Question:
What is reasonable impact factor for skid (about 50-60 ton weight)
during lifting using Allowable Stress Design ASD. Is 25% to low?
Answer:
This depends to a great extent on the environment in which the lifting will be
done. Is this a special item that will be lifted infrequently and with careful
planning and supervision or will it be lifted frequently and in a potentially
rough manner?
What type of crane or hoist will be used to do the lifting? Will the lifting
be done using a conventional mobile crane on a construction site or a derrick
barge at sea constucting an offshore platform? Or perhaps an overhead bridge
crane in a manufacturing facility.
How comprehensive will the skid design process be? Some (most?) lifting
operations impart transverse loads as well as vertical loads to the lifted
item. Is the combination of allowable stress design factor plus vertical
impact factor expected to account for these other forces, or will an effort be
made to quantify *all* forces that will occur during the lift?
If you can provide additional information about the nature of the load and how
it is to be lifted, then we can offer reasonable opinions about design loads
and allowable stresses.
The lifting device will usually dictate the value of the dynamic / impact
factors. The lifting device itself will be covered by a design code which
should also state the dynamic factors for which the lifting device should be
designed. Many of the factors are related to the lifting device's
accelerations and braking effects, slipping of slings etc., but there is
also and underlying minimum for environment, and type of usage.
Whatever the lifting device is designed for can (with judgement) be used for
the design of the skid lifting points.
