Campbell Scientific ST350 Strain Transducer for Structural Testing - Manual (Page 33)

Appendix A. Special Instructions for
using ST350
A.1 Instructions for Using ST350 Strain
Transducer Extensions on Reinforced
Concrete Structures
Special gage-lengthening extensions have been designed for use with the
ST350 Strain Transducers in order to measure surface strains on reinforced
concrete (R/C) structures. The aluminum extensions simply increase the
transducer gage length to allow an “averaged” strain value to be recorded in the
presence of cracks associated with most R/C structures. These units make
available seven additional gage lengths, each one an integer multiple of the
original 3-inch (76.2mm) transducer gage length.
There are three items to consider when selecting an appropriate gage length for
a particular R/C member. The first is that it must be long enough to minimize
the effects of flexural cracks. There are several factors that control crack
formation in concrete, primarily the beam depth, steel ratio, concrete strength,
and bond strength. While there are no precise methods for determining a
minimum crack spacing, it has been determined experimentally that a gage
length equal to the member depth (d) is satisfactory for slabs and rectangular
beams and 1.5 times d is suitable for T-beams. The second item to consider is
that the gage length be short enough that the measured strains are not
significantly affected by moment gradients. An upper limit of 1/20
th
the span
length (L) will usually maintain the gradient below 5%. In general, it is desired
to obtain as long a gage length as possible without exceeding the upper bound.
The following table provides the recommended lower and upper gage length
limits for R/C members.
TABLE A-1. Recommended Lower and Upper Gage Limits
Member Type Lower Limit Upper Limit
Slabs and Rectangular Beams 1.0 x d L / 20
T-Beams 1.5 x d L / 20
The third item is the available strain range of the transducer. As the gage
length is progressively increased, the force on the transducer imposed by the
extension is increased as well for a given amount of strain. This has the effect
of reducing the available strain range for the transducer/extension assembly.
The upper limit of the strain range recommended for aluminum transducers is
approximately
±4000 με. However, to minimize the force in the system and to
avoid the mounting tabs from popping off the concrete members during
loading, BDI recommends keeping the maximum strain in the transducer to
about 1,000
με. Therefore, the following table has been developed to indicate
the maximum strain ranges for each available gage length. Higher strains can
of course be measured. However, special attention should be paid to the gain
settings on the data acquisition equipment being used. If the load is going to be
A-1