Second-Order effects – Axial-Displacement Moments P-D and P-d (as shown above).
Effects of initial geometric imperfections.The direct analysis method is basically accounting for (3) issues: We will now cover the DAM for a computer based approach and a simplified hand method. I don’t want to get off track so I will explain what I mean later (this refers to common FEM models and placing multiple nodes along a member). Neither of these moments will show up in a first order elastic analysis. interstory drift) think of these as global displacements. The second is P-Δ (I may refer to this as P-D) moments which are caused by axial load and the translation of the end of the column (i.e. The first is P-δ (I may refer to this as P-d) which are moments associated with the axial load and deflection due to column curvature (Think of these as local displacements). Essentially we need to address two different types of deflection/deformation associated with axial load that amplify moments in the structure. However in DAM, AISC is assisting you in your analysis to make the design simpler. It is a very interesting approach in that most design codes / manuals do not get into the analysis meaning that they don’t tell you how to get the required/design forces but rather they give an allowable/capacity of the member to which you are designing. Like anything else you need to put some time into learning it but it’s really not so bad in the end. I know many are not used to this new approach and some will say that if it’s not broke don’t fix it but I like the DA Method. In the 14th Edition of the American Steel Construction Manual the Direct Analysis Method (DAM) is moved into the main specification from the appendix. Stability – AISC’s Direct Analysis Method Intro
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