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STRUCTURES USING HOLLOW TIMBER POLES
Mark Batchelar1 & Michael Newcombe2
ABSTRACT: Hollow pole structural elements have now been incorporated in the design of a number of
structures and used as piles, columns, struts, walls and floor systems. Simple structural connections, using
bolted internal steel tubes, post-tensioning tendons, screws and timber notches have been developed for
application with these new structural elements. Water-jets have been inserted into the core of piles to allow ease
of installation, and subsequently grouted reinforcing rods have provided fixity to superstructures. Hollow poles
have been mechanically connected together to form solid timber panels for shear walls and floors. Post-
tensioning tendons have been inserted through the hollow core of rounds within a shear wall panel to provide a
Figure 3: Te Wharehou O Tuhoe: a) Structure Schematic (c.o. Jasmax Architects) b) Photo of Tribal Chamber and Administration Building c) Photo of Tribal Chamber entrance
Using a timber structure minimised the mass of the
building which reduced gravity loads on the
foundation system and correspondingly reduced
earthquake generated lateral forces. Furthermore,
the structural concept incorporates several
innovative solid timber systems in order to achieve
the extreme-event and operational performance
objectives.
2.7.2 Foundations
In consideration of the geotechnical consultant’s
report (from Beca) which identified the
vulnerability of the site to settlement, liquefaction
under earthquake induced ground motion and flood
damage a foundation system utilizing timber piles
was adopted.
Hollow timber piles (TTT MultiPoles) vibrated to
the gravel layers, provide a stable foundation
system. Structure settlement due to earthquake
induced liquefaction or uneven settlement of the
compressible silt/sand layers is therefore avoided.
Durability requirements were satisfied by using
micronized copper azole (MCA) treatment. The
complete penetration of preservative treatment
achieved using hollow timber piles provides an
assured design life.
The core of the piles was concrete grouted with
cast-in reinforcing bars tying the piles to concrete
bearers at ground level. Pull-out test were done to
determine appropriate anchorage length for the
deformed reinforcing tie bars as shown in Figure 4.
Figure 4: Pull-out test of deformed reinforcing bar grouted into hollow core of MultiPole.
2.7.3 Ground floor structure
Traditional timber floor construction was used with
sawn timber joists supported on the concrete
bearers and over-laid with selected timber flooring
(Figure 5). This design reduces the risk of
unevenness due to ground settlement and flood
damage.
Figure 5: Traditional Timber Floor
2.7.4 First floor structure
Consistent with the LBC’s recommendations to
minimize the use of concrete, the first floor of the
administration building is constructed using timber.
Machined round timbers (Multipoles) span between
radiata pine glue-laminated timber primary beams,
which are in turn supported on MultiPole timber
columns as illustrated in Figure 6.
.
Figure 6: MultiPole diaphragm floor
Mechanical keying between individual timber
rounds creates a solid timber floor system that
provides efficient diaphragm action. Loads
developed through the floor diaphragms transfer to
timber shear walls which provide the lateral load
resisting system.
Care was taken in the specification of timber
moisture content for the fabricated MultiPole shear
wall and floor panels to limit in-service
dimensional changes.
2.7.5 Roof structure
All roofs were constructed using an Equus
Duoatherm cladding system with plywood
diaphragm, timber purlins, and glue-laminated
timber rafters. Significant additional load from
photovoltaic panels was accommodated in the
design.
Lateral wind and seismic loads developed at roof
level are distributed by a plywood roof diaphragm
connected to timber shear walls.
2.7.6 Timber shear walls
Solid timber shear walls constructed using kiln
dried hollow timber rounds, interlocked as
illustrated in Figure 7, have been used as the
primary lateral load resisting system for both the
Administration Building and Tribal Chamber.
a)
b)
Figure 7: Multipole shear walls: a) Complete shear wall b) Shear key notches (c.o. TTT Products Ltd)
Post-tensioning tendons were installed through the
hollow timber rounds with stressing tendons
anchored at the top of the walls and within the
concrete foundation beams at the wall base.
The post-tensioned walls provide efficient
resistance to earthquake and wind loads. The
design of the walls focuses on a controlled rocking
mechanism that targets minimal structural damage
and avoids residual deformation of the structure
after an earthquake [4]. The application of this
system and modern displacement-based design
approaches [5] has enabled the seismic response to
be tuned to achieve the desired performance limits.
Extensive research on post-tensioned timber walls
has been performed at the University of Canterbury
(see Figure 8), which has led to the application of
this technology to commercial structures. These
applications have typically used Laminated Veneer
Lumber (LVL) wall elements but by using
mechanically jointed timber rounds, as illustrated
in Figure 9, processing and cost are reduced.
In the Tuhoe project the relatively low tendon
forces enabled Reidbars and modest manual
jacking equipment to be employed. Refer to Figure
9.
Figure 8: Post-tensioned timber walls in UoC test building.
a)
b)
Figure 9: Post-tensioned shear walls: a) Wall with protruding rods b) Manual tensioning
2.8 CONSTRUCTION METHODOLOGY
The structural system was intended to minimise on-
site construction time and cost with pre-fabrication
and modularity maximised throughout the
structure. In addition the inherent light-weight
nature of timber offered savings in transportation
and crane costs.
Key aspects that were considered to reduce
construction time and cost are:
Floor, wall and roof elements designed as
prefabricated panels to enable rapid fixing. The
use of light-weight prefabricated components to
minimise the number of workers required on-
site, and to improve on-site safety.
Solid timber walls providing rapid and effective
temporary construction bracing in addition to
permanent bracing for the structure.
Piled foundations and suspended floor avoiding
significant on-site excavation.
3 HUIA ROAD
The Huia Road project is a 5 storey residential
building located in a high seismicity zone near
Wellington that incorporates MultiPole columns
and wall panels, and a traditional light timber frame
floor (see Figure 10).
Figure 10: Huia Road Project
3.1 SITE & BUILDING CHARATERISTICS
Seismic and wind loading are the dominant natural
hazard for the structural design.
The site is surrounded by a forest park, on a steep
slope with underlying Greywacke.
The site is exposed to high winds, although
surrounding trees provide significant shielding.
Design loads appropriate for an Importance Level 2
structure were used [2] as per NZS1170.0 [2].
3.2 EARTHQUAKE DESIGN
Both Te Wharehou O Tuhoe and Huia Road
incorporated solid timber shear walls with
unbounded post-tensioning (see section 3.4.1) and
were designed to avoid structural repairs and allow
continual occupancy after a design level
earthquake. However, this was achieved by
applying contrasting design approaches within a
displacement-based design (DBD) framework [5].
Unlike Te Wharehou O Tuhoe, the wall panels
were designed to undergo large lateral drift and
exhibit a controlled rocking mechanism under large
(ULS or MCE) earthquake loading. This allowed
elongated of the fundamental period of the
structure and provided a significant reduction in the
design seismic forces.
Stresses within the structural elements at the ULS
loading were limited to the elastic range,
minimising structural damage and avoiding
residual deformations. External and internal
cladding and stairs were detailed to allow for the
required lateral drifts without sustaining damage or
imparting significant lateral resistance.
Under NZS1170.5:2004, an importance level of 2,
a site soil class B and a hazard factor, z, of 0.4 was
used. Drift performance levels under serviceability
(SLS), ultimate limit state (ULS) and maximum
considered earthquake (MCE) are described in
Table 2.
Elastic response (with structural damping of 5%) is
assumed for determining the design earthquake
forces. High strength reinforcement (MacAlloy
Rod) is used for post-tensioning, which is
inherently brittle if overloaded. Hence, a structural
performance factor of 1.0 is considered.
Table 2: Key earthquake design parameters
Performance limit state SLS ULS MCE
Return period (years) 1/25 1/500 1/2500
Risk factor, R 0.25 1.3 1.8
Structural performance
factor, Sp
1.0 1.0 1.0
Drift limitation (%) 0.33 1.5 2.5
As for Te Wharehou O Tuhoe, the ADRS curves
for the walls were generated. This is shown for one
wall in Figure 11. Comparing Figure 2 and Figure
11, it is evident that the Huia Road walls achieve a
significant reduction from the plateau design
acceleration. This is because; a) stiffer soil for Huia
Road limits the period range of the acceleration
plateau, b) the wall elements for Huia Road are
more slender and c) the displacement limitations
for Huia Road are less severe.
Figure 11: ADRS curve for shear wall
3.3 BUILDING MATERIALS
Radiata pine is the predominant structural material
for the superstructure.
Columns and shear walls are constructed using
round timber. Floors, in-fill and internal walls are
light timber framing. A steel UB is used at each
level to support timber joists. Other steel
components are also used for connecting timber
components.
Reinforced concrete footings and piles were used
for the foundations (designed by Clendon Burns
and Park Ltd).
All shear walls were CCA treated (H3.2) for
aesthetic reasons, as the walls were visible from
inside the building. Light timber framing was either
CCA or Boron treated in accordance with
NZS3604:2011 [6] and NZS3640:2003 [7].
3.4 STRUCTURE
3.4.1 Timber shear walls
Similar to Te Wharehou O Tuhoe, solid timber
shear walls were constructed using kiln dried
hollow timber rounds, were used for Huia Road.
However, the system was extended for medium-
rise multi-storey construction.
Wall elements were spliced at each level (see
Figure 12), enhancing the ease of construction and
minimizing material waste. Wall splice joints were
designed to provide adequate shear transfer under
design level earthquake and wind loading, and also
to allow for temporary anchorage of post-
tensioning tendons at each level. This provided
sufficient structural bracing during the construction
phase, avoiding the need for additional construction
bracing.
High-grade MacAlloy post-tensioning provided
moment fixity at the base of the walls and between
joints. Compared to Te Wharehou O Tuhoe, high-
grade post-tensioning was necessary to achieve
higher post-tensioning forces, avoid bar yield at
large lateral drifts and to minimise losses in post-