Two types of sensors were used during short-term structural monitoring sessions performed by graduate students in 2015: BDI ST350 strain transducers and BDI A1521 accelerometers. The ST350 strain transducer has a 3-inch gauge length and a calibrated range of ±2,000 με. The transducer contains four 350Ω foil gauges to form a full wheatstone bridge circuit housed in an aluminum casing. The casing has mounting holes for use with mounting tabs that are adhered to the structure. For the temporary deployments the sensors were clamped rather than adhered so the metallized coating on the bridge would not be disturbed. The clamped mounting points tended to creep over time but gave accurate enough data over short intervals to make design decisions for the permanent monitoring system. Another drawback of clamping is the inability to attach gauges to the webs of the truss members, so only flange strains could be measured. The A1521 accelerometers used during these deployments consist of a nitrogen gas damped micro-machined capacitive sense element contained in an aluminum housing with a mounting hole for use with the same type of mounting tabs as the strain transducer. The accelerometers used for the temporary deployments on the bridge are uniaxial and have a ±2g range. As with the strain transducers, the accelerometers were clamped to flanges during temporary monitoring sessions. In addition, accelerometers were also mounted to steel plates and placed on the bridge deck, which was acceptable for temporary use because accelerations were much less than 1g.
Three types of sensors are used in the long-term structural health monitoring system installed on the bridge. In addition to BDI accelerometers similar to those used for temporary deployments, the system also uses BDI T500 tiltmeters and HPI HBWR-35-125-6-VR strain rosettes. The T500 tiltmeter uses an electrolytic tilt sensor with a ±3° range inside an aluminum housing attached to an adjustable mounting. The strain rosettes consist of 3 350Ω foil gauges with gage lengths of 0.125” bonded to a stainless steel shim at 45° angles from each other and pre-wired with vinyl-coated wire. The rosettes allow for measurement of all three components of 2-dimensional strain at each location on the web of the truss members.
This image shows the strain gauge on the right and the accelerometer on the left during a temporary deployment on the Memorial Bridge. One of the project goals is to have the gauges on the bridge for a long term deployment.
This shows two gauges secured to a bridge member using clamps.
This is an image of an accelerometer secured to a piece of metal. This type of attachment is what would be used for a long term deployment.
This is a stock picture of an accelerometer.
This shows a strain gauge stock picture.