivalent (modal) mass, a
dimensionless variable and () the mode shape
associated with the first resonance frequency. From this,
the horizontal peak acceleration of the structure () can
be written as,
() = ()
(18)
where is a peak factor that relates the mean and the
standard deviation of the response.
3 BASIC MODEL OF THE 22-STOREY
BUILDING
3.1 Structural principles for tall timber buildings
There are of course many ways to design tall buildings.
For steel structures rigid frame systems, with or without
trusses, have been used for buildings up to 70 storeys
and for higher buildings tubular systems are often used.
When using concrete a central core system with concrete
walls are often used together with columns in the
perimeter.
In timber many of the tallest buildings at the moment are
built with platform framing using CLT-elements, that is
a structure that has been used up to 8-10-storey buildings
such as the 8-storey building Limnologen in Växjö,
Sweden, the 9-storey Stadthouse in London, UK
For the tallest timber building so far, the 14-storey high
“Treet” in Bergen, Norway, a glulam beam/column/truss
system was used. For taller timber buildings MGB [8]
suggested a continuous core of CLT or LVL elements
together with glulam columns in the perimeter of the
building. In their concept study, they plan for 20-30
storeys. There are also plans for a building with up to 25
storeys in Vienna, but in that case utilizing a hybrid
structure with a concrete core and CLT-element in the
outer walls.
3.2 Structure
For the concept study in this project the plan is for a 22-
storey building only using timber as structural material
located in Växjö or Stockholm. The maximum outer plan
was set to 20 x 20 meters with a storey height of 3.4
meters. The structure is planned as a residential building.
The first design attempt for this building is a floor plan
that is 17 x 21 meter as outer measurements and with a
total building height of 75 meters. The building is made
with a central core of CLT elements that are continuous
over three storeys and connected with a butt joint. The
core has a measure of 12.5 x 8.6 meters with areas for
elevators, stairs, hallways and bathroom areas inside the
core. The CLT walls used in the design are planned with
the outer layers in the vertical direction. In the outer
perimeter of the building there are continuous glulam
columns at a distance of 4.1 meter. The core and the
columns are connected at each floor plan with glulam
beams. The floors structure which will consist of CLT
plates is planned to be hanging on the inside of the
CLT/glulam beams to have the vertical structure
continuous, see Figure 2.
Figure 2. Sketch of the main structural system.
From earlier calculation of tall timber buildings it is
known that sway is the criterion that is the most difficult
to meet for the structure [2]. The calculation in the
ultimate limit state is therefore in this first step only done
as a hand calculation where the structure is seen as a
cantilever beam with full interaction between all the
elements.
The chosen dimensions of the structural elements are
enough to withstand the snow, gravity and wind loads in
the ultimate limit state. The connections, especially to
the ground, will need more evaluation and development.
The proposed floor plan for the building can be seen in
Figure 3. The inner core (light gray) will be made up of
7-layer CLT elements with a thickness of 230 mm with a
width of either 2500 mm or 2720 mm. The CLT wall
elements are made with the two outer layers and the
middle layer in the vertical direction and with layer 3
and 5 in the horizontal direction. The CLT elements is
planned to be continuous over three storeys and jointed
with butt joints. To avoid joints at the same height the
joints between the CLT elements will be staggered. The
joints between the CLT elements in the vertical
directions will be made with a spline LVL joint which is
a standard connection type for CLT structures.
20930
16940
12500
8616
4100 4100 4100 4100
4100 4100 4100 4100 4100
Figure 3. Sketch of the main structural system, an inner core
with CLT elements and some non-load bearing walls in the
core, glulam columns in the outer facades and glulam beams
connecting the columns and the core structure.
The columns are in the first design made of glulam
GL30c with a dimension of 340 x 540 mm. The columns
are planned to be made as continuous to act as one high
column. The beams in the perimeter and between the
columns and the core are also made of glulam GL30c
and with a dimension of 215 x 405 mm. The glulam
beam-column system is inspired by Moelven Törebodas
“Trä 8” system [9]. The floors are in the first version
made up of a 5 layer CLT elements with a thickness of
200 mm.
The same floor plan /structural elements dimensions are
used for all storeys in this first model.
4 FE-MODEL
The model representing the assembled 22 storey building
was made using MSC SimXpert and the calculations
were made by MSC Nastran version 14.1. which is a
general FE-program originally developed for the
aerospace and automotive industry and therefore has
well developed code for dynamic analysis. The model
consists of 186,836 nodes and 186,340 shell, beam,
bushing and rigid elements; CBAR, CSHELL, CBUSH
and RBE2 respectively.
4.1 Model of one storey
The model is first made as a detailed model for one
storey, see Figure 4. This model is made for studying the
properties of one storey before expanding the model to
the full 22-storey building.
The CLT elements are made as shell elements and with
laminates made up by orthotropic material. The
properties of the laminates are = 12000 MPa in the
fiber direction and = 0 MPa in the direction
perpendicular to the grain, the shear stiffness is set as
= 690 MPa and = 50 MPa. The density is 500
kg/m3. The CLT elements are in this first model fixed to
the ground. The spline connections between the CLT
elements are made with springs with a stiffness of
1.3·106 kN/m in all three directions. In total 10 springs
are used between two CLT elements in the height
direction, the same spring stiffness are used in the
corners of the CLT core. This stiffness is what can be
expected for a spline connection based on tests [10 and
11].
The glulam beams and columns are modelled with beam
elements with an isotropic material model with the
stiffness of 13000 MPa with a density of 500 kg/m3. The
columns are fixed to the ground. The beams also have
pinned connections to the columns and the CLT core.
The floor in the structure is modelled as 200 mm thick
CLT plate with a layered structure with 5 layers. The
material properties for the layers are set to the same
values as for the CLT elements in the core. Inside the
core the model is made with one whole slab attached to
the inside of the CLT elements with pinned connections
on all four sides. In a real structure it will be necessary to
support this floor with beams/walls to avoid too large
deformations. The floor structure is also connected to the
glulam beams with pinned connections at all four sides.
Figure 4. FE-model for the first storey, showing CLT elements
in the core and as floor structures and glulam beams out to the
glulam columns in the facades.
4.2 Model for the 22-storey building
The model was then expanded to a 22-storey building.
The connections between the storeys were made by
assuming rigid connections between the CLT elements
in the vertical direction, the same assumption was made
for the columns.
Figure 5. FE-Model for the complete 22-storey building.
The model was used in a modal analysis calculating the
resonance frequencies and the mode shapes for the total
building. The model was in this first step run without the
gravity field, which results in a slightly higher resonance
frequency than if the gravity field had been used to pre-
compress the structure. In this case the first ten elastic
eigenmodes were extracted The results showed that the
first mode shape is a bending mode in the weak direction
of the building with a resonance frequency of 0.6 Hz, see
Figure 6.
Figure 6. Mode shape for the lowest bending mode in the weak
direction from the FE-model, resonance frequency f1 =
0.60 Hz.
Eurocode 1-4 gives as a rule of thumb a first resonance
frequency for buildings higher than 50 m that is
expressed as:
=46
(19)
Where h is the total height of the building. Utilizing the
building height of 75 m this will mean a first resonance
frequency of 0.61 Hz mainly based on experience from
steel and concrete buildings. The building in this model
showed a result that was surprisingly close to this value.
The bending mode shapes for the first resonance
frequency for a tall slender building is in Eurocode
expressed as:
() =
(20)
Where z is the height above ground and h is the total
height of the building. The shape factor is in Eurocode
1991-1-4 recommended as 1.0 for a building with a stiff
central core surrounded by columns in the façade. The
mode shape will in that case be almost linear with
change in height. Figure 6 show that this is a quite good
approximation for this building. The mode shape shows
a lot of shear deformation, especially for the higher
storeys.
The second mode is a bending mode in the strong
direction with a resonance frequency of 0.8 Hz and the
third is a rotational mode with a resonance frequency of
1.1 Hz, see Figure 7 and Table 1.
Figure 7. Mode shape for the first torsional mode from the FE-
model, resonance frequency f3 = 1.10 Hz.
The first resonance frequency is lower than what is
measured in already built timber buildings, but they are
on the other only a third of the height of this structure.
The 14-storey building “Treet” in Bergen, Norway had
based on a FE-model a frequency of 0.75 Hz but that
building had more mass [2].
Table 1. The three lowest modes for the building, mode shapes
and resonance frequencies for the building according to the
FE-analysis.
Mode
no.
Shape
Resonance
frequency
1
Bending weak direction
0.6 Hz
2
Bending strong direction
0.8 Hz
3
Torsion
1.1 Hz
The results for this structure is, however, not realistic as
there are many issues that are not dealt with such as real
stiffness of the connections to the ground, holes for
openings in the core mass of non-structural material and
so on.
4.3 Accelerations level for the building according to
EC1
The value of the first resonance frequency is one of the
key parameters to calculate the acceleration level on the
top of the building to compare it to the comfort criteria.
In this case the comfort criteria used is the peak
acceleration calculated for a 2-year return period of the
wind [7]. The building is in this case placed in Växjö in
Sweden with a basic wind velocity of 24 m/s and in
terrain type II (area with low vegetation). The
acceleration level is calculated according to the method
in Annex B in EN 1991-1-4:2005 [3]. The parameters
calculated with this method can be found in Table 2.
Table 2. Parameters for calculating acceleration levels
according to Eurocode 1-4 [3] and ISO10137[6]. The rows
with light grey background are input parameters and the dark
grey rows shows the resulting acceleration levels.
Parameter
Value
Unit
Heigth of the building
H
74.8
m
Width of the building
B
20.9
m
Depth of the building
D
16.9
m
Reference height
44.9
m
Equivalent mass, height
20850
kg/m
Equivalent mass, area
996
kg/m2
Basic wind velocity
24
m/s
Roughness length
0.05
m
Minimum height, wind

2
m
Maximum height, wind

200
m
Terrain factor
0.19
Roughness factor
1.4
Wind turbulence intensity
0.15
Mean wind velocity 2 years

19.1
m/s
Force coefficient
1.47
First resonance frequency
0.6
Hz
Damping, mechanical
0.09
Damping, aeroelastic
0.05
Reference length scale
300
m
Alfa
0.52
Turbulence length scale
()
138
m
Non-dimensional
frequency
4.3
Spectral density
()
0.05
Background response
factor
()
0.76
Aerodynamic admittance
0.11
Aerodynamic admittance
0.34
Resonance response factor
()
0.27
Up-crossing frequency
0.20
Hz
Peak factor
()
3.28
Dimensionless coefficient
1.5
RMS of the acceleration
()
0.04
m/s2
Peak acceleration
()
0.13
m/s2
The result for the peak acceleration can then be plotted
into the diagram for acceptable acceleration levels from
the ISO10137 [6]. The result show that the building
having the structure studied now has a too high
acceleration level, see Figure 8.
Figure 8. The peak acceleration level for the structure plotted
into the ISO10137 diagram.
5 CONCLUSIONS
This paper presents a first FE-model able to capture the
dynamic behavior of a 22-storey building. The structure
of the building is made with a core of continuous CLT
wall elements with a seven layer structure and an outer
structure of continuous glulam columns and glulam
beams. The floor structure is made up of CLT elements
that in this first study are pinned to the glulam structure.
The results show that the first resonance frequency is
about 0.6 Hz which might be reasonable for a building of
this height in concrete or steel but combined with the
low mass of the timber structure it leads to acceleration
levels that are too high at the top floors of the building.
The main aim of creating the FE-model is, however, to
use it in further studies were different methods to reach
acceptable acceleration levels will be tested. These
further studies will include investigating such parameters
as:
- Changes in the design and dimensions of the
structural elements such as beams, columns and
wall thickness.
- Changes of the material properties of the timber
elements; create CLT elements better designed
for large shear forces.
- Determine necessary stiffness requirements for
connections in the system.
- Improve the modeling of the mass and its effect
- Test different methods for damping such as
tuned mass dampers or visco-elastic dampers.
- Include bracing systems in the facades as well
as out-rigger structures.
- Study the effect of openings in the CLT-core
and include more stabilizing walls in the core.
- Develop the model for checking also the
ultimate limit state.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the funding for the
project “Tall Timber Buildings concept studies” from
Formas the Swedish Research Council for Environment,
Agricultural Science and Spatial Planning [Dnr: 942-
2015-115].
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K, Olsson, J, Reynolds, T 2015, Building higher
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[3] SS EN 1991-1-4:2005 Eurocode 1: Actions on
structures Part 1-4: General Actions Wind
actions, SIS (2002).
[4] SS ISO 6897: 1984, Guidelines for the Evaluation of
the Response of Occupants of Fixed Structures,
Especially Buildings and Offshore Structures, to
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... Some examples are wind-induced vibrations [3], vibrations from metros [4,5], human-induced vibrations [6,7], and other sources of dynamic loads. For tall timber buildings, wind-induced accelerations are often the limiting factor, which governs the design [8][9][10][11]. Buildings that are considered tall when built in timber, are most often considered as midrise structures when built in steel or concrete. However, such structures are at the frontier of timber engineering where wind-induced vibrations are an important design consideration. ...
... Structures with lower fundamental eigenfrequencies are more susceptible to windinduced vibrations, making these estimates nonconservative. Johansson, Jarnerö and Landel [11] performed a simplified concept study of a 22-storey structure consisting of glulam and CLT elements. In the concept study, the wind-induced accelerations were compared to the serviceability requirements in ISO10137 [26] according to the gust factor approach in EN1991-1-4 [25]. ...
... They found that the peak acceleration did not satisfy serviceability criteria in ISO10137 [26] and that the fundamental eigenfrequency was about 0.6 Hz. In the tall timber structure Treet in Norway, the FE model had a fundamental eigenfrequency of 0.75 Hz [10,11]. In both Treet and Mjøstårnet in Norway, the wind-induced vibrations were on the limit according to ISO10137 [11,13]. ...
Article
Full-text available
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.
... When the height of timber buildings rises in cities around the world, new types of challenges appear for structural designers. One of them is wind-induced vibrations which appears to be annoying for the occupants at lower heights for buildings made of timber than for traditional high-rise buildings [2]. Mass, stiffness and damping matrices (denoted M, K and C) need to be fairly well known to accurately predict responses of structures subjected to time varying excitations. ...
... For tall timber buildings, it is relevant to analyze the accuracy of the mass and stiffness matrices used by structural designers in FE-analysis for serviceability load levels. 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]. ...
... Schmid et al. (2018) compared the lateral stiffness of timber buildings with an internal reinforced concrete core against an internal CLT-core for buildings between 40 and 200 m tall and logarithmic decrements δ s of 0.05 and 0.10. Johansson et al. (2016) carried out a preliminary study of wind-induced vibrations of a 22-storey CLT-building. ...
... In a number of studies, it has been shown that the static dowel connection stiffness, which is calculated in accordance with Eurocode 5 [9], is different from the stiffness calculated from the in-service cyclic tests [8,10]. The common practice in modeling the dynamic response of glulam timber frame buildings is to assign pinned conditions to the connections between glulam elements, [5,6,11,12]. ...
Article
Full-text available
Currently, there is limited knowledge of the dynamic response of taller glue laminated (glulam) timber buildings due to ambient vibrations. Based on previous studies, glulam frame connections, as well as non-structural elements (external timber walls and internal plasterboard partitions) can have a significant impact on the global stiffness properties, and there is a lack of knowledge in modeling and investigation of their impact on the serviceability level building dynamics. In this paper, a numerical modeling approach with the use of “connection-zones” suitable for analyzing the taller glulam timber frame buildings serviceability level response is presented. The “connection-zones” are generalized beam and shell elements, whose geometry and properties depend on the structural elements that are being connected. By introducing “connection-zones”, the stiffness in the connections can be estimated as modified stiffness with respect to the connected structural elements. This approach allows for the assessment of the impact of both glulam connection stiffness and non-structural element stiffness on the dynamic building response due to service loading. The results of ambient vibration measurements of an 18-storey glulam timber frame building, currently the tallest timber building in the world, are reported and used for validation of the developed numerical model with “connection-zones”. Based on model updating, the stiffness values for glulam connections are presented and the impact of non-structural elements is assessed. The updating procedure showed that the axial stiffness of diagonal connections is the governing parameter, while the rotational stiffness of the beam connections does not have a considerable impact on the dynamic response of the glulam frame type of building. Based on modal updating, connections exhibit a semi-rigid behavior. The impact of non-structural elements on the mode shapes of the building is observed. The obtained values can serve as a practical reference for engineers in their prediction models of taller glulam timber frame buildings serviceability level response.
... At serviceability level, the wind-induced vibrations (in both along-wind and across-wind directions) have been highlighted as key problems in taller multi-story wooden buildings [3][4][5][6][7][8][9]. To date, only a few models that correctly predict the response of tall timber structures for habitability have been proposed [10][11][12][13]. Experimental studies focusing on the dynamic responses of timber structures at different scales and serviceability levels have produced valuable data for improving the accuracy of these models. ...
Article
Full-text available
The rise of wood buildings in the skylines of cities forces structural dynamic and timber experts to team up to solve one of the new civil-engineering challenges, namely comfort at the higher levels, in light weight buildings, with respect to wind-induced vibrations. Large laminated timber structures with mechanical joints are exposed to turbulent horizontal excitation with most of the wind energy blowing around the lowest resonance frequencies of 50 to 150 m tall buildings. Good knowledge of the spatial distribution of mass, stiffness and damping is needed to predict and mitigate the sway in lighter, flexible buildings. This paper presents vibration tests and reductions of a detailed FE-model of a truss with dowel-type connections leading to models that will be useful for structural engineers. The models also enable further investigations about the parameters of the slotted-in steel plates and dowels connections governing the dynamical response of timber trusses.
... There is little to be learned from this analysis, as the taller version was not fully modelled, and there are doubts about the accuracy of the earlier calculations, regardless. A subsequent study by Johansson et al. (2016) calculated the response of a 75 m building, (designed by the authors) which also failed the comfort criteria. These isolated studies of different buildings show the difficulty of designing a timber structure that will meet serviceability criteria, but do little to answer how the issue could be solved. ...
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.
... Most literature concerning wind-induced vibrations in tall timber buildings focus on case studies of one or a few buildings. Such literature examples include Feldmann et al. [1], Johansson et al. [2], and Reynolds et al. [3]. More recent works by Vilguts et al. [4] and Cao & Stamatopoulos [5] on moment-resisting timber skeleton frames analyzed a large sample of hypothetical timber structures subjected to wind loads. ...
Conference Paper
Full-text available
Wind-induced vibrations is one of the main design considerations for tall timber buildings due to their flexibility and lightweight nature. Some approaches to solve this problem are the addition of extensive bracing elements, the introduction of semi-rigid connections or the addition of extra mass. In this paper, the response of planar moment-resisting skeleton frames and braced frames with diagonal elements is calculated, and a comparison of peak accelerations between the gust factor approach and a dynamic time-history approach is performed.
... In general, TTBs have sufficient capacity to resist lateral loads for the ultimate limit state and the design is governed by the wind-generated vibrations that cause discomfort or annoyance to occupants (e.g. Edsk€ ar & Lidel€ ow, 2017;Johansson, et al., 2016;Reynolds, Harris, Chang, Bregulla, & Bawcombe, 2015). The amount of sway/acceleration depends on the mass and stiffness distribution across the TTB and its ability to dissipate kinetic energy (e.g. ...
Article
Full-text available
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.
... For a maximum wind velocity with a return period of one year. These two sets of standards will yield slightly different acceptance levels for the same buildings [25]. Examples of studies investigating human perception of accelerations can be found here [26,27]. ...
Article
Full-text available
Over the last decades, the increasing urbanization and environmental challenges have created a demand for mid-rise and high-rise timber buildings in modern cities. The major challenge for mid-rise and high-rise timber buildings typically is the fulfillment of the serviceability requirements, especially limitation with respect to the wind-induced displacements and accelerations. The purpose of the present paper is to evaluate the feasibility and the limitations of moment-resisting timber frames under service load according to the present regulations. The parametric analyses investigate the effects of the rotational stiffness of beam-to-column and column-to- foundation connections, storey number and height, number and length of bays, column cross-section dimensions and spacing between frames on the overall serviceability performance of the frames. Elastic and modal analysis was carried out for a total of 17,800 planar moment-resisting timber frames with different parameters by use of Abaqus Finite Element (abbr. FE) software. Finally, the obtained results were used to derive simple expressions for the lateral displacement, maximum inter-story drift, fundamental eigen-frequency, mode shapes, and acceleration.
Article
An experimental program was performed at IVALSA Trees and Timber Institute on single and coupled cross-laminated (CLT) wall panels with different anchoring systems and different types of joints between adjacent panels. The mechanical properties of CLT walls were assessed and are critically discussed in the paper. The connector layout and the design of the screwed vertical joints were found to markedly affect the overall behavior of the structural system. The in-plane deformations of CLT panels were almost negligible, whereas concentration of forces and deformations mainly occurred in the connections. Advanced analytical models for nonlinear pushover analysis of CLT wall systems were developed and verified against test results. The models take into account all stiffness and strength components of connectors, as well as the bending and shear deformation of the panels. A parametric study of CLT wall systems with different aspect ratios and wall segmentation was performed, showing that segmentation of CLT walls decreases their stiffness and strength but significantly improves their deformation capacity.
Article
This paper presents the results of an extensive experimental programme on typical cross-laminated timber (CLT) screwed connections conducted at CNR-IVALSA research institute. In-plane monotonic and cyclic shear and withdrawal tests were performed on screwed wall-to-wall, floor-to-floor and wall-to-floor CLT connections. Mechanical properties such as strength, stiffness, energy dissipation, ductility ratio and impairment of strength were evaluated. The experimental results showed good performance of CLT screwed joints under cyclic loads when ductile behaviour was achieved. Brittle response occurred only in cases where requirements for end and edge distances were not satisfied. The experimental characteristic shear strength and mean slip modulus of the connections were compared with values obtained using analytical design equations. The Eurocode 5 (EC5) formulas overestimated the characteristic strength values in some cases, while the Uibel and Blaß formulas specifically developed for CLT connections provided more accurate and conservative predictions. In cases where brittle failures were attained, the analytical values overestimated the experimental ones. This issue can be avoided when the requirements for minimum edge and end distances stated by EC5 are fulfilled. EC5 empirical formulas for the prediction of the screw connection slip modulus at serviceability limit state corresponded well with the experimental elastic values. The overstrength factor, which is of great importance in capacity-based design, was also evaluated, and a conservative value of 1.6 can be recommended for screwed CLT connections.
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