Overview on The Design for Stability Chapter in the AISC Specifications

Overview on The Design for Stability Chapter in the AISC Specifications

ABSTRACT

 In the AISC specifications of chapter C, the design for stability, the manual provides a valuable explanation about how to design a member or a whole structure when considering stability. This paper’s review, then, will focus on requirements that need to be considered in the design for stability. To account for such requirements AISC specifications offer three methods: (1) direct analysis method, (2) effective length factor, and (3) first-order analysis. Throughout this paper, I will consider the three methods; however, the first method will be more detailed.

1. INTRODUCTION

Galambos (1998) defined the Stability as “The capacity of a compression member, element, or frame to remain in position and support load, even if forced slightly out of line or position by an added lateral force”.

To understand the behavior and design of metal structures effectively, an engineer needs a fundamental understanding of structural stability. In addition to structures designed with other building materials, steel structures are largely governed by the stability limits. All major international design requirements include provisions based on stability theory. (Galambos 2008).

Instability problems are often catastrophic and most frequently occur during erection. For example, a number of large steel box- girder bridges collapsed in the late 1960s and early 1970s, leading to many deaths among erection personnel. (Galambos 2008).

2. GENERAL STABILITY REQUIREMENTS

Carter (2013) gives good explanation for the general stability requirements. Chapter (C) “Stability Analysis and Design” provides that the design of the structure for stability as a whole and each of its element should consider all of the following: (1) flexural, shear, and axial member deformations – these are the all other component and connection deformations that contribute to displacements of the structure; (2) second-order effects – these are the increases that occur in moments and forces due to displacements of the structure caused by the loads, containing both P- effects(displacement due to sidesway of the structure or the member). (3) geometric imperfection – these are the initial out-of-straightness of the members and the initial out-of-plumbness of the structure; (4) stiffness reductions due to inelasticity – these are the effects of residual stresses; and, (5) variability in component and system stiffness – these are the effects of variations in material and cross-sectional properties of members, as well as the other effects generally accounted for in the resistance factors (LRFD) and safety factors(ASD).

The specification states expressly that any method of analysis and design that accounts all the required effects is allowable, and then presents certain specific approaches that consider for the last four of the listed effects (P-∆ effects, P-δ effects, geometric imperfections, and inelasticity). (Nair 2007)

2.1. DIRECT ANALYSIS METHOD

The direct analysis method (DAM) is essentially considering three issues, Freund (2012): (1) impacts of initial geometric imperfections. (2) second-Order effects – axial-displacement moments P-∆ and P- δ (as shown below figure 1). (3) Effects of material non-linearity – inelasticity due to residual stresses.