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Re: Concrete Crane Beam for 40ton Container Crane

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Tripp,

I am not so sure that a bond failure would be the original cause of the
cracks.

It sounds from your description that the beam is a continuous beam.  Thus,
the top bars would be the ones doing the "work" at the piles (supports
locations) while the bottom bars would be very lowly stressed or not
stressed at all, depending on the span lengths.  In either case (top or
bottom bars), the bond at the pile locations should not be too much of an
issue.  In the case of the top bars, the bond stresses would be more of an
issue as you get "just" away from the support/pile location in each
direction.  In the case of the bottom bars, the stress in the bars should
be so low or non-existant that the bond stresses would be minimal or zero
since the stress in the bars would be going from highest at midspan or so
to lowest/zero as you go to the supports (thus, the bond is "taking" the
stress into the concrete more in the center of the span).

Now all the above is assuming that the beam is in good condition.  With
your description that the bottom cover seems to be spalling off, the
location where the bond stress can be "taken" out could be shifting
depending on where you have effective cover versus where you do not.

The end result is that I would speculate that you may have your causes
reversed.  I would suspect the the primary culprete would be corrosion of
the bottom bars.  This could be further worsened by the location where the
stress can be taken into the concrete being changed as cover spalls
leaving it more difficult to have the bars bonded to the concrete.

Based upon your description, if the beam can be left in place in some
reasonable condition (either after repair or some such), then I would not
be too concerned with the lap lengths for the bottom bars IF they really
are occuring at the pile locations.  This location should be (by your
description of the beam) an area of low stress in the bottom bars should
the fact that they are not completely lapped should not be a big concern
(if this were a new beam being designed by the current ACI code, then the
situation would be completely different).  If, however, the top bars are
also lapped at the same location, then there might be some reason for some
concern since the top bars should be highly stressed at this location
(when load is present).

While replacing the beam is a perfectly reasonable option, I would at
least look at what the repair options are for the beam.  It may be
possible to repair the beam.

HTH,

Scott
Ypsilanti, MI


On Wed, 27 Nov 2002, Tripp Howard wrote:

> I am analyzing an existing (1974 era) concrete crane beam that
> is cracked badly to determine what is causing the cracks.  The
> beam is part of a marginal wharf and supports a container crane
> used to load 40-ton containers onto ships (see sketch below).
>
>            36"Wide
>         +---------------+
>         | ~~~~~~~~~~~~~ |
>         |!o  o  o  o  o!|<--5#10 bars
>         |!             !|
> 38"High |!          -->!|<--2" clear to
>         |!             !|   #4 stirrups
>         |!             !|   on sides and bott.
>         |!             !|
>         |!o  o  o  o  o!|<--5#10 bars
>         | ~~+-------+~~ |   (these basically
>         +---|-------|---+    lay on top of pile)
>             |       |
>             |       |<--20" Octangonal Pile
>             |       |   embedded 2" into cap
>
> The cracks run horizontally along the sides of the beam at the
> level of the bottom layer of reinforcing.  The length of the
> cracks runs almost the entire length of the waterside beam.  The
> concrete has even spalled off the beam over many of the piles.
> The landside beam (same except 6" wider) doesn't have any cracks
> but doesn't see as much load either.
>
> The bottom bars are lapped over the piles.  The original
> drawings call for a lap length of 24 bar dia. (only 31").  My
> copy of CRSI lap length tables for ACI318-71 call for a class B
> lap splice to be 72" long (fc=3000psi, fy=60ksi).  This
> structure used 40ksi bars so that length can be reduced to 48",
> but this is still 50% more than was provided.  For ACI318-95, it
> gets worse.  The required lap length goes to 61".
>
> In addition to the lap length apparently being far too short,
> I'm not sure how much bond can be attained over the piles since
> the bottom bars were essentially placed directly on top of the
> piles (pile embeded 2", cover over reinf. 2", see below).  This
> leaves no concrete at bottom surface of bars for the bars to
> bond to.
>
> I know this is pretty long post, but I wanted to give you all a
> good idea of what I'm looking at.  Right now I think the culprit
> for the cracks is a bond failure of the bottom bars over the
> piles.  I think this opened up the concrete and allowed the bars
> to corrode which caused the cracks to propogate even further.
>
> What do you guys think?  Could I be missing something?  I mean,
> the crane hasn't fallen in the river yet, but I think the beam
> will need to be replaced.  Any input would be appreciated.
>
> Thanks
> Tripp Howard
>
> =====
> Tripp Howard
>
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