COMPRESS performs a preliminary calculation to determine the size of the anchor bolts. This calculation is reported under the heading "Anchor bolt load (operating, corroded + Weight)" and reports a calculated value for bolt load "P".

The preliminary calculation is based on the assumption that the neutral axis of the resisting section is at the center of the base ring. However, the neutral axis actually shifts from the center of the base ring due to the different modulus of elasticity of the steel bolts and the concrete foundation.

COMPRESS reports additional calculations to determine the resisting section's neutral axis location and the resultant tension and compression forces. The actual maximum steel stress in the bolts is calculated based on the shifted neutral axis.

The actual maximum bolt force F is determined as the product of the calculated maximum steel stress and the bolt area. The uncorroded bolt area is applied because this is conservative (the bolt force when bolt is corroded would be less).

This force is applied to the analysis of the compression ring (anchor bolt chair top plate), gusset plate, and skirt thickness check.

If the anchor bolt is preloaded, the force applied to the skirt thickness check will not include the preload because this force is directly supported through the compression ring and gusset plate to the concrete foundation.