18J0.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



251 



his opinion, sufficiently alluded to the numerous other examples 

 of a similar kind existinj,' in this country. It should be borne in 

 mind, that in the roof of Westminster School, of which a drawing 

 was exhibited, it had been found necessary to introduce cross tie- 

 rods to connect the opposite hammer beams. It was a remarkable 

 fact, that tlie flyinjj buttresses of Westminster Hall are not placed 

 exactly opi)osite tlie principals of the roof, and tliat a straight 

 joint is to be seen between them and the wall: this would tend to 

 prove that the stability of the roof is not dependent on them, and 

 that they had been probably added at a late period, when the walls 

 had evinced signs of weakness. 



Mr. MoBRis replied, that the object of his paper had been to 

 show that the arch-ribs of \Vestminster Hall are not mere orna- 

 mental portions of the structure, but that the weight of cither 

 side of the roof is brought to bear upon two of tlie strongest points 

 of each rib. At Eltham the same principle was carried out, some 

 eighty years later than in the present case. 



JMr. T. T. BuEY (Fellow), mentioned, that in the clerestory 

 roof of St. Mary's, at Bury St. Edmund's, tlie hammer-beam con- 

 struction is introduced alternately with the single arch-rib, and 

 produces a varied and good eftect. 



Mr. Bellamy (Fellow), thought that no one could look at the 

 roof of Westminster Hall without feeling assured of its stability; 

 and he was astonished, that after it had stood the test of time 

 during four centuries-and-a-half, any one should venture, not only 

 to decry its beauty, but actually to call in question its principles of 

 construction. 



Mr. C. 11. SiiiiTH stated, that upon a close inspection of this roof, 

 he had ascertained that the foot of the arch-rib did not rest upon 

 the projecting portion of the moulded stone corbel, but that an 

 actual space existed between them; and he had been informed by a 

 competent authority, that this is the case with many similar roofs. 



Mr. Fowler, V.P., said, that he also had an opportunity of 

 closely inspecting the construction of the roof of Westminster 

 Hall at the time of the erection of the lantern, and of the general 

 repair some thirty years ago, and he had observed the expedients 

 adopted to secure the roof, by means of bolts and ties, which 

 compensated for the decay of the pins and tenons of the framing, 

 but were not required from any defect in the principles of the 

 construction. With respect to the paper which had been read, he 

 gave the author great credit for his ingenious explanation of the 

 principle on wliich this roof was constructed — viz., that of equi- 

 poising different portions, and eventually bringing their whole 

 weight to bear upon the points best adapted to receive it. Mr. 

 Smith's observations respecting the corbels did not, in his opinion, 

 tend to disprove the theory advanced by Mr. Morris, but rather 

 showed the prudence of those who constructed the roof; it would 

 certainly have been very injudicious to allow the feet of the ribs to 

 impinge upon the extreme ends of the corbels, weakened as they 

 were, to some extent, by the mouldings. The ribs were, doubtless, 

 continued into the solid of the wall. He was glad to see that this 

 mode of construction was not only admired, but had actually been 

 carried eut in some of our modern buildings. 



Mr. TiTE (Fellow), thought that all the theoretical objections to 

 the principle of construction of such roofs as Westminster Hall 

 were most satisfactorily answered, by the mere fact of theii having 

 stood the test of centuries; and he thought we might be well 

 satisfied, could we assure ourselves that the roofs erected in our 

 time would be in as good a state of preservation 450 years hence 

 as that now covering \Vestminster Hall. He also observed, that 

 all tie-beam roofs are liable to objection, on account of the shrink- 

 ing and deflection of the timbers. 



Mr. Penrose (Fellow), thought that the arched form of the rib 

 had more to do with the appearance than with the stability of the 

 roof. He remarked, that in the roof of Westminster Hall timbers 

 acting as struts had been introduced between the main ribs and 

 the principal rafters; and he was inclined to consider that the real 

 advantage of the ribs consisted in their acting also as struts, and, 

 at the same time, binding the whole frame-work together. It 

 should be remembered that there were many roofs formed on the 

 tie-beam principle, which were w ell worthy of commendation — as, 

 for instance, that of St. Nicholas, at Lynn, in Norfolk, and many 

 others in Somersetshire; and it must not be forgotten, that these 

 roofs can be executed at a far less outlay than those constructed 

 on the arch principle. 



Mr. G. G. Scott (Fellow), considered that the curved, or arched 

 rib, was not useless. Its object was not so much to bear any portion 

 of the weight of the roof as to prevent it spreading outwards. 

 This was done also, to some extent, at King's College. In a roof of 

 68 feet span, without a tie-beam, like that of ^Festminster Hall, 



we had no right to complain of the existence of buttresses; but, in 

 his opinion, they were used rather as a precaution, than from 

 necessity. It miist be remembered that the walls were 300 years 

 older than the roof; and it was very probable that the buttresses 

 were erected with the view of counterbalancing any weakness that 

 might have been produced by such a lapse of time. Had the walls 

 been new, buttresses, in such an erection, could not be condemned. 



The Chairman thought it was wortliy of remark, that the 

 buttresses are so constructed as not to give their resistance at the 

 point where the gi-eatest lateral thrust is exerted — viz., at the level 

 of the stone corbels. 



Mr. G. G. Scott drew attention to the roof of a remarkable 

 ruin at Mayford, in Sussex, which was of 40 feet span, and had 

 stone principals, or gablets; the walls were of moderate thickness; 

 and, although the hall had been in ruins 300 years, these stone 

 principals wei-e quite sound. 



The Chairman. — The roof was counterbalanced by buttresses. 



Mr. Scott. — Yes; but the buttresses are in ruins, while the 

 arches or principals are yet sound. 



The vote of thanks to Mr. Mobbis was then put and carried. 



TO the editor of the c.b. and a. journal. 

 Sir — In the paper read by Mr. Morris at the Institute, on the 

 structural principles of the Roof of Westminster Hall, I have 

 noticed what I consider to be an error in the principles of equili- 

 bration that he assigns to that structure, and on which I have 

 a few remarks to make. I agree with Mr. Morris that the 

 great arch ribs are not merely ornamental additions to, but princi- 

 pal supports of the roof; at the same time I must differ from him' 

 as to the system of equilibrium that he puts forward. If I under- 

 stand Mr. Morris rightly, he says that the weight of the upper part 

 of the roof is transmitted by means of the posts to the inner ends 

 of the hammer-beams, which form levers, being balanced at their 

 centres on the lower part of the arch ribs. Now, if the posts do 

 not rest on the arch ribs, at the points A-,/, this assumption is per- 

 fectly correct. The upper part of the roof being prevented by the 



collar-beam from sj)reading, may be considered as resting on the 

 inner ends of the hammer-beams, by means of the posts d,f, e, jr, 

 and pressing vertically with half its weight on each. iMr. Morris 

 then says: "The lower half of the roof discharges in like manner 

 its weight on the outer end of the same timber, and the equipoise 

 is thus rendered perfect." Now, to this I demur. The weight of 

 the lower part of the roof d, b,f, will act vertically, not at the 

 outer end h, of the hammer-beam, but in the vertical line drawn 

 through its own centre of gravity; and if the great arch rib inter- 

 sect the centre of the hammer-beam, then at the point of intersec- 

 tion. (This is speaking of the roof covering only, without reference 

 to the weight of the "frame; if this be considered as well, the 

 vertical action of the lower half of the roof d., b,J\ will be inside 

 the point A, in consequence of the greater weight of the post rf,/, 

 and the framing on that side of the point.) The lower portions of 

 the roof <Z, h,f, e, g, c, will then each be in equilibrio, or will 

 balance themselves on the points h, i, of the great arch rib : this, 

 I think, is plainly evident on inspecting the diagram. The frame 

 b,d,f, with its proportional weights of covering, would plainly 

 not keep itself in equilibrio if the point h, were nearer the wall, 

 but would undoubtedly turn over on this point, and fall inwards. 

 It is therefore impossible that the lower portions of the roof, only 

 keeping themselves in equilibrio, can also balance the additional 



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