118. The Figures given in Table 2 are the safe bearing strengths of different classes of masonry. The first values may be used when the stonework is of fair quality and good lime mortar is used; the second ones, for the best quality of work with cement mortar.

Table 2. Safe Bearing Loads Of Masonry

Kind of Masonry.

Bearing Value per Sq. Ft.

Concrete...................

5

to

15

tons.

Rubble...........................

5

to

15

tons.

Squared Stone, 1/2-inch joints........

15

to

20

tons.

Sandstone ashlar, 1/4-inch joints......

10

to

20

tons.

Limestone ashlar, 1/4-inch joints......

20

to

25

tons.

Granite ashlar, 1/4-inch joints........

25

to

30

tons.

119. A column of good stone, which is carefully set and has well dressed bearing surfaces, should, if its height is not over 10 times its diameter, safely carry a load about one-fifteenth of the breaking load of stone of the same quality. Table 3 gives the safe bearing values for different kinds of stone columns, when the shaft consists of a single piece:

Table 3. Safe Loads On Stone Columns

Kind of Stone.

Load per Sq. Ft.

Sandstones -

Potsdam, N. Y., best............

40

tons.

Longmeadow, Mass., best........

35

tons.

Manitou, Col., best..............

25

to

30

tons.

Ohio............................

25

tons.

Fond du Lac, Wis................

25

tons.

Limestones -

Glens Falls, N. Y................

35

tons.

Indiana .............................................

25

to

35

tons.

Marble -

Good quality....................

40

tons.

120. A column should not carry a greater weight than 40 tons to the square foot, as, while the stone itself might carry much more, the mortar joint is the weakest part, and hence is most likely to fail. When a column is loaded with over 15 tons to the square foot, it should be set in Portland cement mortar, made of equal parts of cement and sand, which should not be laid within an inch of the face until the building is finished and the mortar has thoroughly set, when the joints may be pointed. If a column consists of several pieces, the joints should not exceed 3/16 inch in thickness, and the bed surfaces should be very finely dressed, perfectly true, and perpendicular to the axis of the column.