Avoiding Torsion in Structures

Introduction

The twisting of elements within structures due to eccentric loading is something that is best avoided as far as is possible. Such actions develop torsion forces in elements against which they were not designed to withstand. This post concerns this buildability and detailing the issue that structural engineers must become familiar with in order to avoid otherwise unforeseen problems that can lead to signifcant remedial works on-site and in some cases failures.

Principles of Torsion

The rotation of a structural member along its axis is something that should be avoided as much as possible. It generates forces within the element that it is rarely efficient at resisting and can result in a significant increase in member size and even change in form. Some structural shapes such as steel channels, T and angle sections are more susceptible to torsion where the shear centre is outside the web.

Other examples are twin beams with unequal loading, curved beams on plan and angles. This post shows how to avoid torsion in structural members and what needs to be done when it becomes necessary for elements to resist torsion.

Torsion Induced by Framing Layout

It is possible to inadvertently develop torsion in structural elements depending on the structural framing layout adopted. This usually occurs due to changes being made to the structure to accommodate services or in respect to the form of the building that has been established during the design process. Figure 1 shows typical examples of structural frame layouts that have been altered to the point where a member becomes subject to torsion.

Fig 1: Revision to frame layout resulting in torsion in a member

It is important to understand that when a member is subject to torsion, this force occurs simultaneously to all other forces, i.e. shear and bending. With this cumulative effect, it is possible that the member may need to be increased in size to resist these additional forces. To prevent torsion from developing, the following rules should be followed when devising and revising a frame layout to a structure:

1. Consider how the forces are transferred from one element to another.

2. Avoid change in direction of forces from within a frame.

3. Ideally, cantilevers should not intersect.

Torsion Induced by Eccentric Loading

Considering more local effects, it is possible for structural elements to be subjected to torsion through the introduction of eccentric loads. While they were originally designed with the assumption that the load placed upon them would be largely within their centroid, changes to the form of the building or passage of services can result in an eccentric load being generated, which leads to torsion. Figure 2 is a pair of examples of such details which have been revised to the point where an eccentric load is being applied to the structural member.

Fig 3: Detailing examples of eccentrically loaded beams

It is usually very difficult to avoid such occurrences as the reasons for the alterations are normally sound. Nevertheless there are measures the structural engineer can take to counter these changes:

1. When developing cladding interface details, set parameters for the rest of the design team with respect to what can and cannot be altered.

2. Highlight the consequences of inducing eccentric loads onto structural members to the design team. This includes an increase in member size, change in form and more complex connections.

Torsion Induced during Construction

Structural engineers are required to consider the condition of the temporary work during the design of any element within the structure. While they are ultimately seeking to design a structural element for the permanent condition, some cognisance must be given to the possibility of a member being subject to torsion during construction. If there is such a possibility then the structural engineer is required to alert the contractor of this possibility and they can then carry out mitigation measures to avoid torsion being induced into the member for which it was not designed. This will include temporary propping of the member or altering the sequence of construction to prevent the torsion from occurring. Figure 3 shows two examples of elements that are subjected to torsion during construction.

Fig 3: Structural elements in torsion during construction

It is uncommon for the design of such elements to be designed for torsion as the contractor is alerted to the issue. The measures they employ can then be implemented to negate the need to design the element to resist a torsional load.

When considering the likelihood of torsion being developed during construction, the structural engineer must:

1. Consider alternative design solutions to prevent this from occurring

2. Assuming no viable alternatives exist, identify the member to the contractor and advise how they can prevent the member from being subjected to torsion

3. Advise the contractor at what point during the construction sequence the member will be subject to torsion

Designing and Detailing for Torsion

There are instances when torsion cannot be avoided and the designer’s only course of action is to allow for it. In such instances there are essentially two approaches: provide an arrest that prevents the element from being subject to torsion in the first place or design the element to resist torsion

If a restraint cannot be provided, then the member must be designed for torsion. For concrete elements this is relatively straight forward as the designer is required to provide additional closed links throughout the length of the beam that are installed in addition to any shear links (Figure 4).

For steel elements, the design to resist torsion is somewhat complex. This is especially with regard to open sections such as I beams and angles. Closed sections such as rectangular hollow elements are less susceptible to torsion but they are not as stiff as their open section counterparts.

Additionally, the designer must pay particular attention to the end connections of steel members that are designed to withstand torsional moments. Simple fin plate connections are not robust enough to support such twisting forces, hence the need to provide end plate connections for members subject to torsion (Figure 5).

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