An earthquake induces dynamic loading, but several of the most commonly used deterministic design codes have procedures for developing a static-load equivalent to represent the effect of earthquake forces. Many designers ignore earthquakes for any location other than parts of the American southwest. The fact remains, however, that many other areas in the United States have experienced significant earthquake shocks within recorded history. The question is one of probability. There are earthquake maps that divide the country into earthquake zones according to potential risk.
For most cranes, probability favors the premise that the device will be unloaded should an earthquake occur. Further, for most earthquake zones, an unloaded machine will be able to withstand the shocks without damage. For permanent, highly sensitive, or long-term temporary installations of cranes with high CGs, however, verification would be in order, otherwise you’ll end up with something like the below video.
In addition to the centrifugal forces previously mentioned, dynamic loads are for the most part those associated with masses undergoing changes in motion as described by Newton’s second law, F = ma.
All crane motions produce dynamic forces as the motions begin and end. This includes hoisting, trolleying, luffing, slewing, and travel motions, as well as counterweight movements for machines so arranged. Finally, there are the rotating masses in the various drive systems producing local dynamic effects, but if the rotating mass is significant in relation to the mass of the entire Crane, this will affect transitory powering up or breaking forces associated with the rotation.
The force required to accelerate or deceleration a mass in linear motion can be found simply by applying Newton’s law if they acceleration rate is known – except for machines with controlled acceleration rate. In many instances, drive and braking systems are capable of far greater force needed, and the operator rarely applies the full force. The extra capacity is required to overcome wind and other nonconstant effects. The appropriate acceleration rate to be used in design is a matter for judgment, but the FEM offers some guidance for heavy lifting equipment. With acceleration time and final (initial) velocity given, average acceleration is velocity divided by the time, of course. Many machines have drive systems that do not provide linear acceleration, but for most work a linear assumption satisfactory since system inertia tends to linearize the acceleration, but for most work a linear assumption satisfactory since system inertia tends to linearize the acceleration.
For cranes with brakes that will always fully engage automatically, the break will be rated for braking force or for torque. When these values are mathematically referred to the point of contact of a wheel with a rail or a rope to his winch drum, the force inducing the acceleration becomes known and acceleration rates and times can become elated.
Preconditions can create significant dynamic loading and a hoisting system: picking a load suddenly from a condition of rest, suddenly stopping a load being lowered, and suddenly releasing a load, such as would be the case during emptying of a clamshell bucket or dropping of a magnetic load of scrap.
When a load is dropped in freefall, the acceleration is retarded by friction at the sheaves and the inertia of the winch drum. Should the brakes fully and suddenly engage, so that deceleration is virtually instantaneous, load will not stop immediately. The hoist ropes will stretch, like sprains, providing some additional movement. Freefall of any loads other than minimal weights can be extremely dangerous. Operators do not often allow other than marginal loads freefall. The resulting impact quickly rises to high multiples of the load. On cranes that permit free fall, it is usual for an operator to ride the break in order to modulate velocity and be in a position to maintain acceptable limits of deceleration for the final stop; this is done by judgment alone.
American mobile Crane manufacturers claim that impact loading is not an important condition for their type of Crane and do not use it in design. They prefer their own deterministic loading conditions, which will be dealt with later. Their position can be defended with a strong logical argument. Q loads lifted by a mobile Crane approach strength covered ratings (most ratings are stability governed) so that impact would rarely pose a structural threat. Loads that are in the structural range are very heavy loads, which are understandably treated with respect by operators. As a short duration dynamic condition, impact has been found to impart too little energy to overturn a Crane except in extreme cases.
A task force of the American Institute of Steel Construction (AISC) conducted a series of tests to determine how best to account for impact in Crane design. In their report they state that “energy applied by motion at the hook is absorbed immediately and simultaneously by all elements of the crane setup and cannot be separately identified in the ensuing elastic vibrations of masses individual elements…” The task force found that simply increasing the live load by an impact factor did not yield correlation with test results. Instead, they recommend increasing axial as well as dead and live load bending stresses (but not lateral or side bending stresses) by an impact factor. This procedure produced values that closely match measured stresses. For lifting. Really based rated loads, a factor of 20% is suggested, but the tests revealed that greater impact should be expected as loads decrease in relation to rating. The tests were deliberately carried out to produce extreme, or upper bound, impact compared with normal, proper production operations.
The AISC task force makes another interesting and subtle observation that is particularly pertinent for cranes. Winches are manufactured so that there is a direct relationship between winch line pole capacity and great capacity. A winch properly matched to age Crane will be incapable of stopping rated loads instantaneously; the brakes will be sized so that a reasonable stopping distance can be expected. However when an overcapacity winch is matched to a Crane, brake overcapacity will present a potential for excessive impact loading.