Cap Design

Substructure is capable to carry out design of cap, column and footing as per CHBDC specification . When service groups are included along with strength groups, it carries out service checks for cap and footing.

When there is at least one Seismic load group selected, program carries out the non-seismic design of cap, column and footing for strength combinations and then also checks these members for seismic combinations. However, if no Seismic load group/limit state is included, program only carries out the non-seismic design. For more details on seismic design, please see the section titled , Plastic Hinging and Seismic Design.

For pier cap and strut designs, Substructure uses the envelopes for ultimate strength, service and fatigue, and the selected design code. For flexure design, the program considers the maximum positive and negative moments. The required steel area for each section is computed based on the maximum positive and negative moments. The effective steel area is obtained after adjusting the gross steel area taking into account development length. The calculated beam capacity (*Mn) is taken from the consideration of concrete material, and any available top and bottom bars.

Required steel as shown in the report is based on the actual moment and the minimum steel criteria for the appropriate code.

In CHBDC, this is based on Art. 8.8.4.3, which states that the minimum section capacity should be the smaller of 1.2 Mcr and 1.33 Mu. 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. If not, at least that capacity must be developed. The program internally checks separately for top and bottom steel. 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. Using the auto design feature, all sections are considered. The steel requirement at each section is determine first by taking the section as singly reinforced. However, if the section cannot be design as a single reinforced section, Substructure computes both tension and compression reinforcements requirements, resulting in a double reinforced design.

Geometrically, the cap beam is divided into spans that are considered to extend from each column centerline to next column centerline with supports at the column centerline. Cantilevers, if present, are considered separate spans. The reinforcement provided over each support is selected when it is at least equal to the required reinforcement in 1/4 of the span on each side of the support. The bars are extended beyond the 1/4 points in accordance with the development length, so the reinforcement is fully developed at 1/4 point. For moment in the middle section of the beam, the provided reinforcement is calculated as the largest requirement for the middle half section of the beam (from 1/4 to 3/4 in each span). The bars are then extended outwards to a length equal to the development length. If the span length is less than 11 ft (3.4 m), Substructure provides continuous reinforcing bars and does not compute separate detailing for reinforcement over the column and midspan.

The calculated area of steel is transformed into required rebar quantity, using the selected rebar sizes. When calculating the number of reinforcing bars, clear spacing of at least 1.5 times the bar diameter or 1.5 inch and specified side clear cover is maintained. If you are using the auto design feature, up to three layers of reinforcement for top and bottom can be created. If a section requires more than three layers, you must manually input the reinforcement, as follows.

To manually input a design (design check), input the appropriate reinforcing pattern and Substructure will compute the required reinforcement for all sections and the section capacity with provided reinforcement. Substructure uses the development length to compute the effective steel at each section at top and bottom and then calculates the capacity based on both top and bottom effective reinforcement.

Program can do the design of cap either for cap moments at centerline of columns or at face of support. For round columns, Substructure converts the round column section to the equivalent square section. The face of the equivalent square will become the critical section. For rectangular chamfered, rectangular filleted, hexagonal, and octagonal columns Substructure determines the size of an equivalent rectangular section maintaining the overall section aspect ratio. When the face of support option is chosen, program ignores moments at all the check points between faces of supports including the centerline of the column values. Such designs mostly result in fewer number of bars at supports.