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7.
DATA REDUCTION
Readings in position E on
GEOKON
microstrain based on the theoretical equation:
2
–3
µε
= 0.391 (f
x 10
)
theory
EQUATION 5: Theoretical Microstrain
Where με is the strain in the wire in microstrain and f is the resonant frequency
of the vibrating wire.
7.1 CONVERSION OF THE READINGS TO STRAIN CHANGES
In practice, the method of wire clamping effectively shortens the vibrating wire
slightly, causing it to over-register the strain. This effect is removed by applying
the batch gauge factor (B) from the calibration report supplied with the gauges.
µε
= (R
– R
)B
apparent
1
0
EQUATION 6: Strain Calculation
Where R
is the initial reading on position E and R
0
– R
Note: When (R
) is positive, the strain is tensile.
1
0
The value obtained from the above equation is required for computing stresses
in equations steps two through four in Appendix B. The stresses thus computed
are the total of those caused by both construction activity and by any
temperature change that may have occurred.
7.2 CONVERTING STRAINS TO STRESSES
Strain gauges measure strain or deformation of the structure, however, the
designer is usually more interested in the structural loads or stresses. This
requires a conversion from the measured strains to computed stresses.
Stresses are computed by multiplying the measured strain by the Young's
modulus for steel, which varies between 190 to 206 Gpa, (28 to 30 x 106 psi).
Loads are computed by multiplying the stress by the cross-sectional area of the
steel member.
Strain changes are computed from strain gauge readings taken at various times,
and by comparison with some initial readings taken at time zero. This initial
reading is best taken when the structural member is under no load, i.e., the
gauges should be mounted while the member is still in the steel yard or
warehouse.
MODEL 4100/4150 SERIES VIBRATING WIRE STRAIN GAUGES | DATA REDUCTION | 23
's readout boxes are displayed directly in
is a subsequent reading.
1
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