Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/510

 488 THE BUILDING NEWS. June 14, 1872. EE 4.—The superintendent of stores, or his deputy, will select materials representing 4 per cent. of the value, from which will be cut pieces 20in. in length, and of plates and sheets 20in. by 18in. These pieces, after being stamped at or near the ends with the superintendent of stores’ stamp, in addition to the maker's brand, will be sent to Mr. David Kirkaldy, Testing and Experimenting Works, The Grove, Southwark-street, London, 8.E., to be tested and reported upon by him to the superintendent of stores. The iron will be accepted, although under the above specified strain, provided the contraction of area at frac- ture is the same percentage higher, or in other words, softer iron than that specified will be accepted. In order to avoid expense and delay arising from rejection of materials, the attention of contractors is particularly requested to the foregoing tests, which will be strictly enforced in all cases. The contractor will be required to supply and deliver the materials, but the cost of testing will be borne by the India Store Department. There are many obsolete notions about iron which still retain their places in specifications, such as “ all castings to be of soft gray cold blast iron,” and instructions laid down which are ignored by manu- facturers, and any deviation from which could not be detected, All that is required with wrought iron is a compliance with the tests for strength and elasticity, and if these are satisfactory, it matters not to the architect or engineer how the metal has been manu- factured, or what brand it bears. By limiting the stress to be put upon it, and then taking the proper precautions to ensure getting a good and suitable class of metal, a great stride will have been made towards a sound method of dealing with iron for constructive purposes. It is useless to specify that girders, when tested under a given load, shall not take more than a certain amount of permanent set. seeing that the manufacturer could, if he chose, take the permanent set out of them beforehand by loading them with the test-load, so that when tried after delivery they would show no permanent set what- ever. [Captain Seddon here pointed to the results of two interesting experiments made by Mr. Kirkaldy, ene on a wrought-iron plate girder 23ft. 2in. between the supports, which broke under a force of 44-1 tons applied at the centre; and the other ona cast-iron girder, 18ft. between the supports, which broke with a force of 45°19 tons. ] In order to ascertain the quality of the cast iron used—for instance, in cast-iron girders—the test bars usually specified to be cast at the same time, and from the same heating, as the girders themselves, should, if possible, be cast on to the girders, and only detached in the presence of the architect or engineer, or person appointed for that purpose. These bars are usually 2in. by lin. and 4ft. 6in. long, and are afterwards tested under transverse stress; but it is a question whether it would not be better to cast pieces to be tested under direct tension and compres- sion. Most people would condemn an iron which showed a crystalline fracture as totally unfit to resist tensile strains or sudden shocks, and yet that crystal- line fracture might be due either to the shape of the specimen experimented on or to the mode in which the breaking force was applied, for iron of a superior quality, even when broken by a sudden shock, will present a crystalline fracture, from the fibres not having time to draw out, and, therefore, breaking short across. As an instance of this, Captain Seddon exhibited a specimen of bar iron showing a erystalline fracture with no diminution of area, having been torn asunder by the explosion of a charge of gun-cotton; whilst another specimen shown, which was a piece of the same bar broken under a gradually applied tensile strain, gave a highly fibrous section, with a very great reduction of area, indicating a soft, ductile iron. The author also exhibited some armour bolts which had been broken under the shock of an eight ton hammer falling from a height of 3ft. Gin. ‘Those which were of a sufficiently soft ductile iron drew out before breaking, which they only did after ten or eleven blows; while those rejected on account of the hard- ness of the metal broke, in some cases, with only one blow. That the best of these irons broke with a fibrous fracture is, however, no proof that under a more instantaneous rupture they would not snap without drawing out, and show a crystalline fracture. It is possible that if the most ductile of these bolts had been broken by the more instantaneous force generated by an explosion of gun-cotton, the fracture might have been crystalline instead of fibrous. Then again, the effects of different degrees of temperature upon the ductility of irons of different qualities is also a subject requiring further in- vestigation. We often hear of the crystallisation of iron under the effects of frost, which means that it is apt to snap and show a crystalline fracture; but this is due merely to a loss of ductility, causing it to break without any extension of the fibres. We know that under great heat wrought iron loses its elasticity, but becomes per- 4 fectly ductile, regaining its elasticity but losing its ductibility as it cools. Different classes of iron no doubt lose their ducility at different degrees of temperature, when they become brittle and liable to break off short under any suddenly applied force. Experiment may in time give us some definite means of deciding the best class of metal to use in hot and cold climates respectively, though we know sufficient at present to prevent our using a hard metal where ex- posed to extreme cold. Wrought iron will also lose its Auctility if, as is often the case in heating rivets, it is burnt by being left in the fire too long. A first-class rivet, when taken out of the fire and allowed to cool may be nicked with a cold chisel and then broken across by a blow of a hammer, and if it has not been in the fire too long will show a fibrous fracture, but if burnt the fracture will be crystalline. If proper attention is not paid to this, much injury may be done to rivetted work by too large a num- ber of rivets being heated at a time, in order to save trouble. In rivetting up the ironwork for forts a foreman of works is appointed specially to watch the rivetting, twenty minutes being the utmost time lin. or din. rivets are allowed in the furnace; some are put aside to be tested, and any showing signs of burning are rejezted. Other causes may, perhaps, also tend to reduce the ductility of iron, such as constant vibration and tension combined; but this, in the absence of any direct proof, is mere specula- tion. Putting aside the quality of the iron as well as the nature of the breaking force, a crystalline fracture may be caused by any sudden diminution in the sectional area of a bar, such as is made by cuttiag a thread on a bolt, so that the same iron, exposed to the same stress, may be made to show a fibrous or crystalline fracture by merely altering the shape of the specimen. The best shape to be given to different materials according to the work they have to do, is another question as yet but little inquired into, but one which is of great importance in designing iron struc- tures, asshown by Mr. Kirkaldy’s experiments, and, practically, in what are termed the Palliser bolts for attaching iron plates to their backing. The use of a minus thread on these bolts, or a thread cut into the bolt, and so reducing its effective area between the threads, had the effect of concentrating the work done upon such a small length of the bolt that the fibres very soon reached their elastic limit, and the bolts gave way. This was remedied by reducing the shanks of the bolts, so as to relieve the weak parts between the threads, by spreading the work over a greater length of fibres, thereby obtaining increased strength, with an actual reduction of metal. Again, a law has been laid down, and universally accepted, derived from one or two experiments made upon iron of a certain class, that iron under tension extends one ten-thousandth part of its length for every ton per square inch put upon it. Is this true for all quali- ties of iron and for bars of alllengths? The author thought there was one person, at least, present, who would without hesitation, answer No. Some few experiments have lately been made by Mr. Kirkaldy on long bars under tension and compression, and the results when published will be very interesting; but it is to be hoped that many more such experiments may be made, and on different classes of iron. With regard to the resistance of wrought iron to compression, we have very little satisfactory evi- dence, the whole question depending on the pressure under which different classes of iron begin to yield and sensibly alter their form; for if too short to bend or buckle as it sets up, the area of resistance is increased, and with it the force required to compress it, which has led to an erroneous idea amongst some that wrought iron is stronger under compression than under tension. In leaving the subject of iron, the author expressed the hope that before long an accumulation of facts, derived from careful experiments, may clear up the many uncertain points connected with it. STONE. Passing on to another material, let us see whether we ought to rest satisfied with what we know about building stones. Here, again, the majority of experi- ments made have been upon very small specimens, such as small cubes under compression, whilst the recorded results vary with each set of experiments, according to the amount of accuracy capable of being arrived at by the machinery made use of, as well as the skilland care with which the experiments were made and recorded. If we take a stone which has been more largely used, perhaps, than any other— namely, Portland—we learn from Barlow that its crushing strength ranges from about 1,3841b. to 4,000lb. per square inch, whilst in the experiments made by the Institute, and recorded in its ‘* Sessional Papers” for 1864, the mean resistance to crushing per square inch arrived at was, for 2” cubes, 2,5761b., for 4” cubes 4,0991b., and for 6” cubes 4,300Ib. According to Rennie its crushing strength may be taken as 3,7291b. per square inch, which has been followed by Molesworth in his ‘‘ Handbook,” whilst in Hurst's ‘‘ Handbook” it is given as 2,0221b. per square inch. Now the many varieties of Portland stone, apart from any different method or course pursued in making the experiments, and the amount of seasoning the blocks had undergone, all points which should be carefully recorded, would fully account for the manifest discrepancies between these results; in addition to which, the direction of the natural bed of the stone, which in a small block of Portland might escape detection, would no doubt make a considerable difference. For instance, ac- cording to some experiments by Mr. Kirkaldy on the resistance to thrust of Doulting stone (a Somer- setshire bolite), which Captain Seddon believed to be the only known experiments on this point—if we except two on York paving and Bramley Fall stone, recorded by Rennie, in which the crushing strength both with and against the strata are given as pre- cisely the same, a coincidence too good to be.true— the advantage of laying the stones on their natural beds is considerable, increasing rapidly with the increase in height of the block, in proportion to its sectional area, which is what we should naturally be led to expect, if we look upon the block as approxi- mating, more or less, according to the amount of lamination in the stone, to anumber of thin columns placed side by side. More experiments, on a larger variety of stones, are much wanted to throw addi- tional light on this subject. With regard to the supposition that the crushing strength of stone in- creases with the size of the blocks under trial, there has as yet been too little proof put forward on which to lay down any law. In fact, the few ex- periments made by Mr. Kirkaldy bearing on this subject go to prove that there is no increase in the resistance to crushing consequent upon increase in the size of the blocks. ; With regard to another of the dolites—namely, Bath stone, there is a good deal of misconception, which a careful series of experiments would soon clear up. For instance, Farleigh Down, being a little more expensive than Box Ground stone, is very generally looked upon as the best and strongest description of Bath stone for outdoor use, and is accordingly very often insisted on in specifications, the fact being that, on account of the stone being more difficult to get out of the quarries, especially in large blocks, the price runs a little higher, whilst in strength or endurance it is not known that it can claim any precedence over Box Ground stone. From the experiments already referred to as recorded in the ‘Sessional Papers” of the Institute, it would appear that Corsham stone is considerably stronger than Box Ground, though this is opposed to the results of other experiments. The durability of Bath stone mainly depends on its being placed on its natural bed, which can only be detected by an ex- perienced eye, or by working the stone; though when not so placed it soon reveals the secret, especially where exposed to the weather, by its cracking and peeling away on the face. Much also depends on its being well seasoned, or air dried, before being put into the work, therefore the stone should only be got from quarry-owners who keep large stocks of seasoned stone on hand. If quarried in the spring of the year, aad stacked at open order during thesummer weather, it is doubtful whether Corsham stone is not well able to resist the weather, though it is generally considered only fit for indoor work. Artificial drying, which has sometimes been resorted to, should not be allowed. In one case a large quantity of picked Bath stone, which had been dried by heat, had to be condemned, and led to a lawyer's bill, in consequence of the breaking up of the stone under exposure to the weather, owing, in all probability, to the unequal contraction and expansion of the dried and hardened surfaces, and the soft and green interiors of the blocks. The author had seen stone, which had worn well when exposed to the weather, crumble away on being shifted to the inside of a house. (To be concluded.) ———_os———_ ALLEGED Fravup By A BurtpEr.—In the Court of Queen’s Bench, on Tuesday, in the case ex-parte “ Henderson vy. Holland,” Mr. Lloyd moved for a rule to set aside an award, on the ground that the arbitrator had been wilfully deceived, The plaintiff was a builder, and the defendant the owner of a cottage on the Cheshire side of the Mersey. The plaintiff had been employed to do some repairs, and in the defendant's absence ran up a bill for £213. The claim was partly referred to the arbitration of the stipendiary of Birkenhead, who made an award. The defendant afterwards took criminal proceedings, which are now in progress. The court granted a rule pending the result of the criminal issue.