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Building and Bushwork fo
Selectors, Squatters, and others.
WRITTEN FOB THE TOWN AND COUNTRY JOURNAL
BY IRED. HARRISON, M.V.I.A., DENILIQUIN.
YI.-CONSTRUCTION OF FLOORS.
THEKE are three kinda of floors: ainele, double and framed floorB. The timbera whioh support the floor aro termed naked flooring. A Bingle floor consista simply of oommon flooring joists, or bridging joists, A double floor oonsista of binding joists, whioh sup port the bridging joists above, and the ceiling joists
below. Framed floors differ only from donble floors, in h&vinsr tho binding j3ist tram ed into girders, which, may be wood or iron. Rules donld be given for calculating the strength of »beams but the following simple rule for finding the actual strength of any beam, will be suffioient for the purpose of this article. The strength is directly as its breadth, multi plied by the square of the depth, and inversely aa the
Tho following may be taken as a constant multi plier for rectangular beams, loosely euppor ed at the ends, and loaded in the middle.
Wrought iron ... ... 2048 Cast iron ... , ... ... 2000 Timber ... ... ... 400
For instance, a beam 12ft long, 6.n deep, and 3in wide. That ia. 410 multiplied by the square of the depth, 36, and again by the width 3, and divided by the length, 12, is i equal to 36001b ; the leneth being taken in feet, the breadth and depth in inches, and breaking weight in pounds. This is the actual breaking weight ; but in praotioo only one-fourth Bhould be applied. It is important to remember this principle, that the strength of a beam is in proportion to its width, but the square ot the depth ; for example,
take's binding joist, 9x4, and 12ft long, and place it j edgeways, it will require, to break it, a weight of
10,8001b ; but if for the purpose of gaining height, we place it flatways, it will break with a weight of 48001b, or loss than one half. Tho strongest beam that oan be out out of a round tree, is that of which the depth is to the breadth. ai tho square root of 2 to 1 or nearly 7 to 5. And tho strength of a square beam out from the same cylinder, or round tree, is to the strongest boam as 101 is to 110 ; but the square beam wonld contain more timbor. Another thing to remember is in the roBiatanoo of a beam, that if it be loaded so as to produoe fracture, this will take place about a central or neutral axis, below whioh the fibres will be torn asunder, and aboye which they will be crushed. This neutral line divides the fibres that are subjected to compression from those in a state of tension. It will ba Been, then, that the shape of« a beam greatly affects its strength ; and in easting iron beams, whioh can easily be oast any shape, the objeot ia to obtain tho strongest form of section with the least metal. The usual section given to oast-iron girders is that of a thin and deep rectangular beam, with flanges or pro jections on each side, at top and bottom; where the
strength of the metal will be most effective, as being at the greatest possible distance from the neutral axis. It it offered equal resistance to tension and compression, the neutral axis would occupy the oentre of the beam, and tho top and bottom flangeB would require to be of an equal strength. The neutral axis, at tho time of fracture, is at about one I Bovonth of tho whole depth of the boam below its
upper surface. Honoo, in girders with flanges, tho thiekness of tho bottom flanges should be six times that of tho uppor ones (supposing them to be tho same width), in order to obtain th« greatest strength with tho least metal. Practically, it would be im possible to oaat a beam thus proportioned, and therefore the top flnngoa aro mad o of tho same thickness, or nearly so, as tho bottom ones, but of a loas width, so aa to contain the same relative quan tity of metal, disposed in a moro convenient form for ousting. Tho difficulty of making sound castings whoro tho posts aro of unequal thickness, also renders it nooosaary to make tho thickness of the middle rib noarly equal to that of tho flanges. Inpraotioo, it
ia better to have the top flange somewhat wider, to prevent bnokling. The efl5eot of a load variée ander different cironmatancea. As already stated, the atrength of a beam supported at the ends, and loaded in the middle, ia inversely aa the distance between the supports. If a beam be Axed at one end and weighted at tho other, itR atrength will be half that of a similar beam of double the length, sup ported at the ends ; or, a parallel oaae would be a beam supported in tho middle and loaded at the euda. In each of the above oasoa the beam will bear double the load if it be equally distributed over its whole length as shown by the dotted linos ; and lastly the strength of a beam firmly fixed at the ends, is to its strength when loosely laid on supports, as 3 to 2,
or ono half moro. Plate A illustrates the above, j whioh may be Bimply expressed aa fol-1 low? : - Suppose a ton weight wouid break j a boam of given length and soantling, j fixed at one end, Pig. 1, and loaded at the other ; then two tons would break the same beam, fixed at one end, and uniformly loaded; four tons would break the same beam, supported at eaoh end, and lojdedin the middle ; six tons would break tho Bame beam, fixed at eaoh end, and loaded in the middle ; eight tons would break the same beam, supported at each end, and uniformly loaded (Fig 2.) 12 tons would break the same beam, fixed at eaoh end, and uni fo mly loaded (Fig. 4.) Timbers exposed to severe strains are sometimes trussed with iron, and this is done in two ways ; first, by inserting oast iron struts, thus placing the whole, or nearly the whole of the woodwork, in a state of tension ; second, by wrought iron tension rods, whioh take the whole of the tension, whilst the timber ia thrown entirely into compression. The latter mode of trussing is now very extensively used in strengthening the oarriages
of travelling oranea, and for eimilar purposes ; and by its nae a baulk of timber, whioh.will barely support its own weight safely without assistance; may be made to oarry a load of many tons without sensible deflection. The practice of sawing a girder down the middle, and bolting them together with the sawn sidoa outwards, ia supposed to strengthen it, but this in faot is not tho case. It. however, exposes any bad parts, and causes it to dry sooner, thus rendering it leBS liable to rot.
Tredgold (to whoso works I am so much indebted) Bays :-" Tho attempt to make a solid beam stronger in the same bulk, without uaing a material stronger than the beam itself, is ridiculous ; yet such has been the aim of most writers on oarpontry." There should always be a tie rod, to prevent the truss from spreading ; for the failure is occasioned by the enor mous compression, applied to a small surface of timoor, at the abutments. The best way of strengthening a limber girder, without increasing;
the depth, is by bolting a plate of-wrought iron on eaoh Bide of a timber beam ; or a Bingle plate, or flitch of iron, ia plaoed between two planks of a beam, out down tho middlo and reversed. Thia is a very suitable form of broastBummer for a Bhop front.
Wrought iron is now extensively used in the con struction of girders and breas taummers, to support walla over shop fronts, heavily loaded floors of mills, BtoroB, &c, and is superseding wooden and oast-iron girders. Cast-iron ÍB liable to oraok from Budden shooks, or from tho sudden application of cold water whilo in a heated state, and in case of fire ÍB littlo botter than wood.
ThoBO boama aro calculated to break with about four times the1 loads in tona hero mentioned, Wrought iron plato girders of tho same strength BB the hox girders given above, may bo mado by increasing tho area of the top and bottom platea $in, the thiokneas of the vertical plates for tho 10ft girders l-8in, and the 15ft and 20ft girdorB 8-10in. In plato iron girders not only tho Bootional areas muBt bo correct, but also the perfoot
-¡- - --. i
arrangement of the joints must be attended to. All j joints Bhonld butt close together, covered with pro- j per jointing strips, and no two joints should ooour in j the same plaoe. In punching the plates and angle
irons, the sectional areas should he diminished, only t to the extent necessary for rivets of tho proper size, | and plates imperfectly punohed should be rejected. Í The rivets should be of Lowmoor, or other Buperior I iron; and where the strain depends on them, their | oolleotive areas should equal the sectional areas of j the plates oonneoted. All girders should be so 1
secured on walls, as to admit of expansion and con- fi traotion. Although box, iron bark, gum, and Btringy | bark (each in the order named), are proved* by expo- .
I riment to be stronger than deal or pine, ox even |
English oak, still the strength of different samples varies BO much, that it is not safe, in practice, to estimate its value mnoh above that of deal ; and in calculating tho strength of beams by the rule given before, 400 may be taken as a safe constant. Floors and ceilings, when first framed, ahould be cambered in the centre. In framinc roofs, the following consideration should be kept in view. It should be BO constructed, KB to have no lateral pres sure on the walls ; but on the contrary, serve to tie them together. The pitch of the roof depends on the nature of the oovering; excepting where tke style of architecture demands a certain pitoh. The eastern custom of making roots flat, as an agreeable plaoe of resort, might be more generally adopted in this oountry. They eould bo covered with stone slabs, cement, conoieto, or asphalted felt, and framed aceordingly. A ridged roof would require little pitoh for asphalted felt-say 3d«g from horizontal ; iron,10deg; slates, usual pitoh, 27deg; shingles, not leas than 35deg ; and thatch, 45deg. In England it IB usual, in addition to the weight of roof oovering, to allow 401b to the square foot, for wind, snow, and other occasional forces, aoting on the inclined sur face of a roof. Hore, it might be rednoed, excepting in exposed situations. Instances are very rare where roof timbers have been broken by the wind, but more frequently they have been stripped of their covering by its getting underneath, or from the vaouum caused by ita passage over the building. In iron roofsjthe meeting points of each truss, or princi pal, are provided with shoes, sockets, and eonneeting plates, into which the ends of the rafters, atruts ana rods aro scoured with screwed bolts and nuts, or giba and keys.
Plate B Bhows two simple forms of iron roof of very easy construction. When the roof is not re quired to support a ceiling, an iron tie rod is prefer able to a wooden beam. There ia no reason why tho prinoiple on which girders are used in thecocBtrno> tion of bridges, should not bo applied to roofs. Mansard, or curb roofs, are now frequently adopted ; the roof ia pitched at two different angles, the lower one being nearly vertical ; and thus affording more spaas for attic rooms. Hoofs constructed on this prinoiple require to be well braced ; and if the trass can also be used aa a partition, it will add consider ably to tho stiffnees.
Mr. Mayoa, in his very useful little work the " Australian Builders' prioe book," gives the following table for wt ought iron girders, without the support of column0. To support walls and heavy maohinery the box girder is the best, but for loaded floors, the plate girder offers many advantages,
Box GIRDER .
10 10 10 15
15 15 20 20 20
15 20 1U 15 20
18 24 20 26 35 ÜÖ 85 46
T ° ti
to o o tn
7 10 0
Sí» fl Si «
In. 3.16 3-16 3-16
Small-pox will go out as tho summer cornea in. Perspiration ia ita deadliest foe. Tho Turkish bath is tho next.
Small-pox is believed by many thinking persons to bo not a torriblo scourge to be dreadod, bat a posi tive bepefit to the porBona nfilleted by it-provided, of oourao, that theyreoovor. A person who hasonoo had Bmall-pox is over after in better health than before* Tho malady is boliovod by this school of thinkers to bring away from tho aystom some im purity that ought not to bo thero, and which natara rojoota by means of small-pox pustules. If thia view bo correct, tho morbid state of public opinion on the subject and tho romodies proBoribod by tho authorities, are moro to bo dreaded than the disease.
PLATE B.-IRON ROOFS. ; -y; *