For flexure design of cap at each location, the program considers the maximum positive and negative moments in design. The required steel area for each section is computed based on the maximum positive and negative moments. This required steel is then checked for minimum steel criteria of Art. 5.7.3.3.2. According to LRFD Art. 5.7.3.3.2, the section is to be designed for at least 1.2 Mcr and 1.33 Mu.

Further, the temperature and shrinkage criteria of Art. 5.10.8 should also be considered. Substructure computes temperature and shrinkage steel as per LRFD Art. 5.10.8.2 even when sections are deeper than 48 inches. The program first checks if the section is adequate for the applied moment from the critical combination. It then checks if the design moment is at least equal to the minimum moment. This has to be at least equal to the smaller of 1.33 Mu or 1.2 Mcr.

You can manually input the modulus of rupture, fr, value in A/D parameters under Analysis tab. By default, the program uses $7.5\sqrt{{f\text{'}}_c}$ for classic approach and $0.37\sqrt{{f\text{'}}_c}$ when phi is selected as per LRFD. If needed, the revised steel is calculated. Then the program checks for temperate and shrinkage steel. When the classic phi approach is used, shrinkage steel is equal to 0.11×Ag/Fy. However, the code specifies to distribute this in two faces. The program internally checks separately for top and bottom steel. It checks if the top and bottom steel is at least equal to 0.5×(0.11×Ag/Fy) individually. When phi as per LRFD 2006 is used, program uses the revised criteria of A_s ≥ 1.30bh / 2 (b+h) fy as specified in LRFD 2006 interims. In certain cases, it is possible that there is no moment in a section. In such a case, zero moment might be reported for combination number 0. However, temperature and shrinkage steel would still be required. Therefore, the program in LRFD mode will show required steel based on temperature and shrinkage.

For capacity calculation, program first computes effective steel area. This effective area is obtained after adjusting the gross steel area and taking development length into account. The beam cross section capacity (ϕ Mn) at every location is then computed using strain compatibility with consideration of concrete material, and any available top and bottom bars.

The calculation of ϕ for flexure design for cap and strut in AASHTO LRFD specification is done in two possible ways. You can choose to do the design as per classic approach (LRFD 2005 interims and earlier) or as per LRFD 2006 approach. This choice can be made on the Analysis / Design Parameters (A/D) screen on Analysis tab. Please see the section Resistance Factor/Strength Reduction Factor ϕ for more information.

When the classic design (LRFD 2005 interims and earlier) approach is selected, the maximum reinforcement ratio as per LRFD 5.7.3.3.1 is checked. If the c/d value for a section is greater than 0.42, section is said to be over-reinforced and is flagged.