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Behaviour of prestressed continuous beams

Lopes S., Harrop J.

Archives of Civil Engineering

1998

Vol. 44, nr 1

89-105

In previous research works on redistribution of moments, beams have generally been constructed in one piece and transported to the test rigs and then placed on the supports. Great precautions have to be taken when leveling the supports because small deviations from the correct position can introduce large values of unwanted moments corresponding to support settlements. Ideally, at least one support should have a measuring device to measure forces and a threaded device should also be provided to adjust the height of the support as the test progresses. The new technique described in this paper consists of joining two separate precast beams over the supports and maintaining the contact surfaces beams/supports undisturbed even when a top flange is cast in situ. This procedure minimises possible inaccuracies caused transportation and placement of the beams and is closer to the conditions of beams in practice. The paper also describes main results obtained from prestressed composite concrete two-span beams and its implication on codes practice.These codes normally allow an elastic analysis of the structure followed by an adjustment on the diagram of bending moments at the ultimate limit state. In the case of a continuous beam the adjustment is obtained by multiplying the bending moments over the supports by a coefficient, delta, and modifying the whole diagram of bending moments to maintain the static equilibrium of the structure.