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CIVIL & STRUCTURAL DESIGNERS DATA PACK

SECTION 10: MOVEMENT AND DEFLECTION

10.1 TOLERANCES

For further information on tolerances refer to TRM 79. Be realistic, not optimistic. Usepublished criteria where possible, eg National Structural Concrete Specification (NSCS),National Structural Steel Specification (NSSS). Particular care is needed when differentmaterials and trades are used; brick cladding supported on stainless steel angles fixed toconcrete casing cast on a steel frame was a real case.

Remember that each 'permitted deviation' is a limiting value, so that cumulative deviationsshould be calculated by the 'root-sum-of-squares' rule, not by straight addition. As anexample, if a column can be set out with a deviation of 10 mm and can then be out of plumbby 8 mm, the deviation of the head will be (10

2+ 8

2) = 13 mm, not 10 + 8 = 18 mm. The rule

can be applied to any number of deviations, eg if the column can be bowed by 6 mm, themid-height can be out of position by (10

2+ (8/2)

2+ 6

2) = 12 mm.

10.2 MOVEMENT

For further information on movement refer to TRMs 109 and 110. The principle categoriesare:

Deflection of beams and slabs from elastic deformation plus creep under sustainedload and shrinkage where appropriate.

Horizontal shortening, usually from shrinkage (including early thermal contraction) butalso from creep in prestressed members.

Vertical shortening and foundation settlement from elastic deformation plus creepunder sustained load and shrinkage where appropriate.

Thermal, combining ambient temperature and solar gain where appropriate.

Wind.

Vertical movements are important for the design of cladding of all types, but usually arisefrom deflection alone with no significant contribution from the other sources. However, someof the deflection will occur before the cladding is fixed; this part should be treated as anadditional tolerance, leaving only the remainder to be accommodated by the cladding asmovement.

Horizontal movements can also be important for cladding design, particularly if the cladding is

inserted inside a structural frame. Wind and thermal can usually be consideredindependently, ie it is assumed that strong winds are unlikely to occur simultaneously withvery cold or very hot temperatures. However, wind or thermal should each be combined witheither of the two extremes of horizontal shortening - immediate (elastic only) and long-term(after final creep and shrinkage).

All these combinations are normally calculated by straight addition, which emphasises theneed to use average properties to get realistic deflection estimates.

It is then important to communicate the findings to other members of the team, usually thearchitect and relevant specialist contractors. Make sure the distinction between tolerances - ie


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deviations occurring before the element is fixed - and movements - occurring afterwards - isclear.

10.3 MOVEMENT JOINTS

10.3.1 General

Joints should be provided to minimise the effects of movements caused by, for example,shrinkage, temperature variations, creep and settlement.

The effectiveness of movement joints depends on their location, which should divide thestructure into a number of individual sections. The joints should pass through the wholestructure above ground level in one plane. The structure should be framed on both sidesof the joint, and each section should be structurally independent and designed to bestable and robust without relying on the stability of adjacent sections.

Some examples of positioning movements joint in plan are given in fig 10.1.

Figure 10.1. Location of movement joints

Reproduced by kind permission of the Institution of Structural Engineers from its publication Manual for the design ofreinforced concrete building structures

Movement joints may also be required where there is a significant change in the type offoundation or in the height of the structure.

Attention should be drawn to the necessity of ensuring that joints are incorporated in thefinishes and in the cladding at the movement joint locations.

10.3.2 Movement joints concrete structures

a) Concrete frame structures: movement joints at least 25 mm wide should normally

be provided at approximately 50 m centres both longitudinally and transversely.In the top storey and for open buildings and exposed slabs additional jointsshould normally be provided to give approximately 25 m spacing.

b) Ground bearing slabs: refer to TRM 67 for guidance

10.3.3 Movement joints steel structures

Table 10.1 gives a summary of recommendations for the spacing of expansion joints insteel framed buildings.


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Single Storey Buildings Generally Note 1 150m

Building subject to HigherInternal Temperatures

Note 1 125m

Multi-Storey Buildings Simple Construction Note 1 100mContinuous Construction Note 2 50m

Roof Sheeting Down the slope Note 3 20m

Along the slope No LimitNotes:1. Where the stress due to the constraint of thermal expansion has been considered in

the member design, no limit is necessary in simple construction.2. Larger spacings are possible if the stresses due to the constraint of thermal expansion

are considered in the member design.3. Longer lengths are possible where provision for expansion is made.

Table 10.1. Spacing of expansion joints in steel framed buildings.

10.3.4 Movement joints masonry structures

Movement joint locations and spacing need to be carefully considered.

Section 7.2 provides recommendations.

The uninterrupted height and length of the outer leaf of external cavity walls should belimited so as to avoid undue loosening of the ties arising from differential movementsbetween the two leaves. The outer leaf should, therefore, be supported at intervals of notmore than every third storey or 9 m, whichever is less. However, for buildings notexceeding 4 storeys or 12 m in height, whichever is less, the outer leaf may beuninterrupted for its full height.

Joint spacing(m)

Recommended jointwidths

Up to 7 10-12 mm

7-11 15 mm

11-15 15-20 mm

Notes:1. Maximum joint spacing specified in clause 20.3.2.2 of BS 5628: Part

3: 1985.2. A shear joint should be a minimum of 10 mm wide, the width of the

sealant being equal to or greater than its depth.

Table 10.2. Recommend widths of movement joints in masonry walls.

10.4 DEFLECTIONS

10.4.1 Deflections concrete structures

The basic span/effective depth ratios for beams are given in Table 5.1. These are basedon limiting the total deflection to span/250 and this should normally ensure that the part ofthe deflection occurring after the construction of finishes and partitions will be limited tospan/500 or 20 mm, whichever is the lesser, for spans up to 10 m.


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10.4.2 Deflections steel structures

Table 10.3 gives a summary of recommendations for deflections of individual numbers insteel framed buildings.

a) Vertical deflection of beams due to imposed load

Cantilevers Length/180Beams carrying plaster or other brittle finish Span/360

Beams supporting glazing Span/500(1)

Other Beams (except purlins and sheeting rails) Span/200Purlins and sheeting rails To suit the type of

cladding

b) Horizontal deflection of columns due to imposed load and wind load

Tops of columns in single-storey buildings, except portal frames Height/300

In each storey of a building with more than one storey Height of thatstorey/300

c) Crane girders

Vertical deflection due to static vertical wheel loads from overheadtravelling cranes

Span/600

Horizontal deflection (calculated on the top flange propertiesalone) due to horizontal crane loads

Span/500

(1) At the design stage, establish with the architect the type of glazing system whichwill be used and confirm with the manufacturer the vertical and horizontal deflectionswhich can be accommodated by their system.

Table 10.3. Suggested deflection limits

Indicative deflection limits for portal frames are shown in Table 10.4. Buildings with cranesshould be considered as a special case, and specialist advice should be sought.

Horizontal Deflection at Eaves Level

Type of Wall Cladding Absolute Limit

Profiled Metal Sheeting h/150

Fibre reinforced Sheeting h/150

Masonry h/300

Vertical Deflection at Ridge

Type of Roof Cladding

Profiled Metal Sheeting h/200

Fibre reinforced Sheeting h/200

Where: h = height to eavess = span

Notes:1. It may be necessary to consider differential deflections relative to adjacentframes.2. Limits are typically based on the worst case of unfactored wind load or unfactored

imposed roof load or 0.8 unfactored (wind + imposed) loads. It may also benecessary to consider the proportion of the dead load relating to services.

Table 10.4. Deflection limits for pitched roof portal frames (pitch 3 degrees)


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10.4.3 Deflections masonry structures

Table 10.5 gives a summary of deflection of members supporting or restraining anexternal masonry cavity wall.

Building Structural member Load SupportedTheoretical deflections

Vertical Horizontal

Singlestorey

Eaves beam Wind only - L/360Eaves beam Wind + SWT

rainwater gutterL/500 L/500

Main columns (sway)1

Roof, dead and wind - h/300Gable post Wind only - L/360

Intermediate post Wind only - L/360Horizontal restraintrail

Wind only - L/360

Multi-storey

Main columns (sway)3

Wind only - h/300 h/600

Intermediate post Wind only - L/360Horizontal restraint Wind only - L/360

Perimeter floor beam Floor (dead + super) L/200(super)

-

Perimeter floor beam Floor (dead + super)+ SWT inner leaf

L/500 (fullload)

-

Notes1. L, is the horizontal or vertical span of the beam or post.

H, is the height of the column.2. Refer to figure 5.5. Maximum theoretical differential deflection between the

braced gable frame and the first portal frame limited to600

diagonaloflength.

3. The theoretical deflections quoted are per storey height. However, the

permissible cumulative deflection at the top of a multi-storey building will probablyfurther limit the deflection of the columns.

Table 10.5. Deflections for members supporting and/or restraining an externalmasonry cavity walls.

10.5 WATCH IT NOTES

C&S 2 - MEMBERS SUPPORTING GLAZING (DEFLECTION OF)

Issued February 1997

On a recent project it has come to light that the glazing system adopted was unable toaccommodate vertical imposed load deflections of more than 2 mm. This meant that for thecolumn spacing of 6 m the deflection limit was span/3000 which is much more onerous thanwe would normally design for. To avoid this problem in the future the following course ofaction is recommended:

At the design stage establish with the architect the type of glazing system which will beused and confirm with the manufacturer the vertical and horizontal deflections which canbe accommodated by their system.

If the architect wants to keep their options open then agree the deflection criteria to beadopted in the design of the supporting structure with them. In the absence of any betterinformation a limit of span/500 is the code recommendation for members supportingbrittle finishes.


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In both cases the deflection criteria adopted must be clearly stated in the specificationand on the drawings which are issued for tender. It must also be clearly stated that thecontractor is to design the glazing system to accommodate these movements.

10.6 FURTHER READING

1. Alexander, S.J. and Lawson, R.M.: Design for movement in buildings, CIRIATechnical Note 107, CIRIA, London, 1981.

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