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Re: Crane Design

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Drew A. Norman, S.E. wrote:
> To all who responded to or followed the recent thread on crane design:
> I forwarded all of the items from the list to my architect client.  From
> this and whatever research he has done independently he says he has
> concluded that (1) the bridge cranes in commercial buildings standard (as
> opposed to essential) occupancy are NEVER designed for seismic loads and
> that (2) such bridge cranes are usually designed by mechanical (as opposed
> to civil or structural) engineers.  He is on this basis taking issue with
> our having specified a design submission including seismic analysis under
> the signature of a structural engineer.  I have suggested that proper design
> of the thirty ton and sixty foot span crane in our building (apx 71 kips
> dead load including rails) requires not only a seismic analysis but a
> DYNAMIC analysis.  My client thinks that I am being overly conservative,
> asking for something that is never done and will cost a lot of money, and
> generally being outrageous.
> Needless to say, my client and I are not seeing eye to eye here.  I have NOT
> drawn the same conclusions from the discussion on the list.  Either he or I
> appear to be misunderstanding the meaning of the posts.  If anyone would
> care to offer a final comment on the subject that might help us to move
> closer together by getting a better grasp on the issues, we would both
> appreciate your efforts.
> Thank you
> Drew Norman, S.E.
> Drew A. Norman and Associates
I assume the original question relates to the crane itself, and not the
runway.  Runways definitely require design to resist seismic
forces/deflections including the crane DL (but not payload mass).

If your bridge crane operates in an occupied structure, and/or is cab
operated (i.e. - occupied space as a part of the crane), and you are
under UBC jurisdiction, it appears hard to exempt those "occupied"
spaces from the seismic provisions of the Code.  What does your local
building official say?

I think a dynamic analysis is probably not warranted unless there is odd
geometry with a high likelyhood that the crane's behavior will effect
the response of the structure (your judgement).  Vertical acceleration
components (@ 2/3 of horizontal +/-) are usually less than the impact
factor crane mfr's use.  Also, low allowable stresses due to fatigue
considerations also enhance overall strength, at least for most of the
crane's life.

If you spec seismic analysis/design for the crane, I would suspect the
mfr can see that seismic loads are less than typically designed for
against normal use.  Forcing them to spend money retaining an outside
structural engineer seems a likely cause of friction and negative
feedback THROUGH the architect.

Perhaps a compromise:  Discuss design loadings with the engineer of
record for the crane mfr (mechanical or structural) and convince
yourself that seismic loads either will or will not control the design. 
This should take less than one hour of your time.  If odd conditions
exist, you and the crane designer will uncover them, and this will
provide a justifiable reason for additional that the
architect and HIS client can buy into.  

Also, a mfr's certification that the crane is designed to resist
specified loads may provide the owner with better protection than an
engineer's seal obtained under duress.  If a problem later arises, he
will have a deep pocket to collect from.

Creeping commercialism vs. professional responsibility.  Try to solve
with a smile.  Good luck.

Larry Oeth