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APPENDIX E.
TEMPERATURE EFFECTS
If the ends of the structural member are free to expand or contract without
restraint, strain changes can occur without any change in the stress reading.
However, if the ends of a steel structural member are restrained by some semi-
rigid medium, then any increase in temperature of the structural member will
result in a buildup of compressive load related strain in the member, even
though the actual strain would be tensile.
The strain gauge would accurately measure the magnitude of this temperature
induced, compressive stress increase because the vibrating wire is not
restrained from expansion, even though the member is restrained. Expansion
would be indicated on the readout box by a decrease in the strain reading equal
to the temperature-induced increase in compressive stress in the member.
These temperature-induced stresses can be separated from any external load-
induced stresses by reading both the strain and the temperature of the gauge at
frequent intervals. Take these readings during a period when the external
loading from construction activity remains constant. When these strain changes
are plotted against the corresponding temperature changes, the resulting graph
shows a straight-line relationship, the slope of which yields an empirical
correction factor, CF
microstrain/degree. This empirical correction factor can
emp
be applied to the total strain and temperature data to remove the temperature-
induced stresses leaving only those stresses produced by changing external
loads, i.e.
External Load Stress = [(R
– R
1
0
EQUATION 12: External Load Stress Only
Note that the correction factor, CF
construction activity, as the rigidity of the restraint may change. It would then be
a good idea to repeat the above procedure in order to calculate a new
temperature correction factor.
In a free field, where no loads are acting and the steel is free to expand or
contract without restraint, then R
steel are given by the following equation:
µε
= (T
– T
) x CF
thermal
1
0
1
EQUATION 13: Free Field Thermal Strains
Where CF
is the coefficient of expansion of steel = +12.2 microstrains/ C.
1
If, for whatever reason, the actual strain of the steel member is required, (i.e.,
the change of unit length that would be measured by a dial gauge attached to
the surface) you can arrive at this using this equation:
µε
= (R
– R
) x B + (T
– T
actual
1
0
1
EQUATION 14: Actual Strain
is the coefficient of expansion of steel = 12.2 microstrains / C. When the
CF
1
ends of the structural member are perfectly restrained then (R
compressive strain induced by temperature change alone would be exactly
canceled by (T
– T
) x CF
, the expansive strain and µε
1
0
1
MODEL 4100/4150 SERIES VIBRATING WIRE STRAIN GAUGES | TEMPERATURE EFFECTS | 31
)B + (T
– T
)CF
] x E
1
0
cmp
may change with time and with
emp
would equal R
and the thermal strains in the
1
0
) x CF
0
1
actual
– R
)B the
1
0
would be zero.
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