Table of Contents
Advertisement
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 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 reason for this is because the member is restrained from expansion but the
vibrating wire is not. An increase in temperature will produce expansion in the
vibrating wire, which in turn will cause a reduction in wire tension. This results in
a decrease in the vibrational frequency. The magnitude of this temperature-
induced, compressive strain increase would be measured accurately by the
strain gauge, and can be determined using Equation 5 on page 16.
These temperature-induced stresses can be separated from any external, load-
induced stresses by reading both the strain and temperature of the gauge at
frequent intervals. These readings should take place during a period when the
external loading from construction activity remains unchanging. 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
T
temperature-induced stress, as shown by the following equation:
σ
temperature induced
EQUATION 10: Temperature-Induced Stress
This can be subtracted, if desired, from the combined load related stress change
using the following equation:
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
the following equation:
σ
= [(R
external load
1
EQUATION 12: External Load Stress
Note that the correction factor (K
activity as the rigidity of the restraint may change. In such a case, it would be
advisable to calculate a new temperature correction factor by repeating the
above procedure.
If, for whatever reason, the actual strain of the concrete member is required
(e.g., the change of unit length that would be measured by a dial gauge
attached to the surface), this is given by the equation:
με
=(R
–R
)B + (T
actual
1
0
EQUATION 13: Actual Strain
Where C
represents the coefficient of expansion of steel = 12.2 microstrain/C.
1
This equation may seem less than intuitive and therefore requires some
explanation. As an example, assume first that the strain gauge is inside a
concrete slab that is perfectly restrained at its ends. If the temperature rises by
one C, then the vibrating wire undergoes an expansion of 12.2 microstrains and
MODEL 4200 SERIES STRAIN GAUGES | MEASUREMENT AND CORRECTION OF TEMPERATURE EFFECTS | 29
= K
(T
–T
)E
T
1
0
= [(R
–R
)B + (T
–T
1
0
1
0
–R
)B + (T
–T
) (C
–C
) – K
0
1
0
1
2
T
) may change with time and with construction
T
–T
)C
1
0
1
) (C
–C
)]E
1
2
(T
–T
)]E
1
0
Advertisement
Table of Contents
