POLES FOR CABLES

Evening Post, Volume CV, Issue 147, 23 June 1928, Page 10

POLES FOR CABLES

CONCRETE OR WOOD? RELATIVE LENGTH OF LIFE ENGINEERS' PROBLEM Several papers dealing with concrete or wooden poles fo carry cables for electrical transmission were read before the conference of the Electric Supply Authority Engineers' Association. ' Dealing with concrete poles, Mr. J. G. Lancaster pointed out that with the development of extensive reticulation systems, throughout the country, involving hundreds of thousands of line supports, the question has naturally arisen here, as elsewhere, whether, under certain conditions, the use of steel oi- reinforced concrete poles may not be a more economic proposition than the adoption of Australian hardwoods. The principal factors forcing this position under the notice' of supply authorities and engineers are:—(l) The extremely .short life found for certain Australian hardwood timbers' in various parts of this country,' resulting, in heavy and expensive renewals after a period of but a few years; (2) the growing scarcity and consequent increasing cost both of' suitable local timbers and of Australian hardwoods; (3) the relatively high'price of ironbark poles in certain parts of tho country, due. to transhipment aitd handling costs, and long .hauls to site of erection; (4) the presence of ample supplies of shingle and sand in most areas; (5) the decreasing cost of steel; (6) the national aspect, that concrete poles can be made by local labour, from local materials with tho exception of the .steel rein' forcement, and the greater part of the money paid for poles can be therefore kept in the Dominion. The first question of importance, he went ,ou to say, was whether concrete poles are-.inherently suitable for'transmission and reticulation work. Proper ly constructed, concrete poles would withstand all the usual loading, ttiough, perhaps, not cyclonic conditions which might play havoc equally with any type of line. "Tho outstanding and as a rule the most surprising characteristic of a concrete pole, at any rate to engineers who are not versed in the properties of the component materials, is the elasticity of the finished product, and this is by no means a disadvantage, when'properly used, in the application of the poles to comparatively long span reticulation work. "Another feature is the ease'with which concrete jiolcs can be transported, snigged over rough country, and erected, provided always that suitable plant, including power hoisting equip: ment, is available. It might bo thought that it would be difficult to secure tho necessary suitable labour for manufacture and erection, but the initial difficulties in respects can soon be overcome and tho speeds of transport and erection are very little, if anything, less than for ironbark poles of similar length and strength.'' LIFE OF CONCRETE POLES. Dealing with the life of concrete poles, Mr. Lancaster said:—"lt will bo found in general that a concrete pole is more expensive, though sometimes by a i very small margin, than an ironbark' pole of equal strength., "Tho economic reason for the adoption of concrete poles is their lower annual cost, due to their longer life as compared, with that of their timber competitor, and the probable, life is; therefore,, a determining factor in their adoption. Experience in New Zealand in. the use of concrete for fence posts dates back some twenty-two years. Reinforced concrete poles were adopted by the Post and Telegraph Department about 1910, and the heavy line between Feathersfon and Master-; ton erected about that time^is still in use "and tho poles have, we understand, shown no signs of deterioration. In some areas the Department has, we know, taken down concrete pole lines, j but this was because the loading increased above tho designed capacity of the poles, and not because of any defect inherent in tho use of reinforced concrete. "Tho frost bogey has been mentioned, but in the case of a structure with practically -vertical sides, it is difficult to understand how water could aecuinulate and freeze in. such a way as to cause disintegration. Experience to date does not justify fears of trouble due to frost, nor is the practice, of constructing concrete buildings and bridges being checked for that reason. It is a common opinion that tho minimum estimate of the life of a concrete pole should not bo less than 50 years and that a fairer estimate would probable be 20 to ,30 years more. "The quesiion has been raised as to whether, in view of possible developments in the distribution of electricity, such' permanent "structures are advisable. The. distribution, measurement, and control of wireless power are problems still to bo solved, and, in the meantime, lines are still being"erected on steel towers and such like more- or less permanent structures." TARANAKI'S EXPERIENCE. Four years ago, said Mr. H. R. Climie, tho Taranaki Electric Power Board commenced the manufacture o± concrete poles. About 3000 were made and have given every satisfaction. "Most of the poles, have now been in service for three years and have been found eminently satisfactory. Although the poles have been struck by lightning, and subjected to severe torsioual and other stresses duo to breakage of wires' through falling trees, and from other causes, there have been no failures or damage to the poles. The poles arc' very flexible, deflecting about three feet under test, and it is this flexibility probably that protects them from damage." ■■ DECAY IN WOODEN POLES. In a paper on "Wooden Polo Loading and the Effect of Decay," the joint authors (Messrs. N. G. M'Leod, M. Amer, and M. Macbeth) said:—\ "Electrical development; in .New Zealand since the inception of electric power boards has been very rapid, especially in the rural districts, and thousands of route miles of line have been installed. Approximately £20,----000,000 has been expended to date by . the Public Works Department and the Electrical Supply Authorities in NewZealand in the development of electricity, and of this-amount about £16,----000,000 has been the expenditure-in the last seven years. A great portion of this expenditure has been made on the erection of overhead, polo lines, the supports being mostly of wood. Steel and ferro-concrete poles have been used and are being used, but'this paper deals only with woodeii poles, of which not only electric supply authorities, but' the Post and Telegraph Department are large users. "Several different kinds of timber Uavo been used for poles in New Zealand, some being native grown and others Australian grown, suitable varieties of the Australian hardwood being most favoured on account of their strength and. durability, ironbark bo ; ing' especially notablo for these latter qualities. Most of the poles erected in New Zealand are natural round poles with the bark stripped off. A number of square poles, dressed poles, and de-sapped poles have been used, however, but mostly in the more densely populated areas. VALUE OF IMPREGNATION; "In very few cases have tho poles [ rpceived any treatment to prevent do-

cay —where any . .treatment has been given it has usually been by brush or by dipping, and sufficient information is not yet available .1s to the effectiveness of this treatment on . har.dwood poles. There does'not appear to be any piant for the treatment of poles by the vacuum and impregnation process in New Zealand, possibly because the expense of the plant and treatment, and possibly because of the doubt as to whether the increased lifo would bo sufficient to justify such treatment for hardwood poles. ~ "The. value of the impregnation of softer timber lias been very definitely proved in Britain, on the Continent, and in America, where poles which without treatment would last only five to eight years, have been made to last thirty to forty years with treatment. This may be the solution for the use of New Zealand grown timbers as polos, but they would require to be obtained cheaply' in their natural state, and in largo enough quantities to', warrant a plant being installed. This matter may be receiving- consideration by the Forestry Department, but if not," it is worthy of it. "Meanwhile the poles in 'New Zealand are decaying. The poles received from Australia are expected to have ,a lifo of not less than about fifteen years, and anything from twenty to thirty years is confidently expected for the better class.of pole. But some poles have decayed to danger limits in five to six years, and there is necessity for continual periodical inspection." D ; uring the subsequent discussion it 'was stressed,by'several delegates that by using concrete .poles money was kept in the country,, which would otherwise be paid for imported wooden poles. For many boards it was too late to start using concrete poles, as reticulation was practically complete. Mr. Henderson (Dunedin) said that the poles imported twenty years or Wre ago were possibly of better quality than those obtainable now. Some of thr Australian wooden poles in his district had been up for twenty years, and were good for at least another ten. Soft-wood poles had to be considered as well as hardwood poles. The Douglas fir, fully creosoted, and-the western red cedar, butt treated, were almost universally used in the United States and in Canada. Some of the latter had been in use iii Dunedin for thirteen years, and were still as good as when they wore put in) in fact, they seemed to be so good that 500 more were being ordered. Another delegate instanced some totara poles which were being re-erected after thirty-five years of use. Experimenting with concrete poles was, in the opinion of Post and Telegraph officers, not finished ■ with. The Department had in 1910 14,000 concrete poles made at a cost of £29,000. Ironbark poles, however, were given preference, because of the cost of transport of concrete poles. In other countries, when it had been found necessary to remove poles, the cost in connection with concrete poles was much greater than that with wooden poles. It had been found that when motors collided with concrete poles the poles came ofi Second best. Falling trees also broke the. poles. Mr. H. U. Toogood closed the discussion by instancing the experience of Christchurch' with matured ironbark poles, the durability of which was unquestioned. Bad results with . wooden poles had been experienced in the past owing to faulty specifications. Ifthey stuck to certain well-tried and wollmatured woods they would not go far wrong. It was a good idea to mark poles as to their species, etc., when they were put into the ground, so that all details about them could be, easily read at any' time.