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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
resilient lateral load resisting system.
This paper describes the application of hollow poles (MultiPoles©) for the principal structural elements in two
building projects Te Wharehou O Tuhoe and Huia Road. Key aspects of structural design and detailing are
highlighted and the performance attributes of each project are appraised.
KEYWORDS: Hollow poles, MultiPoles©, timber structures, post-tensioning, seismic design, Living Building
Challenge.
1 Principal, mlb Consulting Engineers, PO Box 125-258, St Heliers, Auckland 1071, New Zealand.
Email: [email protected].
2 Structural Engineer, mlb Consulting Engineers, PO Box 125-258, St Heliers, Auckland 1071, New Zealand.
Email: [email protected].
1 INTRODUCTION
In New Zealand mlb Consulting Engineers and
TTT Products Ltd have developed hollow timber
radiata pine poles to form structural elements such
as beams, columns, wall and floor panels.
Hollow timber poles have been found to be
dimensionally more stable and exhibit significantly
smaller drying checks than solid round wood [1].
Together with improved material stability the
hollow core provides opportunity for efficient
concealed connections. Bolted internal steel tubes
for column base connections, column-splices and
brace junctions together with post tensioning
tendons for shear wall anchors have been
employed. Reinforcing bars cement-grouted into
the hollow core of timber MultiPole piles provide
positive connections between piles and foundation
platforms.
Preservative treated MultiPole piles for buildings
have achieved a 100 year durability classification
due to 100% treatment of the wood fibre.
Tolerance fit machined scallop connections
between hollow poles have been developed by TTT
Products Ltd to create solid timber wall and floor
panels. These new structural components have been
used extensively in the Te Wharehou O Tuhoe and
Huia Rd projects.
2 TE WHAREHOU O TUHOE
The structural system for the Te Wharehou O
Tuhoe Tribal Chamber and Administration
Building is described. This project had strict design
and performance constraints as it is the first Living
Building Challenge (LBC) development in New
Zealand and is positioned in a highly active
earthquake zone.
2.1 SITE CHARACTERISTICS
Seismic loading was the dominant hazard for the
structural design, as the building site is within 2km
of the Whakatane fault line which runs along the
eastern side of the Taneatua valley (highlighted in
yellow in Figure 1). If fault rupture occurs,
significant vertical and horizontal ground
movement is expected. Approximately 30
kilometres further east the Waimana fault has
strike-slip characteristics where geologically
recorded single event lateral ground movements of
greater than 5 meters have occurred.
The site founding material is compressible alluvial
silts and sands overlying dense gravels at 5m to 8m
below the ground surface. A Class D (deep or soft
soil) seismic classification was identified which
tends to amplify earthquake ground motion and
heighten the seismic demand on the structure. The
potential for soil liquefaction under strong shaking
was identified by Beca Geotechnical Engineers for
soil layers above the dense gravels.
The site is also exposed to high winds and is
susceptible to surface flooding from surrounding
runoff and in extreme weather conditions from
Whakatane-Waimana River flood waters.
Figure 1: Whakatane fault line (shown in yellow)
2.2 BUILDING USE AND IMPORTANCE
LEVEL
Design loads appropriate for an Importance Level 3
structure were used for the proposed development
(AS/NZS 1170.0:2002 [2]). This importance level
defines higher wind and earthquake forces than for
normal dwellings and is suitable for structures that
as a whole contain people in crowds, or contents of
high value to the community.
2.3 EARTHQUAKE DESIGN
The structures are designed for minimal damage
after a design earthquake. This objective aims to
avoid repair to the structure and ensure continuing
occupancy after a seismic event. Under
NZS1170.5:2004, an importance level of 3, a site
soil class D and a hazard factor, z, of 0.3 was used.
Drift performance levels under serviceability
(SLS), ultimate limit state (ULS) and maximum
considered earthquake (MCE) are described in
Table 1.
Solid timber shear walls with unbounded post-
tensioned reinforcement (see section 2.7.6) provide
primary lateral load resistance for both the Tribal
Chamber and Administration Building. The walls
and reinforcement are designed to remain
essentially elastic up to the MCE earthquake.
Therefore, at this level of earthquake, no repair or
replacement of damage components should be
required.
While elastic response (with structural damping of
5%) is assumed for determining the design
earthquake forces, the wall system has inherent
ductility capacity, which will be activated for
earthquake demands greater than MCE. Hence, a
structural performance factor of 0.7 was
considered.
Table 1: Key earthquake design parameters
Performance limit state SLS ULS MCE
Return period (years) 1/25 1/1000 1/2500
Risk factor, R 0.25 1.3 1.8
Structural performance
factor, Sp
0.7 0.7 0.7
Drift limitation (%) 0.33 0.5 1.0
As part of the performance-based lateral force
design, the acceleration-displacement response
spectrum (ADRS) for the walls was generated. This
is shown in Figure 2 for one of the walls, where
acceleration is converted to base moment demand
and displacement is converted to drift demand.
Because of the strict displacement limitations, long
(stiff) walls were required. As shown in Figure 2,
the plateau acceleration (or base moment) demand
was required for the SLS, ULS and MCE limit
state.
Figure 2: ADRS curve for shear wall
2.4 MATERIAL CONSIDERATIONS
Timber is the predominant structural material as it
was understood to be sympathetic with the desire of
Te Uru Taumatua (the client) for a building in
harmony with the natural environment and forest
landscape. In-depth consideration was given to the
use of suitably graded and processed locally owned
and felled timber for use in the development.
Radiata pine was selected as the principal structural
material being consistent with the Living Building
Challenge (LBC) [3] where “all materials in the
built environment are replenishable and have no
negative impact on human and ecosystem health”.
In this regard radiata pine is a renewable
construction material and stores carbon which
offsets carbon emitted in the construction process.
Round timber members were specified for many
structural components as they require less energy to
produce and make optimum use of timbers natural
strength.
Chemical treatment of timber was necessary to
achieve adequate service life durability. Treatments
were selected to avoid harmful contaminants such
as arsenic, creosote or pentachlorophenol. Boron
treatment for interior structural elements and
micronized copper azole (MCA) for exposed
members was used, being suitable treatments that
satisfied the LBC. It should be noted that
substitution MCA treatment for traditional copper
chrome arsenic (CCA) on round wood adds
additional cost.
2.5 BUILDING SERVICES
The structure is designed to satisfy multiple
building service objectives including acoustic, floor
vibration, thermal performance and operational
energy efficiency.
The suspended floor in the Administration Building
has been designed to limit vibration and noise
transfer between levels.
Under the LBC, high levels of energy efficiency
are required for the facility. Low energy passive
ventilation, photo voltaic energy generation and
high levels of insulation have been incorporated.
Solid timber construction assists in providing
significant levels of thermal insulation. This
reduces the requirement for synthetic insulation
around exterior walls and floors. The timber
structural elements also limit thermal bridging and
assist with regulating humidity.
2.6 FIRE PROTECTION
Fire protection for the buildings is provided by
sprinklers with further protection provided by the
inherent charring resistance of the heavy timber
structural sections.
2.7 STRUCTURE
2.7.1 Development of the structural concept
Three main structural concepts were developed and
analysed. The final concept, using mainly solid
timber elements, is illustrated in Figure 3.
The development comprises two main buildings:
reception and administration facilities housed in a
two level structure connected by a link-way to the
main Tribal Chamber.
0
1000
2000
3000
4000
5000
6000
0 0.5 1 1.5 2 2.5 3
Bas
e m
om
en
t (k
N.m
)
Drift (%)
MCE EQ DemandULS EQ DemandSLS EQ DemandULS Wind DemandSLS Wind DemandPushover curve
a)
b)
c)
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-
tensioning force.
0
500
1000
1500
2000
0 0.5 1 1.5 2 2.5 3
Bas
e m
om
en
t (k
N.m
)
Drift (%)
MCE EQ DemandULS EQ DemandSLS EQ DemandULS Wind DemandSLS Wind DemandPushover curve
a)
b)
Figure 12: Jointed shear walls: a) Two wall segments end-on-end b) Lower side of splice joint
3.4.2 Floor system
Traditional timber floor construction was used with
sawn timber joists supported on steel UB primary
beams and LVL trimmers. Lateral wind and
seismic loads developed at the floor and roof levels
are distributed by plywood diaphragms to timber
shear walls.
A key consideration was the design of the floor
diaphragm-to-shear wall connections. Higher mode
response of a medium-rise multi-storey structure
can result in significant amplification of diaphragm
forces. Based on recommendations from
Newcombe, 2012 [4] and NZS1170.5:2004 [8]
(Requirements for Parts and Components) a
maximum floor acceleration of three times the ULS
peak ground acceleration (PGA) was considered.
Furthermore, the location of diaphragm
connections was chosen to minimise the effects
displacement incompatibilities between the wall
and floor systems.
3.5 CONSTRUCTION METHODOLOGY
Speed of assembly is important so that weather
protection for the structural system can be provided
as soon as possible.
To achieve this objective, the following
construction methodology was proposed for the
Huia Road project:
1. The concrete footings are levelled
underneath the walls and columns to
ensure the walls and columns are true,
plumb and positioned at the correct
elevation.
2. The ground-to-first floor wall panels are
assembled with the post-tensioning rods in
place and lifted onto temporary supports
above the foundation beam. The post-
tensioning tendons are connected to the
footings with couplers. The walls are
lowered into their final position.
Temporary out-of-plane bracing is
provided. A small amount of post-
tensioning force is applied to the wall
element to provide construction bracing.
3. Timber columns are placed in position and
braced.
4. The Level 1 floor is constructed.
5. The second to first floor wall panels are
assembled with the post-tensioning rods in
place and lifted onto temporary supports
above the level 1 wall panels. The tendons
from the wall panel above are connected
to the corresponding bars from the wall
panel below. The next level wall panel is
lowered into place and secured with
temporary out-of-plane bracing. A small
amount of post-tensioning is applied to the
wall panel to provide construction bracing.
6. The process is repeated until the structure
is complete.
Note brittle cladding and glazing elements are not
installed prior to final post-tensioning of the wall
elements.
4 CONCLUSIONS
Hollow timber poles have been applied as piles,
columns, beams and braces and have been
mechanically connected to form floor and wall
panels. These structural elements have been applied
on two major projects and are demonstrably robust,
cost effective, sustainable and energy efficient to
produce.
While careful design and detailing is required,
structures using hollow timber rounds have the
potential to compete with traditional construction
materials.
ACKNOWLEDGEMENT The authors extend their thanks to Ivan Mercep of
Jasmax Architects and Linda Mead of Mead
Design for their confident support of the new
structural systems proposed and developed for
these two unique projects.
REFERENCES [1] Batchelar, M. L. (2012). "Innovative Use
of Timber Rounds in High Performance
Structures." World Conference on Timber
Engineering, Auckland, New Zealand, pp.
7.
[2] NZS1170.0. (2002). Structural Design
Actions - Part 0 - General principles, New
Zealand Standards, Wellington.
[3] McLennan, J. F., and Brukman, E. (2010).
Living Building Challenge 2.0; A
Visionary Path to a Restorative Future,
International Living Building Institute,
Cascadia, North America.
[4] Newcombe, M. P. (2012). "Seismic
Design of Post-Tensioned Timber Frame
and Wall Buildings," Doctorate Thesis,
University of Canterbury, Christchurch.
[5] Priestley, M. J. N., Calvi, G. M., and
Kowalsky, M. J. (2007). Displacement-
Based Seismic Design of Structures, IUSS
PRESS, Pavia, Italy.
[6] NZS3604. (2011). Timber Framed
Buildings, New Zealand Standard,
Wellington, New Zealand.
[7] NZS3640. (2003). Chemical preservation
of round and sawn timber, Standards New
Zealand Wellington, New Zealand.
[8] NZS1170.5. (2004). Structural Design
Actions - Part 5 - Earthquake Actions,
New Zealand Standards, Wellington.
.