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TIMBER POLE DESIGN AS/NZS 7000:2010 – Appendix F

Henry HawesFIEAust, RPEQ, CPEng.

Consultant

hhawes@bigpond.com

Timber Pole & X-arm Design Standards

• Before the first C(b)1 in 1962 most utilities had

internal or government design regulations or used

earlier CSIRO work on timber design by Boyd.

• Most utilities have used C(b)1.

• The first limit states version of C(b)1 was produced in

1999 to bring it into line with other design standards.

• AS/NZS 4676 :2000 provided timber pole design

provisions and AS1720.1 used for crossarm design.

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AS/NZS 7000:2010

• Timber pole design clauses Appendix F based

on provisions in AS/NZS 4676:2000.

• Added in Torsional strength and pole-top

deflection comments.

• Major current issue is AS/NZS 7000 is

‘informative’ and AS/NZS 4676 is ‘normative’

but this is being addressed.

Timber pole strengths

• Limited historical research in Australia.

– J.D. Boyd led studies by the Pole Strength Joint

Research Committee, as well as some other

studies.

– Variations in testing methods can cause variations

in observed strength.

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Timber strength properties

• Where do the strengths come from?

– Small clear specimens – Main (old and new)

– Full-size beam tests – Early Work

– Clamped Cantilever – Some (Overseas)

– Free cantilever – Recent, few

– Pull-down – Rare

– 4-point cantilever – More recent, ENA sponsored

and NZ pine poles

Testing pine at the concrete pole plant in Orange.

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Testing pine and TSB Pull-Down at Grafton.

Testing Hardwood at Grafton

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Section 8 – Load Tests

– COV for recent round timber poles full scale

load tests has been in range of 10-20%

NZS3603 & AS1720.1

• Generally pine poles in NZ are proof loaded

• If using NZS 3603 there are a few subtle

differences to AS 1720.1 and AS/NZS 7000.

– Peeling factor & Slenderness factor in Section 7

– Not as many ‘k’ factors in Section 3 equations

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Limit State DesignCl. 6.3.1

Serviceability Limit State (RH/3)Serviceability Limit State (RH/3)Serviceability Limit State (RH/3)

φRn > effect of loads ( Wn + ΣγxX)

whereX = the applied loads pertinent to each loading condition

γx = load factors which take into account variability of loads,

importance of structure, stringing, maintenance and safety etc.Wn = wind load based on selected return period wind or a specified

design wind pressureφ = the strength reduction factor which takes into account

variability of material, workmanship etc.Rn = the nominal strength of the component

Strength Reduction Factor For Timber

Table 6.2

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Characteristic Properties of Timber

• Clause F3:

– Australian East Coast Australian Hardwoods are

either S1 or S2, Tasmania has minimum S4 and

WA doesn’t really use local hardwood poles any

more.

– Slash Pine is S5

– Radiata is normally S6

– Pines tend to vary significantly with location and

elevation, hardwoods not as much.

– The design assumes poles sourced to AS3818.11

Characteristic Properties of Timber

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Characteristic Properties of Timber

Clause F4.1

Capacity Factor φ

• use 0.9 unless the supplier is proof or in-grade testing and you

are confident in the properties.

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Load Duration k1

Load Duration k1

• Table F4:

– Use k1 = 1 for wind load combinations and 0.57 for

permanent loads like transformers and other high

resultant compressive loads.

– Wind and bending combinations may need further

assessment, but normally 0.8 would be used.

– If structure is under significant permanent load

and is deflection sensitive, be sure to use the

characteristic Young’s Modulus (see clause F5.6)

and creep factors from AS1720.1 or NZS3603.

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Degradation factor, kd

Degradation factor, kd

• For most poles Table F5 gives kd

= 0.85 and for

average service life of poles

• Taken from the equations derived for the

“Timber Service Life Design Guide”

(www.timber.org.au).

• Equates to about 55% loss of diameter from

the centre out.

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Shaving Factor, k21

Shaving Factor k21

• Strength of round timber can be reduced when it is machined into cylindrical form as the extreme fibres are shortened.

• Table F7 gives values for this case, however it does not apply to de-barked poles and “dressing”. Therefore in most cases is 1.

• NZS3603 does apply a factor of 0.9 in bending or tension for machine peeling (de-barking) of pine, but this is not included in AS/NZS7000.

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Immaturity Factor, k20

Accounts for a decrease in fibre strength for younger timber.

Processing Factor k22

• Steaming under pressure to remove moisture and

break cells for improved treatment of hard-to-dry

timber can reduce pole strength.

• Not known to be done in Australia, best to check

with manufacturer. If not done k22 = 1.

Note: Steaming under vacuum has been used to

improve treatment fixation time and does not

cause a strength reduction.

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Capacity in bending φφφφM

• Clause F5.1 - Calculation for moment

capacity at the critical section:

φφφφM=φφφφ.k1.k

20.k

21.k

22.k

d.f’

b.Z

– Where Z = π.dp3 / 32 and dp is the diameter at

the critical cross-section ( 200 below GL).

Capacity in bending vs. Tip Load

• To convert from a bending moment

capacity to a tip load capacity is simple.

• Divide the moment capacity by the distance

from the critical section to the tip.

• Make sure it is specified whether you are

using a tip load position at the very tip, or

at 300mm or 600mm below the tip

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Shear capacity φφφφV

• Practically never an issue, even in combined load

checks.

• Simple formula from Clause F5.2 if required.

φφφφV=φφφφ.k1.k

20.k

22.k

d.f’

S.A

s

Compressive Strength φφφφNc

• Clause F5.3:

φφφφNc

= φφφφ.k1.k

12.k

20.k

21.k

22.k

d.f’

c.A

c

– The stability factor k12 is a function of the slenderness factor (Cl. 3.3.3 of AS1720.1), which in AS/NZS7000 and AS4676 is 1.15L/dp where L is the distance between effective restraints and dp is the mid-length diameter between those restraints.

– In NZS3603 the slenderness factor is just L/dp and k8 (equivalent to k12) uses a slightly different formula, but the results are similar.

– kd should be the same as for shear.

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Combined Actions

• Clause F5.4:

(M*/φφφφM) + (N*c/φφφφN

c) ≤ 1

– This combination will govern most designs,

even if there is only cable weights, fittings and

pole self weight in compression.

– Combined bending and tension not normally

an issue because the tension capacity is very

large.

Torsion capacity φφφφT

• Clause F5.5

φφφφ T = φφφφ.k1.k

20.k

22.k

d.f’

s.Z

T

– kd can be the same as for bending

– ZT = π.dp3 / 16

• Note torsion capacity for a timber pole is

normally very high, where the pole is likely to

rotate in the ground before it fails in torsion.

• Must consider where pole has rigid foundations

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Pole Selection

– From design load combinations determine critical load case eg.

φφφφRn > Wn + 1.1 Gs +1.25 Gc +1.25 Ft

– Determine limit state overturning moment

– Determine ultimate pole tip load

– The tip load is then compared to a list of limit states design tip capacities for a pre-determined range of poles characterized by their tip capacity, strength grade and length and the appropriate pole is selected.

Additional Considerations

• Design of cross-arms should be as for sawn

timber from the detailed procedures of either

AS1720.1 or NZS3603.

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Additional Considerations

• Timber poles are regularly inspected, and

allowed to degrade in strength to a set level at

which time they are replaced.

– Hence, the kd

and φ factor for timber poles

• Degradation assessment of reinforced poles

needs careful consideration

Questions