Bramke Bridge: Pier Head Deflection Monitored During Incremental Launching

Construction method

A bridge that moves across the valley segment by segment

The Bramke Bridge is being built by incremental launching. Behind the abutment there is a fixed casting yard. There the superstructure is cast in sections, each new section cast against the previous one. Once a section is complete, hydraulic launching jacks push the entire superstructure one segment length further towards the valley. Then the next segment begins.

The superstructure slides over sliding bearings that sit on every pier head. At its tip it carries a steel launching nose. The nose is lighter than the concrete section and reduces the cantilever moment while the tip projects freely across the span to the next pier. This way the bridge grows across the valley without any falsework having to stand there.

Challenge

What happens to the piers as the superstructure advances

During launching, horizontal forces act on every pier head. The most important is the friction in the sliding bearings: the superstructure slides forward and drags the pier head a little way with it. Depending on the longitudinal gradient of the bridge, a share from the slope is added. The pier is fixed at the foundation and behaves like a cantilever. Its head is deflected in the launching direction, noticeably so on tall, slender piers. This pier head deflection, also referred to as pier top displacement, is what the monitoring tracks.

This deflection is accounted for in the design. It becomes critical when it turns out larger than assumed: because a sliding bearing starts to stick, friction is higher than expected or the superstructure moves off with a jolt. The stress in the pier then rises, above all at the fixed pier base, and in the worst case up to cracking.

On top of this, the situation changes with every segment. The superstructure gets longer, the tip reaches the next pier, the loads are redistributed. A pier that was unremarkable during the last launch can reach its limit during the next. And that is decided in the seconds while the jacks are pushing.

Solution

One live tilt sensor per pier, monitored every second

For this incremental launching monitoring, Infrasolute installed a live tilt sensor on every pier head. It measures the tilt of the pier head, and its change gives the deflection. All sensors are connected to the Infrasolute live gateway. The gateway transmits the readings to the data platform every second, throughout the entire launch.

On the platform, every value is checked immediately against the defined limits, graded into warning and alarm thresholds. If a limit is exceeded, the system raises an alarm automatically and without delay. The information is therefore available on site while the launch is still under way, not only at the next survey.

Result

Every segment launched with data rather than by eye

Throughout the incremental launching, the deflection of every pier was visible at every second. The site team could accompany each launch with readings instead of relying on visual checks and surveys between segments. At the same time, a complete record was created for every segment of how each pier behaved.

The project shows how tilt monitoring works in bridge construction as early as the construction phase. The sensor technology: the Infrasolute tilt sensor. How limits and the alarm chain are set up is described under Monitoring & Alerting; how the readings are evaluated, on the Infrasolute platform. For the bearings in later operation there is bridge bearing monitoring, and for the structure as a whole structural health monitoring.

Are you building by incremental launching, or do you have another construction phase in which piers, bearings or the superstructure need to be watched? Tell us about your project via the project inquiry.