Technical Knowledgebase

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This page is devoted to frequently asked questions (FAQ) related to design.

 

On this page:

General

How do I specify a certain design code?

Answer: The design code may be specified by selecting Design > _ Frame Design > View/Revise Preferences. Under Item 1 Design Code, select the appropriate code from the drop-down list.

Why do some members always differ during design iterations?

Extended Question: I have done iterative analysis and design many times. On the first iteration, 2000 members differed between analysis and design, and now that it is down to 100, there is no further reduction, and sometime the number increases. Is there an explanation?

Answer: This indicates that design has converged to the best solution available to the sections presented on the Auto List. With each iteration, design changes cause load path to redistribute. As a result, each additional iteration may again slightly change member selection and the resultant load path.

When convergence occurs, it is best to review the results, then if necessary, make changes manually. To report zero differences between analysis and design sections, fewer members must be provided to the Auto List, especially for sections available to the lateral systems.

Can different performance measures (D/C ratio) be assigned to various objects during steel frame design?

Answer: Yes, design overwrites may be applied to various structural members within a single model. For example, to apply AISC 360-05, select Design > Steel Frame Design > View/Revise Overwrites, then item 44 of the menu allows specification of the Demand/Capacity Ratio Limit. Overwrite values will then supercede the D/C ratio defined within design preferences.

Why is the moment obtained from analysis different from the moment used for design?

Answer: Non-zero Pattern Live-load Factors may have been applied during design, which would redistribute negative moment and cause design-moment discrepancy. Please see the Pattern live-load factor page for additional information.

Modeling

When a frame is modeled using multiple segments and not a single segment, why are design results different?

Answer: The axial and flexural capacity of a frame member is dependent upon its effective length factor, which is based on unbraced / unsupported length. When a frame is divided into multiple segments, connected by nodes, each unbraced length is only as long as the corresponding subsection. If lateral bracing does not actually occur at these points, frame properties should be overwritten to reflect the behavior of the entire length. Similarly, when a frame connects to another object which serves as bracing, the frame should be divided accordingly, or its properties should be overwritten to simulate the effect of bracing points.

To divide a frame into multiple segments, use either the Edit-Divide Frames or the Automatic Frame Mesh commands.

How are composite and noncomposite sections designed in the same model?

Extended Question: I am using auto selection to design a steel structure with both composite and noncomposite sections. Composite design returns the correct results, but the noncomposite design for the gravity system does not generate results except for columns. Noncomposite member check is shown as composite design. How is this resolved?

Answer: The design procedure may be changed by selecting the appropriate members, then switching from Composite Beam Design to Steel Frame Design through Design > Overwrite Beam Design.

For steel-angle design, how is orientation considered?

Extended Question: The steel design procedure for angles seems to consider local axes and not principal axes. This discrepancy in orientation causes significant differences in the formulation of BS-5950:2000 clause 4.3.8. Is there an explanation?

Answer: Flexural stresses are calculated according to properties about principal axes. Principal axes coincide with geometric axes for sections which include I, T, box, rectangular, circular, pipe, and channel. For single-angle sections, design considers the principal properties. For general sections it is assumed that section properties coincide with principal directions. If this is not the case, manually rotate the frame local axis to match that of principal directions.

Of use may be the SAP2000 and ETABS Steel Frame Design Manual > Chapter VII, Check/Design for BS 5950-2000. This resource is available within software documentation. All details regarding design algorithm and implementation are presented.

Formulation

How reliable is the formulation for unsupported length?

Extended question: Why does the software not seem to consider body constraints, which laterally support a main girder, as support points for unbraced-length calculation?

Answer: Constraints are useful for many modeling applications, though they do not always represent lateral support. Consider a simply-supported composite beam, for example, where body constraints are assigned at each shear-stud location to connect a frame-element girder to a shell-element deck. If the distance between constraints represented unsupported length, the value would be underestimated.

Based on the Lateral Torsional Buckling section (pages 3-5) of the AISC Steel Construction Manual, 13th Edition, unbraced length is simply described as the distance between braced points. However, Appendix 6.3 then provides an exhaustive definition of braced point, covering lateral bracing systems, torsional bracing systems, whether or not inflection points can be considered as braced points, etc. Unsupported length is more than a matter of simple calculation, and often requires substantial engineering judgement. Further, sufficient information may not always be available to the computational tools which determine unsupported length.

It is recommended that users review Appendix 6.3 and other relevant material. Also, designers should always review software-derived values for unsupported length to verify correlation with design intention. These values are ultimately at the discretion of the designer, and overwrite options are integrated into software such that users may ensure the application of appropriate unsupported-length values.

How are allowable capacities calculated in the design module?

Answer: The calculation of allowable capacities for steel frame design according to AISC 360-05 is explained in Context Help (Section 3.5 – Calculation of Nominal Strengths), available through Help > Documentation > Design.

Can the design equations used in SAP2000 be customized?

Yes, tabular results may be exported to Excel using the Display > Show Tables > Design Data command. The intermediate values reported within the Excel file may then be used to calculate D/C ratios and other pertinent data.

How should the Response Modification Factor (RMF) be applied to bridge components per AASHTO seismic requirements?

Answer: The RMF factor may be applied as a coefficient for a particular load case within a load combination. Since different substructure elements may have different modification factors, separate load combinations should be created for each group of components.

Reporting

How are stresses obtained for individual load cases?

Answer: It is recommended that users establish design combinations for each individual load case and load combination to be checked. The design summary and performance ratios for all members and load cases may be displayed in a single table available through Display > Show Tables > Design Data > Steel Frame > Steel Summary Data > Table: Steel Design 1 - Summary Data.

This approach is adequate when the performance ratios (or stresses) from individual load cases are additive, which is the case for codes based on allowable stress design (ASD).

Can SAP2000 plot interaction diagrams for reinforced-concrete columns?

Answer: Yes, interaction diagrams may be plotted by either: 1). running concrete design and reviewing the results; or 2). modeling sections and then displaying the interaction diagram in the Section Designer. Please see the Interaction diagram page for additional details.

Troubleshooting

Why do the steel and concrete frame-design applications not run?

Answer: To ensure proper frame design:

  • Set the desired Design Type (other than No Check/Design) on the Section Definition menu shown below in Figure 1.
  • Verify that the frame is assigned to the proper Design Procedure, as shown below in Figure 2.

 

Why does resistance not increase when I add reinforcement?

Answer: If the section is over-reinforced, additional reinforcement will not increase resistance. When section design provides for a compression-block depth which exceeds prescriptive criteria, section depth or section width should be increased to achieve additional capacity.

See Also