## 311 Introduction

The concept of the effective length factor has been well established and widely used by practicing engineers and plays an important role in compression member and column design. The essence of the concept is to estimate the interaction effects of the whole frame on an individual compression member. In the development of design interaction equations for beam-columns, much discussion has been focused on the need and validity of using the effective length factor K in the equations (Cheong-Siat-Moy 1986; Liew et al. 1991; ASCE 1997; White and Clarke 1997a,b; Schmidt 1999). Although attempts were made to formulate the general interaction equations without K factors, it was found that this was almost impossible if the interaction equations were to be versatile enough for a wide range of slenderness ratios and load combinations (Liew et al. 1991). It is well known that the effective length factor approach introduces inaccuracies into the design process; the simplicity of the approach, however, is likely to still make the approach an important part of compression member design in the foreseeable future (Hellesland and Bjorhovde 1996). The most structural design codes, standards, and specifications worldwide have provisions concerning the effective length factor.

The aim of this chapter is to present a state-of-the-art engineering practice of the effective length factor for the design of compression members and columns. In the first part of this chapter, the basic concept of the effective length factor is discussed. Then, the design implementation for individual columns, framed columns, crossing bracing systems, latticed members, tapered columns, crane columns, gable frames, columns in fire, space frames, truss-type highway sign support structures, precast concrete skeletal frames, and steel moment frames is presented. The determination of whether a frame is braced or unbraced is also addressed. Several detailed examples are given to illustrate the determination of the effective length factor for different cases of engineering applications.

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