. STATE GEOLOGIST. 145 
Steel concrete floors and beams were designed to meet tnis objec- 
tion. Contractors have become bolder and bolder as experience and 
science have taught them the strength of this combination, until it is not 
unusual now to find four inch floors of steel concrete spanning 18 by 22 
foot bays. These floors are made of excellent concrete having iron rods 
or bars imbedded in the lower third of the mass and supported upon 
steel concrete beams or girders. Figure 82 shows the section of such a 
floor. 
“This type of flooring is similar in every way to the other System 
B floors (Roebling’s) except that the flat steel bars are bent or crooked 
downward 2 inches or more at the center of the span and imbedded in that » 
position in the concrete. ‘This type is particularly well adapted for floors 
of light capacity and where special conditions make it necessary to econ- 
omize as much as possible in the structural steel. Where the distance be- 
tween the steel members or supports is more than 9 or 10 feet, this form of 
flooring will be found more economical than any other. It may be em- 
ployed to span directly from girder to girder, dispensing with the cus- 
tomary intermediate beams. Type 5 has been installed successfully in 
spans up to 22 feet. Under ordinary conditions, however, considering 
both the steel work and fire-proofing the most economical results will be 
obtained when the girders are spaced 14 to 16 feet apart. The weight of 
the concrete and imbedded steel bars as shown in the illustration is 43 
pounds per square foot; plaster (two coats), 7 pounds.” 
English Practice in Floor Construction. —Mr. Frank Caws, who has 
had thirty-two years experience, gives the following rules for concrete 
floor construction, in an article written for the Journal of the Royal 
Institute of British Architects. 
1. Obtain old cement. 
2. Use good broken brick aggregate and not sand, body concrete 
to be I part cement and 4 parts brick and the surface to be 1 part 
cement and 3 parts crushed granite. - 
3. Use as a precaution, “Sheep-wire netting’ as a base and steel 
bars of 114 pounds per foot in weight spaced 3 feet apart. 
4. Consider a slab to feet square by 4 inches thick capable of bear- 
ing 900 pounds per foot in weight including its own weight and reckon 
for every slab, more or less than goo pounds per foot directly in propor- 
tion to the square of the thickness and inversely as the cube of its span. 
When the span is rectangular the minimum. span is taken. 
5. Avoid casting slabs in frosty weather. 
6. Cast large areas at once, leave no partially cast slab over night. 
7. Insist on strong centering—leave it up at least five weeks. 
STAIRWAYS. 
If stairways and elevator shafts are of wood then fire will be carried 
from floor to floor, so that these, too, are now constructed of steel con- 
OSS. G: 
