=41. Column Footings.=—With factory buildings of more than five or
six stories in height, great pressure is transmitted to the soil from
the base of the bottom column, and as it is necessary with soils of
even fairly good bearing capacity to have footings beneath the piers
supporting columns of from 6 to 10 feet square, adequate means of
providing these footings must be obtained. In Fig. 19 (_a_) and (_b_)
are shown two types of footings for concrete columns. In (_a_) is
indicated a reinforced-concrete column with a steel core. In such an
instance, all the load is transmitted by the steel core through its
angle plates and webbing at the foot to grillage beams. These grillage
beams are, however, not made sufficiently large to transmit the load to
the soil, but merely to distribute the load on the bed of concrete. The
spread portion of the footing is reinforced with steel rods _a_, _a_
crossed each way, and longitudinal shear is taken up in the footing by
means of stirrups _b b_. This is the usual type of footing construction
under reinforced-concrete factory columns.
Where, however, the column is not reinforced with a steel core, but
is merely a pier, footings may be designed as illustrated in Fig. 19
(_b_). Here the base of the column is enlarged in order to better
distribute the load on the several steps of the footing, and where the
bottom step has a considerable overhang, it is reinforced with steel
rods and stirrups, as indicated.
=42. Detail of Slab and Girder Reinforcement.=—In the previous article,
the general construction of the floors and column supports of a factory
building was explained. By referring to Fig. 20, it will be shown how
the girders and beams are reinforced with the steel bars. In this
figure, a plan is indicated at (_a_) and an elevation at (_b_). The rod
reinforcement of the slab is shown in the plan at _a_, _a_. It will be
noticed that over every other beam these rod reinforcements lap, or
break joints, and that some additional tie or reinforcement is placed
over the girders, as indicated by _b_, _b_. These latter rods tend to
tie in the floor slabs still more rigidly than can be accomplished with
their individual reinforcement.
Referring to the elevation (_b_), it will be noticed that all the
reinforcement of the beams is not usually carried along the lower
portion of the girder for its entire distance, but that some of the
reinforcement is bent up at a point about one-quarter of the span
from the abutment, in the form of a camber rod. By arranging the
reinforcing rods in this manner, an additional stirrup action, or tie,
to the girder supports is provided, and the oblique section made by a
horizontal line passing through these rods tends to provide additional
resistance to the horizontal shear in the beams and also provide for
negative bending moment produced in the beams near the support. To
further provide for this, shear stirrups are placed closer together,
toward the abutments, as indicated at _c_, _c_. These stirrups are
ordinarily light pieces of bar iron bent in a =U=-shape, and sometimes
bent around the rod reinforcement, a detail of this stirrup being shown
in Fig. 20 (_c_).
[Illustration: FIG. 20]