Notional Loads to Represent Imperfections

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In the AISC 360 – 14th Ed. C2.2, the direct analysis method requires the consideration of initial imperfections to be taken into consideration. The important imperfection of recognition is column out-of-plumbness. According to C2.2a, the direct modeling of imperfections is one method to account for the effect of initial imperfections. However, in many situations, the expected displacements may not be known or easily predicted.

Section C2.2b states another allowable method to account for imperfection is with the use of notional loads. Notional loads are permissible for structures that support gravity loads primarily through vertical columns, walls, or frames. The magnitude of the notional loads which are applied as lateral loads at all levels is in accordance to C2.2b(1) where Yi is the applied gravity load at level i for the load case considered.

RFEM has the ability to calculate and apply these notional loads to a member, set of members, or list of members. Under ‘Insert’ → ‘Loads’ → ‘3.14 Imperfections’ → ‘Dialog Box’ will bring up the notional load calculation options according to equation C2‑1. The user can select ‘According to Standard’: ‘ANSI/AISC 360‑10 (current loading)’ or ‘ANSI/AISC 360‑10 (gravity load combination)’ where a load case or load combination respectively can be used for gravity load application.

The notional load coefficient is set to 0.002 as a default based on an out-of-plumbness of l/500. However, section C2.2b(3) allows the engineer to adjust this factor if desired. The alpha factor must be set depending on LRFD or ASD analysis considerations.

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