The thermal expansion for a model is based off the temperature difference between the Load Case temp and the ambient temp. Therefore, if ambient was 70F and the Load Case temp was 100F, the thermal expansion (and thus thermal load) would be based of 30F difference.
The thermal expansion in code compliance is based on the moment calculations of the corresponding load case (e.g. Amb to T1). These moment values are provided in the Forces and Moments report and represent the difference between operating and ambient temperature. If a separate combination exists that compares two operating load cases (e.g. T1 to T2), the temperature difference shall be between T1 and T2.
The axial force in code compliance is taken from the axial force evaluated during analysis of the model.
AutoPIPE thermal expansion is calculated using the equation found in the link here.
Loads and Load Sets - AutoPIPE