EARTHQUAKE EFFECTS ON DERRICK AND CRANES BASIC AND TUTORIALS


Studies of earthquake effects on cranes are few, and code development in this area is in its infancy. Generally, permanent installations such as bridge cranes and port cranes can be subjected to seismic analysis using the same principles as those used for other fixed structures.

A decision to analyze a crane seismically should be based on the degree of risk as weighed against potential consequences of a loss. Risk may be assessed by study of earthquake maps. In areas of low or moderate earthquake risk, seismic study may be demanded only for the most-sensitive applications, such as nuclear work.

In adopting a philosophy for earthquake resistance, the crane analyst or designer might consider one or more of three risk mitigation levels, or limit states.

1. The earthquake design does not cause structural damage to the crane. All stresses remain in the elastic range. The crane should remain serviceable.


2. The design earthquake may result in some damage that could be readily repaired and the crane restored. Failure may occur in components that are not part of the main force-resisting system. Component failures cannot put workers or the public at risk, and significant collateral damage to surroundings is not permissible.

3. Controlled ductile yielding may result in the complete functional loss of the crane, which would be replaced, but the avoidance of a catastrophic failure leaves the public, workers, and surroundings protected.

A designer might choose to calibrate the design to only one of these states or, alternatively, consider associating each of them with a different magnitude earthquake event. As all three imply a high level of life protection, this decision would be based only on economic considerations.

Except for those few industrial cranes that are in near-constant use, probability favors the premise that the device will be unloaded if an earthquake should occur. Nearly all cranes used in both general industry and construction will have a substantial load on the hook only a small percentage of the calendar year.

With few exceptions, then, earthquakes might reasonably be evaluated only for out-of-service consideration. Though earthquakes are dynamic events, the simplest methods of seismic analysis make use of equivalent static loads.

These methodsare suited to areas of low or moderate seismicity or for structures that are relatively simple in their response to excitation. Other methods in the toolboxes of seismic engineers may be applied for more complex situations or where the level of risk warrants the investment.

A freestanding tower crane may respond well to moderate earthquakes because its long period of oscillation will not resonate with the higher-frequency ground motion. However, the crane could be at risk in a severe earthquake due to base shear or from vertical acceleration acting on the counterweights.

In some soils, liquefaction could pose a risk. On a tower crane that is secured to a building, tied to the outside or mounted within, the interaction with the building can lead to higher seismic loads compared to those expected for standard freestanding erections.

Generalized assessment of earthquake risks for a mobile crane can be difficult because a typical machine changes location frequently and its boom disposition changes constantly. Overall exposure should not be great. There could be vulnerability under certain conditions, however.

For example, ground acceleration might induce an unloaded machine with a short boom at a high angle to tip backward.


Perhaps the most studied seismic event with respect to cranes was the Kobe, Japan, earthquake of 1995. Documented failures included

• Total collapse due to foundation failures probably caused by liquifaction
• Overstress of towers from horizontal shear, with resulting diagonal and connection failures
• Leg tension failures due to overturning moment on pillar cranes and tower cranes
• A bridge crane girder lifting off its supports
• Overturning of a rail-mounted gantry crane

In 2002, two tower cranes toppled from the 60th floor of a steelframe building under construction in Taipei, Taiwan, during a moderately severe earthquake. The failures were not caused directly by the earthquake, but rather by the cranes oscillating in resonance with the building.


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