TIMBER FOR POLES

Waipa Post, Volume 32, Issue 1789, 31 July 1926, Page 7

TIMBER FOR POLES

THE VALUE OF HARDWOOiDS. AN INTERESTING PAPER. The gradual reticulation of the 'whole of New Zealand by hydro-electric schemes has produced an acute problemI—the 1 —the question of the best kind of poles to use for the transmission line/s. Considerable differences of opinion exist as to the best kind of timber to use, if timber should be used at kll. The whole question as to what kinds of timber are most suitable for transmission poles was thoroughly ventilated at the meeting of the Electric, Supply Authority Engineers’’ Association in Wellington last week, when an interesting paper on the subject was submitted by Mr J. R. Templin, of Christchurch.

“ Poles can be divided into three general classes—wood, concrete, and iron or steel —and each of these into various divisions and sub-divisions,” said Mr Templin. “As the majority of poles used iu New Zealand are of Australian hardwood, this paper will deal chiefly with this class of timber, and an endeavour will be made to give ■the results obtained with various kinds of hardwood poles. Unfortunately, the majority of poles have not been in use a sufficient length of time to obtain * complete record, but there is sufficient information/ available to arrive at some general conclusions. “ Before giving details of the results obtained with various hardwoods, it will be of interest to set oujt the opinions of -a number of Australian authorities concerning the class of poles which have been sent to New Zealand, and their recommendations for future supplies. Having these :in min'd;, and, analyising the results obtained with poles purchased to various specifications, it should be possible to drawsome conclusions which will he of benefit in ordering future requiremenls't. One authority states that ironbark is by no means the most durable of hardwoods in the pole sizes, as its quality depends 'upon the growing conditions. The sapwood of all hardwoods is practically useless, ironbark particularly so, as it often carries very thick sap. To have a durable ipo'le, •the tree must have a comparatively thin sap, which means that it must grow more slowly; .that is to Isay, in barren, hilly country, or be suppressed by other trees. This authority recommends the following timbers a& being suitable for poles: Ironbark grey gum, white mahogany, tallowwood, grey box, and bloodiwood. SEASONED POLES NEEDED.

“.From observations made in Australia by memberls of our own firm, the following conclusions were drawn: Spotted gum, black bhjtt, or stringy bark should not be used for poles. The sap is of no value -m considering the life of timber. Ironbark grows rapidly in good ground, and contains from 50 to SO per cent of sap in small pole size, but on poor ground the sap is much 1 less. The mixed hardwood timbers mentioned above were considered satisfactory. The heart of well-mat-ured ironbark trees was considered the best lasting timber imported from Australia. It may be stated here that in discussing timbers supplied to New Zealand with one authority ,it was re. marked that some of the poles rotted within six or seven years after being erected, the reply being that it was surprising they had lasted so long. t One forestry inspector recommended the 'Uise of ironbark poles, but that they should be larger sizes than is generally specified in New Zealand. “The general consensus of opinion concerning poles shipped to New Zealand is that in the past we have been .using poles cut from immature trees and, too small sizes. Concerning the use of blood wood it should not be used for dressed poles, als it, is inclined to bleed and become unsightly. Also, if it is grown! on good country it carries heavy sapwood. The •effect of dd-sap-ping poles before they are properly seasoned is to make them more inclined to crack, and if this occurs at the ground line decay is hastened instead of delayed. The general advice is that poles should be cut only during the winter and that they .should be carefully seasoned at least a year before being erected. From act Ail results noted in the bush the sap of ironbark is much less durable than a number of other varieties of timber. HARDWOOD POLES.

“From the results being obtained with hardwood pole s in use by the above distributing authorities, the following conclusions can be drawn: “Firist, and most important, the sapwood of any of the Australian hardwoods used for poles should not be taken into consideration in '(Determining their strength and durability. However, experience with dressed poles shows that the sapwood above the ground line fjs of some value in preventing poles bending due, to side strain. This advantage may be nullified by the extra bending movements diuje to wind. Again, the results obtained with brush treatment of preservative over sap shows that it is of lit'tle or no value, even though the tefet line of poles erected in Waimairi County had 35 per cent, sound after being in use seven years. Furthermore, the use of natural round iron-

bark poles in restricted, areas where appearance must be considered is not good practice, as in the cour'se of time the sapwood will peel off above the ground line. “Ironbark poles tbajt are squarehewn and cut from mature trees appear to l.e it he most durable timber, a' though sufficient evidence is not available with other timbers concerning this type cf .pole to draw a definite conclusion. In view of the fact that practically all of the poles considered in t'hils report were not seasoned, it is impossible to make any statement concerning this point. However, as Australian authorities strongly recommend that poles should he seasoned and cut in the winter in order to obtain better durability, it is suggested that supply authorities carry out tests along these lines. It is only very .recently that (supply authorities have been removing the decayed sapwood and treating the heartwood with wood preservative, and up to the present time it is impossible to draw any conclusion whatsoever as to the benefit of this treatment.

“The principal reason for the unsatisfactory (service of poles in some localities can be traced definitely to the lack of heartwood, showing that the trees were either ,immat'U|re or cut on open country. As it is impossible for a (pole inspector to get a contractor to guarantee that all trees are fully matured and grown on hilly country, .the only safeguard, that the purchaser .lu-Js is to insist on a small percentage of sap in comparison to the heartwood. “No more contentious question than ithe durability o;f hardwood poles exists,” remarked the chairman. “They seem to be a bugbear in New Zealand.” 'During the subsequent discussion it ,'wajs suggested that if New Zealand •timber was located satisfactorily .there would be no need to go to Australia, for supplies. What a pole would stand up to in Australia was no criterion as to its behaviour in New Zealand. With regard to the use of New Zealand timber, such a)s beech or totara, fear was expressed as to the exorbitant price that might have to be paid.