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, CFemp microstrain/degree. This empirical correction factor can 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 = [(R1 – R0)B + (T1 – T0)CFcmp] x E
Equation 12: External Load Stress Only
Note that the correction factor, CFemp may change with time and with 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 R1 would equal R0 and the thermal strains in the steel are given by the following equation:
µεthermal = (T1 – T0) x CF1
Equation 13: Free Field Thermal Strains
Where CF1 is the coefficient of expansion of steel = +12.2 microstrains/ C.
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:
µεactual = (R1 – R0) x B + (T1 – T0) x CF1
CF1 is the coefficient of expansion of steel = 12.2 microstrains / °C. When the ends of the structural member are perfectly restrained then (R1 – R0)B the compressive strain induced by temperature change alone would be exactly canceled by (T1 – T0) x CF1, the expansive strain and µεactual would be zero.