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2005 AISC Direct Analysis Method

Steel frame design may be performed in accordance with the 2005 AISC Direct Analysis Method (AISC 360-05 / IBC 2006) using SAP2000, CSiBridge, and ETABS. Attached is a Technical Note which provides an overview of this process.

Is stiffness automatically reduced during Direct Analysis Method application?

Extended Question: Does the software automatically reduce stiffness according to AISC 360-05 - App 7, clauses 3 and 4?

Answer: Yes, but not in the initial analysis. Since the Direct Analysis Method is a second-order analysis, these reduction factors are applied in subsequent analyses. To implement the Direct Analysis Method, clauses 3 and 4, select the option for “Tau-b Variable” or “Tau-b Fixed” under Stiffness Reduction Method (item 14), as shown in Figure 1.

Figure 1 - Steel Frame Design Preferences

How is the Direct Analysis Method used for a response-spectrum load case?

Answer: The two options available include the following:

Use the generalized 2nd-order option of the Direct Analysis Method. Here, response-spectrum load cases should be based on modal load cases which use stiffness properties from nonlinear-static P-Delta load cases. This load case may include dead and live loads which represent the gravity contribution of the load combination which includes the response-spectrum load condition. Moment amplification due to the gravity portion of axial force will be included in response-spectrum results. Any additional amplification or de-amplification due to axial forces in the response-spectrum case itself are not included, however, since response-spectrum analysis is inherently linear. This may not be significant except for taller structures, though still subject to engineering judgment.

In the end, a load combination must be created which includes the nonlinear P-Delta gravity load case and the corresponding (dependent) response-spectrum case. If amplification of seismic axial forces is of concern, nonlinear time-history analysis should be considered as an alternative to response-spectrum.
Use the amplified 1st-order option of the Direct Analysis Method. In this case, however, only B1 is calculated. B2 must be determined and applied as a design overwrite for specific members. Depending on the model, this may be a time-intensive process.

Attachments

AISC360-05 Direct Analysis Method Technical Note (PDF)

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