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Re: Existing Bridge--Steel girders cast directly into concrete piers and abutments

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I am not a bridge engineer, but here is a thought: can you have the
concrete strength tested to verify f`c?  You might be able to justify a
higher bearing strength and get it to work without all the other
headaches.

-- 
Jake Watson, E.I.T.
Salt Lake City, UT


> Mattesons wrote:
> 
> We have been asked to certify that an existing 50-or-so-year-old
> bridge can support a "water-tender" for our rural fire department.
> The 12-foot wide bridge consists of two steel S-shape beams running
> longitudinally, set 6'-0" apart and spanning 15 feet max, with an
> 8-inch thick cast-in-place reinforced concrete deck slab on top of
> them.  The beams are embedded in the concrete bridge abutments and
> piers.  The deck is poured around the top flanges of the beams, and
> also on top of the piers and abutments.
> 
> The bridge has held up admirably (I wish all of our infrastructure was
> this durable);  the steel beams have only slight rust, the concrete
> deck has only a few BARELY noticeable cracks, very minor scour, etc.
> The owner also points out that this bridge served a gold mine for many
> years and has supported concrete trucks carrying 15 yards of
> concrete--maybe with the extra 5 yards of concrete in the cab with the
> driver?--and possibly train locomotives (OK, kidding here...).
> Initial calculations show that the bridge is adequate in most
> respects.
> 
> I see two problems:
> 
>     1.  Shear in the deck.    This can be mitigated (with the Fire
> Department's approval) by installing curbs to keep heavy vehicle tires
> within "d" of the center of the beams, which thankfully are set at the
> same spacing as the tire tracking width.   Or:   pour another layer of
> concrete on top of the existing deck, which would worsen problem 2.
> 
>     2.  The steel beams bear directly on the concrete at the piers and
> abutments; indeed they are embedded into them an unknown distance.  My
> "old" reference (1924 Structural Engineers Handbook, by Ketchum,
> McGraw Hill) gives an allowable bearing of 600 psi on concrete.  This
> seems reasonable to me for the existing materials.   If I assume (and
> then verify) that the beams are embedded fully through the piers, and
> the full width of the bottom flange is effective in spreading out the
> beam load (which it is not because of longitudinal flange bending) I
> still do not have enough bearing area for dead plus live loads.
> 
> This would not be such a problem if the beams could simply be jacked
> up and bearing plates and pads slipped under them.  However, in this
> case the beams are embedded in the piers AND the deck on top of them
> is cast with no sign that the deck can be raised up off  the piers and
> abutments.  Casting new corbels under the beams would not help unless
> the beam load that is already on the existing piers could be
> transfered onto the new corbels (maybe by filling between the corbels
> and the beam with expanding grout?)   I would love to get an
> elastomeric pad in there, but with the beams "locked" into the piers I
> don't see how to do so economically.
> 
> Another option, if we verify sufficient embedment of the beams into
> the piers, is to consider that the bearing load is carried by both the
> top and bottom flanges of the beams, since the concrete piers are
> poured around the entire beam.
> 
> Suggestions, thoughts, comments?
> 
> TIA,
> 
> Thor Matteson