Capturing data makes bridges last longer

Minho University is undertaking a long-term project to improve the durability of Portuguese bridges by using HBM’s equipment to monitor permanently fitted sensors. The information will be used to prepare long term monitoring and maintenance plans and will be useful for modifying the design of future bridges.

The 120 m long bridge is on a gentle curve of the highway as it crosses the Ave River on the A11 between the cities of Braga and Guimarães in the hilly northern Portuguese region of Minho. The recently opened bridge is constructed from pre-stressed, pre-cast, high performance concrete girders. The cast-in-place, concrete deck slab is reinforced with mild steel. Eight V piers, made of ordinary concrete, support the bridge.

Notes Professor Paulo Jorge de Sousa Cruz, Director of Minho University’s Civil Engineering Department: “The bridge over Ave River is probably the most innovative bridge in Portugal constructed using this method. This is because of the pre-stress construction system and the complexity of the construction sequence”. The bridge has one of the longest spans constructed from pre-cast girders in Portugal.

Using pre-cast girders is quite common in Portugal and there are many other bridges, underpasses and overpasses made with this method. To help prolong their service life it was decided to monitor the Ave River bridge during construction and while in service to evaluate its durability.

Minho University gathers the data needed for monitoring the bridge using HBM’s MGCplus fitted with ML801 cards and catman® software. A total of 256 sensors are installed in the bridge comprising 74 full-bridge strain gages on the reinforcement, 42 embedded strain gages in the concrete, and 88 temperature sensors. An additional 52 corrosion sensors are checked separately.

The bridge is fitted with five access boxes holding the cable ends terminating in connectors that can be easily plugged into the portable MGCplus at any time. Notes Professor Paulo Jorge de Sousa Cruz: “The data is used principally to determine deformations of the structure under imposed loads. It is also possible to examine the sectional load distribution and how these are affected both by time and temperature”.

HBM’s equipment was also used during the initial construction to monitor the bridge’s behaviour by taking measurements during the proof load test before entering service. These tests were related to the program of durability monitoring and were not needed to verify the structural solution.

Deflection measurements taken during the proof load test on the bridge showed absolute deflections of between 0.5 mm and 5 mm with the maximum deflection recorded under a load of about 112 tonnes. Only a few readings have been taken so far as the structure is new and future readings will be taken at periodic intervals during the structure’s life.

Comments Professor Paulo Jorge de Sousa Cruz: “We chose HBM’s equipment because it is highly versatile and allows users to select the correct cards needed for different types of sensors. The electrical connections between the sensors and the cards are quite easy to prepare and the catman® software is a consistent way of measuring. Most important of all, this equipment guarantees proper measuring. In practical terms I think that the ease with which the sensors can be connected to the MGCplus and the catman® software makes our work much easier”.