List of Abutment Design Standards: [Show]
Current practice is to make decks integral with the abutments. The objective is to avoid the use of joints over abutments and piers. Expansion joints are prone to leak and allow the ingress of de-icing salts into the bridge deck and substructure. In general all bridges are made continuous over intermediate supports, and decks under 60 metres long with skews not exceeding 30° are made integral with their abutments.
Full height integral abutments (DfT BA 42/96 call Frame Abutments) are generally used for the shorter spans (< about 20m).
Integral abutments with piled foundations (DfT BA 42/96 call Embedded Abutments) usually incorporate steel H piles in a single row; the H piles are orientated so that bending occurs about their weaker axis. These abutments are suitable for the larger span decks.
Integral abutments with spread footings (DfT BA 42/96 call Bank Pad Abutments) should only be used where settlement due to consolidation of founding strata is minimal.
Where decks exceed 60 metres long or have skews exceeding 30° then movement joints and bearings usually need to be provided.
Open Side Span with Bank Seats
Solid Side Span with Full Height Abutments
Usually the narrow bridge is cheaper in the open abutment form and the wide bridge is cheaper in the solid abutment form. The exact transition point between the two types depends very much on the geometry and the site of the particular bridge. In most cases the open abutment solution has a better appearance and is less intrusive on the general flow of the ground contours and for these reasons is to be preferred. It is the cost of the wing walls when related to the deck costs which swings the balance of cost in favour of the solid abutment solution for wider bridges. However the wider bridges with solid abutments produce a tunnelling effect and costs have to be considered in conjunction with the proper functioning of the structure where fast traffic is passing beneath. Solid abutments for narrow bridges should only be adopted where the open abutment solution is not possible. In the case of wide bridges the open abutment solution is to be preferred, but there are many cases where economy must be the overriding consideration.
Loads transmitted by the bridge deck onto the abutment are :
Vertical loads from self weight of deck.
Vertical loads from live loading conditions.
Horizontal loads from temperature, creep movements etc and wind.
Horizontal loads from braking and skidding effects of vehicles.
These loads are carried by the bearings which are seated on the abutment bearing
The horizontal loads may be reduced by depending on the coefficient of friction of the bearings at the movement joint in the structure.
However, the full braking effect is to be taken, in either direction, on top of the abutment at carriageway level.
In addition to the structure loads, horizontal pressures exerted by the fill material against the abutment walls is to be considered. Also a vertical loading from the weight of the fill acts on the footing.
Vehicle loads at the rear of the abutments are considered by applying a surcharge load on the rear of the wall.
For certain short single span structures it is possible to use the bridge deck to prop the two abutments apart. This entails the abutment wall being designed as a propped cantilever.