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
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
> 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?
> Thor Matteson