Notional loads for the detection of imperfections

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According to the standard AISC 360, 2016 Edition, chapter C2.2, the direct imperfections must be considered for the direct design method. The deviation from the perpendicular for columns is the important imperfection that must be taken into account. According to C2.2a, the direct modeling of imperfections is a method to consider the effect of the initial imperfections. However, in some cases, the expected displacements may not be known or may be difficult to predict.

In Section C2.2b, we explain another allowable method by which the imperfection can be taken into account by using the notional loads. Imposed fictitious loads are allowed for structures where the weight loads are mainly supported by vertical columns, walls, or frames.

The size of the notional loads applied as lateral loads on all stages is determined in accordance with C2.2b (1), where Y i represents the applied weight load on stage i for the considered load case.

In RFEM (and RSTAB), it is possible to calculate these notional loads and apply them to a member, set of members, or a member list. "Insert → Loading → 3.14 Imperfections → Dialog Box" shows the possible calculations of the notional load according to Equation C2-1.

The user can either select According to standard "ANSI / AISC 360-16 (Current loading)" or "ANSI / AISC 360-16 (Weight load combination)" to use a load case or a load combination for the application of the weight load.

The notional load coefficient is set to 0.002 by default on the basis of a perpendicular of l / 500. However, according to Section C2.2b (3), it is allowed for the engineer to adjust this factor if necessary. The factor α must be set depending on whether the design method is applied according to LRFD or ASD.

Keywords

Imposing fictitious load Imperfection Weight load Column

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RFEM Steel and Aluminium Structures
RF-STEEL AISC 5.xx

Add-on Module

Design of steel members according to the American standard ANSI/AISC 360

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1,480.00 USD
RSTAB Steel and Aluminium Structures
STEEL AISC 8.xx

Add-on Module

Design of steel members according to the American standard ANSI/AISC 360

Price of First License
1,480.00 USD