Some Design Improvements for Integral Bridges

The design of integral bridges is currently limited to lengths of no more than 60m in the UK because the behaviour of longer bridges is not yet fully understood. Longer bridges experience larger thermal length changes and abutment movements. These lead to: (i) a continuous flow behaviour in the backfill, leading to progressive settlements (close to the abutments) and, (ii) greater lateral soil/structure interface stressing. Current design methods provide only an empirical upper limit to the lateral stress escalation. They do not address the flow-induced settlement in the backfill at all.

Attention is focussed here on stabilizing the strain behaviour of the backfill while maintaining the integral form of construction, such that bridges of almost any length may be constructed. This is achieved by introducing into the beams a load bearing compression expansion joint – called a displacement compensation unit (DCU) – designed to operate at a load which maintains backfill stability and prevents the development of granular flow within. These units act as expansion joints operating under an essentially constant design force.

Model studies on a 1/12 scale stiff retaining wall have been used to simulate bridge lengths up to 120m. Recorded pressures on the face of the retaining wall and backfill soil movements over many cycles are presented. These studies are then compared with results for a model bridge of 60m length containing DCUs to accommodate the simulated thermal movements. The effectiveness of the DCUs is then quantified in relation to their effectiveness for longer bridges.

Advantages of the technique include: (i) definable maximum lateral stresses on the abutments, (ii) reduced road surface settlement close to the abutments, (iii) reduced stressing at the junctions of deck beams and abutments by eliminating hogging bending moments caused by repeated expansion/contraction of the deck, and (iv) an extension of integral form to long bridges.

• Abstract

• Introduction

• Behaviour of Granular Backfill and Previous Results

• Requirements of the DC Us

• Results from new tests, with and without DCUs

• Discussion and Conclusions

• Acknowledgement

• Reference