=20.= In Fig. 7 is designated an economical type of mill construction,
which is much in use. This construction is slow burning in every
respect, and is exceedingly simple, withal being substantial and
presenting a good appearance on the interior of the building. It
will be observed that the column supports of the floor consist of
yellow-pine posts, varying from 20 inches square to 8 or 10 inches
square, the latter being used for the support of the roof. The drawing
shows columns and wall construction suitable for a six- or seven-story
factory building, and the size of post indicated in the basement is
about the maximum.
The girders consist of 6" × 20" yellow-pine pieces bolted together
with ¾-inch bolts, though it is suggested that ⅞-inch bolts would be
preferable. The purpose in bolting up a girder in this way is that the
thinner planks are much more readily obtainable, they are more likely
to be thoroughly seasoned, and a girder built up in this manner is
usually stronger than a solid beam, from the fact that there is less
likelihood of hidden defects existing in the timber and much better
stock can generally be obtained.
=21. Post Caps and Base Plates.=—These girders just described are
supported on cast-iron post caps, similar to the Goetz-Mitchell
construction. These post caps, where they support girders in one
direction only, are usually known as two-way caps. If they support
girders in two directions—that is, transverse and longitudinal—they
are known as four-way caps. These caps are generally cast of ¾-inch
metal, and the girders bear on them at least 4 inches.
For the basement columns, it is usual to provide a cast-iron base
plate, as indicated at _a_. The timber column is sized into the socket
of the base plate, and it is best to carry the top edge of the cap well
above the floor, so that any moisture from leakage or in washing the
floor will not be allowed to penetrate to the wood. Owing to the fact
that this base plate must transmit the entire load on the column to the
brick, it must be heavily webbed on the sides and corners, as indicated
in the plan at _b_.
=22. Concrete Footings.=—It is usual in designing the foundations for
mill buildings to use concrete footings, as indicated in the plan.
It will be noticed in this particular instance, and it is the usual
practice, that the concrete footings are 12 inches in thickness. When
the footings are stepped, as indicated, under the wall of the building,
each footing is made about 12 inches in thickness, with a projection of
not more than 6 or 7 inches.
[Illustration: _Section of finish flooring_