Torsional Buckling Of Circular Beams: Causes And Resistance

Methods of Predicting the Torsional Buckling

The torsional buckling in beams occurs when the load applied leads to both twisting and lateral displacement of a member. The failure observed, in this case, is normally observed when an applied load is directed towards unconstrained beam with dual flanges acting diversely, one under tension and the other under compression. Torsional buckling can happen in an unrestrained beam. A beam can be defined to be unrestrained if its flange of compression is free to rotate or displace laterally. When the load applied leads to both twisting and displacement of a member torsional buckling has happened.

This report paper focuses on the torsional buckling of the circular beams by discussing the reasons behind the torsional buckling of circular beams and also examples of constructions where the torsional beam buckling has one particular time occurred. The steel members exposed to loads which leads to the production of bending at the primary axis of the cross-section will ultimately experience huge deformation of torsional bucking which under some specifications may lead to the primary source of failure (Bird, 2014, p. 247). It is in order to idealize the condition and determine the loading at which a perfect member can undertake both buckled and unbuckled shape that have been deflected.

The torsional buckling load of such a perfect member normally provides an estimation of the final strength of the real member. The torsional buckling can happen when the member is still elastic or if the structures of steel of dimensions that are practical when the section of the member have yielded (Hendy, 2017, p. 187).

There have been numerous researches which are aimed are finding critical expression of the load for unsupported circular beams under diverse conditions of loading which include the opposite and equal moment of bending at the terminals of the beam, uniform load distribution in the whole span, and concentrated load at the middle of the span. The equation of the critical beam moment subjected to opposite and equal moments at the terminals

Where M_cr, W_cr and P_cr are the critical lateral unsupported beam moment, critical load unit and critical concentrated load respectively. L is the length that is unbraced of the beam, GC and B are the torsional rigidities and out-of-plane flexural of the beam. This assumption made do not regard the behaviour of torsional buckling of the beams (Communities, 2014, p. 217).

In case all the fibres of compression in the whole of length and depth of the beam are stressed beneath the limit of proportionality of the beam at the time of buckling, the beam undergoes elastic torsional buckling. For the elastic buckling, the preliminary tangent magnitude of elasticity of the beam gives a perfect estimation for the rigidity of every fibre of compression in the beam. A graph of stress against strain plotted at this instance minimizes the strain and stress rise more than the limit of proportionality (Dowswell, 2012, p. 158).

Liable to the stresses attained during the beginning of buckling process, values of modulus of elasticity for fibres that high stress at the furthest section of the zone of compression may be considerably lower than the preliminary tangent of the concrete’s modulus of elasticity, minimizing the whole of modulus of the beam utilized in the determination of the rigidity of the lateral bending. Only a small percentage of the beam beneath the centroid of the reinforcement of the tension is ignored in the calculation of the critical load (Galambos, 2010, p. 179).

Resistance to Torsional Buckling

Hence, the extension of the flexural cracks in the region of tension should be established well to evaluate the portion of the beam giving buckling against rigidity. The utilization of d in the calculation of the critical load may overestimate the portion of the beam efficient in resistance to lateral buckling at the moment of failure (Gupta, 2014, p. 194).  

The section of the beam efficient in resisting the torsional buckling can be evaluated according to the distribution of strain in the section and in the span of the beam at the buckling’s onset. The fibres in the zone of tension strained beyond the strain of cracking of concrete are not put into the contemplation in the calculation of the critical load (Hassan, 2015, p. 184). According to the levels of moment attained before buckling process, the material and properties of cross-sectional of the beams, the utilization of the value of a or b or c between them may be more appropriate to account for the efficient beam’s portion in the resistance to buckling as shown in the following expression of torsional  

However, the utilization of c in the calculation of the critical load results in the lower estimation of lower buckling that the utilization of d.

In theory of double modulus for the torsional buckling of beams, the distribution of strain along the width and depth of the section of the midspan at the moment of buckling (Hendy, 2017, p. 257). The assumption made is that the beam does not experience bending deformation of out-of-plane before the buckling 

The deformation out-of-plane after the introduction of strains and tensile stresses by the buckling from the out-of-plane and in-plane moments of bending add up in the concave section of the region of compression, while the tensile strains triggered by the lateral bending cancel the strain of compression consequential from the vertical bending in the side that is concave of the region of compression is known as reversals of strain (Johnson, 2012, p. 157).

A load directed above the centre of the shear leads to torsional moments, in the similar direction as the torsional rotation that is existing because of instability. 

The torsional rigidity of the concrete beam that has been reinforced in the early stage of post-cracking, which means that the development of diagonal cracks of tension because of torsion. The analytical ways of predicting torsional buckling loads of beams considered the stress-strain and elastic-inelastic behaviour of the beam, the shear and longitudinal contribution to the stability, and the cracking due to flex determines the resistance of the beams to torsional buckling (Trahair, 2013, p. 287).

The numerous equations governing the torsional buckling of a circular beam can be determined by considering a bar of the constant circular cross-sectional area subjected torsion

In this instance, the plane cross section typical to the member's axis persists in the plane after twisting which means that there is no warping. The torque is resisted through shear stresses of the circumference as a result of torsion of St. Venant. The modulus of this torsion differs as its distance from the centroid (Kitipornchai, 2016, p. 176). The torsion of the circular section is given by:  

Torsional Buckling Strength of Beam

Where z = direction along member’s axis

I_p = the polar inertia moment

T_sv = Torsion of St. Venant

G = Rigidity modulus

? = Twist angle

The vertical load applied leads to tension and compression in the section flanges, the flange of compression makes an attempt to laterally deflect away from its initial location, whereas the flange of tension makes an attempt to maintain the member to be in a straight line (Klopper, 2015, p. 268). 

The lateral bending of the section leads to the restoration of forces which prevent the movement due to the section tends to persist being straight. The forces restoring are not sufficient enough to halt the section form laterally deflecting, however, together with the lateral tensile forces component, the beam’s buckling resistance can be determined. The behaviour of load-deformation can be affected by the torsional buckling through the reduction of the rotational capacity and strength. The behaviour of post-buckling deformation is very critical of the dual issues, however, no possible treatment method it is as yet accessible (Galambos, 2010, p. 147).

The effects of the torsional buckling can be illustrated in the graph of the relationship between strong axis ration of slenderness L/r_x and Mo of a member subjected to a force that is axial in nature and equivalent end moment is noted. It has been noted that the torsional buckling of the beam is responsible for a serious strength reduction. The factors affecting the torsional buckling of beams include:

Applied Load Location: The perpendicular distance between the shear centre of the section and application point of the load affects the section’s susceptibility to the torsional buckling of beams effects. In the process of application of the load at the position on top of the section of shear centre, it is more vulnerable to torsional buckling than if the load’s application was done through the centre of shear. The application of the load at the position beneath the centre of shear of a section minimizes the vulnerability of the section to the impacts of torsional buckling (Nethercot, 2012, p. 185).

The application of the load above the centre of shear is referred to as destabilizing load, with applied loads below or at the centre of shear known as non-destabilising loads. The impacts of load destabilization are considered by the utilization of effective lengths provided, where the lengths that are effective are longer for loads destabilization compared to the loads that are non-destabilizing. 

Applied Bending Moment Shape: The resistance of buckling for a section issue to a constant distribution of bending moment along its length is less than the resistance of buckling gotten for the similar section subjected to the diverse distribution of bending moment. These factors are involved in the guidance of the design to enable for the impacts of diverse distribution of bending moment (Coutie, 2017, p. 219).

Conditions of End Support: The conditions of the end support considered during the growth of primary theory for the moment of buckling are equal to web cleats which halt the web from deflecting twistingly and laterally. For the conditions of end support where extra confines are provided to the section the moment of buckling increases, with the moment of buckling reducing the support at the terminal which provides less restraint to the section. 

Reasons Behind Torsion Buckling of Beams

Cross-sectional Size: The resultant interaction strength curves are not sensitive to the variations in the size of the cross-section for shapes of wide-flange that have been rolled, and a single set of curves for any specified value of the ultimate ration of the moment is enough. It is possible that the torsional buckling will be near the final strength even before the cross-over for any section (Morchi, 2016, p. 268).

Level of Yield Stress: It has been proved that for a common straightening and rolling processes, the distribution of residual stress does not significantly vary with the increase in the yield stress. The elastic section of the beam in the level of yield stress shifts to the left the elastic torsional buckling. This shows that at higher strength member, elastic torsional buckling would happen at a length that is shorter than that of the member.

Residual Stress Influence: The major influence of increasing the residual level of residual stress is the reduction in the value of moment applied at which the limit of elastic is attained. In the range of inelastic, the relationship between the slenderness ratio and moment applied are approximately linear for n-both the levels of residual stress. Therefore, increasing the pattern of residual stress minimizes the member’s elastic limit moment and hence causing a minimization in the strength in the range of inelastic (Miranda, 2011, p. 174).

During the building of the bridge of steel girder, the girders to be positioned are collected by the crane.  It is the responsibility of the contractor to place the girder without any damage. During the construction of this bridge, the girder was positioned by the two cranes concurrently picking the girders up from both ends. The build-up plate-girders weighed about 96,000 pounds, 7.5 feet deep, and 197 feet long (Lue, 2013, p. 235). Due to such a long length under consideration, the torsional buckling occurred. During the process of lifting the first girder, it was likely that the failure of the girder was imminent as the span at the middle started bowing over which is the initial sign of buckling. Previously, the major intentions of this projects were the determination of necessary forces to adequately stiffen the flange compression of the girder hence preventing torsional buckling (Klopper, 2015, p. 197). 

The construction of Washington Metro in Washington area of US in 1976 faced the torsional buckling during its operations. In 1982, the Washington Metro faced derailment at the crossover of the Triangle station which was triggered by the torsional clasping of the rails leading to the car sliding off the track and hitting the tunnel.  The Amtrak passenger railroad service which was constructed in 1971 and is situated in the US also faced the problem of torsional buckling in the past. This torsional buckling cause derailment of the bridge leading to the formation of a barge on the rail in 1993. The other construction which has been affected by the torsional buckling of the beams include Metrorail train that derails off tracks in 2012 near Hyattsville in Maryland where the torsional buckling affected the rails (Klopper, 2015, p. 213).

Conclusion

This research paper is about the torsional buckling of beams. The torsional buckling in beams occurs when the load applied leads to both twisting and lateral displacement of a member. This report paper focuses on the torsional buckling of the circular beams by discussing the reasons behind the torsional buckling of circular beams and also examples of constructions where the torsional beam buckling has one particular time occurred. This torsional buckling cause derailment of the bridge leading to the formation of the barge on the rail in 1993. The other construction which has been affected by the torsional buckling of the beams include Metrorail train that derails off tracks in 2012 near Hyattsville in Maryland where the torsional buckling affected the rails.  

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