Brittle Behavior of Precast Segmental Bridges?

brittle behavior of precast segmental bridges

Continuous longitudinal reinforcement contributes to the flexural capacity of bridges cast in-place on falsework or with form travelers or movable scaffolding systems. In precast segmental bridges, reinforcement is interrupted at the joints and the flexural capacity of the cross-sections is therefore lower.

As thoroughly discussed in Short-Line Match Casting of Precast Segmental Bridges (42 pages), cast-in-place bridges typically feature tension-controlled flexural failure. According to AASHTO-LRFD, therefore, the resistance factor to be applied to the nominal flexural strength is 1.0 and the factored design strength corresponds to the full flexural capacity. The resistance factors of precast segmental bridges are smaller: 0.90 for epoxy joints, and 0.75 for compression-controlled sections.

The combined loss in factored design strength (less flexural capacity, and smaller resistance factors) is significant, and in the absence of continuous longitudinal reinforcement, increasing post-tensioning and/or the strength of concrete are the only remedies. This may lead to brittle deck behavior.

The epoxy joints are weak points in precast segmental bridges, and brittle compression-controlled failure starts at the joints. The photograph shows the recent brittle failure of the Skytrain Bridge in Sydney.

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