i STATE GEOLOGIST. 151 
inch top slabs and 1% inch ceiling slabs resting on concrete beams 
spaced from 17 to 24 inches apart. No steel is used in the floor and ceil- 
ing slabs, but steel bars are used in the top and bottom of the beams. 
In the rotunda there is a circular light-well 14.5 feet in diameter, 
around which the upper floors are supported by a circular concrete girder 
1034 inches wide and 203 inches deep, having a single 34 inch steel bar 
imbedded in the concrete at the top and bottom. From this girder radiate 
other concrete girders to the side walls, thus giving the circular girder 
support. The floor in the rotunda is 3% inches thick made in 8 by 10 
foot panels, reinforced with %4 inch bars spaced three inches apart and. 
bedded in the lower portion of the concrete about 34 inch from the lower 
face. | 
The exterior walls are hollow, with occasional ribs connecting the 
outer and inner wall slabs. The hollow spaces contain heating and vent 
ducisee Nhe outer walleclabiis 3s "inches! thick: and! the imner) 2 inches; 
with 15 inch air spaces between. The concrete used was made of 1 part 
cement, 2 parts sand and 4 parts crushed stone. 
The roof is like the floors, but is lighter and has no ceiling slab be- 
neath the concrete rafters, except over the large court room where the 
vaulted ceiling with a clear span of 49 feet has a 2 inch shell connected 
with the 2 inch roof shell by 5 inch transverse ribs spaced 3 teet apart 
in the clear. A shrinkage joint is provided at the ridge to care for 
expansion and contraction. 
The dome roof has a solid shell 3 inches thick without ribs and 
reinforced by 4 inch steel bars, both horizontally and radially, the hor- 
izontal bars being 12 inches apart and the radial bars 12 inches apart 
at the base of the dome. The rods are 34 inch from the outside of the 
shell at the base of the dome and cross over at about three-fifths of the 
height of the dome to 34 inch from the inside of the shell. Plaster 
forms were used for the outer part of the dome and for the decorative 
features, while wooden forms were used for the inner surface of the 
dome and the work in general. 
The building contains 70,000 cubic feet of concrete and 80,000 
pounds of steel, and cost about $106,000.00. The specifications required a 
sectional area of one square inch of steel and four square inches of con- 
crete for each 2,000 pounds of tensile and compressive strain respectively. 
Unscreened broken stone, having a maximum diameter of 34 inch, 
with Atlas Portland cement was used in the concrete. Figures 87 and 88 
illustrate this work. 
To illustrate the Hennebique system of concrete floors and roofs, a 
section of Mr. W. C. Sheldon’s five story residence on Fortieth street, 
New York, is presented in figure 80. The extreme lightness of the 
construction is clearly shown. 
