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232 STRENGTH OF MATERIALS.
The preceding Table gives the safe load in long tons corre-
sponding to a square post of the dimensions of sides given at
top of the columns, and lengths given in the first column. For
round posts the load should be 0.75 to 0.6 of the given load de-
pending upon the length of post.
Example.
What size of post is required, with 10 as factor of safety,
to support a load of five tons, when the length of the post is 16
feet?
Solution
:
In the column headed "Length of post in feet" find 16,
and in line with 16 find the numbers nearest to five tons, which
are 4.34 and 6.43. Thus, a post 16 feet long and 8 inches square
will support 4.34 tons, and a post 16 feet long and 9 inches
square will support 6.43 tons. It is, therefore, best to select a
post 9 inches square.
To Calculate the Strength of Rectangular Posts from
the Table.
Find, in the Table, the strength of the post according to
its smallest side, and increase the tabular value in proportion
to the largest side of the post.
Example.
What is the strength, with 10 as factor of safety, of a spruce
post 10 feet long, 6 inches thick, %]/
2 inches wide, with square
ends well fitted, calculated by Table No. 29.
Solution
:
In the Table we find the strength of a post 10 feet long
and 6 inches square to be 3.09 tons. Therefore, when the pillar
is 6 inches thick and 8% inches wide its corresponding strength
will be 3.09 X ^f = 4.38 tons.
It is a waste of material to use a post of rectangular cross-
section. For example, this post is 6 X 8% inches = 51 square
inches of cross-section and will support 4.38 tons, but a post of
the same length and 7X7 inches = 49 square inches of cross-
section, will support 5.03 tons. ( See Table No. 29).
To Obtain the Weight of Pillars in Kilograms per Meter
when the Weight in Pounds per Foot is Known.
Multiply the weight in pounds per foot by the constant
1.4882, and the product is the weight in kilograms per meter.
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