Ársrit Verkfræðingafjelags Íslands - 01.01.1914, Blaðsíða 33

Ársrit Verkfræðingafjelags Íslands - 01.01.1914, Blaðsíða 33
33 for concrete. Would it not be possible to make some satisfactory provisions as to the quality required of cement fit for use? Those foreign building regulations which I have seen, have not contained any provisions regarding the substances used for concrete, with exception of the building acts of New-York City. There it is so ordered, that the biggest pebbles of the grovel may have a diameter not surpassing 5 cm, and that only such a cement may be called Portland cement, as — unmixed — is able to sustain the weight of 8,5 kg/cm2, without being crushed, and having been ex- posed to the air for one day; after one day in the air and six days in water, it should be able to bear 21 kg/cm2, still without being quite crushed. Olher sorts of cement should be able to bear proportion- ally 4,2 kg/cm2 and 8,5 kg/cm2. — In the building reg- ulations now in force it is allowed to mix the con- crete for house walls up to 1:5: 10. I find this con- crete far too poor, and think that in this country it could never be made from such materials or with snch a care, as would be necessary to prevent it from being soaked with water, and it is unsafe to rely on the plastering of the walls as the sole pro- tection against damage by water. To get good walls, we must use a stronger concrete. Of course there can hardly be question of making the whole walls of a house watertight just by concreting them, for that purpose they had to be made from a concrete of 1:2:4, or not much poorer. Neither is this neces- sary. I have observed that walls made from a con- crete of 1:4:7, are nearly waterproof, if carefully prepared. It is probably more common to mix 1:4:8, and that will do very well, only the other concrete is safer and comparatively hardly more expensive. I do not think it is advisible to permit a poorer concrete than 1 : 4 : 8, — (1 : 4 : 7), for the bulding of usual dwellinghouses or other buildings of equal importance. During the last years people have beginned to mix the concrete with stone — for economical reasons. The Germans allow this to the extent of 40°/o of the volume of the concrete. This is of course economical, but must be limited, otherwise it will turn out badly. The permission of using stone in this way should depend on a certain thickness of the walls, not less than 40 cm should be required — and on the breath of the piers between doors and windows, which should be considerable and could be more closely defined in the building regula'tions. AIso should the size of the stones be limited, it should not exceed tlie half of the thickness of the walls and there should never be a less interval between the single stones or the stones and the surface than Í1/^—2 times the size of the biggest chips in the concrete. Stones might be mixed in concreted walls, which only had a certain pressure for instance 7 kg/cm*, but it would probably be too complicated. When the queslion is of dwellinghouses, I think it is doubtful, whether it is prudent or not to save the concrete by mixing it with stone. Very likely the thermal conduct- ivity of the walls would be increased by this. The thickness of the walls of onestoried houses, and of the walls of the highest stories in manystoried houses, need not be more than 23—24 cm. This would be sufficie.nt as strength regards, if the houses were not extraordinary big. This thickness is appoin- ted in our building regulations, but no difference is made between small and big houses. In most foreign towns, as far as I know, the minimum thickness of outer walls must not pass below l1/2 time the lenglh of a brick or 35—38 cm. But then not only the durability is considered, but the possible shelter against storm and rain as well, as water alvays may be expected to force its way through a wall which only is one brick thick, or is cut across with transverse ligaments of brick. But, to my knowledge, in foreign towns wooden walls are not allowed as sheltering linings of the outer walls, except in special cases, for instance in houses, where only a single family lives. Therefore, in my opinion, the provisions of the building regulations, should be adapted to the strength and density of the concrete which would be allowed and chiefly used — I suppose, that would be con- crete of 1 : 4 : 8 (— 1:4:7) — the size of the houses being duely considered. In foreign building regulations the thickness of the walls is governed by the size of the brick and running in V2 brick sizes (= width of a brick). This is natural in countries, where people ahnost exclusively build from bricks. Here it is different, where it is an easy matter to proportionate the thick- ness of the concrete walls to the strenftth required, which in ordinary cases is determinated with refer- ence to the height and the breath of the building. Therefore it seems to me most natural, that the pro- visions of the build. reg. should adjust the thickness of the walls to the size of the house. Fortunately there are some rules to be found, poinling towards this and approved of by learned professionals. I mean the rules of Bondelet, which he made afler having carefully measured all kinds of houses. Summed up his rules run as follows: The sidewalls of houses, which have internal supporting walls, should have the thickness of Vís out of the sum of the brealh of the house (measured inside) and the height up to the roof. In houses, which have not such supporting walls, the thickness of the walls should be Vsí of the brealh of the house -f- V^s of the height up to the roof. In bolh cases we speak

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