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DIAPHRAGMATIC BEHAVIOUR OF HYBRID CROSS-LAMINATED TIMBER STEEL FLOORS

Authors:

Abstract

The diaphragmatic behaviour of floors represents one important requirement for earthquake resistant buildings since diaphragms connect the lateral load resisting systems at each floor level and transfer the seismic forces to them as a function of their in-plane stiffness. This paper presents an innovative hybrid timber-steel solution for floor diaphragms developed by coupling cross-laminated timber panels with cold-formed custom-shaped steel beams. The floor consists of prefabricated repeatable units which are fastened on-site using pre-loaded bolts and self-tapping screws, thus ensuring a fast and efficient installation. An experimentally validated numerical model is used to evaluate the influence of the; i) in-plane floor stiffness; ii) aspect ratio and shape of the building plan; and iii) relative stiffness and disposition of the shear walls, on the load distribution to the shear walls. The load transfer into walls and lateral deformation of the construction system primarily depend on the adopted layouts of shear walls, and for most cases an in-plane stiffness of floors two times larger than that of walls is recommended.
DIAPHRAGMATIC BEHAVIOUR OF HYBRID CROSS-LAMINATED
TIMBER STEEL FLOORS
Cristiano Loss1, Filippo Gobbi2, Thomas Tannert3
ABSTRACT: The diaphragmatic behaviour of floors represents one important requirement for earthquake resistant
buildings since diaphragms connect the lateral load resisting systems at each floor level and transfer the seismic forces
to them as a function of their in-plane stiffness. This paper presents an innovative hybrid timber-steel solution for floor
diaphragms developed by coupling cross-laminated timber panels with cold-formed custom-shaped steel beams. The
floor consists of prefabricated repeatable units which are fastened on-site using pre-loaded bolts and self-tapping
screws, thus ensuring a fast and efficient installation. An experimentally validated numerical model is used to evaluate
the influence of the; i) in-plane floor stiffness; ii) aspect ratio and shape of the building plan; and iii) relative stiffness
and disposition of the shear walls, on the load distribution to the shear walls. The load transfer into walls and lateral
deformation of the construction system primarily depend on the adopted layouts of shear walls, and for most cases an
in-plane stiffness of floors two times larger than that of walls is recommended.
KEYWORDS: Hybrid floors, Prefabrication, Modular members, Load path, Numerical Analyses, Sensitivity analysis
1 INTRODUCTION
123
Earthquake-resistant structures consist of an ensemble of
vertical and horizontal elements designed to achieve
specific safety and performance requirements under
lateral loads [1]. Floors, which act as diaphragms, carry
lateral loads and transfer them to the vertical bracing
elements.
The effect of the diaphragm’s deformability is
significant when designing the structure for seismic
loads, influencing the amount of load transferred to each
shear wall. In general, when the in-plane stiffness of
floors is significantly larger compared to the lateral
stiffness of the shear walls, diaphragms can be assumed
rigid, and the seismic analysis is simplified assuming
masses lumped at each story centre of mass. Otherwise,
the actual stiffness of the diaphragms needs to be taken
into account to perform the seismic analysis of the
structure. Most building codes provide provisions for
judging the rigid diaphragm assumption in case of
concrete or composite concrete floors, as well as
simplify methods of analysis of flexible diaphragms
when they can be idealised as deep beams. Focusing on
wood, ASCE 7 [2] and other codes provide similar rules
in case of wood light-frame buildings. More in detail,
ASCE 7 gives a scale of classification distinguishing
among flexible, rigid, or semi-rigid wood diaphragms.
1
Cristiano Loss, Integrated Wood Engineering, University of
Northern British Columbia, cristiano.loss@unbc.ca
2 Filippo Gobbi, University of Trento, Italy,
filippo.gobbi@studenti.unitn.it
3 Thomas Tannert, Integrated Wood Engineering, University of
Northern British Columbia, thomas.tannert@unbc.ca
A limitation is that current codes do not provide criteria
for permitting the classification of other wood-based
construction systems, such as cross-laminated timber
(CLT) or hybrid timber floors. Furthermore, although
CLT is gaining popularity in residential and non-
residential applications [3], little research on the in-plane
stiffness and strength of CLT-based floor diaphragms
has been conducted [4,5]. In particular, little is known
about the response of CLT plates in the context of open-
space hybrid construction systems, such as those having
primary steel or reinforced concrete frameworks. Some
previous works addressed the in-plane load-carrying
capacity and shear stiffness of timber-based floors [6-8].
Ashtray et al. [6] showed that the in-plane behaviour of
CLT floors is mainly influenced by the response of the
panel-to-panel connections, length-to-width ratio of the
floor and aspect ratio of the CLT panels.
1.1 OBJECTIVE
This paper deals with an innovative hybrid timber-steel
solution for floor diaphragms developed by coupling
CLT panels with cold-formed custom-shaped steel
beams. It represents a dry sustainable solution where the
materials usage, CLT panels and steel beams, is
optimised in order to design lightweight and robust
floors. This paper also includes results of a sensitivity
study performed to evaluate the influence of the in-plane
floor stiffness, aspect ratio and shape of the building
plan, as well as relative stiffness and disposition of the
shear walls on the load distribution into the shear walls.
Complementary studies on the in-plane response of these
floors are published in [9,10].
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
Loss, C., F. Gobbi, and T. Tannert (2018). “Diaphragmatic behaviour of hybrid cross-
laminated timber steel floors.” Proceedings of the 15th World Conference on Timber En-
gineering (15WCTE), Seoul, Republic of Korea, August 20th-23rd
6.2 2D ANALYSES OF PHASE I
The results summarised in Figure 11 and Table 2 show
that for floor stiffness at least twice that of shear walls
(k*≤0.5), the difference in loads between the internal and
external walls remains below 10%, independent of the
floor aspect ratio and the wall arrangement. Conversely,
when the stiffness of the floor is comparable or lower
than that of the walls (k*≥1), the wall positions
significantly affect the load distribution; particularly
evident for stretched floor plans. In the worst case
scenario (A5), the load distribution to external and
internal walls can differ up to 61% compared to that of a
rigid floor.
Even though these findings are limited and do not cover
every possible design situation, they suggest general
practical design rules which consist first in the avoidance
of remarkable difference between the spacing of
subsequent shear walls and second in limiting their
distance. In other words, the diaphragmatic behaviour of
the floor can be improved placing shear walls following
a uniform pattern inside the plan and, when possible,
reducing their tributary area (or increasing their number).
These results are in good agreement with the effective
behaviour observed in case of light wood-frame
buildings in which traditional floors are continuously
supported and connected to all the perimeter and internal
walls, which all tougher contribute in providing the
lateral-load resistance of the system.
6.3 3D ANALYSES OF PHASE II
Figure 12 and Table 3 show that when kD* increases, the
distribution of the load between the external and internal
shear walls tends to diverge from the ideal solution
which has equal transfer among the bracing elements, as
ensured by rigid floors. This effect is partially mitigated
by the layout of shear walls chosen in the B4 case. On
the contrary, the difference of the load in the worst case
scenario of B3 can rise to 32% when the kD* is assumed
equal to 10, which means stiffness of shear walls ten
times higher than the design condition herein studied.
The analysis of capacity curves shown in Figure 13 gives
an overview of which is the impact of the floor at the
structural level. As general conclusion, it is difficult to
judge the influence of floors from these charts since the
lateral bearing mechanism of the structure changes
considerably at varying the stiffness of the shear walls.
More representative are results expressed in terms of
story displacements included in Table 4. In fact, it can be
shown that the stiffer the system, the more is the lateral
deformation of the floor. Besides, the deformability of
floor affects particularly the first floor, where in the
worst case scenario the displacement of floor represents
49% of the total measured. Interesting also to note that in
the design condition (kD*=1) the influence of floor in-
plane behaviour on the
d
i,tot of the first floor is between
15% and 20% independently of the shear walls layout
chosen.
7 CONCLUSIONS
This paper contributes to the ongoing research on new
hybrid wood-based solutions for the building market.
Specifically, the diaphragmatic behaviour of hybrid
cross-laminated timber steel floors was evaluated
performing a sensibility study. A numerical approach
based on an experimentally validated model was used. In
order to assess the impact of the floor on the structural
response, parameters varied in the analyses were
stiffness and layout of shear walls, as well as the aspect
ratio of the floor plan.
Based on the results, the following conclusions can be
drawn:
the numerical model developed has the potential
for other application such as for studying other
hybrid floor systems or different building’s
lateral load-resisting systems, as well as for
carrying out non-linear dynamic analyses;
the arrangement of the shear walls and the
relative stiffness between floor and shear walls
are fundamental parameters that guide the
distribution of the horizontal load into the
vertical bracing systems;
as practical and useful design rules: an in-plane
stiffness of floors two times larger than that of
walls is recommended; uniform distribution of
the vertical bracing systems avoiding large
spacing is also recommended;
in general, the actual in-plane stiffness of the
hybrid floors provide a reduction of the lateral
stiffness of the building in the range between 20
and 30% compare to the case of rigid
diaphragms;
the influence of the actual stiffness of the floor
on the lateral deformation is higher at the first
story compare to the other levels;
the increase of the stiffness of the shear walls
gives in general a reduction of the drift of the
building when floor act as rigid elements, while
can be useless in case of deformable floors,
because of the increase of its lateral deflection;
results encourage the adoption of two
coefficients of correction: the first to adjust
design loads of shear walls compare to the ideal
case of rigid floors, the second similarly to
account for the increase of the lateral
deformation in the evaluation of the inter-story
drift of the buildings;
focusing on the results for the design
configuration (kD*=1), damage requirements
resulted more affected from the actual
diaphragmatic behaviour of floors than the
ultimate limit state, having measured maximum
differences of 18% and 11 %, respectively;
second-stage analyses dynamic-type related to
further assess the influence of the arrangement
of the shear walls and the relative stiffness
between floor and walls on the load distribution
among the vertical bracing system has started
and results will be presented in future papers;
ACKNOWLEDGEMENT
The experimental research program included in this
paper was supported by the Regional Development Fund
of the Province of Trento as part of the ‘Live to Live’
Project: Integrated sustainable construction systems with
a steel-timber structure for industrialised buildings
(http://www.dicam.unitn.it/en/293/live-to-live). The
analytical part of the work was supported by the British
Columbia Innovation Council through funding to the BC
Leadership Chair in Tall Wood and Hybrid Structures
Engineering.
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buildings and other structures. Reston, VA, 2010.
[3] Brandner R., Flatscher G., Ringhofer A.: Cross
laminated timber (CLT): Overview and
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New Test Configuration for CLT-Wall-Elements under Shear Load
  • T Bogensperger
  • T Moosbrugger
  • G Schickhofer
Bogensperger T., Moosbrugger T., Schickhofer G.: New Test Configuration for CLT-Wall-Elements under Shear Load. In 40th CIB-W18 Meeting, Bled, Slovenia, 2007.
In-plane stiffness of cross-laminated timber floors
  • S Ashtari
  • T Haukaas
  • F Lam
Ashtari S., Haukaas T., Lam F.: In-plane stiffness of cross-laminated timber floors. In: 13th World Conference on Timber Engineering (WCTE2014), Universitè Laval, Quebec City, Canada, 2014.