> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
> To: "=?ISO-8859-1?Q?=87?=" <seaint(--nospam--at)seaint.org>
> >The owner has replaced many of the structural
> >bolts, because they'd "worked loose". The steel (crane girder & columns)
> >does not show any immediately obvious distress -- cracked welds; cracking
> >in the top flange cap channel... The crane itself is being replaced,
> >because rehab will cost nearly as much as replacement.
So, if the crane is so worn that it is not economical to replace, why
should anyone expect the structure to be in any better condition?
Presumably, they were designed/constructed for balanced operating life
Mag cranes are on the high end of heavy service duty and impact loading.
Is there some control to prevent overload lifts?
You are doing some numbers to determine if the
girder/bracing/tiebacks/stops are anywhere near their fatigue limit.
Within +/- 25% I would make a strong case to the owners for expensive
inspection vs replacement/reinforcement during the shutdown, despite the
lack of visible distress.
Note that the crane may have operating cycles that do not affect all
areas of the runway similarly. Therefore, it is possible that portions
of the runway are not distressed. I recently handled a crane that had 3
separate operating cycles: Bracing exposed 100%, girders 33% or 66% and
not all were in lift zones.
> You'll want to check the structure against CMAA-70, which is the design
> standard for overhead cranes. This is one of those instances where
> If it were me I'd replace all the bolts and mag particle check the
> welds--at least the major connections. Presumably you're going to blast
AISE Technical Report #13, Guide for the Design and Construction of Mill
Buildings, section 5.17 recommends inspection techniques for weld types
in various locations (radiographic, ultrasonic, dye, magnetic, etc.).
AISE publication, Classification of Cranes, for service condition
assessment. This was a paper presented at an AISE conference and despite
their disclaimers, they obviously thought highly enough of its intent to
publish it under separate cover.
Also, if you can find one (!), Crane Runways (1990), WG5-IIW Commission
XV (whatever that means), provides an impressive review and application
of international standards and research on runway design. When the
typical standards say, "using appropriate analytical techniques", this
publication can help to fill in the gaps and clarify terminology. Ever
heard of compression fatigue cracks?
> I'd also prepare myself with a regime of fasting and prayer. You can
> never tell what sorts of things actually happened during the previous
> service, and primary metals producers are characteristically hard on
> equipment. Just 'worked loose' may actually cover anything from 'It
Scenario: Nothing is repaired/replaced, now; you suggest to the owner
that there is possibly 3-5 years left (they WILL budget for 5 years);
something fails in 2.9 years; they have an unplanned shutdown. What will
be their lost production cost? How happy will their lawyer be when it
calls your lawyer?
Paul Ransom, P. Eng.
Burlington, Ontario, Canada