Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/105

 N Fre. 2, 1872. wholly and entirely thrown in a vertical direction upon the wall by the wall plate, and the tendency to spread being counteracted by this grip, given by the direct load. It will be seen that the greater the vertical weight shown here, the greater is this counteracting force; and that if a corbel were in- serted to carry this wall-piece and brace, the weight thereby being taken off the top of the wall, the sole counterpoise to the lateral thrust would be taken away. Starston Church, again (Fig. 22), of similar construction, has yet a further interest in the manner jn which an arch curve is employed instead of a straight tie for the upper portion of the roof with a pendant king-post. We now come to the roof in question, that of the great hall at Eltham Palace, and it will be observed how great is the number of pins employed, showing the provision made for a tensile strain. The illustration given by Dunnage and Laver, plate XIX., in their ‘‘ Account of Eltham Palace,” published in 1828, shows all the joinery and notching of the various timbers. Here the pendant-posts or queens are shown as being in one length, and the arched brace on either side framed in. Itis said in the letterpress that the principals, resting upon the wall ends of the horizontal beams, more than counterbalance the weight lodged upon the pendants. This is plainly a mistake, for the principal rafters rest upon a line projecting actually beyond the inner face of the wall, and must have rather a tendency to depress the hammer-beams than to act as a counterpoise to them. It is then said that great strength is given by the arch being con- tinued in one entire mass, falling almost in the same line with the horizontal beam, and at last terminat- ing its abutment against the external wall and act- ing asso many struts in its progress. Verily these are struts pushing out the wall, and this roof ought to have relied more wholly upon trussing, and not at all upon the corbel bracketing which acts upon the wall. In this roof the real construction is much concealed by the amount of moulding planted on, some of it running into the mouldings workedin the solid of the timbers themselves. But these mould- ings are in some instances of sufficient scantling to add strength to the timbers. This makes it some- what difficult to ascertain accurately the mode of jointing in some portions of the work. The diaw- ings just referred to, apparently made with great care, differ materially from the rough sketches which Mr. White took. They show the arch-brace framed with great regard to structural strength into the pendent posts or queens. Mr. White’s sketch was taken from the westernmost bay, at the back of the principal against the gable wall, accessible by a ladder put up by Mr. Bloxam on the occasion of the visit of the Association last autumn. In this principal at least the brace is in one piece, and the upright is in two pieces framed into it above and below. This seems evident from the manner in which the framing has separated: a depression in the brace has impinged upon the outer edge of the upper upright, and upon the inner edge of the lower. It was this that first of all called Mr. White’s attention to a manifest defect in the construction. Had this been in one piece it would have afforded by its tensile power great assistance in holding up the end of the hammer ‘beam, as well as by its breaking strain contributing to the strengthening of the brace itself. Possibly the other principals, or some of them, may be con- structed in the manner shown in that publication, but the probability is this was put in from sup- position and guesswork, without that careful exami- nation which such a record imperatively requires, and which makes it comparatively, if not absolutely worthless as an authority, however it may show its authors’ appreciation of true construction. Mr. White was the more confirmed in this view by the drawings presented to the Institute only three years later than the date of the work, and which the Secretary and the Librarian of the Institute had kindly lent him for the purpose of illustrating his paper. This section showed the main brace running through in one piece, without any elaboration of mortice, and tenon, and pins, and it showed the up- right not in two, but dotted through, as though possibly it might pass the brace on its other side. It may be that at the time of making these draw- ings so much of the separation had not taken place as is now visible. The open joint now shown is i}in. on the upper edge, and nearly as much on the lower. This, then, Mr. White conceived to be one of the probable causes of failure in this roof, but it was by no means all. The second cause of failure may be the absence of king and queen posts, which hold an important place is the upper spandrel of Westminster Hall roof, and give support to the collar. Certainly a third cause is the unequal distri- ution of pressure, whether as tensions or as supports, afforded by the four-centred or elliptical form of the THE BUILDING NEWS. main arched rib. It is true that the brace is con- tinuedin the more regular curve behind this down to the hammer-beam, one small member being continued through or beneath it. A fourth cause is the want of tie or direct tensile force between the point of the arch and the end of the collar-beam, irrespective of the absence of the main arch, which arch is so conspicuous in Westminster Hall. A fifth cause is the bringing of the foot of the principal so nearly upon, and indeed almost actually over, the inner face of the wall, by which means a great portion of the body of the wall is deprived of its direct vertical pressure, and the very slightest tendency to depress necessarily forces the wall outwards. But above all is the sixth cause, viz., that in addition to the want of tensile support, or suspension of the hammer-beam, the hammer-beam is corbelled out by semi-elliptic arched brackets from the wall as a main support, and not, as in the case of the Westminster Hall arch, with its supplemental cusping by way of braces. Now Mr. White maintained as a principle that if the whole superincumbent load of a roof of this nature is thrown vertically upon the wall plate and wall a considerble proportion of lateral thrust by mere leverage will do no harm whatever. The direct vertical weight of the roof acts as a real counter- poise to the lateral pressure given by any tendency of the upright wall-piece to drop; but if this upright wall-piece is shored up or carried by a corbel to such an extent as materially to relieve the pressure from the wall-plate, the lateral thrust is enormously increased, whilst the counterpoise which, by direct vertical pressure, would have counteracted it. is taken away. He also maintained that this is the case to some extent in the roof at West- minster Hall, for although the main arched ribs appeared to be bedded in the wall, and may possibly have some bearing, yet by very far the pre- ponderance of bearing is vertical upon the wall plate through the omission of the trussing force of this arch, and by the subsidiary timbers. And he was convinced that unless there were an enormous sus- pensive power in what he called the queen-post of this roof, the designer would never have accumulated on to it the whole mass of intermediate weight, as is done by the longitudinal struts and braces. In con- cluding, Mr. White said that only on the previous day he had paid a visit to Eltham in order to assure himself upon some of the points as to which he could not satisfy himself. He would give the results of this visit without any modification of his paper already prepared, although in one or two respects it would prove that his observation had not been suffi- ciently full or sufficiently accurate. He exhibited two drawings, showing by dark shadow various openings of the joints, and showing also the nature and average extent of the mischief, which varied materially in the different principals. He found that the main principals and wall principals are constructed differently; braces and queens are halved together, but in the other principals the queen-posts are in one piece, and the braces tenoned into them, if they do not pass through them, which seems scarcely possible, although they are but Sin. thick, and the queens are 18in. square. They are, however, supplemented by moulded pieces on each side 5in. thick, which are keyed and pinned into them laterally. It is plain that these braces are not sufficient for the office which they have to perform, and that the line which they take at their foot is one calculated to throw lateral pressure upon the wall in case of the slightest failure or depression. The stone corbels have ful- filled their office nobly—whether that office was a legitimate one or not. They have carried a large portion of the whole weight of the roof. The upper part of the wall has actually separated from them, and left them and the under part of the inner wall- face mainly in their vertical position, but at what a sacrifice may be seen by the crippled state of the wood bracket over. This bracket, instead of being, as it ought to have been, a brace acting in conjunc- tion with the upper brace and the crown of the arch, and again with the heads of the rafter by a king, has been converted into a compressive arched support, which has buckled up and literally thrown the roof and the wall off their bearings. This is more especially the case on the side which is exposed to the pressure of the south-westerly gales. Then there are evident signs of the depression of the main collar, which has opened with the sagging of the principals at the lower part of the joint. The upper collar, not being tied up by a king, has been depressed by the uprights of the tracery. The rafters, haying a tendency to slide down, as well as to spread, haye separated at their apex. This evidently arises from their bearing hard upon the studding of the tracery. These are tightly jammed upon the upper collar, but the lower collar has sagged con- 89 _————— ooo siderably more; so much so, indeed, as to leave an inch and a half space below the tracery filling. This lower collar has sunk from them, dragging down the braces. These braces have opened both at the point of the arch and at their haunches, showing the manner in which they have spread, and showing, at the same time, the nature and direction of their tensile strain. From all that he had advanced, Mr. White said it was apparent that there was ample room for a considerable amount of further investi- gation into the scientific principle of the construc- tion of these roofs. A spirited discussion ensued, the speakers being the President, Mr. T. H. Watson, Mr. J. D. Mathews, Mr. G. R. Redgrave, Mr. S. F. Clarkson, and others, and the thanks of the meeting having been unanimously accorded to Mr, White for his paper, that gentleman replied to some of the points raised by the speakers, and the proceedings termi- nated. —__g_—. COMPETITIONS. Dersy.—The designs of Mr. Coulthurst have been chosen by the local School Board, for the new school in Gerard-street, after competition. The second premium was awarded to Messrs. Giles & Brookhouse. Tur Lourn New HosriraAn AND DISsPENSARY. —On Friday a general meeting of the governors and subscribers to the projected hospital was held at the Townhall. The committee had been authorised to obtain designs, but it was decided that the selection of one out of the three recommended by the com- mittee should be left to the decision of a public meet- ing. The sum of £1,000 hed been fixed on as the amount to be expended on the building, and the secretary had informed the various competitors of the decision of the committee, so that each one started fair in that respect. Nine designs had been sent in, and out of them three had been selected, which were to be submitted to the decision of the meeting. The three plans selected were, according to the Lincoln Gazette, those of Mr. J. Fowler, Louth, £1,750; Mr. Albury (lately an assistant to Mr. Fowler), £1,150; and Mr. Buck (also lately in the employ of Mr. Fowler), £1,050. They were respectively marked “Triangle,” ‘Red Star,” and ‘‘ Well Considered.” During the morning the plans had been exhibited in the committee-room, and were inspected by a con- siderable number. When the meeting had been opened, Dr. Sharpley expressed his opinion that as the committee had publicly announced £1,000 as the sum intended to be expended on the building, the plan at £1,750 was excluded, and that their choice must lay between the two others, more especially as the medical men had reported in their favour. During a conversation that followed Mr. Fowler advocated the claims of the ‘‘ triangle” plan, which, considering it was his own, was, to say the least of it, extremely indiscreet, and was so considered by most present at the meeting. It was proposed by Mr. Allison, seconded by Mr. Sharpley, and carried, ‘‘ That look- ing at the limited amount which the institution had at itscommand for building purposes, it was not prudent to expend a sum much in excess of that originally proposed, viz., £1,000, and that no esti- mate should be accepted of a greater amount than £1,400 or £1,500.” As this decision excluded Mr. Fowler’s plan from further competition, the other two were referred back to the committee, which was authorised to select the one considered most suitable A great deal of useless mystery was kept up, as it was well known to whom the three plans belonged, and the friends of each competitor had a natural bias in favour of one or the other. —_—_ 5 —— SCHOOLS OF ART. Sarrspury.—The annual distribution of prizes of the Salisbury School of Science and Art was announced to take place last night (Thursday), the mayor in the chair. Dusuiw.—The Lord Lieutenant of Ireland dis- tributed the prizes on Monday evening, at the Royal Dublin Society’s theatre, to the successful pupils in the Dublin School of Art. The report of Mr. Lyne, the head master, to whom the success of the school is mainly attributed, was satisfactory. It stated that the total number of pupils attending the school during the past year was 415. Of these 218 were males and 197 females. The proportion of artisan students was 280. The fees amounted to £417 9s, A growing interest has been displayed by the students in those branches of art education which are more immediately connected with manu- factures, and local manufacturers have given some little encouragement in this direction. [ive pupils were awarded prizes in the national competition. The works executed by twenty-six of them have
 * been selected for the next national competition.