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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 for 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: Reduction  Yes, but not in the initial analysis. Since the Direct Analysis Method is a second-order analysis, these reduction factors are automatically applied to section EI and EA properties when 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” is specified Fixed” under Stiffness Reduction Method (item 14), as shown in Figure 1. Reduction factors are not implemented during the initial analysis, but then once design is run, reduction factors are used and remain within subsequent formulation. This means that an additional analysis and design iteration must be executed.

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Figure 1 - Steel Frame Design Preferences

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

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1. 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.
   
   
2. 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 labortime-intensive process.

Attachments

• AISC360-05 Direct Analysis Method Technical Note (PDF)