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Re: Cast-in-place Segmental box bridge

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Apparently top slab midspan reinforcement is inadequate to resist the action
effects developed in this section.
Since the cracks have been observed along the entire length of the job, they
should be attributed to actions that stress the top slab in the same way
along the bridge length, irrespective of the relative location, i.e at
midspan, between the piers, where torsional effects are not so pronounced or
near the supports where transverse torsion is maxinized.
The fact that the cracks extend full depth means that the effect of
transverse axial tension in the top slab should be investigated.
One reason could be the presence of big side cantilevers extending from the
box section which can cause considerable transverse tension of the top slab
even with no live load on them. Of course live load on the cantilevers adds
to the same effect.
Another effect of the side cantilevers is that they minimize the bending
moments in the midspan of the top slab thus leading to an underestimation of
the bending reinforcement, especially if axial tension has been neglected.
In this case, slab negative moments above the webs are rather big and
reinforcement design is not so sensitive to axial tension. This
reinforcement could easily resist transverse tension even if not taken into
account.

HTH

Stathis Papageorgiou
Athens - Greece

----- Original Message -----
From: "Troy Madlem" <tmadlem(--nospam--at)kenherceg.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, April 08, 2003 9:48 PM
Subject: Cast-in-place Segmental box bridge


> Do any of the bridge engineers out there have any thoughts or know of a
good
> resource in any of the state DOTs to offer some insight into the following
> problem?  Any suggestions or contact information would be greatly
> appreciated.
>
> BACKGROUND:
> We are working on a rehab job of a CIP segmental box bridge, constructed
> originally in the early 1980s, that is comprised of nearly identical twin
> structures approximately 1 1/4 miles in total length (mainline road and
> several access ramps).  General layout of the bridge is as follows:
Typical
> spans between 180'-265' long utilizing single boxes (9' tall x 17' wide)
and
> double boxes (9' tall x 35' wide) reinforced with mild steel and
> longitudinal PT strands.  Typical boxes have 15" thick webs, 9" thick top
> slab (thickening over 4'-0 length to 18" thick at the webs) and 8" thick
> bottoms.  Project is located in the Midwest and subject to heavy snow and
> deicing salts in winter.
>
> Most of our work is straightforward, consisting of upgrades to barrier
> rails, cleaning/repairing drains and replacement of expansion joints.  The
> one unique aspect of the job is how to deal with longitudinal hairline
> cracks we are observing in the top of the boxes.  These cracks are located
> mid-span of the top slabs, extend full depth thru the box's top slab and
> topping and occur along the entire length of the job.  So far we've gained
> access inside the boxes on one of ramp sections and have observed
> efflorescence and minor rust stains along the cracks.  We've observed
these
> longitudinal cracks in both the dead of winter and the heat of summer and
> they always appear to remain hairline in nature.  Subsequent to our
> inspections, one of the ramps was rehabbed at which time the entire
topping
> was removed and replaced.  Since that time the longitudinal cracks in the
> box tops migrated thru the new topping slab with no noticeable difference
> between the rehabbed section and the original structure and topping.
>
> One of our initial thoughts on the origin of these longitudinal cracks is
> excess deflection of the top slab.  After some additional research, its
been
> suggested that perhaps a temperature differential due to the sun hitting
the
> outside of the boxes while the inner half is shaded by the adjacent twin
> structure could also be causing these cracks.  To maintain the useful life
> of the structure and avoid any further deterioration of the top slab
> reinforcing, we are considering injecting the cracks with a flexible
> sealant.  However, there are now questions being raised whether any
> remediation to the boxes is necessary at all.  The toppings are all in
> excellent condition with maybe 0.2 % delamination and the only real
> structural problem evident is the hairline cracks.  The rest of the job is
> all maintenance work and modifications to bring the bridge up to current
> design standards.
>
> At first look, sealing the cracks appears to be the best answer since
> cutting off water from migrating into the boxes prevents deterioration of
> the reinforcing from occurring.  However, we're talking about a lot of
money
> to remove the topping, install injection ports along the cracks, inject
the
> sealant and finally replace the topping (which didn't need replacement to
> begin with) not to mention the maintenance of traffic nightmare we would
be
> creating.
>
> QUESTION:
> What are the ramifications of waiting a few years before doing anything
with
> the cracks?  Water will continue to migrate thru the crack but the cracks
> remain tight after 20+ years and all that has resulted is some
efflorescence
> and minor rust staining.  Are longitudinal cracks commonly found on
> segmental boxes and shouldn't be a real concern?  Do they possibly signify
a
> larger problem that we've all overlooked?  Most importantly, our DOT
client
> is interested in finding precedent from similar projects before deciding
how
> to proceed: are there any?
>
> Thanks for any insight you might have.
>
> Troy Madlem, P.E.
>
>
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