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Flat Slab Bridge

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David Finley wrote:

I've got a project with several C.I.P. flat slab bridges with spans =
lengths of 13' / 30' / 13'.  With such short end spans, there is uplift
at each end of the bridge if a continuous deck is used.  There is too =
much uplift to simply thicken the deck at the ends.

Rather than use simple spans to eliminate the uplift, I'm considering =
using integral abutments to resist the uplift.  I know integral =
abutments have been used sucessfully on steel bridges, but I've not =
heard of them being used on flat slab bridges.  A further complication =
is that the agency requires 24" sq. conc. piles.  Obviously, they will =
be much stiffer than the H-piles normally used in integral abutments.  I
don't need a lot of movement in order to accomodate thermal =
expansion/contraction, but I do need some.

>From your further posts, it sounds like you're pretty constrained in
free-thinking this problem.  However, to step out of the box for a
moment, I load-rated a couple of bridges some years ago with spans of
about 11' / 30' / 11' for the Forest Service.  They had only two
supports (pile bents), with 11 foot cantilevers.  At the ends of the
bridge there were backwalls attached to the ends of the beams to prevent
soil from spilling between them.  There were no abutments.  These seemed
to work pretty well.

The integral abutment idea does not bother me much, although those
piling will be pretty stiff.  What is your thermal range, maybe 90 or
100 degrees?  While it doesn't get quite as hot here in Minnesota, our
temperature range is around 150 degrees for design, and I've designed
slab bridges with integral abutments for the D.O.T.'s in Minnesota,
Wisconsin, and Iowa with lengths out to about 120 feet, and prestressed
beam bridges out to 300 feet.  Minnesota requires that these be designed
as rigid frames, which leads to some rather large moments at the ends
(really large, with your piling).  Wisconsin and Iowa just ignore it.
They've had good success for at least 20 years with this.  If the slab
wants to crack near the end, it doesn't matter structurally because it's
deflection driven rather than load driven; it won't fall down.  A little
crack with rebar through it is preferable to an open joint in my book.

Out of curiosity, what would happen if you just design a normal seated
abutment?  You say you have some uplift, but how far will it pick up?
Maybe a quarter inch? Maybe it's not too big a deal.  Put a full-length
rubber pad on the abutment.  Nobody smart enough to complain to the DOT
is going to tailgate a truck that's big enough to approximate an HS20

The longest simple span bridge slab I've ever designed, if I remember,
is 30 feet.  It was not the most efficient animal; I think it was 2 feet
thick.  Continuity is your friend.

Mike Hemstad, P.E.
St. Paul, Minnesota

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