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SEISMIC STRENGTHENING OF EXISTING BUILDINGS WITH
CROSS LAMINATED TIMBER PANELS
Iztok Sustersic1, Bruno Dujic
2
ABSTRACT: This paper deals with the issue of seismic retrofit and energy sanation of existing older buildings. A
possibility for solving both problems at once by applying a new outer shell made of cross laminated timber (crosslam or
XL) plates and effective aerogel insulation is presented. A seismic strengthening case study is presented on a 3 story
reinforced concrete frame building. Thermal insulation properties of the insulated panels are presented.
KEYWORDS:timber, seismic retrofit, energy
1 INTRODUCTION 123A majority of the buildings on seismically active areas
built before the nineteen sixties or seventies have two
major problems first they are seismically unsafe
because of the lack of seismic design codes at the time
they were built. And second, they have a high energyconsumption because of lack of insulation, proper details
etc. The proposed retrofit system deals with bothproblems at once; a new outer cross laminated timber
wall stabilises a building against horizontal shear forces
that are caused by earthquakes on one hand the timber
panels have a low mass and therefore dont contributemuch to seismic forces, but are very stiff on the other
hand [1] and provide high shear resistance. In addition
the new outer wall if combined with an effective
insulation provides a very good thermal insulation of
the building a combination of a 95 mm cross laminated
timber plate and 60 mm thermal Aerogel Spaceloftinsulation gives a U factor of 0,19 W/m2K [2]. Timber
panels also store CO2. A 170 mm outer shell (including
a facade) could therefore provide sufficient buildingthermal insulation and strengthen a reasonably sized
building against earthquakes. The new outer shell couldbe integrated with windows, doors and a facade already
in the manufacturing plant. Than the panels could be
transported to sight and rapidly attached to the building
(with proper detailing, subjected to earthquake
demands). The system is still in development but so far
seems to be most suitable for buildings up to 4 or 5floors, even floor plans, structures with stiff floor
1Iztok Sustersic, CBD d.o.o. , Lopata 19G, 3000 Celje,
Slovenia. e-mail: iztok.sustersic@cbd.si1dr. Bruno Duji, CBD d.o.o. , Lopata 19G, 3000 Celje,
Slovenia. e-mail: bruno.dujic@cbd.si
membranes and access to all outer walls. Another
positive aspect of the outer shell is that there are no
harsh interventions to a building and that people dont
have to move out during the construction phase (unlike
when using most of conventional methods for seismicretrofit). All together it makes a unique system that
solves two major problems in older buildings on
seismically active areas and therefore prolongs the
lifespan of constructions, contributing to sustainability.In the following chapters the system is presented more in
detail. Crosslam timber panels [1, 3, 4] are presented aswell as the Aerogel Spaceloft material [2]l. The
background of seismic analysis and the performance
based design N2 method [5] used for the evaluation of
the seismic resistance of the case structure are discussed.
Thermal properties of the outer crosslam shell combined
with Spaceloft are presented as well as a comparisonwith conventional insulation.
2 MATERIAL CHARACTERISTICS2.1 CROSS LAMINATED TIMBER PANELSCrosslam timber panels have been developed in Austria
in the late 1990s. The panels are glued together from
several (min. 3 and up to 9 layers for standard setups)
layers of spruce boards where each layer (Fig 1) runs
perpendicular to the neighbouring two (or occasionally
the most outer two layers run parallel to achieve greater
strength and stiffness in one direction).
Due to the high stiffness, strength and in-plane stability,
crosslam quickly gained popularity among architects due
to the possibilities the system offered in construction
design.
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Figure 1:A 5layer crosslam panel
The first publications regarding the panels mechanical
properties and proposed methods for calculation were
published in the beginning of this century. The first
seismic tests were conducted in 2005 [1] and the firstmulti-storey (3-story) crosslam building (Fig 2) was
tested in full scale on a shaking table in Japan in 2006
[3] and another 7-story building in 2007. Unfortunately
to date crosslam is not yet covered with Europes current
standards and codes of practice [6, 7] for timber andseismic design.
2.2 AEROGEL SPACELOFTSpaceloft is a product of American Aspen Aerogels. It is
classified as one of the so called nano insulations. It is
basically a combination of a polymer fabric and the
silica aerogel. With the term aerogel we generally defy amaterial that is formed directly from a fluid gel in a
process that replaces the liquid in a gel with air. The
polymerisation of molecules in the solution forms
scattered nano-particles. Under the influence of catalysts
the particles form chains that create a net of nano pores.
In such state the liquid can no longer disperse though out
the structure. It is removed in a process of super critical
drying. The whole process is applied directly to the
polymer fabric and dried in the end.
Figure 2:Aerogel Spaceloft
Table 1:Aerogel Spaceloft technical characteristics
Margin Value Unit
Thickness (basic) 0,01 m
Density 150 kg/m3
Thermal conductivity 0,014 W/m K
Heat capacity 1046 J/kg K
Water vapour diffusion 4,51 Ng/Pa s m
Fire classification C -
Compression strength 70 kPa
Dynamic stiffness 23,7 MN/m3
Table 2: Comparison of basic technical characteristics ofinsulations
Aerogel
Density kg/m3 150
Thermal conductivity W/m K 0,014Heat capacity J/kg K 1,046
Water vapour diffusion Ng/Pa s m 4,8
Polistyren Rock wool
Density 15 200
Thermal conductivity 0,041 0,041
Heat capacity 1,26 0,84
Water vapour diffusion 25 4
3 SEISMIC RETROFIT CASE STUDY3.1 SPEAR STRUCTUREIn the paper a case study of seismic retrofit shall be
performed on a three-storey plan-asymmetric structure
(Fig. 3). The structure was conceived as a representative
of typical non-earthquake-resistant older constructions in
Southern European countries. It was designed for
vertical loads only, with the construction practice and
materials commonly used in Southern Europe in the
early 70s. This structure was pseudo-dynamically tested
at full-scale and analysed within the scope of the
European project SPEAR
Figure 3: The SPEAR structure [8]
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The typical reinforcement in columns and beams is
shown in Figure 5. For the analysis in the paper the
structure was initially modelled without any additional
retrofitting systems. An elastic modal spectral analysis
and more important a nonlinear static (pushover)
analysis were performed. The latter serves as a basis for
the application of the N2 method a performance baseddesign method with which structural damage can be
assessed for different types of earthquake intensity. The
N2 method is discussed more in detail in the next
section.
Figure 4: Floor plan and cross section of the SPEARstructure analysed
Figure 5: Typical reinforcement in means and columns
3.2 SEISMIC ANALYSISCurrent codes of practice suggest two different
approaches for design of ductile structures in
earthquake-prone regions [9]. The first approach, well-
known and widely used [7], is referred to as the Force-Based Design (FBD) method since it mainly focuses on
designing the strength of the structure. The objective is
the evaluation of the behaviour factor q, which is
employed to transform the elastic demand spectrum into
an inelastic design spectrum. In this way a non-linear
structure can be designed using a linear-elastic static or
dynamic (modal response spectrum) analysis under
seismic action, with the structural ductility only
implicitly considered when evaluating the behaviour
factor q. The second approach, which explicitly refers to
the structural ductility in addition to the strength, is
based on a Non-linear Static Analysis (NSA) procedure.
The purpose of this approach is the evaluation of theactual structural response mainly in terms of ductility
demand and, hence, ultimate displacement induced in the
structure by the earthquake ground motion [4].
The NSA procedure is more complex than the FBD,
however it allows the designer to take into account the
actual dissipative behaviour of the structure.Furthermore, it can be used for Performance-Based
Design (PBD), where the design is achieved for different
performance levels such as no damage, limited structural
damage, important structural damage without collapse,
etc. Each level is generally linked to the structural
displacement by defining a damage index (i.e. for r. c.
beams and columns) and by assigning a limit value forevery performance level.
The NSA procedures are generally based on the
evaluation of the push-over curve, which represents the
response of the structure under a lateral loading
distribution schematising the seismic action. A numberof different methods have been proposed, including the
modified version of the Fajfars N2 method [5], which
has been adopted by the Eurocode 8 [7]. The aim of such
methods is the evaluation of the seismic displacement,
which is linked to the damage control of the structureand has to be kept below some reference values. The N2
method considers a performance point defined in terms
of both strength and displacement, where the structuralcapacity is compared with the demand of the seismic
ground motion. The base shear force and the top
displacement of a Multi-Degree-of-Freedom (MDOF)
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system are first computed by means of a non-linear
Push-Over Analysis (POA) and then converted
respectively to the spectral acceleration and
displacement of an equivalent Single-Degree-Of-
Freedom (SDOF) system. The demand of the seismic
ground motion is represented through the response
spectrum in terms of pseudo-acceleration anddisplacement. Such an inelastic spectrum depends upon
the cyclic behaviour of the SDOF system and the
characteristics of the ground motion (peak ground
acceleration and shape), and can be obtained from the
elastic spectrum using suitable reduction factors. The N2
method was found to provide the best approximation
among various NSA methods for SDOF systems with
different hysteretic models and for MDOF systems [10],
however the N2 was never developed for the design of
timber buildings with specific hysteretic behaviour.
Therefore it must be noted that the results derived in this
study could be non-conservative, because hysteresis
loops with pinching, slip and strength degradation(typical for connections in timber structures discussed
more in detail in the following sections) dissipate less
energy than bilinear plastic loops with the same ductility.
Figure 6: Relation between R T
Nevertheless, it should be also pointed out that for both
analysed retrofitted setups, the SDOF systems equivalent
to the multi-storey building have periods longer than Tc
which is usually the value from where the reduction
factor (R) and ductility factor () are considered to be
the same (Fig. 6), regardless the type of hysteresis loop.The results of these analyses should therefore be
considered as a preliminary study aimed to investigate
the effect of different retrofit panel setups on the seismicresistance of the case reinforced concrete frame building.
3.2.1 Modelling of crosslam panelsCurrently Eurocode 8 [7] does not provide extensive
seismic design guidelines for timber buildings. Crosslam
structures are not even included in it as well as in the
Eurocode 5 [6]. There are, however, some papers dealing
with modelling of crosslam [1, 4, 11, 12].
The complex panel layout can be modelled using anorthotropic, homogenised orthotropic or homogenised
isotropic material, depending on the possibilities offered
by the FEM software.
Figure 7: Proposed [4] reduction coefficients formodelling crosslam wall panels
The proposed [4] homogenised-orthotropic-plane stress-
reduced cross section method, which is based on the
reduction of a multilayer to a single layer section using
the coefficients k3 and k4 in Figure 7 to modify the
stiffnesses and strengths, is precise enough for the needsof seismic modelling, where building behaviour mostly
depends on the behaviour of connections.By assuming a plane stress state, only two moduli of
elasticity (E0 and E90), one shear modulus (G12) and
one poissons coefficient (12) need to be defined. The
thickness of the panels (finite elements) remains thesame as does the shear modulus. If the adjacent boards
of individual layers are not glued along their thickness
(i.e. for KLH panels), a 10% reduction in the shear
modulus is suggested. The aforementioned method was
used to defy the crosslam panels used in this study. 120
mm (layers 40-40-40) plates were taken into account.
3.2.2 Reinforced concrete plastic hinge definitonIn the FE model used for the analysis, the plastic hingesthat form at the ends of beams and columns are set in
accordance with Eurocode 8, part 3, that deals with the
assessment and retrofit of buildings. The hinges in theFE model in SAP2000 software [13] and their plastic
rotation capacity is set in accordance with the following
expression:
0,0145 (0,25) ,(,)
, ,
,
25
(1,275) (1)
The parameters in the expression mean the following: el
for primary seismic elements is 1,8 is the normalised
axial force in an element, is the mechanical
reinforcement ratio of the tension and compression
longitudinal reinforcement, fcand fyare the compression
strength of concrete and the tension strength of
reinforcement, Lvis taken as half of an elements length,
h is the width of an element, is the confinement
effectiveness factor, sx ratio of transverse steel
parallel to the direction x of loading, fyw is the
tension strength of the shear reinforcement and d theratio of the diagonal reinforcement. An additional
reduction factor of 0,375 is used due to the use of
smooth reinforcement bars.
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Figure 8: Definition of limit states in relplastic hinges
The plastic hinges are defined with a bil
rotation-bending moment. The crac
sections and a drop in load capacitcollapse (NC) state is neglected. The li
following: DL (damage limitation) pre
of reinforcement or the maximal elastic
cross section. At this point the plastic
remains 0. SD (significant damage) is
of the full plastic rotation of a cross(near collapse presents a full plastic r
section. A value of 3 um,pldefies the T
state, though in our study it is merely
only consider states up to NC.
3.2.3
Characteristics of connectionsconcrete frame and crosslam p
The connections are modelled on the
experimental response of a BMF 105 a
ten 60 mm long 4 mm diameter nails.
the response of the 3rd cycle is taken a
slip-force relationship in a non-linear l
in the FE model for connecting the R
retrofitting XL panel.
Figure 9: Calibration of the non-linear Fbackbone curve of the 3rd cycle of theresults of BMF105 brackets with 60 mmto shear force.
tion to bending in
inearised relation
ing in the RC
y after the nearmit states are the
sents the yielding
capacity of a r. c.
rotation (um,pl )
efined with 75%
section. The NCtation in a cross
C (total collapse)
theoretical as we
between theanels
slipshear force
ngle bracket with
The backbone of
the input for the
ink element used
C frame with the
EM link on thexperimentalnails subjected
Figure 10:A BMF105 angularpresumed XL panel r.c. plate cinside of the building is on the
The reason for picking theaccount the accumulated da
angular bracket. It is visible f
1st cycle yields about 30% hi
to the 3rd cycle. The connecti
the same response is model
horizontal direction. The cas
structure being retrofitted (thethe possibility of accessing
inside since the structure has
a case of a real residential co
probably have to be changed,
it from the outside.
3.2.4 Seismic resistance of tWith the use of the N2 metho
building in the X directions
The basic SPEAR structure,
an earthquake with peak grou
0,2 g has been retrofitted in toption was to try strengtheni
panels. A BMF 105 bracket
panel to the main structure at
That resulted in a not partic
the behaviour of the panels
the short leverages to the co
deformations. As a resultacceleration is raised by rough
hand the long panels with l
connectors resulted in an
allowable ground acceleratioeasier to assemble as the pan
doors as well as the facade
production plant and hence n
needed on the building.
buildings floor height is ov
transport can be necessary to
bracket (left) and theonnection (right). Theright side.
3rd cycle is to take intomage that occurs in the
om Figure 9, that i. e. the
gher peak strength oppose
on is simplified and hence
led for both vertical and
e study presumes that the
SPEAR construction) hasthe connections from the
o outer walls or infills. In
struction, the detail would
in order to allow access to
he SPEAR structure
d seismic resistance of the
is assessed.
hich could itself withstand
d acceleration of less than
o different ways. The firstg it with shorter crosslam
as used to attach the outer
every 30 cm of the panel.
larly stiff structure, since
as mostly in bending and
nections caused extensive
the allowable groundly 21 percent. On the other
nger leverages and more
almost 90% increase in
. Such a system is alsols allow the windows and
to be assembled in the
mayor additional work is
owever if the existing
r 2,95 m, an exceptional
et the panels on site.
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Figure 11: The basic SPEAR structureboth cases of the seismically retrofittedmiddle the option with short panels andoption with long panels.
n the left andtructure. In the
on the right the
Figure 22:AD format elastic sspectra and capacity diagramsdifferent retrofitted constructiosystems written next to capaci
Table 3: Comparison of maxiacceleration with basic and ret
Basic structure
Retrofitted with short pane
Retrofitted with long panel
4 THERMAL INSULOF A CROSSLAM4.1 CROSSLAM PANELS
SPACELOFT
In Table 4 a comparison of d
attached to a crosslam wall i
thick XL wall panel was in
polistyren, 18 cm thick rock w
Spaceloft. Mass of a wall se
(U), phase shift (t), temperat
surface temperature (Tn,surf)
Table 4: Comparison [2] of difwall insulation
m [kg]
Polistyren 18 cm 65
Rock wool 18 cm 98
Spaceloft 18 cm 89
[-]
Polistyren 18 cm 117
Rock wool 18 cm 199
Spaceloft 18 cm 1340
The results show that when co
with insulation just on the out
of Spaceloft is primarily its s
rather small mass, the pha
ectra (for 0,3 g), inelasticfor the basic and the twos. Period values of SDOFy curves.
um allowable groundrofitted structures
Maximum
allowable
ground
acceleration [g]
0,197
ls 0,239
s 0,374
ON PROPERTIESACKET
INSULATED WITH
fferent types of insulation
s presented [2]. A 95 mm
sulated with 18 cm thick
ool and 6- and 18 cm thick
up (m), heat conductivity
re damping () and inner
re compared.
erent types of a crosslam
U [W/mK] t [h]
0,19 6,69
0,19 12,37
0,07 17,81
Tn,surf [C]
19,27
19,27
19,72
mparing a basic wall setup
r side, the main advantage
mall thickness. Due to its
se shift and temperature
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damping are not as good as with conventional insulation.
However it still performs better than polistyren, though
the latter has an advantage in even lower mass (an
important issue for seismic retrofit).
But a choice between one of the insulations when using a
crosslam outer jacket would significantly be influenced
by the primary structure being retrofitted. If a heavymasonry structure is being considered, phase shift is not
a really a problem due to the thermal capacity of the
basic structure. In such a case one strives for a highly
insulating and thin outer shell (not to push the windows
even deeper into the building), which is exactly what a
Spaceloft setup can provide.
4.2 CROSSLAM PANELS COMPARED WITHOTHER TYPES OF WALL SETUPS
INSULATED WITH SPACELOFT
Zrim [2] has optimised each of the outer wall type setups
(brick, concrete, timber), depending on the position and
thickness of the outer and/or inner insulation. Thepreferred setups for each of the basic wall materials arepresented and evaluated in the following table. Mass
(m), heat conductivity (U), phase shift (t), temperature
damping () and inner surface temperature (Tn,surf) are
compared in Table 5.
Table 5: Comparison [2] of optimised wall setups fordifferent basic wall materials
m [kg] U [W/mK]
Masonry wall 418 0,15
Crosslam wall 83 0,09
Reinforced concrete wall 776 0,13
t [h] [-] Tn,surf [C]
14,68 2840 19,40
14,09 1644 19,64
12,04 4628 19,48
The crosslam setup was generally pointed out as the
most desirable, since it performed best on 3 out of 5
compared criteria. It came out second on phase shift and
finished last on temperature damping.
The crosslams low mass is extremely favourable forseismic retrofit, so we dont unnecessarily add more
mass to the structure being retrofitted and hence enlarge
the seismic forces.
5 CONCLUSIONSIn the paper we have presented a new system for seismic
retrofit and energy sanation of existing buildings. As far
as the preliminary studies show it is most advisable to
use longer (if possible) crosslam panels instead of
shorter segments. If shorter segments are used, it would
be advisable to join adjacent panels together on the
vertical sides as well, though this option has not yet been
explicitly analysed. As the crosslam jacket does not
influence the structures ductility, just its stiffness and
strength, it would be advisable to use stronger instead of
more ductile connections. The ductility of the basic
structure is still limited with the capacity of the primary
construction system. There is of course a possibility to
neglect the contribution of the latter and assume that all
load is transferred to the outer crosslam shell though
that option has not been investigated yet. The crosslam
panels can be upgraded with various insulation types.
The most efficient in terms of thickness is the AerogelSpaceloft. The small thickness is desirable when dealing
with existing structures with thicker walls. In that case
the temperature damping (which is a drawback of the
light Spaceloft system) does not present a problem any
more. Due to its light weight that does not contribute
much to seismic forces, polistyren presents an interesting
(and cheaper) option as well.
ACKNOWLEDGEMENTThe research support provided to both authors by the EU
through the European Social Fund 'Investing in your
future' is gratefully acknowledged. The kind cooperationof Mr. Zrim is also acknowledged.
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