Effective Length Method
The direct analysis method as described in chapter C of AISC 360-16 is explained below on how to do it within SCIA Engineer. In summary the following steps need to be followed:
-
Define the imperfections
-
Run nonlinear analysis and execute the steel code check
1) Define the imperfections
Imperfections as specified in the commentary are applied on the structure in the form of:
-
Frame out-of-plumbness equal to H/500 (if needed: only for combinations with gravity loading), where H is the story height.
First a set of functionalities to enable to introduce these imperfections needs to be activated, activate the following functionalities:
1.1) Frame out-of-plumbness
Global imperfections can be used to introduce the frame out-of-plumbness of H/500 (if needed).
This can be achieved as follows:
Go to Libraries > Structure & analysis > Initial deformations
Define under the "Pos" column the height of the structure, and under "Defor" column the result of height divided by 500.
Next assign this inclination function to the nonlinear combinations in the 4 directions. This means that all your nonlinear combinations need to be duplicated in the 4 directions in order to assign for each set the inclination function in +X, -X, +Y and -Y direction of the global coordinate system.
2) Refine the finite element mesh
In order to properly account for the 2nd order effects the number of mesh elements per member needs to be increased. A value higher than 3 is the bare minimum, a value of 10 or higher is advised in order to obtain accurate results. This can be done within the mesh settings as follows:
3) Run nonlinear analysis and execute the steel code check
All necessary preparations for the 2nd order analysis have been performed.
Run a nonlinear analysis on your structure which is in fact a 2nd order analysis according to the Effective Length Method and execute afterwards the steel code check with the nonlinear combinations to validate your structure.