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APPENDIX G. MEASUREMENT AND CORRECTION OF
TEMPERATURE EFFECTS
If the ends of the structural member are free to expand or contract without restraint, strain
changes can take place without any change in stress. On the other hand, if the ends of the
structural member are restrained by some semirigid 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.
Because the member is restrained from expansion but the vibrating wire is not, the thermal
expansion of the vibrating wire will cause a reduction in wire tension, which results in a decrease
in the vibrational frequency. The magnitude of this temperature induced, compressive strain
increase, would be measured accurately by the strain gage, and can be given by Equation 5 in
Section 6.5.
These temperature induced stresses can be separated from any external load induced stresses by
reading both the strain and temperature of the gage at frequent intervals. These readings should
take place during a period of time when the external loading from construction activity remains
constant. When these strain changes are plotted against the corresponding temperature changes,
the resulting graph will show a straight-line relationship, the slope of which yields a factor K
microstrain/degree. This factor can be used to calculate the temperature induced stress, as shown
by Equation 10.
This, if desired, can be subtracted from the combined load related stress change using Equation
11.
σ
combined temp and load related
Equation 11 - Combined Temperature and Load Related Stress
To give that part of the stress change due to construction activity loads only use Equation 12.
σ
external load
Note that the correction factor (K
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.
σ
temperature induced
Equation 10 - Temperature Induced Stress
= [(R
= [(R
-R
)B + (T
–T
1
0
1
Equation 12 - External Load Stress
) may change with time and with construction activity as the
T
= K
(T
-T
)E
T
1
0
-R
)B + (T
–T
) (C
1
0
1
0
) (C
–C
) - K
(T
0
1
2
T
T
–C
)]E
1
2
-T
)]E
1
0
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