The program has the ability to check adequacy of a section (shape) in accordance with the requirements of the selected design code. Also the program can automatically choose (i.e., design) the optimal (i.e., least weight) sections from a predefined list that satisfies the design requirements. To check adequacy of a section, the program checks the demand/capacity ("D/C") ratios at a predefined number of stations for each design load combination. It calculates the envelope of the D/C ratios. It also checks the other requirements on a pass or fail basis.
If the capacity ratio remains less than or equal to the D/C ratio limit, which is a number close to 1.0, and if the section passes all of the special requirements, the section is considered to be adequate, else the section is considered to be failed. The D/C ratio limit is taken as 0.95 by default. However, this value can be overwritten in the Preferences (Chapter 3). To choose (design) the optional section from a predefined list, the program first orders the list of sections in increasing order of weight per unit length. Then it starts checking each section from the ordered list, starting with the one with least weight.
The procedure for checking each section in this list for adequacy is exactly the same as described in the preceding paragraph. The program will evaluate each section in the list until it finds the least weight section that passes the code checks. If no section in the list is acceptable, the program will use the heaviest section but flag it as being overstressed. To check adequacy of an individual section, the user must assign the section using the Assign menu. In that case, both the analysis and design section will be changed. To choose the optimal section, the user must first define a list of steel sections, the Auto Select sections list.
The user must next assign this list, in the same manner as any other section assignment, to the frame members to be optimized. The program will use the median section by weight when doing the initial analysis. Refer to the program Help for more information about Auto Select section lists. Analysis sections are those section properties used to analyze the model when the analysis is run. The design section is whatever section is used in the steel frame design. It is possible for the last used analysis section and the current design section to be different.
For example, an analysis may be run using a W18X35 beam, and then in the design, it may be found that a W16X31 beam worked. In that case, the last used analysis section is the W18X35 and the current design section is the W16X31. Before the design process is complete, verify that the last used analysis section and the current design section are the same. Refer to the program Help for more information about completing this task. For each design combination, steel frame members (beams, columns, and braces) are designed (optimized) or checked at a number of locations (stations) along the length of the object.
The stations are located at equally spaced segments along the clear length of the object. By default, at least three stations will be located in a column or brace member, and the stations in a beam will be spaced at most 0.5 meter apart (2 feet if the model has been created in US units). The user can overwrite the number of stations in an object before the analysis is made using the Assign menu. The user can refine the design along the length of a member by requesting more stations. The design load combinations are the various combinations of the prescribed analysis cases for which the structure needs to be checked. The program creates a number of default design load combinations for steel frame design.
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