=48.= In factory construction, the headroom is seldom available to
support beams on the girders, as indicated in Fig. 23 (_a_). It is
usually necessary, in order to cheapen the construction of mill
buildings, to keep the distance between the clear headroom and the
finished floor level to the very minimum, and consequently the tops of
the beams are most always brought flush, or nearly so, with the top of
the girder.
A common construction is to use some of the various forms of
wrought-iron hangers, as shown in Fig. 23 (_b_). The type of hanger
shown is a single stirrup, and is probably the best of any on the
market; where beams enter the girder on both sides, the hanger is
designed double. While it is popularly supposed that this hanger
would readily fail by the bending of the metal at _a_, it is usually
proportioned to safely carry any reaction imposed under ordinary floor
loads. This hanger is obtained stamped out of steel plate or formed
from bar iron.
[Illustration: (a)]
[Illustration: (b)]
[Illustration: (c)FIG. 23]
=49.= Where it is not desirable to use wrought-iron or steel hangers,
a simple and inexpensive form of construction may be adopted as that
shown in Fig. 23 (_c_). Here the beam _a_ is supported on a wooden
strip _b_, which extends the full length of the girder, and is bolted
near the bottom with through bolts. Such a construction provides
sufficient strength for the support of the average factory floor, but
its strength is difficult to figure with any degree of certainty, and
some surer form of connection is generally considered preferable.
In all instances, it is good practice to tie together the opposite
floor-beams butting on a girder by means of an iron dog, or tie-plate,
_c_.
=50.= In Fig. 24 (_a_), (_b_), (_c_), and (_d_) are indicated other
methods of supporting the secondary floor-beams on main girders in the
construction of factories. In Fig. 24 (_a_) is shown an =I=-beam girder
supporting heavy timbers of a floor of slow-burning construction. It
is always necessary in this construction to bring the top edge of the
timbers above the upper flange of the =I= beam, and to span the space
_a_ thus created with a piece of timber for a tie and for the support
of the floor planking. By providing this space between the ironwork and
the wooden tie, any shrinkage that may occur in the secondary timbers
will not cause the floor to ride on the top of the steel beam and thus
make a ridge evident in the finished floor at this place. The timbers
forming the secondary girders may either be supported on angle-iron
brackets, or on angle irons extending the entire length of the girder.
The latter method is only pursued when it is necessary to keep the end
of the timber a few inches away from the steel beam, and the angle,
consequently, being subjected to a greater bending moment, must have
more resistance by increasing the width of the section of the bracket.
[Illustration]
[Illustration: FIG. 24]
Sometimes, the secondary beams are supported on double stirrup hangers,
as shown in Fig. 24 (_b_). When it is not desired to use steel beams,
resort is frequently had to flitch-plate girders. They are, however,
held in some disfavor by the building departments of the several
cities, who do not consider that the combined strength of the timber
and metal can be taken, and will only permit the strength of either the
timber or metal to be used.
=51.= The building departments of several of the large cities
stipulate that buildings of the second class, which includes factory
construction, shall not have steel girders that are not fireproofed
supporting brick walls or floors. When this construction is required,
the secondaries must be supported as in Fig. 24 (_c_). In this view is
two angle brackets riveted or bolted to the steel beam, and extending
through the concrete for the support of the wooden beams. While there
is some danger of heat being transmitted to the beams through the
projecting ends of these brackets, nevertheless it is considered better
construction than that shown in Fig. 24 (_d_), where stirrups are used
over the concrete fireproofing. In this latter construction, there is a
liability of the stirrup bending at _a_, _a_, and crushing the concrete
beneath. Where the reaction from the end of the girder is great, this
undoubtedly is likely to occur, and such stirrups should be provided
with a bearing plate on top of the concrete, so that their bearing at
the edge will be distributed over a considerable area.