42 THE BUILDING NEWS. Jan. 12, 1872.
The timber in the centre stage is as follows — Cub. It, 28 verticals, 12in. by 12in., and 9in, by 9in...... 2157 45 transverse wallings, 12in, by 12in., and 9in.
by 44in.... 5 86 longitudin: ne by 43in, 1443 44 platform joists igs, Jin. by 44in...... 320 184 tranverse diagonals, 12in, by 12in., and Qin. WAY GUM sa sncnarpanetnnehsieumnsaacececeossatecet tence 1949 152 longitudinal diagonals, 12in. by 12in., and Sin iby Olnascevscstee once 1620 5 platform floors, 3in. thick., 780 4 sets of timber slides........ 160 3 sets of horizontal bracing 72 Battens ‘and joint-pieces........00. aver 154 10,155 RVIDURLECUD EN, scsescsacavscrsussssancocecceckcs evecnsecess 16,679.
Total in the whole stage ......... 26,834 Timber Slides.—The timber slides, one of the most important, and as they proved in practice the most useful and handy part of the apparatus, are made of 12in. square timber; at the outer end a jaw somewhat like that of a vice is worked on them; the 12in. timber has a packing piece of 2in. thick on each side of it, and outside of these are the pieces forming the jaw 12in. by 4in. The flange of the main ribs being 16in. wide, these packing pieces are fitted so that when the jaw pieces are screwed up they hold the rib firmly in its place, The 12in. square piece is bolted firmly down by 4in. by 1iin. bolts; these bolts do not pass through the slides. The slides are made so as to be capable of adjusting ; a cros3- piece is placed on the top of the slide, the bolts pass through this and outside the slide, but touching it; & cross-piece is also placed under the wallings or plat- form timbers, and the whole bolted down. Two of such bolts and cross-pieces are fixed at each end of the alide. It is formed outwards by the folding wedges which abut against the vertical timbers. Having described the hoisting jibs or derricks and the steam crab, Sir George proceeded to describe the mode in which the whole of the apparatus was ac- tually used, first expressing his thanks of Mr. John Clarke, the foreman, to whose care the use of this great heavy machine and the erection of the floor and roof was entrusted, for the skilful and energetic way in which the work was executed, The method adopted in putting up the main ribs, purlins and intermediate ribs were this :—It was ordered that the box girders and plated part of the main ribs should be erected first on balks of timber, and wedged up to the proper position and then be under built. The same system was adopted for the intermediate ribs, they were hoisted to their proper position, wedged up with timber, and under built. Sir George had there- fore to depend on the staging not only for hoisting and keeping the whole of the roof in the proper shape, but also to impose on it the duty of providing lateral support tothe whole structure; no support could be got from the walls until the walls were up, and the wind ties fastened to them. The design of the roof generally is bold and excellent; the ribs are self-sup- porting from the very floor ; the side walls have only to carry the ends of the intermediate ribs and the wind ties. Until these side walls, however, were built and the wind ties fastened to them, the stability of the whole depended upon the staging. The sup- port being obtained as it passed from one station to another, never leaving its hold until the last rib had been fixed. The mode of commencing the work was this :—The first and second box girders and plated part of the main ribs to be erected being fixed, the whole stage was brought with its timber slides oppo- site to this part. A centre line was marked on the plated part, and a centre line set out with a theodo- lite on the stage itself, to indicate the exact position on the centre of the main rib, the whole staging of course being bolted firmly together and also bolted to the floor. The timber slides were then carefully ad- justed, with their centre lines coinciding with that aet out by the theodolite. The slides were so adjusted as to keep the rib to a larger curve than it wou d re- present when finished, because it was necessary to keep the segments out or open to allow the last seg- ment to be got into its place. This being done, the temporary screws were withdrawn, the slides were drawn a little inwards to allow the joints to come together, and the riveting began. Two ribs were thus being fixed together, but only the two first ribs, the purlins and intermediate ribs were also fixed and riveted up; when all was complete the timber slides were slacked a little, and the ribs were allowed to take a portion of their own weight. The staging was now ready to move; the wedges of the wings were knocked out; the holding-down bolts on the sides of these wings slacked, and the slides drawn, entirely inwards; the bolts and chocks which held the wings to the floor and to the centre compart- ment removed; the centre compartment was kept screwed fast to the floor, and the slides were also kept fast. The wings were then removed until their
leading slides were in line with the third rib—the centre compartment all the while holding fast to both ribs. When the wings had been moved to their pro- per position they were screwed down to the floor ; by their slides they held fast to No. 2 rib, the slides being forced well outwards, and carefully fixed sideways. The slides of the centre compart- ment were then drawnin; it was drawn up into line with the wings; the whole was screwed fast as before, and all the three compartments took hold of No. 2 rib, No. 1 being supported laterally by its purlins, which were riveted to No. 2, but carrying its own weight as well as that of half the purlins and intermediate ribs. When No. 3 rib was fixed and finished, the same operation was repeated for No. 4 rib, and all the others to the end of the station, the stage never leaving its hold until all was completed. A brief discussion followed, in which Professor Kerr, Messrs. T. Morris, Dawson, Pain, and the chairman took part, and the thanks of the meeting having been accorded to Sir George Alleyn for his paper, the hon. bart. acknowledged the compliment, and the meeting adjourned. [Erratum.—lIn the discussion on Mr. Cates’s memoir of late Sir James Pennethorne, reported on p. 480 of our !ast vol., in line thirteen from the bottom of the third column, for ‘‘ Mr. Ferrey” read ‘“‘ Mr, Talbot Bury.”] ———————— HAMMER-BEAM ROOFS. A T the usual fortnightly meeting of the Archi- tectural Association, on Friday, Dee. 29, Mr. Rowland Plumbe, the President, in the chair, Messrs. Joseph Taylor and W. F. J. Crisp were elected membrs, and Mr. W. Wuirte, F.S.A., paper on F.R.I.B.A., read a HAMMER-BEAM FAILURES, ELTHAM PALACE. Mr. White said that during the visit of some of the members of the Association this summer to Eltham, he made a few notes and sketches of what struck him as of much importance in the construc- tion of the roof of the great hall. The features and causes of failure were best visible to those who took the trouble to mount the ladder placed at the end of the building for the use of visitors. Numerous causes had contributed to the failure of this roof, arising chiefly from the ignorance or the neglect of the principles which guided the builders of earlier roofs of the same class, and which in many instances were kept up to some extent in the later roofs. The traditions which had kept alive the development of art generally for five or six centuries were gradually dying out at the time the roof was constructed, and hammer-beam construction pre- sented no exception to the universal decadence. The difference between the earlier and later work con- sisted chiefly in this—that in hammer-beam con- struction the earlier architects paid some attention to the system of trussing and tying, as well as to that of corbelling and strutting. It was denied that they knew anything about ‘trussing. But whether they theorised upon the matter or not, it was certain they did happen, somehow or other, in very many instances in the earlier periods, to hit upon the most practical and practicable manner of effecting what they had performed, and that they did this to some extent by means of a sort of truss- ing. Fig. 1 shows a truss of early construction. It does not cease to be a truss merely because in the multiplication of its parts it ceases to be a simple one, Then again, in later days, when strutting and corbelling failed, men were fain to have recourse to a more strictly scientific and exclusive system of trussing than ever before prevailed. But they now took it up in a merely mechanical way, without any reference to architectnral treatment or artistic effect. In all roof construction the two great forces to be counteracted are, of course, the tendency of the principal rafters to deflect or sag, and their feet to spread. The spreading is universally reoognised, but in theconstruction of the hammer-beam roof the provi- sion made against sagging seems to have beentoo much overlooked, not merely by some of the architects of the later Middle Ages, but also by the moderns; and hence we have various theories propounded for what is, after all, a mere common-place and matter- of-fact cause of strength or of failure in these roofs. Even Mr. Thomas Morris himself, who has written from time on these subjects, in his theory of the construction of the roof of Westminster Hall, has been driven to account for its stability by a theory of equipoise, which, howsoever intelligible and satis- factory to himself, has not been equally so to some who have investigated it ; whilst others, again, have not agreed together as to what hereally meant. Mr. Morris's view failed to satisfy Mr. White, who could not see the sufficiency of Mr. Morris's theory to account for the fact of the wonderful strength and stability of the roof. Moreover, on the one hand, it will be found that a system of trussing is fully suffi-
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cient to solve the mystery, and therefore it is need- less and superfluous, besides being inexpedient, to attempt to explain it in any further way. On the
