Sister Bar Strain Gauges
Sister bar strain gauges are used to monitor strain in steel-reinforced concrete structural elements. Datasheet
Sister bar strain gauges with range optimized to measure compression midrange, and tension.
This rebar cage is instrumented with sister bar strain gauges and thermal wires for TIP testing.
The location of each sister bar is important when strains are analyzed. Note the tape measure being used.
Applications
Sister Bar applications include:
- Determining the distribution of stress in bored piles.
- Monitoring the performance of slurry walls and diaphragm walls.
- Monitoring strains in slabs and footings.
Installation
Sister bars are generally tied to axial members of the rebar cage. In typical applications, sister bars are installed in pairs on opposite sides of the axial members, so that strains due to bending moments can be separated from strains due to axial loading.
Signal cables from the sister bar are carefully routed to the top of the cage. The rebar cage is then lifted, installed into the boring or trench, and concreted.
Operation
The sister bar is a vibrating wire strain gauge built into in a three-foot length of rebar. The length of the rebar ensures good contact so that so that strains in the concrete are fully transferred to the strain gauge.
Readings are obtained with a data logger. Changes in strain are calculated by subtracting the initial reading from the current reading and multiplying by a gauge factor, which is supplied with the sister bar.
Advantages
Easy to Install: Sister bars are simply wired to the axial members of the steel reinforcement. No welding or protection is needed.
High Survival Rates: The durable rebar body and simple installation give sister bars high survival rates.
Reliable Performance: The 3-foot span of rebar ensures transfer of strains even if there are local cracks or mixed size aggregate in the concrete.
Predictable Response: Sister bars measure strain in the steel rebar, which has a controlled diameter and modulus.
Specifications
Sensor Type: Vibrating wire.
Range: 3000 microstrain, maximized for the application with strain gauges set to measure compression, tension, or at midrange.
Resolution: 0.4 microstrain.
Accuracy: ± 0.25% F.S.
Temp Sensor: 3k ohm thermistor.
Temp Sensor Accuracy: ±0.5 °C.
Rebar Diameter: #5 (16 mm).
Rebar Length: 3 feet (914 mm).
Shrink Wrap Color Coding: Blue for mid-range, yellow for compression, red for tension.
Cable: Factory attached at specified lengths. Four 22-gauge conductors in two twisted pairs, drain wire, and PU jacket.
Sister bar strain gauges installed on steel beams using welded-on tie-points.
