Strength of class 3 steel W sections in weak axis bending.(NOTE)(Report)

Abstract: This technical note considers weak axis moment capacity of wide-flange steel members of different section classes. In CSA S16-01 Limit states design of steel structures, there is a disconnect in moment capacity of laterally supported members between Classes 2 and 3: when the section crosses the Class 2 boundary, its calculated capacity drops in the ratio of the elastic to plastic section modulus. This effect is relatively minor for strong axis bending but is rather significant for weak axis bending. A rational theory is presented that explains the phenomena on the transition of the two Classes and proves that the noted gap in the design capacity does not exist. An improved design formula is proposed to mitigate this problem.

Key words: bending, class, flange, local buckling, steel beams, strong axis, weak axis.

Resume: Cette note technique aborde la capacite de resistance au moment de l'axe de moindre resistance de membrures en acier a large bride de differents profils. La norme CSA S16-01 Regles de calcul aux etats limites des charpentes en acier presente differentes capacites de resistance au moment de membrures avec support lateral pour les Classes 2 et 3: lorsque le profil traverse une limite de Classe 2, sa capacite calculee chute en fonction du rapport au module de section plastique. Cet effet est sensiblement mineur pour la flexion de l'axe fort, mais il est important pour la flexion de l'axe faible. Une theorie rationnelle pour expliquer les phenomenes de transition entre les deux classes est presentee et prouve qu'il n'existe aucun intervalle de capacite portante. Une formule de conception amelioree pour palier ce probleme est presentee.

Mots cles: flexion, classe, bride, flambement, poutres d'acier, axe fort, axe faible.

[Traduit par la Redaction]

Introduction

Modernization of framed steel structures, such as piperacks in petrochemical plants, often subjects existing Class 3 beams and columns to large weak axis moments (owing to a mismatch of bracing points on two sides or other irregularities). If the resulting force combination exceeds the structural strength of the member and additional bracing cannot be installed to reduce the weak axis moment, then the members must be physically reinforced, which is difficult and costly.

The …