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: mark.batchelar@mlb.co.nz.

2 Structural Engineer, mlb Consulting Engineers, PO Box 125-258, St Heliers, Auckland 1071, New Zealand.

Email: michael.newcombe@mlb.co.nz.

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

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t (k

N.m

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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.

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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.

.