1841. 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



285 



prevent the capillary tubes from becoming perfectly saturated with the solu- 

 tion of corrosive sublimate. 



March 1 6. — The President in the Chair. 

 John Hambley Humfrey was elected an Associate. 



" Description of the Methods adopted for raising and sustaininr/ the sunken 

 Roof (if St. George's Church, Dublin." J3y Robert Mallet, Assoc. Inst. C. E. 



St. George's parish Church, one of the finest ecclesiastical edifices in the 

 city of Dublin, was completed in the year 1802, from the designs of the late 

 Francis Johnston, Architect to th« Board of Works at that time, at a cost of 

 about i;90,000. 



The church had not been built many years, before the roof, which was 

 covered with tun slating and copper, gradnally sunk in several places, by which 

 the cornices at the flank wall were pushed several inches outward. The sub- 

 sidence slowly but continually increased. The ceiling cracked in various 

 places, the ornaaiental stucco work began to drop off, and in the year 1836 

 the state of the roof was such, that the church was deemed unsafe for use, 

 and was shut up. 



Messrs. John and Robert Mallet were consulted as to the practicability of 

 restoring the roof. In November, 1836, they reported that they considered 

 the ceiling might be preserved, and described the manner in which they pro- 

 posed to accomplish it. 



The mode proposed consisted in interweaving with and adapting to the 

 timber framing of tlie roof, a system of metallic framing, so arranged, that all 

 strain or stress should be removed from the former, and borne by the latter, 

 as well as removing all lateral pressure from the walls of the building. 



A careful survey of the roof showed that the ends of several of the princi- 

 pals were unsound. A small hole was then bored through the ceiling, close 

 to each queen-post, and a deal rod, J an inch square, dropped through each. 

 These rods were all of equal length, and their upper ends were secured level 

 with the top surface of the tye-beam of each principal ; then with a levelling 

 instrument placed in the gallery, observations were taken, and the exact 

 amount of the deflection of the framing ascertained. The variation was con- 

 siderable, but the greatest amount of depression was found to be 5J inches. 

 The whole roof was strained and distorted, and was so unsafe that the sUghtest 

 effort caused vibration throughout. 



The causes of this failure a]ipeared to be threefold : a radical want of 

 strength in the framing of the roof; secondly, the employment of unfit tye- 

 beams, which having been constructed during the Continental war, when 

 timber was scarce and dear, were formed almost wholly of short lengths, 

 averaging not more than 20 feet, lapped and scarfed; thirdly, in the queen- 

 posts having been badly constructed and ill placed. 



The stone corbels, which supported the oak cantilevers, being originally 

 cut almost through to receive the wall-plate, were nearly all broken in the 

 middle. It was proposed, therefore, to remove the oak cantilevers and stone 

 corbels, and to cut away the timber wall-plate beneath each principal, to level 

 up the wall, placing a suitable cast-iron abutment piece at each end, and to 

 spring from side to side a cast-iron arch, in " double flitches," connected 

 through the spaces of the timber framing by hollow distance pieces, and also 

 by a certain number of equidistant cross-heads, from which should drop down 

 vertical suspending rods, capable of being adjusted in length, and connected 

 with the tye-beam of the principal, so that being drawn up straight, and the 

 respective parts secured, the weight of the whole roof would be transferred 

 through the framing to the tye-beams ; whilst they being hung from the sys- 

 tem of suspension rods of the cast-iron arches, which would thus sustain the 

 ■whole load, and their abutments being held together by the tye-bars in the 

 chord line, the load would bear vertically upon the walls. 



It was then determined to raise the roof and ceiling by forces applied from 

 below ; to cut away the rotten ends of the principals and to reconnect them 

 with the walls by a modification of tlie cantilever bracket, invented by Mr. 

 Alfred Ainger, and described in the Transactions of the Society of Arts (vol. 

 42). The whole of the oak cantilevers and stone corbels were to be removed 

 as useless incumbrances. 



The total weight of the roof being about 133 tons, each framed principal 

 would sustain about I65 tons, and each vertical suspending rod about I4 ton. 



Although the weight of material in this roof and ceiling may be considered 

 uniformly distributed, it was impossible to foresee what change might be 

 effected in the framing by forcing the ceiUng and roof up to a level line, or 

 what amount of force might bear upon particular points, from the elasticity 

 of the materials being thus forcibly constrained. It hence became a matter 

 of prudence to provide in all parts a large surplus of strength, bearing in 

 mind that, in any complete system, " the strength of the whole is limited by 

 that of the weakest part, and thus that partial strength becomes total weak- 

 ness." The dimensions of the scantling were accordingly so calculated that 

 the utmost strain upon it should not exceed 4-5 tons per square inch, con- 

 sidering 9 tons to be the practical limit to which wrought iron might be ex- 

 posed. 



After giving the formula; for calculating the strains upon the different 

 parts of the roof, with the reasons why the theoretical dimensions were in 

 some instances departed from, the autlior apologises for entering so much into 

 detail of the construction, quoting at the same time the writings of Smeaton 

 and Telford, as abounding in the richest details of theoretic deduction, modi- 

 lied by practical judgment. He then proceeds to describe the means adopted. 



Immediately beneath each of thefourteen queen-posts of the roof, an aper- 



ture of 30 inches square was cut through the floor of tlie church, and a piece 

 of brick and cement built up from the arches of the vaults beneath to the 

 level of the floor ; on the top of each, a i)late of cast iron was bedded, and 

 upon each plate a block of oak timber about 4 inches thick. 



Fourteen straight whole balks of Memel timber, each 3 feet shorter than 

 the height of the church between the floor and the ceiling, with their ex- 

 tremities cut square and smooth, were placed vertically upon the blocks ; up- 

 on this level a platform was laid ; across the tops of the vertical balks, pieces 

 of oak scantling were placed ; fourteen powerfid screw-jacks were then fixed, 

 one beneath each queen-post, and the ceiling cut away for the points to bear 

 directly upon the beams. 



During the progress of these operations, the whole of the ceiling and roof 

 framing had been carefully examined. The dust was removed from the joints 

 and open mortices, &c., of the framing, and the cracks in the ceiling were 

 cleared out by passing a fine whip saw through them, so as to permit their 

 closing when the ceiling was raised to a plane surface. 



The preparations being completed, the word was given to heave simultan- 

 eously u])on the screw-jacks ; the roof rose slowly and steadily, and as soon 

 as any one of the small deal standard rods had reached the level plane, the 

 motion of the screw-jack at that spot was stopped. In about two hours, the 

 whole roof, together with the ceiling, was brought up level, without any acci- 

 dent or injury to any portion of the ceiling. The cracks in the latter as well 

 as the joints and mortices of the framing were found to be nearly all closed. 

 Some slates were broken, and the copper of the platform, which before was 

 wrinkled and loose, was now found to be drawn tight over the timber sheath- 

 ing. 



The roof being thus supported from beneath, the masonry was cut out 

 round the ends of the principals ; the oak cantilevers and corbels of granite, 

 and the rotten ends of the timbers, within a few inches of the inside face of 

 the walls, were also removed. 



The cantilever and abutment castings were now applied and bedded with 

 lead and oil putty, on blocks of stone set at the level of the under side of the 

 tye-beams, on footings of brick and cement. The chord bars were next placed, 

 and temporarily adjusted by means of their screw nuts. The arch segments 

 were put up in succession, their centre or key pieces bolted in, and the seg- 

 ments adjusted to them by means of wedges of African oak : the suspending 

 rods were then hung on from the top shackles, and the junction made good 

 with the tye-beams, by means of the lower cross-heads, stirrups, and shackles. 



As soon as the whole system of the seven arched frames was complete, and 

 the cantilevers adjusted to the ends of the decayed timbers, standing lengths 

 of pine rods were placed in right lines from centre to centre of each pair of 

 abutment cross bolts, and all the chord bars and suspending rods were brought 

 up by means of their adjustment screws, until the united effort of the whole 

 system had lifted and supported the entire roof and ceiling from the screw- 

 jacks, on which they had previously rested, so that these latter all became 

 loose. 



The whole was now left quiet for some days, in order that every part might 

 take its bearing, and that the sufficiency of the structure should be proved 

 before the final removal of the screw-jacks, &c., which remained within about 

 J of an inch of the blocks beneath the tye-beams, by which means, in case of 

 accident, the amount of fall would have been limited to that small distance. 

 The entire work, including the repairing the cracks in the ceihng, occupied 

 little more than four months, and has never since required either alteration 

 or repair. 



The total amount of the contract for this work was 1362/. 6s. The repair 

 of the injury done to the ceiUng only amounted to 33/. 0.«. 8af., and the 

 damage done to the slating, platform, flooring, &c., did not amount to more 

 than an equal sura. 



The total amount of cast and wrought iron in the stnicture was 21 tons 

 10 cwt. 2 qrs. 19 lb. 



The communication is illustrated by five elaborate drawings on a large 

 scale, showing the general arrangement and modes of proceeding, and also 

 the details of the construction of the roof and of the cast and wrought-iroa 

 works used in the repairs. 



ROYAL INSTITUTE OF BRITISH ARCHITECTS. 

 July 3. — Earl db Gret, President, in the Chair. 



A paper was read by Professor Willis, of Cambridge, lion. Mem. F.R.S. 

 &c., On the construction of the Vaulting of the Middle Ages. 



The vaulting of the Gothic architects differs essenti.illy from that both of 

 ancient and modern times, inasmuch as it consists of a combination of ribs, 

 each forming an independent arch, both laterally and diagonally, with the 

 intermediate spaces filled in upon the extrados of the arches to form the 

 spandrils, whereas, according to the ordinary system of vaulting, the whole 

 is soHd and keyed together. The principles of this latter mode of construc- 

 tion were first developed by Philibert de I'Ornie, who, in his celebrated trea- 

 tise, lays down the rules for drawing the vaults and setting out the voussoirs 

 — but of the practice of the Gotl-.ic architects we have no iiccount, and it 

 remains to be inferred from ,-in cx.imination of their works. That they pro- 

 ceeded by geometrical methods there can be no doubt, though they were 

 probably extremely simple, differing greatly in that respect from those ex- 

 pounded by PhiUbert dc TOrme. One thing to be especially observed in the 



