Bridge design for differential settlement of foundations 1

[visitor]

I am designing a continuous bridge. What should be the amount of differential settlement of supports of the bridge should be designed for? Is it required to consider
the full traffic load when this differential settlement is analysed for

 

[Qshake]

There should be no, repeat, no settlement as part of an inital (or new bridge) design. In case there is something mentioned in a geotech report concerning this then you should design your foundations to mitigate settlement. Often this requires a recommendation from the appropriate geotechnical resource.

I've seen plenty of rehabilitations where this issue (differential settlement, mine subsidence etc.) is addressed and it is very messy. But never in a new design.

 

[Ingenuity]

If you have a thorough geotech investigation and your geotech engineer recommends/states that there is liklihood of differential settlement then i think it prudent to make due allowance for this differtial magnitude and determine its effects on the superstructure in the original design.

Sure we strive to have no diffential settlement in our substructure design, but the reality is that setlement can and does occur in certain areas, and if a magnitude is allowed for in the design then remedial action is not always required if settlement does occur of a magnitude that is less than designed for.

In Australia in the early 80's our Road and Traffic Authority (equal to a USDOT) designed and constructed a 220m span single cell continuous box girder road bridge in a mining area where differential mine subsidence of 95 mm vertically was allowed for, with corresponding differential rotations. The rotations are assumed to be non-conurrent with the differential displacements.

However, I do agreee with Qshake that remedial/construction work solving differential settlement problems of older bridges are messy.


HTH

 

I am working on a project in which we are going to extend
Prestressed Concrete Bridges, both substructure and superstructure.
I wanted to know the following:
1. Is it better to provide a longitudional joint or make the two
slabs monolothic?
2. Is it mandatory to Provide dowels between the two transoms
and make them monolithic too?
3. What if we maintain the two decks separately with a longitudional
joint in between?
4. I am extending @ 2.6 meter deck with two girders on a transom
and single pile support.
The width of longitudional joint is 13 mm.
Is it better to provide single girder?

2.6m 8.5m

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|----------| |--------------------/
|New Transom | Existing Transom /
|----------| |------------------/

Extension Existing


Cross section of bridge

 

[austim]

I don't entirely agree with Qshake (that may be a first, by the way).

If you have a site which is liable to significant foundation movements (mining subsidence or the like) then you have to make a choice between adopting fully statically determinate superstructures or designing for the effect of differential settlements in continuous structures (or locating the bridge somewhere else entirely ).

Statically determinate structures basically require one deck joint for every pier. There is a school of thought that aims to minimise deck joints wherever possible (because of ongoing joint and bearing maintenance, poor road surface geometry, etc).

Continuous structures may offer some gains in structural economy and will reduce the number of deck joints, but at the risk of some sensitivity to settlements.

The choice between the two basic alternative structure types is not really all that simple. (Two of us spent about 6 weeks analysing a possible continuous design for Medway Bridge, UK [500 foot span] in the late 1950's before it had to be abandoned because of uncertainty regarding possible settlements in the founding chalk stratum. This was before there was any Code of Practice for prestressed concrete in the UK, and we were working mostly to allowable stress design - a different decision might be made today).

If you have a mining-settlement-prone site which also is subject to seismic activity and your bridge spans a known fault line, like one I worked on in New South Wales in 1977, shortly before Ingenuity's example, then all this gets just that much worse.

The amount of differential to be designed for must be considered for each particular site; this is where a good dose of sound engineering judgement is called for. If there is really good site data, and the behaviour of the foundation strata is well known, then maybe about 25% of the calculated maximum settlement would be reasonable.

If your structure is sufficiently ductile, (permitting all of the necessary redistribution of load before collapse), you MAY be able to ignore the effects of settlement in the ultimate strength condition.

However, for serviceability limit states (crack widths, etc), you should treat the adopted differential settlement effects as just another permanent load, to be combined with all other design loads, factored according to your local Code of Practice.

Just a reminder if you are designing a concrete superstructure. Since settlement is likely to be a very slow and long term effect, creep will be significant. You should base your settlement analyses on a reduced (long-term) value of Elastic Modulus for your concrete.

 

[Qshake]

Well, AUSTIM, perhaps I was a bit harsh. But I prefer to mitigate, if at all possible, the settlement issue. Good advice as usual though and some interesting work there in New South Wales.

Guest JVD, you should really post this by itself either in the bridge or structural forum.

Bridges are widened all the time as you describe and most are monolithic (that is to say without a longitudinal joint). In removing the old parapet or barrier curb you will have an opportunity to open the edge of the deck and expose the reinforcing. With a little more saw cutting and hammer work you can obtain sufficient development length.

Longitudinal joints (like compression joints) are not used for many reasons that run from maintenance headaches to hazards for motorbikes.

Some agencies like to see widenings supported on more than just a beam with one column (hammerhead) or pile and counting on the shear for support where the new beam is placed against the existing. Sort of the idea that the widened structure is completly independent of the existing, except of course where the bridge deck is concerned.

 

[ishvaaag]

Spain's code IAP for bridge design instructs to use in pertinent loadcase combinations the settlement values established in a preceptive geotechnical study, and where the evolution of the settlement in time be determined difficult to establish for a given instant, to use for such instant the max and min values between which it is expected to fluctuate.