Auto Design: Cross Section and Composite Action

If Calculation Approach is set to "Design" in the composite setup, an optimized cross-section and degree of composite action for each cross-section group can be determined. The auto-design procedure is executed based on the Design Approach also found in the setup. A maximum height restriction can also be placed on the designed member in the setup.

Procedure

The procedure used to select the optimum member cross-section and degree of composite action is as follows:

1. The maximum internal forces and deflections for each stage are found for the current member using the cross-section and target degree of composite action specified in the model. Deflection constants, which are used to predict deflections using other cross-sections and target degrees of composite action subsequently in the auto-design process, are also calculated. These deflection constants are calculated as follows:

• For construction stage deflection:

o Deflection Constant = (Deflection at construction stage) / (Moment of Inertia of Cross-Section)

• For final stage deflections:

o Deflection Constant = (Deflection at final stage) / (Lower Bound Moment of Inertia of Cross-Section)

2. A table of cross-sections of the same type as the current member (e.g. W(Imp)) is created and sorted by weight. If a height restriction is imposed, only cross-sections with a height less than the maximum set by the user in the setup are included in this table.

3. A target degree of composite action based on the design approach is set as follows:

• For the "Optimize Beam" approach, a target degree of composite action of 100% is used.

• For the "Optimize Studs" approach, a target degree of composite action of 30% is used.

• For the "Balanced" approach, a target degree of composite action of 55% is used.

• For the "User Defined" approach, the target degree of composite action entered by the user in setup is used.

4. All checks (listed in the "Strength and Serviceability Checks") are performed on the initial current member (i.e. the one that exists in the model when the check/design is run) using the target degree of composite action. For deflection checks, if camber is set to an absolute or relative value, the value of camber specified by the user is included in the check. If camber is set to "design", then the maximum camber value set by the user is included in the deflection check and the final camber value will be designed according to the procedure described in "Camber Design" section above.

5. Optimization of Cross-Section: If the initial current member fails to pass any of the checks, the next strongest cross-section in the table (i.e. a cross-section which has a plastic section modulus higher than the current member) becomes the current member and all checks are performed on it in the same way the initial current member was checked according to Step 4 above. The program continues to iterate through stronger cross-sections until a cross-section is found which passes all checks at the target degree of composite action. If the initial current member passes all checks, the first cross-section in the table (i.e. the cross-section with the least weight) becomes the current member. If this member fails, the program iterates through stronger cross-sections until a cross-section is found which passes all checks. The end result is a cross-section which is optimized to meet the design requirements

Notes on the cross-section selection process:

• The plastic section modulus is used to determine a stronger cross-section since the bending check is the one which generally governs the strength checks
• In cases where all strength and serviceability checks pass for the cross-section being considered, but the number of studs required at the target degree of composite action does not allow minimum spacing requirements to be met, the program will attempt to lower the target degree of composite action as low as possible before iterating to the next strongest cross-section. This ensures the minimum spacing requirement is met while preventing the cross-section from becoming over-designed. This case is most common when the design approach is set to "optimize beam," which uses a target degree of composite action of 100%.
• The deflections used in the serviceability checks are predicted by multiplying the deflection constants (See Step 1 above) by the moment of inertia and lower bound moment of inertia for the construction and final stages, respectively.

6. Optimization of Degree of Composite Action: Once a cross-section has been found which meets all strength and serviceability requirements at the target degree of composite action determined by the design approach, the program then attempts to decrease the target degree composite action as low as possible using the bisection method. The minimum degree of composite action is generally taken as 25%. However, in cases where “Negative flexural strength determined using steel section alone” is set to “Yes”, and the negative moment values exceed the positive moment values, it is possible to have a lower degree of composite action. In this case, the degree of composite action is taken from the minimum spacing requirements.

7. If a new cross-section and degree of composite action can be found which meet design requirements, they can be used to update the model. If no cross-section or degree of composite action are found, a message is displayed in the detailed output. The most common reason a cross-section may not be found is if the height restriction is turned on restricting the number of cross-sections that can be considered in the design. A second way reason a section may not be found is due to minimum spacing requirements. In this case consider allowing 2 rows of studs on the member (see "Composite Setup").

.