=2.= Classified according to their construction, factory buildings may
be divided into three types, which, for convenience, may be designated
as _first-_, _second-_, and _third-class buildings_. A similar division
to this is also frequently made by the state or municipal laws for the
regulation of the construction of factory buildings.
=3. First-Class Buildings.=—Buildings of the first class are those in
which the walls, floors, columns, girders, beams, partitions, and roofs
are of stone, brick, terra cotta, concrete, steel, iron, and such other
fireproof materials as have been proven to be efficient. Buildings of
this class may be considered as constituting an entirely fireproof
building, which means that while the contents of the building may burn,
the building itself will remain intact, unless subjected to the severe
action of a prolonged conflagration.
=4. Second-Class Buildings.=—Buildings of the second class are
considered to include what is known as slow-burning, or the typical
factory-construction, type, in which all posts or girders must be
of heavy and massive timber, and the floor construction at least
3 inches in thickness, and of solid planking. In buildings of the
second class, while it is permissible to use combustible materials,
they must be of such sizes and of such slow combustion that the
security of the building will be insured for a reasonable time after
the conflagration has commenced. It is usual, therefore, in this
class of building, to limit the size of the wooden posts to not less
than 8 inches square, though their strength may be greatly in excess
of the load they are required to support, and girders and beams are
used whose least dimension is 6 inches or more. In buildings of this
character, it is frequently necessary to use steel beams and columns in
order to obtain the strength for the great floor loads to which these
members are liable to be subjected. When such steel or iron columns
are used, however, they must be fireproofed, because even though made
of incombustible material, they would not have the same endurance in
a fire as have heavy wooden girders or posts, and their failure would
precipitate the fall of the floor. Wooden girders and posts, even
when charred part way through, have still sufficient strength for the
support of the load for which they were designed.
=5. Third-Class Buildings.=—Buildings of the third class are not
particularly recommended for the construction of factory buildings, for
the floors of these may be of the ordinary joist and finished floor
construction. Such buildings are readily ignitible and burn rapidly,
not only because the timber work in them is light, but because of the
numerous air spaces that exist between joists and in the furring of the
walls. No building with air space surrounded by combustible materials
can be considered as slow burning.