69. Bedding

As footing stones are generally very rough, being left as they come from the quarry, they cannot be made to bear evenly on the bottom of trenches without being bedded either in a thick bed of mortar, or, if the soil is sand or gravel, by washing the sand into the spaces by means of a stream of water. As a rule, the only safe way is to specify that the stones shall be set in a thick bed of cement mortar and worked around with bars until it is solidly bedded.

70. Offsets

The projection of the footings beyond the wall, or the course above, is a point that must be carefully considered, whatever be the material of the footings.

If the projection of the footing or offset of the courses is too great for the strength of the stone, brick or concrete, the footing will crack, as shown in Fig. 30.

The proper offset for each course will depend upon the vertical pressure, the transverse strength of the material, and the thickness of the course. Each footing stone may be considered as a beam fixed at one end and uniformly loaded, and in this way the safe projection may be calculated.

Table V. gives the safe offset for masonry footing courses, in terms of the thickness of the course, computed by a factor of safety of 10.

table V.

70 Offsets 10030

Fig. 29.

70 Offsets 10031

Fig. 30.

KIND OF FOOTING.

R. IN LBS. PER SQ. IN.*

OFFSET FOR A PRESSURE, IN TONS PER SQUARE FOOT ON THE BOTTOM OF THE COURSE, OF

0.5

I

2

3

5

10

Bluestone flagging..............

2,700

3.6

2.6

1.8

1.5

1.2

.8

Granite.............................

1,800

2.9

2.1

1.5

1.2

1

.7

Limestone...................

1,500

2.7

I.9

1.3

1.1

.9

.6

Sandstone................

1,200

2.6

1.8

1.3

1.0

.8

.3

Slate..........................

5.400

5.0

3.6

2.5

2.2

1.5

1.2

Best hard brick........................

1,200

2.6

1.8

1.3

1.0

.8

.5

Concrete

1 Portland................

150

0.8

0.6

0.4

2 Sand...........

....

......

........

3 Pebbles.............

Concrete

1 Rosendale..........

80

0.6

0.4

0.3

2 Sand..............

....

....

....

3 Pebbles..........

* Modulus of Rupture, values given by Prof. Baker in " Treatise on Masonry Construction."

It should be borne in mind that as each footing course transmits the entire weight of the wall and its load, the pressure will be greater per square foot on the upper courses, and the offsets should be made proportionately less.