Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/80

 64 THE BUILDING NEWS. Jan. 26, 1872, The thickness of an abutment may therefore be calculated, when we know the horizontal thrust, on the principles explained in a former number of this series. In practice it ranges from about one-third to about one-fifth of the radius of curvature of the arch at the crown. To determine the thickness of the arch by calculation would require an almost intermin- able number of figures ; but the practice may be taken to be this, that the thickness of the arch at the corner is a mean proportional be- tween the radius of curvature at that part and a constant which does not yary much from “12 in single arches and -17 in the arches of a viaduct. Viaduct and bridge, although criti- cally meaning the same thing, have come to mean two different structures—a bridge mean- ing one or a few arches and a viaduct many. This rule is more concisely expressed thus : Let D = the depth of the arch at the crown in feet; R = the radius of curvature at the crown in feet ; then, for a single arch— D= V-12R and for an arch of a series— D= V:17R This rule is derived from examples of bridges of masonry taken indiscriminately, whether of brick or stone. Bricks will not sustain so great a compressive strain as some kinds of stone ; but the compressive strength of either is not nearly approached, so that in such a case it is of less consequence whether the material be brick or stone. Only in one well-known example has the actual strength of brickwork been nearly approached, and that is in the bridge carrying the Great Western Railway over the Thames at Maiden- head. Here Mr. Brunel has put the crushing strength of bricks to the test to a greater degree than in any other known example. The ratio of actual strain to the whole strength of the bricks is about as 1 to 3. In most other examples the ratio is from one- twentieth to one-fortieth of the crushing strength of the material. The crushing strength of bricks varies so much with the different kinds that a general statement can hardly be made, but good bricks should bear 90 tons per square foot ; ordinary bricks about 30 tons; while inferior sorts will not bear nearly so much as that. These are the crush- ing weights. In each case the brick begins to crack, and therefore practically to fail, with half these weights. The proportions of the Maidenhead bridge are shown in Hig, 9. The bridge consists of two arches of brick- work in cement, each 128ft. span and 24ft. 3in. rise. The thickness of the arch at the crown is 5ft. 3in.; midway between the crown and the springing 6ft., and at the springing 7ft. llin. The radius of the in- trados at the crown is 169ft. The depth at the crown caleulated by the rule above given would be 536ft., while the actual depth is 5-25ft. It is impossible to make a rule of this kind agree exactly with every example in practice, but it agrees nearly with many examples of large arches, : We may call the above an example of a bold arch on a large scale. Here is one equally bold on asmall scale. It is from the warehouse floors of St. George's Dock, at Liverpool. The arches are 10ft, span, with lft. rise, the thickness being 4iin., the spandrels filled in with sand, and the floor paved with bricks, laid flat. All the arches are tied together with wrought iron tie rods, 1jin. diameter. As the thickness of an arch advances by half-a-brick at once, it may be useful to calculate out the radii for several thicknesses. Taking the above-named rule the radii should be as follow :— Number of tADIUS, half-brick rings. Single arch, Series. eee. csce LEG, eee 0-7dft. me | ex dS) eee 3 me asl e LO; ees 5 Se ae sce LS joer. 15 San Ane cece: 30s, ee 20 GT et. ae 5 30 PLUMBING.—I. (Continued from page 32.) N our last we treated of plain semi-circle rhones, which may possibly be considered to partake more of the character of an ex- erescence and a necessary evil than an orna- ment to the building. In this case, however, we come to treat of ornamental iron rhones or gutters, which are necessary tothe completion of the design, and which, as the old saying has it, kill two birds with one stone, as at one and the same time they serve either as the whole or part of the cornice mouldings as well as runs or gutters for the rain-water. These ornamentaliron gutters donot, generally speaking, require hooks, as provision is made for them being screwed to the woodwork of roof, a board about lin. or 1}in. thick, and about same depth as back of rhone, being fixed up along back of where they go for that purpose. In some cases all the weight of rhone is borne by the screws, as in Fig. a. In other cases great part of bottom of rhone Fig. dD. lies on wall-head, as in Fig. b. Insome cases they are upheld by ornamental iron brackets, as sketch, which are of various styles to suit the place. These ornamental iron rhones or gut- }) ters require to have close ends (unless in cases where they go all round the building, as in some cottages) and drops, just as the plain nalf-circle ones do, only they are not put up with a declivity to drop, for that would spoil their appearance ; they are fitted up quite level or horizontal, but owing to their being several times deeper than the half- circle rhones, they can dispense with a declivity. The depth of a 43in. semi-circle rhone is only about (or scarcely) 2in., whereas the depth of ornamental rhones may be 3in., 6Gin., or 8in., &c., with a corresponding breadth, according to circumstances, and as the size of roof or the character of building may require. In order to keep an unbroken line of frontage, ornamental iron gutters haye no faucits, the plain end of rhone acting as the faucit, while the end of other length iscon- tracted so as to slip into it (see figure). They Fig. a. ag ) >» =< a l are put together with putty or red lead, and rhone bolts, only the head of bolt is kept outside, and the hole countersunk so as to leave as little as possible to catch or to be offensive to the eye. Therhone bolts in this case are also a little thicker, as well as longer than for the semi-circular rhones, In some cases an ornamental iron gutter is put up along the front of building, while a plain half-circle one is put up along the back- eave. ‘This is often done where the building has two gables, and supposing it to be done at a house in the country, or wherever the water is scarce, instead of merely allowing the rain to run off into the drains, provision is made to catch as much as possible either by collecting it ina tank, cistern, or rain- water barrel. In such a ease the barrel, we shall say, is put up at one of the back corners of the house, with the rain-water, both from back and front, led into it as sketch. In this case the water from front gutter E is conveyed along gable into back conductor F, by means of an iron, lead, or zine pipe, as shown at G, which pipe is upheld by holdfasts if iron, or if of lead it may be upheld by wings or bands, of which more anon. As will easily be understood, an overflow pipe, H, requires to be put into barrel (or it may be done as per I), so as to make provi- sion for carrying away surplus rain-water when the barrel is full, The water-tap or bib-crane, J, is for drawing off water for use. Of course there are many little differences in detail which start up ; but to one who under- stands the principle these are easily mastered. In some cases, as in that of the supposed house in former article, which had nothing but plain half-cirele rhones on front eave as well as back, the front rhone is simply carried along gable also, where it is upheld by holdfasts (as here shown) made to fit it, and it runs, say, into a rain-water head, which receives the water both from it and the back rhone, and sodown con- ductor into barrel. There are so many patterns of these orna- mental iron gutters, from the plain square to iit those with the most elaborate mouldings, that we can only refer to them here ; but the above serve to show the principle. The plumber haying put up his rhones, during the time he has been seeing after his pipes the slater has not been idle, for he has got the roof all slated, and the plumber may now put on the ridge, which may be either of lead, zinc, oriron. The lowest breadth, gene- rally speaking, of either lead or zinc ridges is one foot, which allows a cover of about 4 in., or fully that, on each side over slates; but in many cases a greater breadth is used and found necessary, in order to give the slates sutlicient coyer. If it is to be lead for the ridge, then a ridge about 48 feet long can be put on, say, in three pieces (or as lead at hand will allow), an overlap of about twoinches being given where the pieces meet. After the lead has been rolled out and beaten into its place with the usual wooden QF “dresser” (see sketch), a lead-headed — or now, perhaps, a galyanised—slate-nail is driven in ata distance of, say, every two feet or so into each side of top of ridge. Sheet lead for ridges may be from, at lowest, 5lb. up to 7lb. or more per square foot. When less than dlb., it is easily torn up in a gale of wind, and even although (as is done Oe in some cases where the Me wind) galvanised iron ridge (see sketch) straps are put on every three feet or so, yet if the lead be lead is much exposed to thieves as well as to the too thin it is sometimes torn up between the straps.