A formula for obtaining the strength of cast-iron columns having flat or square ends is,

S =

Cast Iron Columns 231

in which

S = breaking strength of column in lb.

per sq. in. of section; l= length of column in inches; *R2= square of the least radius of gyration; † U = ultimate compressive strength of cast iron in lb. per sq. in. Having calculated the value of S, the safe strength per square inch may be obtained by dividing by the factor of safety. The safe load on the column is then found by multiplying this value by the sectional area of the column in square inches.

Example

Find the proper working load for a 10-in. square cast-iron column, 20 ft. long, using a factor of safety of 6, the thickness of the metal being 1 in.

Solution

The ultimate compressive strength U of cast iron in lb. per sq. in., according to Table VIII, page 70, is 81.000; the length l equals 20 ft. X 12 in. = 240 in.; and the formula for determining R2 for a hollow square column is, according to Table XII, page 83,

R2=(b2+b' 2)/12.

Substituting figures,

R2=(102+82)/12=(100+64)/12= 13.6.

Upon substituting these values in the formula, there results,

S = Cast Iron Columns 232 = 81,000 / 2.17 = 37,327 lb. per sq. in.

Using a factor of safety of 6, the safe load per sq. in. of section is 37,327 + 6 = 6,221 lb. Since the net area section is

* For values of R2, see page 83. †See page 70.

100 sq. in. - 64 sq. in. = 36 sq. in., the entire load that the column will safely sustain is equal to 6,221 lb. X 36 sq. in. = 223,956 lb.

Design Of Cast-Iron Columns

Since cast-iron columns are usually more or less in a state of internal strain, due to the unequal cooling of the metal in the molds, and also because of the uncertain nature of the casting, a factor of safety of from 6 to 8 should be used in designing them.

Fig. 10 shows a design for a circular cast-iron column. A shows the elevation for the cap and brackets supporting steel floorbeams. Special care should be exercised in the design of the bracket a, the web being made, as shown at A, to extend to the edge of the plate m. and with, the general outline of its front edge forming an angle of about 60° with m. If the web is made as shown by full line at a in B, and the beam takes a bearing upon the edge of the plate m, the tendency will be to fracture the edge of the bracket. It is well to have at least 1/4" fillets in all the corners of the, casting, and also to thicken the metal in the column adjacent to the brackets, as shown at 6.

The bolt holes f should be always drilled, either in the casting or in the steel beams after the latter are in place, because, if the holes are cored in the casting and the holes punched in the beams at the mill, it is likely that the beams will be supported entirely by the shear of the bolts, without bearing upon the bracket at all. The bolts should fit the bolt holes f closely, and it is best, if practicable, to drill the holes in both beams and cast-iron flange; this will insure as rigid a connection as is possible with this form of construction.

The strengthening webs of the base should be placed in the most effective position, that is, on the diagonals, as shown in Fig. 10; if placed on the diameters, the corners will have a tendency to break off, thus reducing the bearing surface of the base.

Cast Iron Columns 233

Fig. 10.

In designing cast-iron columns a good rule to observe is to have the thickness of the metal in the body of the column not under 5/8 in. If made less than this, the difficulty of obtain-ing sound castings is increased because the metal, in flowing into the mold, is liable to cool before completely filling it, and thus weak spots and dangerous flaws are formed.