HARDWOOD POLES

Evening Post, Volume CV, Issue 124, 26 May 1923, Page 13

HARDWOOD POLES

FOR POWER TRANS, MISSION LINES AN ENGINEERS' PROBLEM THE PRESERVATION PUZZLE. (Written for "The Post.")

Wherever electric power is to be carried, poles are required to carry ths transmission lines; and poles are an expensive item in the capital account of the power scheme. New Zealand is using almoßt exclusively poles of Australian hardwood, of which the chief is ironbark. The poles are bought by the various power boards to meet their needs from time to time, aud a great deal of interest has been aroused among the boards and among interested persons and authorities by the different standards and methods adopted in the specifications and orders. It might be supposed that thero has been sufficient experience in the use of such poles to have enabled some central authority to lay down a standard specification that the boards could have adopted,- but that'supposition is discounted by those who should know best. The Government engineers have, in fact, drawn up specifications intended to be standards for use throughout the Dominion covering undressed ironbark and hardwood poles. And these are useful as far as they go. But the power board_ engineers are now. raising the question of dressing, squaring, or "desapping" the poles in an attempt to attain a cheaper or more satisfactory result, and the question under debate is whether they have succeeded in this endeavour. v It is impossible for New Zealand engineers to draw on the experience of (America and Europe for guidance, be- . cause the timber poles there used are ,radically different material, generally soft woods. In America, for instance, the timbers commonly called "Oregon ■ pine" in this country are used, and their very short life is extended by treating them with some preservative. The chief users of poles in New Zealand hitherto have been Government Departments, and while their experience is valuable, it has ! ;not exhausted all the possibilities, and the operations of the power boards are regarded as a form of research work" of high value. If it is objected that research work on so large a scale involves ■ the risk of great loss, it can be answered , that the margins at stake are riot in fact very large. The differences are not those between success and failure; they can only modify the finances of a power distribution system, and the cost of the experiments is not of great importance with the benefit that will follow in the long run. TIMBER HOEDING ITS OWN. There is no great controversy as to whether timber or 6ome other material should be used. The essential factor ia the cost of the poles, and that is inevitably mixed up with their life. A very cheap pole, that will last five years is a poor substitute for a pole that costs five times as much and lasts 30 years. There are only two rival materials—steel and concrete. Steel poles for a given strength cost approximately "five times as much as timber. Their life is not easily determined, depending upon 'their quality and the care given them, ; but the first outlay imposes too'great a jburden upon the system. Reinforced (concrete poles cost between two and three (times as much as wooden ones, and they <have a life of indefinite duration. Prac r Jtically they will last for ever, unless they >are broken; and here again the practi-. )cabilitv of using them depends' on whether the heavy first,cost can, be faced. Generally speaking the method of the New Zealand authorities is to adopt, except in special positions, the temporary 'wooden pole and replace it when it decays, after from ten to thirty years. Timber being, then, the standard pole material, and hardwood the favourite timber, the important question is how; best to use it, having regard to first cost and durability. Here it is that the doctors differ. The' differences are not now on big fundamental .points, but in regard to comparatively minor matters that affect, in particular, the cost. INFLUENCE OF SAPWOOD. Up till about two years ago, it seems, New Zealand engineers did not pay aufileient attention to the difference between tho soft outer portion, or supwood, and tho heart of hardwood polos. About ©no-third to one-quarter of the contonts of a natural polo is «apwood, which in Borvios doos not last more than about half R8 long as tho heart, Tho zone of fastest decay ia "between wind and ) -'' op sbeut the ground, level, and U happens th&l this is the plaoe whoro thg gi'Bßteat strains are imposed en the pete and the greatest strength is essential, As ogles are. in the first plaog paid, for eufc pf lean, whloh is. usually wiped ,put by means of a einldng fond, in fibeut thirty^ years, the ideal pole, ia. one which will last pi least as Jong as tho Joan, THa best haartwaed can be relied upon tg do, po; but ifc is. obviously bud l>HBin??8 id put up poles which, though etilj sound, in the heart, are put eu| pf. BcMon in eight years ov so because, 'the loss, of the sapwood. has brought their jrtrengf'h below the required standard. Fqil that reason & is obyiovis that, if poles ara not go impregnated, with pro. eevvative thftt tha, Bapwood. is immune Iwm decay, the strength Q f a pole must in practise ba calculated from' the size pf the heartweoci, gapweod cannot be depended upon to last more than six or eight years, whereas heartwood will remain sound for from fiixteon to thirty years, depending upon the species of timber chosen. Engineers are therefore concerned with the method.that is most economical for dealing with a tree that in its original state contains say onethird sapwood. Should it bo merely converted into an oversize polo with the Bapwood left on; should tho sapwood be cut off, leaving a round stick of heart alone; or should the heart be further trimmed into a, square shape? The answer depends, not wholly but chiefly, upon the first cost of the pole landed on the job, and is not at all a simple matter to decide beforehand. It is not necessary here to go into all the details, but some of the arguments used may be indicated. CONSIDERATIONS OF COST.. The Government system in the past has been to use natural poles, ns being obviously the cheapest to produce at the forest. These poles soon show signs of decay, especially near the'ground,* and the resulting ugliness has led many people to condemn the poles in use, and declare that they are rotting away. It !s claimed, however, that they are none the worse, that in the course of several years all the sapwood will have decayed, and that the poles will recover their good appearance and retain the strength which was in view when they were specified. This statement of the case in regard to natural poles has, of course, nothing to do with the question whether in some cases the poles ordered were, too small and. that the heartwood alone is therefore not strong enough. Advocates of fclie round "desapped" heartwood poles contend that the cost of cutting off' the sapwood is more than

set off by the lower freight on tho dressed poles, with indeed a considerable margin to spare. It is contended that on the basis of one power board's requirement of nearly 12,000 poles, the cost of. "sapping" will be approximate-' ly £5000, and that the freight on the amount of sapwood left behind in the forests in Australia would be, say £10,----000, so that there appears a saving of about £5000. Ordering poles on this system also does away with any risk, such as is involved in the use of natural poles, of getting sticks that look big enough but have not enough heart to give permanent strength.

THE CASE FOR SQUARED POLES.

The case for ths square pole is rather more subtle. To obtain a square pole of the same strength as a round one means that a bigger tree must be cut: a pole of 10 inches square is roughly equal in strength to a round one 12 inches in diameter, but requires to be cut out of a round one 14 inches thick. Thus, not only the sapwood but a large amount of heart has to be cut off in the forest, and the resulting pole is dearer on account hoth of the larger size of tree and of the extra work involved. Such poles when landed in New Zealand cost, therefore, a little more than round ones. Mr. Waters, engineer to the Manawatu-Oroua Electric Power Board, in an exhaustive paper on the subject, gays that the c.i.f. cost of. a 10-inch square pole 31 feet long 1 is £3 7s 6d, and that of a 12-inch round pole, 35 feet long, is £3 Is id. The difference will be a little greater with poles of the same length. Some engineers have, however, advanced the rather unexpected argument that this difference in cost is reduced or abolished by the fact that railway freights ' and handling charges on square poles are notably lower than on round poles.. It will be realised that the points involved in the discussion are not to be settled out of hand, so as to arrive at a standard practice. They may be modified in special cases by local conditions or fluctuations in prices or charges, so that each case may hav6 to be dealt with on its merits.

It appears that the discarding of the sapwood is a grave waste of good timber. It is strong enough for the .work, but it will not last. Desapped poles are much more costly than natural ones of the same size, and as it proper, -experts are looking carefully into the question of making the sapwood permanent. Unfortuntely, though the timber is one of the oldest of structural materials, at.tempts to lengthen its life are relatively new. They have been forced upon timber users by the fact that consumption of timber has grown far faster than the supply, so that timber is harder to get and higher in cost than it was a few years ago.

The most generally used "pickle" is creosote, which is an oil obtained from wood-tar. It seems to be more effective than anything else that is easily applicable, perhaps because, being of direct vegetable origin, it is more at home. The problem is to drive it well into the wood, so that the loss of a superficial ■ layer will not allow the agencies of decay to get at the untreated wood. FIELD FOR PERMANENT MATERIALS. Steel poles or towers, as already stated, are roughly five times as costly as wooden poles of similar strength. Concrete in this country averages about two and a half times as dear as timber. With such a large difference in favour of wood, it is more economical to go on renewing wooden poles than to use the permanent ones, even though when these are paid for the financial burden of interest charges is done with for good. At the same time, ■concrete towers can be made of any desired strength, and that means that they can be more widely 6paced than wooden poles. This has the effect of reducing, in some cases, the whole cost of the transmission line, because_ a saving is effected by the reduc- J tion in the number -of insulators'. ■ This 1' effect is especially noticeable when power is transmited at very high voltages and the insulators are correspondingly costly. There is little doubt that the future is with_ concrete poles, if cement can be obtained cheaply. New Zealand is not likely _ to bo able to buy steel so cheaply that it will be a successful competitor, as_ it is in Europe. Concrete may he stimulated, too, by a modification in the method of using it. Already there is a hint of such a reform. Before the war a centrifugal process was devised for cheaply making hollow concrete poles without any internal form, and though it has not yet made itself felt, this or some similar method will probably become effective before long. ■ In the meantime, hardwood poles hold the field. Some of the power boards are looking into the question of growing their own hardwood timber, which flourishes in New Zealand, and though perhaps with rather less strength than in Australia, grow? much faster than in its own country. It is probable that this rapid growth is its undoing as far as durability is concerned, and. there is a big opportunity for a successful process of preservative treatment. Nativegrown timber would, of course, be far cheaper than imported, because .there would be less freight to pay. There is a nig opportunity for a successful proems of preservative treatment, and so far as imported timber is concerned, it seems that "pickling" can most economically be done in large establishments at the ports of entry.