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TIMBER POLE DESIGN AS/NZS 7000:2010 – Appendix F
Henry HawesFIEAust, RPEQ, CPEng.
Consultant
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
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