MNL 116-21 Addendum – Thermal Corrections during Stressing
Remove the existing Standard and associated Commentary for Sections 5.3.10.5 and 5.3.11.5 and replace with the following:
5.3.10 Elongation Calculation and Corrections
5. Thermal effects. For abutment anchorage setups where strands are anchored to abutments that are independent from the form/mold, thermal adjustments are required if the temperature of the strand at the time of tensioning differs by more than 25°F (15°C) from the estimated fresh concrete temperature at the time of casting. Consideration shall be given to partial bed length usage and adjustments made when the net effect on the length of bed used exceeds the allowable force in the strand. The thermal coefficient of expansion of steel shall be taken as 6.5 × 10-6/°F (12 × 10-6/°C).
C5.3.10 Elongation Calculation and Corrections
5. Thermal effects. This item is important for abutment beds as the abutments are not affected by the temperature rise of the strand from the temperature at the time of tensioning on a cold morning to the concrete temperature at the time of placement in the forms/molds at midday or in the afternoon. The actions would be reversed for strand tensioned at elevated temperatures with cooler concrete cast around it.
Because tensioned strands are held at a fixed length, variation between ambient temperature at the time of tensioning and concrete temperature at the time of placing results in changes of stress. Lowering of strand temperature increases force, while a temperature rise results in force loss. For strands tensioned to approximately 70% to 75% of the strand ultimate tensile strength, a temperature variation in the strand of 10°F (5°C) will result in a variation of 1%. Allowance should be made in the tensioning by understressing or overstressing at the rate of 1% for 10°F (5°C) of anticipated temperature variation, depending respectively on whether a reduction or rise of temperature is anticipated. When calculating adjustments using partial bed length usage setups, the length of the strand affected by the temperature differential should be considered.
Most self-stressing beds are not affected by this phenomenon because the bed itself undergoes a similar change. As the strand is warmed and expands, the bed does as well. Because the strand is anchored to the bed, the force in the strand is relative to the bed length. As the strand is expanding and trying to relax, the bed is expanding and holding the strand at its desired value. There are some self-stressing beds that are independent of the forms/molds. For these setups, the self-stressing bed and strand undergo similar changes as the ambient temperature changes between the time when the strands were tensioned and when concrete is placed. However, when the concrete is placed in the form/mold, the temperature of the concrete will affect the strand but not the self-stressing bed that is independent of the form/mold.
Sample tensioning calculations with elongation corrections due to thermal effects are shown in Appendix F.
5.3.11 Force or Gauge Corrections
5. Thermal effects. For abutment anchorage setups where strands are anchored to abutments that are independent from the form/mold, thermal adjustments are required if the temperature of the strand at the time of tensioning differs by more than 25°F (15°C) from the estimated fresh concrete temperature at the time of casting. Consideration shall be given to partial bed length usage and adjustments made when the net effect on the length of bed used exceeds the allowable. The thermal coefficient of expansion of steel shall be taken as 6.5 × 10-6/°F (12 × 10-6/°C).
C5.3.11 Force or Gauge Corrections
5. Thermal effects. This item is important for abutment beds as the abutments are not affected by the temperature rise of the strand from the temperature at the time of tensioning on a cold morning to the concrete temperature at the time of placement in the forms/molds at midday or in the afternoon. The actions would be reversed for strand tensioned at elevated temperatures with cooler concrete cast around it.
Because tensioned strands are held at a fixed length, variation between ambient temperature at the time of tensioning and concrete temperature at the time of placing results in changes of stress. Lowering of strand temperature increases force, while a temperature rise results in force loss. For strands tensioned to approximately 70% to 75% of the strand ultimate tensile strength, a temperature variation in the strand of 10°F (5°C) will result in a variation of 1%. Allowance should be made in the tensioning by understressing or overstressing at the rate of 1% for each 10°F (5°C) of anticipated temperature variation, depending respectively on whether a reduction or rise of temperature is anticipated. When calculating adjustments using partial bed length usage setups, the length of the strand affected by the temperature differential should be considered.
Most self-stressing beds are not affected by this phenomenon because the bed itself undergoes a similar change. As the strand is warmed and expands, the bed does as well. Because the strand is anchored to the bed, the force in the strand is relative to the bed length. As the strand is expanding and trying to relax, the bed is expanding and holding the strand at its desired value. There are some self-stressing beds that are independent of the forms/molds. For these setups, the self-stressing bed and strand undergo similar changes as the ambient temperature changes between the time when the strands were tensioned and when concrete is placed. However, when the concrete is placed in the form/mold, the temperature of the concrete will affect the strand but not the self-stressing bed that is independent of the form/mold.
Sample tensioning calculations with force corrections due to thermal effects are shown in Appendix F.
Approved 08-31-2023