float bushings. The friction forces due to this outward or inward movement,
called spreading and squeezing forces, are applied to all travel wheels
horizontally (and perpendicular to the rail axis) at the contact points on the
rails. The spreading and squeezing forces are taken as 12 percent of the maximum
wheel load caused by dead load and rated load. Designing a portal base for
lateral loads of this magnitude (concurrently with other operating loads) has been
found to give it sufficient lateral strength at the corners.
Additional Loads on Floating Cranes. Floating cranes experience list and
trim under operating conditions, causing the vertical axis of the crane to not be
parallel to the true vertical axis. The angle between the vertical axis of the
crane and the true vertical axis, in the direction perpendicular to the hook
radius, is called the sidelead angle, and the resulting component of force is the
sidelead force. The angle between the vertical axis of the crane and the true
vertical axis, in the direction parallel to the hook radius, is called the offlead
angle, and the resulting component of force is the offlead force. The offlead
force is usually neglected for design purposes.
A horizontal side load is applied to floating cranes for design purposes.
The horizontal side load is a combination of the acceleration forces due to rotate
motion, and the sidelead force due to the maximum list and trim.
Floating cranes are required to be towed at sea, and must be designed for
the wave motion that they will experience. The resulting roll, pitch, and heave
are determined by the selected sea state; and their dynamic effects are calculated
by a rational method of analysis, which must be approved by the NCC.
Seismic Forces. NAVFAC Manual P-355, Seismic Design for Buildings,
requires that OET cranes and underrunning cranes be designed for seismic forces.
The effects of seismic forces on certain types underrunning cranes is considered
to be negligible due to a combination of flexible runway, end trucks, and trolley
mounting. Top running OET cranes should be analyzed for seismic forces if they
are located in seismic zones 2B, 3, or 4. In lower seismic zones, seismic forces
historically have not governed the design of these types of cranes. Container
cranes are usually analyzed for seismic forces. Portal cranes are sometimes
analyzed, depending on their location and their use.
Portal cranes and container cranes are usually analyzed by the methods
used on buildings. The seismic load is applied at the travel wheel treads. The
crane structure is to be treated as a steel moment resisting frame and analyzed by
the methodology of any of the following codes NAVFAC Manual P-355, American
Society of Civil Engineers (ASCE) standard ANSI/ASCE 7-95 Minimum Design Loads for
Buildings and Other Structures, or Structural Engineers Association of California
(SEAOC) Manual Recommended Lateral Force Requirements. Other widely recognized
codes may be accepted by NCC, provided that the analyses are presented in a format
that permits an efficient review by structural engineers. The seismic zone and
site conditions at the crane's location determines the various factors used in
Design of Structural Components. The design criteria follows the
established industry standards and practices when they are available; deviation
from them is made only in exceptional situations peculiar to Navy requirements or