[Illustration: FIG. 25]
=52. Planning for Traveling Cranes.=—In designing factories or mill
buildings in which traveling cranes are to be installed, it is
important to observe that the track of the crane can be properly
supported, and also that there is sufficient headroom under the floor
or roof construction to permit the trolley of the crane and the
traveling mechanism of the crane girder to move underneath.
In Fig. 25, there is shown the upper portion of a steel-mill building.
The columns _a_ support the girder carrying the runway of the crane.
A convenient means of supporting the roof is to splice to this column
a similar column _b_, which is incorporated in the design of the roof
truss and rigidly braced with the truss by means of a knee brace
at _c_. In the design of such a building, it is very important to
determine the distances _x_ and _y_ required by the makers of the
traveling crane. These distances _x, y_ depend on the size of the
crane, that is, whether it is designed to carry 5, 10, 15, or more
tons. Usually from 9 to 12 inches is sufficient for the measurement
_x_, while the measurement _y_ varies from 5 to 8 feet.
=53. Cranes Supported on Reinforced-Concrete Walls.=—Frequently, in the
latest types of construction, the runway for the crane is supported
on reinforced-concrete walls, which construction is shown in Fig. 26
(_a_). It will be observed that the pilasters supporting the crane are
strongly reinforced in all directions from which stresses are likely to
be created from the eccentric load imposed by the crane track.
Where cranes are supported on reinforced-concrete columns, as in Fig.
26 (_b_), it would be good practice to put additional rods in the far
side of the column as at _a_, in order to supply a greater resistance
to bending, and thus counteract the effect of the eccentric load
produced by the reaction from the crane track. Where cranes handle
heavy rails or cumbersome material that might, by swinging, impose a
blow on the reinforced-concrete columns, it is good construction to
protect the edge of the columns with an angle iron as indicated at _b_.
This angle iron may be fastened in the forms and anchored by means of
pronged anchors back into the concrete when it is tamped.
[Illustration: FIG. 26]
[Illustration: FIG. 27]
=54. Detail of Track Construction.=—Many crane failures have been
due to the spreading of the track between supports. It is better,
therefore, to supply considerable lateral rigidity to the beam
supporting the track or traveling crane. Where loads are heavy and
plate girders are used for the runway tracks, the flanges of the girder
are sufficient for this purpose. Where =I= beams are used, however, for
the support of the crane track, it is good practice to place on the
top of them and rivet with countersunk rivets, spaced about 18 inches
apart on each flange, channel irons as indicated at _a_, Fig. 27. By
means of these channel irons, which are drilled with open holes _b b_,
the rail _c_ may be readily clamped in place by means of wrought-iron
clips and bolts, and the rails nicely aligned and adjusted by wedging
between these clips and the track.
=55. Maximum Stress on Track Girders.=—The principal calculation for
the construction of the runway of cranes exists in determining the
maximum bending moment. The maximum bending moment on a runway girder
occurs when the wheels of the traveling crane are in the position
indicated in Fig. 28. It will be noticed that the center of the girder
is midway between the center of the near wheel and the center of the
crane trolley, that is, the distance _a_ is one-half the distance _b_.
The following formula will give the maximum bending moment on a crane
girder when the load is in the position indicated in Fig. 28:
_w_(_l_ - _a_)²
_M_ = ---------------
2_l_
in which _M_ = bending moment, in inch-pounds;
_w_ = load on one wheel of crane, in pounds;
_l_ = span of girder from center to center of
support, in inches;
_a_ = distance, in inches, marked in Fig. 28.
[Illustration: FIG. 28]
In order to illustrate the application of this formula, assume that
the wheel load _w_ equals 10,000 pounds; that the distance from center
to center of supports of the runway girder is 15 feet, or 180 inches;
and that the distance _a_ is 12 inches. By substitution,
10,000 × (180-12)²
_M_ = ------------------ = 784,000 inch-pounds
2 × 180
From this bending moment may be found, by the methods given in _Design
of Beams_, the proper size girder to use.