Campbell Scientific ST350 Strain Transducer for Structural Testing - Manual (Page 40)

Appendix B. ST350 Accuracy Verification
differences in load (truck) placement. Thus, one can expect that every field test
can have an error of two micro-strain. This, of course, is insignificant for
quantifying the behavior of a large civil structure.
B.1.4 Temperature Effects
The ST350 Strain Transducers have been designed for recording Live Load
strains only. Hence it is assumed that there will be little to no temperature
change during any short time-span testing sequence. For example, most
highway bridge tests (a truck passage at crawl speed) can be completed in less
than one minute, usually not enough time for ambient air temperatures to
change significantly. If the sensor is to be mounted on the structure for a long
period of time, it will need to have its "zero" reset periodically as it drifts
around with temperature changes. The primary reason that these sensors drift
with temperature (even a steel transducer on a steel structural member) is due
to large difference in thermal inertias. Because of the relatively small mass of
the transducer compared to a typical structural member, the rate of temperature
change and therefore thermal expansion of the transducer is much greater.
When a transducer is attached to a structure, it is forced to have the same
deformation as the structure. However, if a temperature increase (or decrease)
occurs, and since the ends of the sensor are "anchored", the transducer will
expand between the end blocks and register compression. The same goes for a
drop in temperature which will register tension. It is very difficult to separate
the temperature effects on the gage from the actual temperature-induced
strains, particularly on statically indeterminate structures.
If the transducers are exposed to direct sunlight during live-load tests, such as
on truss members or on top of a concrete slab, significant temperature drift can
be experienced during short periods of time due to changing cloud cover.
Covering the gages with rags or packing material can usually reduce or
eliminate this problem.
B.1.5 Specimen Type and Size
Often, the first verification test to be performed is either on a bending beam or
compression/tension specimen in some kind of laboratory testing machine,
with the results compared to the output of a foil strain gage or the theoretical
strain value. Some of the items to consider during such tests are listed below.
B.1.6 Items for Consideration
1) Remember that these sensors are designed to measure "axial strain".
Flexural bending on structural members can be determined via axial strain
measurements as long as the applied curvature is relatively small such that
the small angle theory is applicable (SIN
θ = θ). This means that if
bending stresses are to be measured, it is best to use a beam with a
minimum depth of approximately 12" or more, since the transducer will
actually be offset from the beam surface slightly due to the thickness of the
mounting tabs. However, with the beam depth of 12" or more, this
difference is minimal. Another thing to watch out for during a beam
bending test; is that it is very difficult to apply the load to the beam
without inducing some kind of torsion or lateral bending. This occurs
because the beam was not perfectly "straight" or because the end
conditions are not perfectly level with one another. To minimize this, the
transducers should be mounted with the tab/adhesive technique to the
center of the flanges, rather than with C-Clamps on the edge of the flanges.
B-2