Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/59

 Jan. 12, 1872. THE BUILDING NEWS. 43 other hand, it will be seen it is the lack of proper trussing, the want of due attention to the relative tensile and compressive and breaking strains, and the making of the main brace too nearly to that of an arched compressive support, which has been the cause of failure in the case of the Eltham roof. Mr. Morris’s theory is as follows :—(1.) Each half princi- pal, kept in place by the collar which ties the two halves together, is poised, as it were, upon a support tending upwards and inwards towards the purlin or chief bearing of the roof, in the manner indicated in Fig. 2 by the dotted line A. (2.) This support is a compound one, eventually carried down on to the walls, in the spandrel below the wall-plate, through the means of the large arch rib. This arch rib (Fig. 3) rests on or rather within the side walls, and forms the grand support of the roof, after the manner of the stone arches of Mayfield and Ightham. (3.) Upon this arch (Fig. 4), at the point B, as a fulcrum, is carried the hammer-beam, which again carries the FIG roof. (4.) Upon this hammer-beam is thrown,equally poised on its outer and inner ends, the weight of the principal rafter and of the vertical post or queen, the principal rafter representing the lower mass of the roof-load A (Fig. 5), and the vertical post the upper part B. (5.) The wall w, as it were, keeps the beam of the balance from oscillating downwards, and theoretically is not essential in carrying the mass of theroof. (6.) Thus the main arch, supported at the feet only, and resting on (or rather we must suppose within) the walls, bears by its strength and rigidity the nicely-balanced superstructure, (7.) The superstructure being thus soadmirably balanecd, the arch gives no lateral thrust upon the walls. Mr. White said that in giving the foregoing analysis of Mr. Morris’s theory, he had followed as carefully though as brietly as possible the process by which Mr. Morris arrived at his result. It must be admitted that it was an plausible one. Mr. White, however, said he could not admit that it was the true explanation. He considered it to be one which was likely to mislead, and if acted upon without a thorough knowledge of practical timber construction generally, to bring about disastrous results. In the first place, he believed the main weight of the roof to be thrown vertically, by the principal rafter, upon the wall-plate and top of the walls, and not upon the corbels or into that part of the wall on which the main arch rests, although some of the load might be there distributed. Indeed, it was known that when the roof was repaired the corbels themselves were found to be free of the curved ribs. Secondly, he could not conceive it possible that the hammer-beam would, either in theory or in fact, bear the strain of the whole of such a roof thrown upon it, at either extremity, in the manner described, even thongh strongly supported by the rib which formed the eusping. Nor, thirdly, could he see how the hammer-beam was in this case supported in its horizontal position, unless it was by the angels carved on the ends of the hammer-beams which formed so prominent a feature in the design, and which were introduced doubtless to suggest the idea of carrying the vast structure in the absence of such other structural support as was found in common roofs. Fourthly, he could show satis- factorily that the system was one of trussing, gain- ing, no doubt, immense strength and rigidity as well as counter-support from the form and structure of the mainarch. The main arch, in fact, acted, and was intended to act, at the same time both as an arch and as a brace, according to the various loads and strains and tensions applied to it. And it might be seen that its structural power as a brace was the greatest which could have been devised, the arch embracing, as it were, the other timbers within its grasp. They actually passed through the body of the arch, which, being composed of many pieces framed and notched together in various ways, laterally and longitudinally, and having counter-pieces above and below it, could have but little tendency to straighten. But even on the supposition that the arch does carry the hammer-beam, and that this, again, carries the whole superincumbent weight of the roof itself resting upon it, the idea ef counterpoise was neither more nor less than a fallacy. This would be seen by a very simple process of analysis. It was plain that in a common rafter framing, such as that shown in Fig. 6, wherein the outer edge of the foot of the frame rests on a wall plate, the whole weight of the superstructure above the collar is in the direction of the inner edge of the wall. This will depress and thrust out the foot. But in the case of Fig. 7, where the inner foot of framing rests on the plate, the weight of this same superstructure will be car- ried down vertically, and having no tendency to depression, there is no lateral thrust, the direct weight being in the direction of the upright, not of the slope of the rafters, because the lateral thrust is wholly neutralised by the cross-tie, Anything, therefore, intermediate to these in the thickness of the wall must still be a preponderance in the direction of the weaker element, and not a rectifica- tion of two forces by equipoise; and anything beyond the face of the wall—as the hammer-beam —would be but an exaggeration of it. An outer bearing on a wall-plate would, of course, be need- ful, not only to steady it, but to relieve any irregu- lar or accidental thrust from the opposite side. Of course, if the tie was raised, asin Fig. 8, another element of weakness was introduced, inasmuch as the rafter then had a distinct tendency to sag. An outward spreading thrust was thus communicated to the top of the lower framing, and transmitted by it to the wall on which it rested. But any secondary process by which that thrust may be counteracted, such as an efficient tie, as Fig. 9, would restore the direct force to its vertical direction. This was a very simple and self-evident matter, and Mr. White’s reason for bringing it forward was to be able to point to the original cause—viz., that in the face of this, a fallacy actually had been set up to sup- port an unsound theory. Mr. White said he had sometimes been told by practical carpenters that a piece of timber would exert its force at the same time both in compression and in tension—as a straining piece and a collar— asa brace and a strut, an arch and a tie, between the same timber, and that from hence arose the great advantage of timber beams and iron tie-rods. O£ course it could not be so in the same particles of the same piece of timber at the same moment. But the front and the back of a brace, for instance, or the top and bottom of a bracket, might be the one in tension and the other in compression, at the same time, so much so that in Fig. 10 a pin was not ingenious theory, and perhaps in some 1espects, a | needed at the lower tenon, the lower part being in compression. In like manner it was plain that such forces might be reversed in their several parts, by the alteration of a superincumbent load, or by side pressure, as by wind upon one side of aroof. And in a brace such as we had in these roofs there would be in the extrados line a strain, and in the intrados line a tension, in counteraction to each other, and giving rigidity to the framework. In the case of the later waggon roofs of the West of England, the ordinary-braced construction commonly failed, in consequence of the brace being so quick as to in- volve its being in two pieces with a joint between, thus affording no actual tension where most required, and throwing the weight on the middle of the rafter and on the inner face of the wall (Fig. 11), which, in consequence, is pressed outwards. In this case, the tendency of the foot of the rafter is to turn upwards, and in many instances this was 8in. or 10in. off the plate. Indeed, it would almost look as if, sometimes, it had not been attached to the plate at all. In others it evidently had rested on the wall-plate, but not being secured by a hori- zontal wall-piece or sill, it had nothing to hold it down, (To be continued.) ee eee THE LAW OF STREET SCAVENGING. Vy HATEVER may be the cause, the condition of the streets during the muddy season is (says the Law Journal) everywhere a striking example of local mismanagement. In London the blame is gene- rally laid on the police, who neglect to enforce the law (2nd and 3rd Vic., c. 94) requiring all house- holders to keep the adjoining footways clean. To cleanse the whole street by making every one sweep before his own door sounds well as a proverb, but it is an absolute failure as a principle of police. To enforce it effectually would require as much labour as to sweep the street twice over. To require every householder to sweep once in two hours for about eight months in the year would be a most vexatious addition to the duties of property, while from an economical point of view the whole idea is most un- reasonable. If the street cannot be kept clean by the central authority, what is the use of belonging to a society atall? One would have supposed that the street was the very type of what is common to all men, and exclusively in the jurisdiction of the the public. If every man is to sweep his own six yards of frontage, why not make him pave it and light it? The result of enforcing the Act will be that special policemen will every morning proceed to ring all the bells in the street, accompanied by a mob of dirty little boys with brooms, who will demand as payment for sweeping the frontage, so long as the policeman’s fervour lasts, much more than would be a sufficient contribution to keep the whole pavement clean all the year rcund. The fact is that the authori- ties of most places have power, under the Publis Health Act, to employ scavengers and charge it to the rates. Thecommon fault of all local manage- ment provisions, however, pervades the present case. The Acts are discretionary, not imperative. Other- wise by employing the crossing-sweepers, whose pre- sent occupation is little more than an excuse for beg- ging, as scavengers of the whole footway, including crossings, we might be rid at once of the nuisance of being dunned for halfpence and the nuisance of mud. ns CHIPS. M. Aurele Robert, painter, and brother of Leopold Robert, died lately in retirement in Berne, aged sixty-eight. Mr. Joseph Gillott, the steel died of pleurisy on Friday last. He was the first to use machinery for making steel pens. He leaves behind him one of the finest private art galleries in the country, valued at from £80,000 to £100,000. The Corporation of Warrington have purchased from Colonel Wilson Patten, M.P., his Warrington residence and eighteen acres of park and garden land for a townhall and public park. The price is £22,000. The first sod of a new railway which will connect the town of Presteign with the Leominster and Kington branch of the Shrewsbury and Hereford line was turned on Thursday week. Mr. Robert Willey, M.R.I.B.A., of Ludgate Cham- bers, Ludgate-hill, has been appointed Surveyor to the Hand-in-Hand Insurance Office. pen manufacturer, The Church of S. Andrew’s, Holborn, will be closed during the next six weeks for repairs and alterations. Tho Rey. Thomas Hugo, rector of West Hackney, and well known as an archsologist, has been dangerously ill during the last few days, but is now, we are glad to say, in course of recovery.