580 PROCEEDINGS OF SECTION J. 



material itself has not the fire-resisting qualities of cement con- 

 crete. Wooden ceiling joists and plaster are adopted if a flat 

 ceiling is required, thus reaucing the value of the system as a fire 

 proof one. 



d. Rolled iron joists filled in with Portland cement concrete 

 formed the next step in advance, and this mode is still much 

 used owing to its simplicity and economy. For wide spans and 

 light loads the concrete may be arched, for heavy loads where 

 the rolled iron joists must be closer a flat ceiling is preferable, 

 (PI. XXXIX., fig. 9.) If the boarded centering be kept 1" to 2" 

 below the lower flanges of the joists, the concrete partially or 

 wholly covers them. Then the ceiling may be formed by cementing 

 direct on the concrete, thus completely covering the iron joists. 

 This is a great gain, as in case of fire, the loM^er flanges are slightly 

 protected, and do not so readily heat, stretch, and fail. If there are 

 any iron girders carrying the joists they can be protected by binding 

 galvanized wire netting around them, tied to strips of hoop iron 

 and then filling in with fine cement concrete between boards, 

 giving a thickness of at least 2" over the iron in every part. 

 I am adopting this system in several works at the present time, 

 and find it sim))le and comparatively inexpensive. For obvious 

 reasons, the lighter the concrete the better, hence, coke 

 breeze is much used both for casing the girders and for filling in 

 between the joists. It was, I believe, first introduced in some of 

 Allen's Woikmen's dwellings in London, is incombustible when 

 mixed with cement, and is, moreover, less likely to fly to pieces 

 from heat than gravel or broken stone concrete. The addition of 

 a small proportion of sand varying with the fineness of the breeze 

 is advantageous. 



e. For small rooms and light floors, Mr. Allen used plain bars 

 of iron as joists, crossed by i" rods laid the other way. The 

 breeze concrete 6" in depth, was then filled in on boarded centering 

 making a very thin, but fairly strong fire-proof floor. The iron is 

 entirely protected and the cross rods hold up the concrete. A 

 pi'eferable alternative is to use deeper plates, punch them and 

 thread the rods through the centre on the neutral axis of the 

 plates, (PI. XXXIX., fig. 10), the rods have a greater depth of 

 concrete above them, and hence there is less chance of Iracture. 



/. Another form of floor, much used in Melbourne at the 

 present time, is a German develo|)ment of the corrugated iron 

 type, called " Traegerwellblech," (PI. XXXIX., fig. 11.) It con- 

 sists simply of sheets of tough iron rolled into deep corn-.gations 

 or channels, and filled in above with concrete. It is not so fire- 

 proof as the floor described in clause d, as the lower flanges of all 

 the iron joists are exposed ; but it will carry a considerable weight. 

 When the load is light, a much cheaper floor may be made by sub- 

 stituting ordinary twenty-four guage corrugated iron, cambered 

 from joist to joist, and then filled in with concrete. Another form 



