Introduction integral bridge abutments

Expansion joints and bearings are conventionally installed in bridges to accommodate the movement of the bridge superstructure due to thermal expansion and contraction,  creep strain of concrete under sustained load and shrinkage of concrete with aging.

A major problem in this construction is corrosion and deterioration cause expansion joints to perform poorly, leading to high maintenance and replacement cost

Integral bridges are designed without any expansion joints between spans or between spans and abutments , When the deck joints and the associated bearings are eliminated by casting superstructure integrally with the substructure, the horizontal movement induced from temperature changes, creep, and shrinkage should be absorbed by other means. Benefits offered by integral abutment bridges include reduced initial costs, lower long-term maintenance expenses,  elimination of problematic expansion joints and bearings, less deterioration, lower impact loads, improved riding quality, simple construction procedures, and structural continuity to resist overloads.

The disadvantages of such construction include subjecting the superstructures to large secondary stresses caused by the response of continuous superstructures to thermal and moisture changes and gradients. These cyclic movements and stresses must be addressed at the bridge abutment (ratcheting behavior).

Types of Integral Abutment bridges

Based on British Design Manual BA42, integral bridge abutments are divided into the following categories:

1.Frame abutments

2.Bank pad Abutments

3.Embedded abutments

The Frame Abutment which supports the vertical loads from the bridge and acts as a retaining wall for embankment earth pressures. It is connected structurally to the deck for the transfer of bending moments, shear forces and axial loads and supported on foundations. It may be assumed that the abutment will rock bodily on its foundation for the purposes of calculating thermal movements and earth pressure.

The Bank Pad Abutment, which acts as an end support for the bridge, moves horizontally during thermal expansion and contraction of the deck.  The bank pad must have adequate weight, and the end span have adequate flexibility, to avoid uplift from live loads or from differential settlement.

The Embedded Abutment, such as a diaphragm wall, which extends to a depth below the retained fill and is restrained against rocking by the length of embedment.

Specifications Integral Abutment Bridges

However, the design and construction of integral abutment bridges are primarily relying on empirical guidelines and no national standards or uniform policy exists. There is little national consensus on allowable limitations, such as the maximum bridge total length,  individual span length, skews, and curvature. The nonlinear interactions among soil, backfill, pile, and abutment are unclear. The analytical and numerical model of the integral abutment bridges remains uncertain.

Interaction Integral Abutment Bridges

Since it does away with issues related to joint and bearing expansion, the integral bridge idea has generally been shown to be less expensive to build for a wide range of span lengths. It has also been demonstrated to be successful from a technical standpoint. On the other hand, geotechnical problems could cause problems as a result of the complex soil structure reacting to the relative movement of the surrounding retained soil and the bridge abutment, especially in long span ,If the movements are large, the soil response may be nonlinear.

Soil-structure interaction in integral bridges is categorized into two components: soil-pile interaction and backfill-abutment interaction

Backfill_Abutment Interaction

In modeling of the backfill-abutment interaction behaviors,. interaction curves, F-Δ  are developed to simulate the soil-abutment interaction, where F is the backfill soil pressure and Δ is the abutment movement. Design curves for backfill-abutment interaction have been recommended by the National Cooperation Highway Research Program (NCHRP 1991) and Canadian Geotechnical Society

Soil_Pile Interaction

There is  common approaches used to idealize the soil-pile interactions for laterally loaded piles: the p-y  curves approaches. The p-y approach idealizes the interactions using a series of lateral springs distributed vertically over the depth of the pile.  The spring stiffnesses are defined by p-y curves, or the relationship between the soil resistance per unit length of the pile (p) and the lateral deflection of the pile (y).

The process to determine the spring stiffness and structure response to analyze an integral abutment bridge using the p-y approach is iterative .It normally requires two computational tools: (1) a soil-structure interaction analysis to generate the p-y curves and evaluate the soil stiffness, such as LPILE and, (2) a structural analysis to evaluate the structural responses, such as SAP2000 or CSI bridge.

As mentioned earlier, integral abutment bridges have been in use for many years, but there is still no comprehensive design procedure. For instance, in the US, each state highway department manages its own integral abutment program and establishes guidelines with regard to design and construction.

The following criteria are utilized in providing integral abutment bridge designs for the state of Iowa:

All abutment substructure units shall be supported on a single row of piles. Cast in- place (C.I.P.) or steel H piles may be used for structures with span lengths of 165 ft. (50 m) or less.  Only steel H piles should be used for structures with span lengths over 165 ft. (50 m)

steel H piles are used, the web of the piles shall be perpendicular to the center line of the beams regardless of the skew. This will facilitate the bending about the weak axis of the pile.

References:

BA- 42 . The Design of Intergral bridges. Design Manual for Roads and bridges, 2003 ,The Highway Agency . The Stationary Office , UK

Dicleli, M. (2000). Simplified Model for Computer-aided Analysis of Integral Bridges, Journal of Bridge engineering.

Jimin Huang  and et al“Behavior of Concrete Integral Abutment Bridges” 1996-2004 , Minnesota Department of Transportation Research

George Z. Voyiadjis and et al “Integral Abutment Bridge for Louisiana’s Soft and Stiff Soils”,2016  Louisiana Transportation Research Center