1-21 how to avoid torsion

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TheStructuralEngineer36

Technical Guidance Note

Technical

December 2012

Note 21 Level 1

991290

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 effi cient

at resisting and can result in a significant

increase in member size and even change in

form Some structural shapes such as steel

channels are more susceptible to torsion

where the shear centre is outside the web

(Figure 1)

How to avoid torsionIntroduction

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 Technical Guidance Note concerns this buildability and

detailing issue that structural engineers must become familiar with in

order to avoid otherwise unforeseen problems that can lead to significant

remedial works on site and in some cases failures

983127

983127 Applied practice

983127 Further reading

ICONLEGEND

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 2 shows typical examples

of structural frame layouts that have been

altered to the point where a member

becomes subject to torsion

It is important to understand that when a

member is subject to torsion this force

occurs simultaneously to all other forces

ie 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

bull Consider how the forces are transferred

from one element to another

bull Avoid change in direction of forces from

within a frame

bull Ideally cantilevers should not intersect

Principlesof torsion

Other examples are twin beams with

unequal loading curved beams on plan and

angles This note 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 layoutIt is possible to inadvertently develop torsion

in structural elements depending on the

structural framing layout adopted This

983118 Figure 1A steel beam in torsion

983118 Figure 3Detailing examples of eccentrically loaded beams

983118 Figure 2Revision to frame layout resulting in torsion in a member

Original layout Revised layout

Secondary beams shifted along a primarysupporting beam removing continuity

Cantilevering section of structure partiallyremoved leaving two cantilevering beams

supporting one another

Original detail

Original detail

Revised detail

Revised detail

Inner skin of blockwork wall movedoutwards while the primary structure

remains in place

Lateral support system to claddingaltered from being fixed to centroid of primary element to being eccentric



wwwthestructuralengineerorg

37

sequence of construction to prevent the

torsion from occurring Figure 4 shows two

examples of elements that are subjected to

torsion during construction

It is uncommon for the design of suchelements 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

bull Consider alternative design solutions to

prevent this from occurring

bull Assuming no viable alternatives exist

identify the member to the contractor andadvise how they can prevent the member

from being subjected to torsion

bull Advise the contractor at what point during

the construction sequence the member will

be subject to torsion

Designing and detailing for torsionThere are instances when torsion cannot

be avoided and the designerrsquos 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 5)

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 To find out

more on this you are directed to The Steel

Construction Institutersquos Design of Steel

Beams in Torsion 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 6)

Torsion induced via eccentric loadingConsidering 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 placedupon 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 3 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

It is usually very diffi cult 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

bull When developing cladding interface

details set parameters for the rest of the

design team with respect to what can and

cannot be altered

bull 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 constructionStructural engineers are required to consider

the temporary works condition during thedesign 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

983118 Figure 4Structural elements in torsion during construction 983118 Figure 5

Shear links vs closed links 983118 Figure 6Fin plate connection vs end plate connection

Floor slabinstalled

on one sideof a beamthat wouldfail due toeccentricloading

condition

Masonrycladding

constructed prior to floorconstruction

thus preventingrestraint to primary

support beam

Glossary andfurther reading

Permanent condition ndash The state of an

element at the completion of construction

Temporary condition ndash The state of an

element during construction

Torsion ndash Rotational force along the axis of

a member

Further ReadingIles D C Hughes A and Malik A (2011)

Design of Steel Beams in Torsion Ascot

Steel Construction Institute

Eurocode 0

Applied practice

BS EN 1992-1-1 Eurocode 2 Design of

concrete structures ndash Part 1-1 General rules

and rules for buildings

BS EN 1992-1-1 UK National Annex to

Eurocode 2 Design of concrete structures ndash

Part 1-1 General rules and rules for buildings



wwwthestructuralengineerorg

37

sequence of construction to prevent the

torsion from occurring Figure 4 shows two

examples of elements that are subjected to

torsion during construction

It is uncommon for the design of suchelements 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

bull Consider alternative design solutions to

prevent this from occurring

bull Assuming no viable alternatives exist

identify the member to the contractor andadvise how they can prevent the member

from being subjected to torsion

bull Advise the contractor at what point during

the construction sequence the member will

be subject to torsion

Designing and detailing for torsionThere are instances when torsion cannot

be avoided and the designerrsquos 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 5)

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 To find out

more on this you are directed to The Steel

Construction Institutersquos Design of Steel

Beams in Torsion 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 6)

Torsion induced via eccentric loadingConsidering 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 placedupon 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 3 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

It is usually very diffi cult 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

bull When developing cladding interface

details set parameters for the rest of the

design team with respect to what can and

cannot be altered

bull 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 constructionStructural engineers are required to consider

the temporary works condition during thedesign 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

983118 Figure 4Structural elements in torsion during construction 983118 Figure 5

Shear links vs closed links 983118 Figure 6Fin plate connection vs end plate connection

Floor slabinstalled

on one sideof a beamthat wouldfail due toeccentricloading

condition

Masonrycladding

constructed prior to floorconstruction

thus preventingrestraint to primary

support beam

Glossary andfurther reading

Permanent condition ndash The state of an

element at the completion of construction

Temporary condition ndash The state of an

element during construction

Torsion ndash Rotational force along the axis of

a member

Further ReadingIles D C Hughes A and Malik A (2011)

Design of Steel Beams in Torsion Ascot

Steel Construction Institute

Eurocode 0

Applied practice

BS EN 1992-1-1 Eurocode 2 Design of

concrete structures ndash Part 1-1 General rules

and rules for buildings

BS EN 1992-1-1 UK National Annex to

Eurocode 2 Design of concrete structures ndash

Part 1-1 General rules and rules for buildings

1-21 how to avoid torsion

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