MIL-HDBK-1038
The height correction factor, applied to portions of the crane that are more than
30 feet above the ground, is determined using the following formula:
Ch = (h/30)2/7
(3)
EQUATION:
Where
h = height above the ground in feet
For portions of the crane that are 30 feet or less above the ground, Ch is equal
to one. Other acceptable procedures are those of FEM, Rules for the Design of
Hoisting Appliances; and ANSI/ASCE 7-95, Minimum Design Loads for Buildings and
Other Structures. The maximum wind gust velocity for a particular geographic
location is obtained from the data in MIL-HDBK-1002/2 or ANSI/ASCE 7-95. When the
wind area of the rated hook load is specified, it must be included in the
determination of the loads due to the operating wind case. The wind load
direction is selected to produce the maximum stress for the applicable load
combination.
The older portal cranes have been designed (for both strength and
stability) to withstand a non-operating (uniform) wind pressure of 20 psf, with
the boom at minimum radius and the upperworks rotated to the least stable
position. The newer cranes are designed for maximum wind gust velocity at the
geographic location; however, the configuration of the crane varies as follows:
a)
The crane must meet stability requirements for a non-operating wind
velocity of 80 mph, with the boom at minimum radius and the upperworks rotated to
the least stable position. If the maximum wind gust velocity at the crane's
location is 80 mph or less, the crane structure must be designed for an 80 mph
non-operating wind velocity in this configuration.
b)
If the maximum wind gust velocity at the crane's location is greater
than 80 miles per hour, provision shall be made for putting the crane into a more
stable stowed configuration for maximum wind gust conditions. This may include
setting the boom at a greater radius, using a more stable rotate position, and
providing slide protection or tie-downs, if required. In its stowed
configuration, the crane must meet the strength and stability requirements for its
maximum non-operating wind velocity.
5.1.1.5
Acceleration and Deceleration Forces. The acceleration and deceleration
of the travel drives and rotate drives (if applicable) produce horizontal loads
that must be considered in design. For portal cranes, floating cranes, and
cantilever cranes, the rate of acceleration is based on accelerating from a
standstill to the rated speed of the drive in 8 seconds. The rate of
deceleration, caused by loss of power or "panic stopping", is based on the ratings
of the brakes and is considered an emergency condition. Rates of acceleration for
OET cranes and underrunning cranes are given in the applicable industry standards
(CMAA #70, CMAA #74, and MH27.1).
5.1.1.6
Spreading and Squeezing Forces. The portal base of a portal crane is
subjected to spreading and squeezing forces. When a portal crane travels into a
curve, the effective gauge of the rail changes, and the travel trucks slide on
their
113
Previous Page
