Modular Expansion Joint for Bridges with Differential Settlement

Differential settlement between adjacent bridge spans creates additional movements at the expansion joint that must be accommodated in addition to the thermal and live load movements. The joint must be designed for the worst-case combination of all movements. Differential settlement occurs when the foundations on either side of the joint settle by different amounts. This can be caused by different soil conditions, different foundation types, or different loading histories. For bridges on soft soil, differential settlement of 20-50 mm is not uncommon over the design life. Vertical movement accommodation in modular joints is provided by the flexibility of the support bars and the EPDM seals. The support bars can deflect vertically to accommodate differential settlement, while the seals maintain waterproofing. The maximum allowable vertical movement depends on the joint design and the seal geometry. Horizontal movement from differential settlement occurs when the settlement causes the two bridge spans to rotate relative to each other. This rotation creates a horizontal movement at the joint that adds to the thermal movement. The joint must be designed for the vector sum of the thermal and settlement-induced horizontal movements. Monitoring of differential settlement is recommended for bridges on soft soil. Settlement monitoring points on the bridge deck near the joint allow the settlement to be tracked over time. If the settlement exceeds the design value, the joint gap may need to be adjusted to prevent the joint from closing or opening beyond its design range. Foundation improvement measures can reduce differential settlement and the resulting joint movements. Ground improvement techniques such as deep soil mixing, stone columns, or preloading can reduce the compressibility of the foundation soil. Pile foundations that extend to a competent bearing stratum can eliminate differential settlement entirely.