Second order moments in torsion members

Abstract

This paper is concerned with the elastic flexural buckling of structural members under torsion, and with second-order moments in torsion members. Previous research is reviewed, and the energy method of predicting elastic buckling is presented. This is used to develop the differential equilibrium equations for a buckled member. Approximate solutions based on the energy method are obtained for a range of conservative applied torque distributions and flexural boundary conditions. A comparison with the limited range of independent solutions available and with independent finite element solutions suggests that the errors in the approximate solutions may be as small as 1%. The predicted linear elastic buckling torques may be used to approximate the second-order bending moments caused by torsion in members under more general loading. A method is developed for approximating these second-order moments. This is used as the basis of a method of estimating when these second-order moments may be significant by comparing the actual member slenderness with a reference value. Reference values of slenderness are calculated for two examples involving an equal angle member and a circular hollow section member (both simply supported), and the importance of second-order torsion effects in an I-section member is estimated. The reference values of slenderness are found to be very high, and it is concluded that second-order moments caused by torsion in typical structural steel members with slenderness ratios L/r_y<300 are very small and may be neglected.

Introduction

Second-order moment components of applied torques are usually ignored in structural analysis, but in the most extreme cases, they may lead to flexural buckling. While it is possible that these components are very small in most practical cases and may be neglected, it is nevertheless important to be able to determine when this is not so, so that these second-order effects may be accounted for. The purpose of this paper is to present solutions for the linear elastic flexural buckling of torsion members, and to use these to develop a method of estimating the importance of second-order moment effects of applied torques in members under more general loading.

The flexural buckling of torsion shafts of circular cross-section is known to mechanical engineers, who prevent this from occurring in long shafts by using intermediate bearings to decrease the unbraced length of the shaft. The most common application is to a shaft of length L which is simply supported at both ends and has a concentrated torque M applied at one end and reacted at the other, as shown in Fig. 1(a). It has been reported that such a shaft may buckle elastically [1], [2], [3] under a torque

$$\text{M}{}_{\mathrm{<mtext>o</mtext>}}\text{=2πEI/L}$$

in which E is the Young's modulus of elasticity and I=I_x=I_y is the second moment of area of the circular shaft. The buckling mode involves lateral deflections u, v of the longitudinal axis of the shaft in the X, Y directions, which are given by

$$\text{u=U}\text{sin}\text{(2πz/L)}$$

and

$$\text{v=U(}\text{cos}\text{(2πz/L)−1)}$$

in which U is an undetermined magnitude, or vice versa. These deflections correspond to a spiral-like buckled shape without any buckling twist rotations.

Early research on flexural buckling under torque has concentrated on the mechanical engineering problem of circular cross-section shafts with concentrated end torques and axial compressions, and has been summarised by Timoshenko and Gere [1], Ziegler [2], and Bazant and Cedolin [3]. A number of discussions have been made [2] of the nature of concentrated end torques. In particular, it has been supposed that an axial torque may be non-conservative when it acts at a simply supported end of a shaft which is able to rotate u′ or v′ (`≡d/dz). As a result, a number of other types of end torque have been postulated which are conservative, such as “quasi-tangential” and “semi-tangential” torques [2]. More recently, Teh and Clarke [4] have proposed a new type of conservative torque.

Ziegler [2] has presented solutions for a number of the flexural boundary conditions indicated in Fig. 2 for a number of different types of concentrated end torque. Barsoum and Gallagher [5] have developed a finite element method of analysis and used it to confirm Ziegler's results. Goto et al. [6] have studied the effects of pre-buckling twist rotations on the buckling of shafts built-in at both ends, and have investigated their post-buckling behaviour.

However, much of this research has little relevance to civil engineering structures, where the torsion actions result from eccentric loads acting between the end supports, rather than from a concentrated end torque. In addition, the members of civil engineering structures are commonly of non-circular cross-section (although an extension by Grammel in 1923 to non-circular shafts is referred to by Timoshenko and Gere [1]).

In this paper, the nature of conservative end torques is first discussed, and then the energy equation for flexural buckling under conservative tangential torque is presented and used to derive the general differential equations of equilibrium. Approximate solutions are obtained for the linear flexural buckling of members of non-circular cross-section under concentrated end torque, uniformly distributed torque, and central concentrated torque (Fig. 3) for the flexural boundary conditions illustrated in Fig. 2. These solutions are used to develop a method of estimating the importance of second-order moments caused by torsion in members under combined bending, torsion, and compression actions.