STRUCTURAL CHARACTERIZATION OF MULTI-STOREY CLT

BUILDINGS BRACED WITH CORES AND ADDITIONAL SHEAR

WALLS

A. Polastri1, L. Pozza2, C. Loss3 & I. Smith4

1
Trees and Timber Institute - National Research Council of Italy (CNR IVALSA), Italy
2
Department of Civil, Environmental and Architectural Engineering, University of Padova,
Italy
3
Department of Civil, Environmental and Mechanical Engineering, University of Trento,
Italy
4
Faculty of Forestry & Environmental Management, University of New Brunswick, Canada

This paper was originally published for the 2015 INTER Meeting.

KEYWORDS

CLT structures, core structures, seismic design, shear walls

1 INTRODUCTION al. 2013). Multi-storey building superstructures

in which beam-and-column frameworks resist
In last twenty years the CLT panels have
effects of gravity loads and cross-braced or
become widely employed to build multi-
core substructures and exterior CLT shear walls
storey residential and mercantile buildings.
resist effects of lateral forces from earthquakes
These buildings are often characterised by the
or wind have been found structurally effective,
presence of many internal and perimeter shear
and fail in predictable stable ways if overloaded
walls. Such structures have been widely studied
(Smith et al. 2009). Advantages of such systems
through experimental and numerical simulation
can include creation of large open interior
methods. The most comprehensive experimental
spaces, high structural efficiency, and material
investigation to date on seismic behaviour of CLT
economies.
buildings was carried out by CNR–IVALSA, Italy,
under the SOFIE Project (Ceccotti 2008, Ceccotti Recently innovative connection solutions that
et al. 2013). Other important investigations create discrete panel-to-panel, or panel-to-
have been conducted at the University of other material joints have been developed in
Trento, Italy (Tomasi and Smith 2015). European Italy (Polastri and Angeli 2014). The method
seismic performance related tests have also results in point-to-point mechanical connections
been conducted at the University of Ljubljana, that only connect corners of individual CLT
Slovenia where the behaviour of 2-D CLT shear panels in ways that fulfil hold-down and lateral
walls with various load and boundary conditions shear resistance functions (Gavric et al. 2013).
were assessed (Dujic et al. 2005). FPInnovations This has the advantage of making the load paths
in Canada has undertaken tests to determine the within superstructures unambiguous. Different
structural properties and seismic resistance of connectors have also been tested to find the
CLT shear walls and small-scale 3-D structures best ways to make point-to-point connections
(Popovski et al. 2014). Those and other between CLT panels and steel structures (Loss
studies have enabled characterisation failure et al. 2014).
mechanisms in large shear wall systems (Pozza et
During recent years connector designs had

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evolved considerably making them suitable for
much large systems that place high capacity
demands on connections, with emphasis on
requirements for high seismicity areas (Polastri
et al. 2014). During such development attention
was paid to avoiding the possibility of brittle
behaviour of joints to CLT panels having many
nails.
Structural performance issues not fully studied (a)
are those related to using CLT building cores
as replacements for one constructed from
reinforced concrete or masonry. Pertinent issues
relate to vertical continuity between storeys,
connections between building core elements and
elevated floors, and building core-to-foundation
connections.

2 MECHANICAL CHARACTERIZATION OF THE (b)

CONNECTORS
Figure 1: Typical tests results: hold-down (a) and angle
2.1 Experimental studies bracket (b).

The mechanical behaviour of connection description of the mechanical behaviour of

systems for CLT structures that employ thin trends representing elastic phase and post-elastic
metal elements fastened to panels with nails phase responses (Piazza et al. 2011). However,
or other slender metal fasteners is well known, as this paper deals with Linear Dynamics Analysis
as demonstrated by numerous scientific papers of superstructure systems only the parameters
(Ceccotti et al. 2008, Pozza et al. 2013). The that characterize the elastic properties of
behaviour of such connectors is determined connections (ktest) and the maximum load at
largely by the elastoplastic response of the failure (Fmax) are reported here, Table 1.
fasteners, and to a lesser extent by the response
2.2 Analytical definition of stiffness according
of steel elements. Stiffness and capacity
to Eurocode 5
values implemented into the numerical models
described in Section 4 were calculated directly The Finite Element (FE) model presented in
from experimental data. Section 4 implements hold-down Rothoblaas
WHT 620 (EOTA 2011) and angle brackets
The first study was carried out at CNR-IVALSA
TITAN TTF200 (EOTA 2012) connectors joined
(Gavric et al 2011), the second study at the
to CLT panels manufactured from class C24
University of Trento (Tomasi and Smith 2015).
wood boards using 32 4x60 or 30 4x60 Anker
In both cases tests were conducted according
nails. The initial stiffness of connectors was
to the European standard EN 12512 (CEN 2006).
calculated taking into account the stiffness of
The CEN 2006 protocol provides a load history
the steel-to-timber nailed joints in shear and
characterized by load cycles of increasing
hold-down connections. Deformation of steel
intensity and is intended to apply to structures
parts within the connections are very small,
in seismic regions. As suggested by the standard,
compared deformation of nailed joints, and
a preliminary monotonic test was undertaken to
was therefore neglected. Characteristic load-
define the magnitudes of cyclic load excursions,
carrying capacities, Fv,Rk, and slip moduli, kser,
Figure 1.
were calculated based on Eurocode 5 (CEN
Initial stiffness was calculated according to 2014), Table 1.
‘method b’ specified by EN 12512 that permits

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Table 1: Experimental and Eurocode 5 derived connection properties

Connection type Elastic stiffness (kN/mm) Capacity (kN)

Test (ktest) EC5 (kser) Test (Fmax) EC5 (Fy,Rk)
TITAN TTF 200 8.2 23.1 70.1 35.5
WHT 620 12.1 24.8 100.1 85.2

3 ESTIMATION OF T1 AND DESIGN OF a superstructure. During design engineers are

CONNECTIONS required to solve iteratively to find the principal
natural frequency (f1 = 1/T1) using a scheme such
A crucial issue in the design of a CLT building
as that in Figure 2. Under the shown scheme: (1)
under horizontal seismic action, is the definition
the stiffness of the connections influences the
of the principal elastic vibration period (T1) of an
global stiffness of the building and therefore its
entire superstructure (CEN 2013). Such vibration
principal elastic period; (2) the external force
period depends on the mass distribution and
induced by earthquake in each connection is a
on the global stiffness of the buildings. In
function of the principal vibration period; (3)
a CLT structure the global stiffness of the
the load bearing capacity of the connection
buildings is highly sensitive to deformability of
must be compatible with the external force; (4)
the connection elements (Pozza et al. 2013).
the strength and the stiffness of the connection
Consequently for a precise control of the vibration
are linked through the effective number of
period of the building it is crucial to define the
fasteners.
stiffness of each connections used to assemble

Figure 2: Calculation process for design of connections

An efficient approach to design a CLT structure to be compatible with external static forces.
is to start from a preliminary definition of the This allows estimation of the connection elastic
external force induced by earthquake in each stiffness (ktest or kser), and therefore realistic
wall panel according to the common equivalent preliminary estimation of T1. Then T1 can be in
static force linear static analysis approach (CEN modal analyses to calculate the effective forces
2013). This does not involve the definition of T1 induced in connections by earthquakes. Obtained
accounting for effects of connection stiffness. connection forces may or may not be compatible
Once static forces on each CLT wall panel are with the connection strength, and if not it is
defined connection capacities can be designed necessary to redesign connections. Afterward it

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is possible to perform a more iterative precise storey CLT buildings braced with cores and
frequency analyses until solutions, including additional shear walls from the seismic design
connection designs, are convergent. perspective based on effects of varying design
parameters. Varied design parameters are:
4 NUMERICAL ANALYSIS OF CORE TALL
number of storey (3-5-8), lateral shear wall
BUILDINGS
panels width, construction methodology, and
4.1 Case study buildings regularity of connectors as a function of the
The aim is to characterize behaviour of multi- height within a superstructure, Table 2.

Table 2: Examined building configuration

Case Study ID 3(5-8) A R 3(5-8) A I 3(5-8) B R 3(5-8) B I 3(5-8) C R

Graphical
schematization of
building cores (ex.
3-storey case)

Joint free wall

Panel assembly Joint free wall panels Jointed wall panels
panels
Elevation
Regular Irregular Regular Irregular Regular
regularity
Construction
Platform system -
methodolgy

4.1.1 Geometric configurations 3m in all cases, resulting in total superstructure

heights of 9m, 15m and 24m respectively. All
Examined case-study building superstructures
CLT panels in the core walls have a thickness of
have footprint dimensions of 17.1m (direction X)
200mm. CLT panels in perimeter shear walls are
by 15.5m (direction Y). Seismic Force Resistant
154mm thick, except for those in the lowest four
Systems (SFRS) include a building core that is
storeys of the 8-storey SFRS which are 170mm
5.5m by 5.5m on plan, and partial perimeter
thick. Floor diaphragms are composed of 154mm
shear walls constructed from CLT panels with a
CLT panels in all cases.
total base length of 6m, Figure 3. Storey height is

(a) (b)
Figure 3: SFRS wall configurations of case study buildings (a) and typical FE model (b).

4.1.2 Design method (the highest value for Italy) with a building
importance factor of λ = 0.85. [*Deep deposits
The earthquake action for these case study
of dense or medium dense sand, gravel or stiff
buildings was calculated according to Eurocode
clay with thickness from several tens to many
8 (CEN 2013) and the associated Italian
hundreds of meters]. The seismic action was
regulations (MIT 2008) using design response
calculated starting from the elastic spectra
spectra for building foundations resting on
and applying an initial q-reduction factor of 2
ground type C*, assuming the PGA equal to 0.35g

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(CEN 2014). The coefficient kr was taken equal 4.1.3 Finite element (FE) models
to 1.0 for regular configurations and 0.8 for
Numerical models of the investigated building
non-regular configurations. Figure 4 shows the
were realized using the finite-element code
adopted design spectra and other relevant
Strand 7 (2005). The illustrative FE model in
design parameters. The figure shows T1 values
Figure 3 (b) uses linear elastic shell elements
determined by simplified formula and numerical
to represent CLT panels and link elements to
frequency analyses methods for configurations A
simulate the elastic stiffnesses of connectors.
R 3-5- 8.
Beam elements with pinned end conditions
Connections were first designed using the force were used to represent beam members
pattern obtained applying linear elastic static interconnecting perimeter shear walls and shear
analysis (CEN 2013) and the seismic action defined walls in the building core at the top of each
by taking T1 = T1_EC8. Connection designs were storey. Horizontal slabs elements in floor and
then refined using the rotation and translation roof diaphragms were assumed to be rigid in-
force equilibrium approach described by Gavric plane.
et al. (2011) and Pozza and Scotta (2014) and
All the 15 building configurations have been
the iterative design process in Figure 2.
modelled respecting the geometrical features
and connection stiffnesses in Tables 1 and 2.
4.2 Analysis results
Results presented here were obtained by modal
response spectrum analyses of case study
buildings, Tables 3 to 6 and Figure 5. Those
tables and figure show calculated building
principal elastic periods (T1), base shear forces
(v) on angle brackets at the Ultimate Limit State
(ULS), uplift forces on base hold-down anchors
at ULS (N), and the maximum inter-storey drift
values (δ) at Damage Limitation State (DLS).
The alternative values given represent effects
of taking connection stiffnesses (kconn) equal
to values derived from Eurocode 5 (kser) versus
values derived from experiments (ktest). Plus in
the case of T1, the simple formula value T1_EC8
is included (CEN 2013). Inter-storey drift was
(a) calculated for each case study building using the
Modal Response Spectrum Analyses and the DLS
design spectrum.
Observing Figure 5 it is apparent that can be
seen that in most cases use of experimental
connection stiffnesses (kconn = ktest) leads to
much larger T1 values than those predicted
based Eurocode 5 based estimates of connection
stiffnesses (kconn = kser). Similarly using the
simple formula given by Eurocode 8 leads to
low estimates of T1 values. Interestingly use of
(b)
Eurocode 5 based estimates of kconn results is
Figure 4: Input data for seismic analysy (a); design spectra
estimates of T1 relatively close to simple formula
and calculated periods (b).
values. However results suggest that neither of
those approaches are reliable ways of estimating

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Table 3: Predicted principal elastic periods (T1)

[s] 3AR 3AI 3BR 3BI 3CR 5AR 5AI 5BR 5BI 5CR 8AR 8AI 8BR 8BI 8CR
T1_EC8 0.26 0.26 0.26 0.26 0.26 0.38 0.38 0.38 0.38 0.38 0.54 0.54 0.54 0.54 0.54
T1_kconn.=kser 0.22 0.19 0.22 0.20 0.18 0.37 0.34 0.38 0.35 0.30 0.73 0.69 0.73 0.68 0.50
T1_kconn.=ktest 0.47 0.39 0.44 0.37 0.33 0.80 0.61 0.73 0.57 0.45 1.40 4.14 1.30 1.08 0.66

Table 4: Predicted base shear per unit of length (v)

[kN/m] 3AR 3AI 3BR 3BI 3CR 5AR 5AI 5BR 5BI 5CR 8AR 8AI 8BR 8BI 8CR
V_kconn. = kser 28.2 33.2 32.7 40.3 35.8 42.3 51.7 51.8 64.2 50.4 50.2 64.6 61.6 82.6 79.7
V_kconn. = ktest 28.1 34.9 33.4 36.4 29.2 29.7 49.3 38.4 58.4 50.5 34.0 47.4 42.7 76.3 61.6

Table 5: Predicted free edge base uplift forces (N)

[kN] 3AR 3AI 3BR 3BI 3CR 5AR 5AI 5BR 5BI 5CR 8AR 8AI 8BR 8BI 8CR
N_kconn. = kser 128.1 170.9 152.0 178.1 161.3 316.0 420.6 355.4 447.3 373.2 511.6 884.1 559.5 759.7 897.0
N_kconn. = ktest 138.5 185.7 149.4 232.1 138.5 246.2 403.8 260.1 455.7 366.9 334.4 527.3 339.5 516.1 678.8

Table 6: Predicted maximum inter-storey drift (δ)

[mm] 3AR 3AI 3BR 3BI 3CR 5AR 5AI 5BR 5BI 5CR 8AR 8AI 8BR 8BI 8CR
δ_kconn. = kser 2.9 1.8 2.5 1.9 1.4 5.6 4.4 5.1 4.3 2.6 9.9 8.8 8.8 7.8 4.7
δ_kconn. = ktest 12.3 7.8 10.9 6.8 3.8 15.6 9.7 14.0 8.8 5.1 21.3 13.9 18.5 12.4 5.6

Figure 5: Comparison of estimates of principal elastic periods and base free edge uplift forces

principal natural periods of buildings having and the out of plane stiffness, provided by the
SFRS consisting of CLT cores and perimeter shear interposed floor slabs, are neglected. On the
walls. Consequences of discrepancies in kconn other hand, FE models did not take into account
values from those found by testing varied in nonlinear deformability or large displacements
their effects on v, N and δ values, but in general effects.
results show that how connection stiffnesses are
It is possible to achieve large vertical reaction
estimated can alter design force and lateral drift
forces using a group of hold-down anchors
estimates by substantial amounts. For example,
working “in parallel”. To obtain the required
estimates of δ were especially sensitive for
uplift force resistances, that can be greater
eight-storey buildings.
than 600kN (configuration 8CR), it is necessary
It is important to underline that the adopted to use more than eight hold-down anchors;
FE model is a limiting condition representing however it is not demonstrated that the hold
the maximum deformability of the system since downs, disposed in the aforementioned group
the interaction between the orthogonal walls configuration, are able to spread the total force

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between the different reaction elements. the construction system varies significantly
compared to other common CLT structures. In
such cases the mechanism of deformation of the
5 CONNECTION SOLUTIONS FOR INNOVATIVE floors under in-plane actions can increase the
DIAPHRAGMS level of shear forces in the connections, due
Diaphragms are an integral part of the building to the distance between the supports and the
any SFRS and if they have high stiffness and number and placement of shear walls around the
capacity any non-linear behaviour of the entire perimeter of the building. Consequently standard
structure is primarily defined by the response of connection techniques for CLT elements can be
the vertical bracing elements and complexity inadequate in terms of capacity and special high
of the seismic analysis is reduced. CLT multi- performance connections are then required,
storey buildings are erected using panels with e.g. Figure 6 (b). Discussion here addresses
limited dimensions because of production and use of two innovative high-capacity connection
transportation limitations (FPInnovations 2011). technologies suitable for the purpose.
In floors and roofs, the different CLT panels are Figure 7 (a) shows a slab made of CLT panels
commonly joined together at the edges using joined together by steel beams, with the beam-
dowel-type mechanical fasteners like self- to-panel connections designed and engineered
tapping screws, Figure 6 (a). from the perspectives of mechanical behaviours
of the materials, installation tolerances,
feasibility of on-site assembly, and cost. The
load-slip curve (F-δ) for these connections
measured by tests is shown in Figure 7 (b) based
on Loss et al. (2014). The operating principle of
such floors is similar to a truss system in which
each pair of steel beams is braced by the CLT
panel and related to characteristics of the beam-
to-panel connections. In Figure 7 it is presented
a beam-to-panel connection solution obtained
(a)
by the use of steel plates welded to the beam
and glued to the CLT panel.

(b)
Figure 6: Typically floor-floor panel connections (a) and
new X-RAD connector (b).

The in-plane behaviour of the horizontal floors

constructed from CLT panels and connections
is mainly affected by the response of panel-to-
panel connections, with the overall length-to-
width ratio of the floor and aspect ratio of the
CLT panels playing primary role (Ashtari et al. (a)
2014). More generally, the in-plane behaviour of
CLT floors depends on the building system, the
location of the bracing walls and their stiffnesses
(Loss et al. 2015). For multi-storey CLT buildings
with cores and additional perimeter shear walls

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 (a)

(b)
Figure 7: Innovative hybrid floor system (a) and tests
results (b).

The second innovative connection method

discussed here employs X-RAD connectors, Figure
(b)
6 (b) that create discrete panel-to-panel joints.
Figure 8: Innovative hybrid floor system (a); tests setup
This method results in point-to-point mechanical
and test results (b).
connections in ways that fulfil hold-down and
lateral shear resistance functions (Polastri and the stiffnesses of connections during structural
Angeli 2014). As for shear walls, making point-to- analyses from which T1, peak dynamic forces
point interconnections lessens the chances that flowing through wall and connection elements
structural systems will fail in unintended ways and inter-storey drift are estimated.
if overloaded. Figure 8 (a) shows use of X-RAD For buildings having three to eight storeys T1
connectors in a floor diaphragm with the result estimates, shear and uplift forces at bases of
being ability to transfer very large forces and wall panels, and inter-storey drift can all be
achieve very high stiffness (Polastri and Angeli miscalculated by substantial margins, Tables 3
2014). As shown in Figure 8 (b) the load capacity to 6. The remainder of this discussion assumes
was 171kN and the elastic stiffness 23.6kN/mm. connection stiffnesses derived from test data
Although results are not reported here it is to be (kconn = ktest) are the most accurate and therefore
mentioned that the authors are currently studying correct estimates of how connections embedded
use on the described innovative connectors as within SFRS actually behave. Although designers
ways of creating next generation of CLT floor could also estimate connection stiffnesses
diaphragms. It is anticipated this will enable in many other ways, the authors believe it
new applications of CLT like construction of tall reasonable to suppose that estimating stiffnesses
building having large footprints and braced by will often be based on information in Eurocode
one or more building cores and perimeter shear 5 and similar international codes (i.e. kconn = kEC5
walls. in case studies).
Case studies suggest T1 values being
underestimated by up to 50 percent is a
6 DISCUSSION AND IMPLICATIONS FOR DESIGN realistic scenario unless designers use test
PRACTICE data to estimate connection stiffnesses. Large
As the case studies demonstrate, hold-down errors occurring during subsequent calculation
and shear connections at the bases of CLT wall of shear and hold-down forces and inter-
panels largely determine the behaviors of SFRS. storey drift is also highly feasible. In capacities
It is therefore crucial to properly represent terms estimation of design forces and sizing

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shear and hold-down connection the likely on proposals here dealing with the structural
consequences of how connection stiffnesses analyses of CLT shear wall systems.
are characterized are lesser, with connections
being somewhat overdesigned being normal (i.e.
based on assuming kconn = kEC5). However, errors ACKNOWLEDGEMENT
in estimation of inter-storey drift are likely to The authors wish to thank Eng. Davide Trutalli,
be much greater. As results in Table 6 show, student Matteo Pasin of University of Padua and
interstorey-drift was estimated to be up to four Arch. Francesca Paoloni trainee at CNR-IVALSA.
times larger assuming kconn = ktest than assuming
kconn = kEC5.
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