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Chapter 2.9 Torsion Structures and Ring Beams

Figure 2.9.1 Torsion

Torsion structures and ring beams are not common to everyday structural engineering in buildings. When loads are applied eccentrically to structures causing twist special attention needs to be taken. It is more common to design structures to avoid torsion than to check sections for torsional stresses.

This chapter begins with the basics: the fundamental torsion equations for shear stress and rotation. Closed hollow sections are compared with open sections, such as channels, for resisting torsion. A practical balcony handrail problem illustrates how torsion and bending are often combined; examining the issues of relative stiffness and how torsion might be ‘designed out’. Radial or circular beams are reviewed, showing how to design out torsion or to put it to good use.

Figure 2.9.15 Curved Beam With Rotation End Restraints

Finally the principle of ring beams is examined with examples: restraining the outward thrust from domes and in a basement structure capping beam around the top of perimeter piles.

Torsion is analogous to bending. It is the application of an eccentric or offset load to a structure which causes a twisting effect. The curved beam in figure 2.9.18 has torsional restraints at the supports. The beam is also supported vertically but the ends can rotate with respect to the bending in the beam.
Estimating Torsion
In example illustrated by figure 2.9.18 the torsion may be estimated by:

1. Measure the chord offset from the beam centre line (maximum distance from the line shown in figure 2.9.17).

2. Work out the total loading on the beam (W x L (C) ).

Find torsion. Half of this is assumed to act in calculating the torsion (i.e. T = W x L(C)/2). L(C) is the length of the chord on the curve.

Find Torsion

Total load for figure 2.9.17,

Where….x = 0.60m, L(C) = 3.1m, w = 10kN/m

Torsion at ends (approximately)

= 10 kN/m x 3.1m x 0.6 /2

= 9.3kNm

Output from a computer model, using the same parameters gave M= 8.3 kNm

(overestimate compared with computer model)