For AASHTO Standard load factored design and for AASHTO LRFD classic approach for φ calculation, user specified values of φ are used in computing the capacity of the section from nominal strength. User may change these φ values on the A/D parameters dialog if needed. For cap and footing flexure design, the φ values for flexure are used. For column biaxial capacity calculations, values specified for axial compression are used along with flexure φ. Program allows two values, one for tied columns and the other for spirally reinforced columns. Depending upon the column transverse steel, it uses the appropriate value of φ. However, if the axial force in the column is less than 0.1 f'c Ag, program uses an interpolated value between the φ for axial compression and that for flexure. When the axial force is zero, program uses the flexure φ. For shear and torsion, program uses the φ value as specified for shear.

Resistance factor calculations per AASHTO LRFD comply with Art. 5.5.4.2 of the latest version of LRFD. The following is based on the old version of LRFD only for users' information. As per AASHTO LRFD 2006 interims approach, program computes the resistance factors φ using Art. 5.5.4.2. As per this approach, φ for a section in bending and compression should be based on the net tensile strain εt. The sections should be classified as either of Tension Controlled, In Transition or Compression Controlled. For this classification, LRFD suggests to use the net tensile strain εt in the extreme tension steel (farthest from the most compressed fiber). Depending upon the net tensile strain εt, in this tension steel, the section should be classified. ACI 318-05 also suggests similar approach for section classification. However, that code also provides the approach that this classification can be done based on c/dt ratio. Where c is the depth of the neutral axis and dt is the distance from extreme compression fiber to the center of the farthest tensile steel. As per ACI 318-05, if the c/dt is less than 0.375, the section is classified as Tension Controlled and the φ for tension controlled sections shall be applied to determine the section capacity in φMn. However, if the c/dt is greater than 0.60, the section is classified as Compression Controlled and the φfor compression controlled section shall be applied. In case the c/dt is in-between 0.375 and 0.60, program shall classify this as In-Transition and will use the linear variation to compute the φ for that specific case. The c/dt = 0.375 corresponds to net tensile strain εt = 0.005 and c/dt = 0.60 corresponds to the net tensile strain εt = 0.002. Substructure program uses the computed c/dt to classify sections and determine φ for each section. It provides even more flexibility and lets user specify these controlling c/dt values at which the φ will change along with user specified values of φ at those locations. When user values are specified, program will use those for classification and φ calculation. The interpolation equation applies to code specified values with code specified net tensile values. If customized values are specified, program will internally compute and use equation depending upon user input. Program will report the classification of the section in output also.