FEATURE
SAUPA ’ s role in treating wooden poles
By SAUPA , edited by Leon Louw
South African Utility Pole Association ’ s ( SAUPA ’ s ) role in the promotion development and maintenance of the wooden pole as the preferred utility pole for electrification and communication .
Over the past 15 years SAUPA has been proactive in working with end-users all over Africa to promote suitable wooden pole treatment standards , the correct drying and treatment as well as the handling of wooden poles to ensure the poles purchased and used by utility companies last the required 35- 40 years . Eskom can be credited for the positive contribution they have brought to the utility pole industry in South Africa specifically .
Currently , the Eskom specifications are drawn up to achieve their objective of less than 3 % failure at 10 years and , with a suitable wooden pole maintenance programme , the poles last 35 to 40 years .
SAUPA has been working with utility companies all over Africa to find ways to improve the quality of the supply of wooden poles . Several critical factors have been identified that need to be addressed to achieve the objective of the wooden poles lasting the required 40 years .
STRENGTH REQUIREMENTS OF EACH SPECIES
The biggest challenge in African specifications is the lack of testing and data on the fibre strength of the different species grown in Africa . It is vital in the design and purchasing of wooden poles that different specifications specify test methods for determining the bending strength and modulus of elasticity in bending .
SIGNIFICANCE AND USE
Tests of wood poles are made to determine :
• Data for use in establishing ultimate fibre stress ;
• Data upon which to base economical line design ;
• Data on the strength properties of different species in pole sizes ;
• Data as to the influence of defects on the strength properties of poles ; and
• Data for correlating the strength properties of full-size poles .
Unfortunately , the only specification in Africa that has been updated is SANS 754 Eucalyptus poles , cross-arms and spacers for power distribution and communications systems .
All wooden pole suppliers need to have test facilities and equipment available at the plant to prove the fibre strength and MOE of poles being supplied to utility companies .
DRYING REQUIREMENTS BEFORE TREATMENT
Wooden poles need to dry before treatment for the following reasons : a . To achieve maximum penetration and retentions in the available sapwood ; b .
Suppress any checking after treatment to avoid untreated heartwood being exposed while in use .
Most wooden poles in Southern and East Africa are from the Eucalyptus species . Generally , the pine timber in Africa is fast growing and does not have the strength requirements for transmission poles .
Eucalyptus timber has a smaller sapwood zone than pine timber . The result of the bigger heartwood zone results in the timber drying slowly .
It has been identified that post-impregnating splitting of poles treated with CCA and to a certain extent creosote is the biggest cause of early pole failure due to untreated timber being exposed to termites and fungus which results in decay at ground level . The cause of postimpregnating splitting is due to the heartwood portion not being dried down to a sufficient level to prevent any further splitting .
ESKOM IDENTIFIES NEED FOR KILNS
Research done in South Africa on air seasoned poles indicate very high mc % in the heartwood even after the 4 to 6 weeks ’ drying period . Only some drying has taken place in the sapwood , whereas further away from the pole surface , the mc % is like what one would expect in a freshly felled tree .
The sapwood dries rapidly and can dry after 12 to 20 weeks . In certain sizes the moisture has fallen to 30 % and below . In contrast , very little moisture movement and loss has taken place at a depth of 20-60mm in the heartwood . The moisture can still be around 50 % at 60mm . If the poles are air seasoned to the correct moisture in the heartwood the poles will need to be stacked for between 4 to 12 months depending on the diameter of the pole .
Air seasoning times are dependent on climate conditions . The advantages of air seasoning include the need for minimal equipment and the absence of direct effects on wood properties . These aspects are offset by some negative attributes . First , air-seasoning requires large areas of land and requires that a company hold onto untreated stock for long periods .
This increases material costs . By far the largest negative aspect of air-seasoning is the risk of decay or insect attack . Freshly harvested trees remain at risk of fungal and insect attack from the time they are cut until the wood dries below 20 % moisture content .
Removing the bark reduces the risk of insect attack , but also exposes large amounts of stored sugar to fungal attack . Field trials suggest that poles should not be air seasoned for more than 3 to 6 months and that nearly every air-seasoned pole will have some decay fungi after
www . timberiq . co . za // AUGUST / SEPTEMBER 2021 19