The questions involved in designing the floors of a mill are of great
importance, contributing in no small measure to elements concerned in the
successful operation of the mill, and to a greater extent to its standing
as a fire risk, and therefore affecting the constant expense of insurance.
In the case of a building designed merely for sustaining of loads, as in a
storehouse, a floor would naturally be designed on the basis of
considering the breaking strength of the timber. But in the case of a
mill, the limitation is the amount of flexure allowable under the
circumstances; and therefore the floors of the building are made more
nearly rigid than would be required merely from the consideration of the
ultimate strength of the structure.
The books on the subject, repeating over a constant which was first, I
believe, given by Brunel in testimony before a parliamentary commission,
have held that one four-hundredth of a span is the proper ratio of
flexure. This may have been a very good rule to give to the parliamentary
commission, but it is hardly the practical method of limitation for a
matter of engineering construction, because the flexure of a loaded beam
is in the form of a curve, and therefore its law is that of a curvilinear
function, and not of a straight line. I have examined a great number of
precedents of good construction in this connection, and for mill use have
deduced the formula for deflection in inches, _d_ = 0.0012 L squared, in which L
is the length of span in feet. It will be readily recognized that the true
constant of deflection of span is measured by the radius of curvature
which will give a uniform and allowable distortion to the floor in either
direction to the limit of the radius upon which this formula is based,
which is 1,250 feet.
I do not propose to offer to you on this occasion any remarks in regard to
the treatment of the mathematics of the problem of applied mechanics
concerned in the questions of transverse stress, knowing that you have
certainly received instruction upon these subjects. But referring to the
questions of mill floors, I would state that Southern pine beams of solid
timber twelve by fourteen up to fourteen by sixteen inches are used; and
instead of attempting the use of one piece of timber, it is preferable to
use two pieces of the same depth and of half the breadth. These should be
bolted together, with a space of an inch or so between them left by
placing small vertical pieces of wood between the timbers when they are
bolted together. In this manner one is more sure of sound timber, and in
the process of seasoning there is less liability of dry rot in the
interior, or of injurious checking, warping, or twisting.
The end of the beams should rest upon iron plates in the masonry, and
should be secured by means of a tongue upon the plate entering a groove
across the lower side of the beam. It is not feasible to make this groove
to a close fit with the tongue; but it is cut a great deal larger, and the
whole brought to a firm bearing by means of pairs of wedges or quoins
driven into the groove each side of the iron tongue.
The outer end of the plate contains ribs or tongues reaching down into the
brickwork. In this manner the timber is securely fastened to the
brickwork; and yet in time of accident or of fire the falling of the beam
in the middle of the mill will raise it up sufficiently so that it will
clear the tongue and fall without tearing the wall down, which is the case
whenever the beams are secured by bolts entering the end of the beam from
the face of the wall.
At the points of support in a line of columns, the beams should be free
from all compressive stress, transmitted through the lines of columns from
floors above, by means of iron pintles between the cap of one column and
the floor of the next one carrying this load.
A faulty method of construction, quite frequently used, consists in
covering each column with a bolster of timber, four or five feet long,
reaching out under the floor beams.
The transverse contraction of wood in seasoning after it is in position in
the mill varies from three-eighths of an inch to double that quantity per
foot; and the aggregation of such shrinkage amounts to a very considerable
distortion or settling of the floor in a mill of several stories.
Moreover, the resistance of timber to transverse crushing has been shown
by experiments on the testing machine at the United States arsenal at
Watertown to be about three times the resistance to longitudinal crushing.
Iron columns for mills have been entirely displaced by those of timber, as
it was found that the latter were more reliable in resistance to fire,
were freer from defects in construction, and possessed less tendency to
vibration. A series of tests on full-sized mill columns of various forms
of construction and age, made in the experiments referred to, at the
Watertown arsenal, showed that resistance to crushing of Southern pine
columns was about 4,500 pounds to the square inch, and remarkably uniform
as to the different results. In white oak there was a wider range, owing
to the difference in the grain of the various samples, the generality of
the specimens being of somewhat less resistance than that of Southern
pine.
It was furthermore found by these experiments, on comparing the crushing
resistance of a full-sized column with that of a portion of the same,
perhaps two feet in length, that the results were practically identical,
likewise that within the limits of construction used for these columns the
question of flexure did not enter at all in the problem, but they gave way
by direct crushing, and that the resistance to crushing was proportional
to its load upon the minimum cross section.
The precedents of safe construction in this matter show that wood columns
in mills have successfully sustained for many years a load of six hundred
pounds to the square inch without deterioration. As the resistance of
such columns is proportional to the cross section, the results of these
experiments have changed the practice of mill engineers in the matter; and
square columns are of almost universal use, which interfere with no
greater area on the floor than the round column of the same diameter,
while they furnish an increased resistance of a little over twenty per
cent. in excess.
Along the axis of such columns a hole of about one and one-half inches in
diameter is bored, and near each end a couple of transverse holes,
generally half an inch in diameter, furnish means of ventilating the
inside of the column for the prevention of dry rot and also checking, due
to contraction and seasoning.
There are several methods of laying the floor plank upon these beams,
which are placed from eight to ten feet apart, according to the dimensions
of the machinery to be placed in the mill. The first floor of three-inch
plank, planed on one side and grooved on both edges, is laid planed side
down, and the hardwood splines are inserted into the grooves before the
planks are pressed up and spiked to the beams. An agreeable finish is
sometimes arranged underneath by plowing a rabbet in each of the corners,
and inserting a bead in the groove thus formed, which is secured by nails
driven diagonally into the plank on one side only, because if the nails
were driven into both sides, the bead would be split by the contraction of
the plank.
These planks should be cut to sufficient length to cover two bays of the
mill; and their transverse resistance is that of a beam fixed at one end
and supported at the other, or one and three-fifths as much as a plank of
the same size but half the length would support; but it should be
remembered in this connection that, if evenly distributed on the floor,
five-eighths of the load would be carried by every alternate beam unless
the planks are so laid to break joints at convenient intervals of about
three feet.
The top flooring is generally laid directly upon the floor plank, with one
or two thicknesses of roofing paper interposed; but the preferable method,
which deadens the sound and vibration, and also greatly increases the
fire-resisting qualities of the structure, is to lay a coat of mortar on
the floor plank, preserving the uniform thickness by means of furring
placed about sixteen inches apart, and then to lay the upper floor upon
this.
For these upper floors hardwood plank, one and one-fourth inches thick,
and not over four inches wide, is used. The black birch is considered by
many to possess the greater resistance to wear; and Southern pine is
ranked next, although the latter wood gives trouble by stringing,
especially when trucks are rolled over it. White maple forms an excellent
top floor, although not so hard as others, especially where the floor is
likely to be exposed to water, as in paper mills and bleacheries.