109. Strength Of Cast-Iron Columns

We owe our knowledge of the strength of cast-iron columns chiefly to the experiments of Mr. Eaton Hodgkinson in the year 1840.

These were very numerous, and to a certain degree comprehensive, embracing over two hundred examples.

As deduced from these experiments it was found that where cylindrical cast-iron columns were shorter than thirty external diameters, the weight required to break them by bending is so great that the crushing force becomes sensible, and the column yields to the combined effect of the forces. But in a long column (where the length exceeds thirty external diameters), although the pressure contributes to break it by crushing as well as by flexure or bending, yet the column yields from bending with a weight which is insufficient to sensibly affect it by crushing alone. It was found that when the pressure on the column exceeded one fourth of the breaking weight, a change or derangement of the metal took place. Therefore one fifth the crushing weight is as great a pressure as can be put upon cast-iron columns without having their ultimate strength decreased by incipient crushing: provided the thickness of metal in column is uniform, with turned ends, secured top and bottom and bolted through flanges.

If the column is secured by an uncertain method, it is safer to use one sixth the crushing weight.

It is obvious, therefore, that it will not do to take the table on page 62 as a guide unless the columns are of uniform thickness throughout, of good metal, with cores made in one piece, castings reasonably perfect and straight, the ends turned off true in a lathe in planes at right angles with their axis, and set up perpendicularly in the building.

Mr. Hodgkinson, in his experiments, found that columns with rounded ends can sustain only about one third the weight of those with flat ends carefully fitted, with the ends at right angles to the axis of the column. In the ordinary mode of chipping off (cutting with a chisel) the ends of a column in an unfinished state, the inequalities of the bearing surfaces cause the weight to rest on a few points on the ends, and it is almost impossible that the ends shall be at right angles with the axis. The safe weight a column can sustain in such cases is considered to be about two thirds that of one turned true.

110. Weight To Be Estimated For Any Use

In computing the weight to be sustained by a column, it is not sufficient to consider only the weight appropriate to that particular use for which it is intended; but the weight should be estimated for any use to which the building may be applied, with full allowance for floors and the weights to be placed thereon. It is not safe to take the average weight sustained on each column, as some columns will have more or less on them than the average, and will be loaded more on one side than the other; besides, they are subject to concussions, from bodies falling on a floor above, or may receive a lateral blow from goods falling against them in transmission.

Great allowance should also be made for columns that are subject to vibrations caused by machinery, etc.

111. Strength Of Hollow Cast-Iron Columns

The following table gives the ultimate strength of round and square cast-iron columns, in pounds per square inch of sectional area.

The numbers in column l/r = the length divided by the least diameter, each taken in inches.

112. Factors Of Safety For Cast-Iron Columns

(a) If column is accurately turned to a true plane and its bearing surfaces are perfectly true, take one fifth of ultimate strength.

(b) If column has turned ends and is set with the usual care as in ordinary buildings, take one sixth of ultimate strength.

(c) If the ordinary mode of chipping off ends as with a chisel is employed, take one eighth of ultimate strength.

Example 1

What safe load will a 12-inch-diameter column I inch thick, 15 feet long, support with a safety factor of 5 or one fifth the ultimate strength?

l/r = 180/12 = 15

Opposite this number for round columns is 51,200 pounds, and dividing this by 5 we get 10,240 pounds, safe-load per square inch of sectional area. For the exact area in square inches, refer to the table of "Areas of Circles".

A 12" dia. area = 113.10 sq. in.

" 10" " " = 78.54 " "

34.56 = area of a 12" dia. column 1" thick.

Then 34.56 inches X 10,240 = 353,894 pounds or 177 tons, total safe load the column will support.

Example 2

What safe load will a 10-inch-square column 1 inch thick, 10 feet long, support, with a safety factor of 6 or one sixth the ultimate strength?

l/r = 120/10 = 12.

Opposite this number for square columns is 62,110, which divided by 6 gives 10,352 pounds, safe load per square inch of sectional area.

Area of column = 36 inches X 10,352 = 372,672 pounds or 186 tons, the total safe load the column will support.