Results

Movement joints should be provided in the masonry to accommodate its expansion and/or contraction due to changes in temperature and the moisture characteristics of the masonry units. Joints should be built-in as work proceeds.

When designing movement joints in separating walls, party walls or compartment walls, the reduction in the efficiency of the wall as an insulator of sound, or as a fire barrier (see 5.8), should be taken into consideration.

Where necessary, slip ties or dowels strong enough to provide lateral stability should be incorporated. They are usually of metal rod or flat strip and one end is anchored, or otherwise fixed, into masonry on one side of a movement joint. The other end is built-in but debonded to allow the joint to open and close while preventing movement in any other direction.

5.4.2.2 Slip planes

Slip planes should be designed to allow parts of the construction to slide, one in relation to the other, thus reducing shear stresses in the adjacent materials.

The design and positioning of movement joints and slip planes should be carefully considered, to ensure that in addition to accommodating movements, such joints or planes do not impair the stability of the wall or any of its functions.

In external wails, movement joints and slip planes should be sealed, protected or otherwise designed to prevent water penetration (see 5.4.4).

Fixings and services should not interfere with the performance of the joints or slip planes, e.g. by bridging them. Finishes should be discontinuous at movement joints and slip planes, and fixings and fittings should not tie across joints.

5.4.2.3 Provision of movement joints 5.4.2.3.1 General

These empirical recommendations are applicable to the majority of situations.

It is not necessary to provide movement joints where the length of internal walls or the inner leaves of cavity walls in dwellings is relatively short.

The spacing of the first movement joint from an external or internal angle should be not more than half the general spacing, and preferably less, where the masonry is continuous at the angle, due to the effect of end restraint of the wall panel.

When more detailed information is needed on basic data and design to accommodate movement, see Annex B. The material manufacturers should also be consulted.

5.4.2.3.2 Spacing and width of movement joints in clay masonry

In general, unrestrained or lightly restrained unreinforced walls, e.g. parapets and non-loaded spandrel panels built of membrane-type d.p.c.s, expand 1mm/m during the life of a building due to thermal and moisture movement changes. The spacing and width of movement joints to control expansion in such walls is governed by the compressibility of fillers and the performance of appropriate sealants (see 5.4.4).

Designers are recommended to consult sealant manufacturers wherever possible, but as a general guide, the width of the joint in millimetres should be about 30 % more than the numerical value of the distance between joints in metres. For example, movement joints at 12 m centres should be about 16 mm wide.

Where a manufacturer can show evidence from experience that the product expands by less than 1 mm/m during the life of a building due to thermal and moisture movement changes, the foregoing guidance may be modified at the designer's discretion.

Experience shows that the expansion of normal storey height walls, as opposed to unrestrained walls, is somewhat less than 1 mm/m during the life of a building and that expansion often reduces with increasing restraint. However, to avoid cracking due to thermal contraction, the spacing between movement joints should never exceed 15 m in unreinforced walls. Closer spacing may be necessary for walls that are less well restrained, e.g. parapets. Where bed joint reinforcement is used, spacings greater than 15 m can be satisfactory, but expert advice should be sought.

Present evidence suggests that vertical movement of unrestrained walls is of the same order as horizontal movement.

5.4.2.3.3 Dissimilar clay units

Where clay bricks made from different types of clay are built in the same wall to produce decorative effects, e.g. contrasting bands of colour, differences in their movement characteristics can dictate that more

5.4.2.3.4 Short returns in clay masonry

If a return in the length of clay masonry is less than 675 mm and either adjoining length of masonry exceeds 6 m, the masonry should be interrupted at the return to prevent the development of a mechanical couple and a risk of cracking. This can be effected by the introduction of a vertical, compressible joint or a

§slide-by¨ detail. Returns of 675 mm or more should be regarded as having sufficient inherent flexibility to accommodate the stress caused by the opposing forces (see Figure 15).

5.4.2.3.5 Spacing and width of movement joints in calcium silicate masonry

Where possible, calcium silicate masonry should be designed as a series of panels separated by movement joints to control contraction. The ratio of length to height of the panels should generally not exceed 3:1.

As a rule, vertical joints to accommodate horizontal movement should be provided at intervals of between 7.5 m and 9 m. Movement joints need not generally exceed 10 mm in width. They should be sealed where necessary.

In external walls containing openings, movement joints may be needed at more frequent intervals or the masonry above and below the opening may need to be reinforced in order to restrain movement (see 5.4.5).

The design should pay particular attention to long low horizontal panels of masonry, e.g. those under windows.

External walls of calcium silicate masonry should have a compressible joint to allow for thermal expansion at not more than 30 m intervals.

5.4.2.3.6 Spacing and width of movement joints in concrete masonry

Where possible, concrete masonry should be designed as a series of panels separated by movement joints to control contraction. The degree of movement is dependent upon unit type and, as a rule, vertical joints

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to accommodate horizontal movement should be provided at intervals of between 6 m and 9 m. The ratio of length to height of the panels should generally not exceed 3:1.

In external walls containing openings, movement joints may need to be provided at more frequent intervals, or the masonry above and below the opening may need to be reinforced to restrain movement (see 5.4.5). Particular attention should be paid to long low horizontal panels of masonry, e.g. those under windows.

External walls of concrete masonry should have a compressible joint to allow for thermal expansion at not more than 30 m intervals.

5.4.2.3.7 Spacing and width of movement joints in natural stone masonry

To accommodate horizontal movement, and in the absence of specific calculations (see Annex B), vertical joints not less than 10 mm wide should be provided at intervals no greater than 15 m to 20 m, and located no more than 8 m from an external corner.

5.4.2.3.8 Placing of movement joints

Features of the building which should be considered when determining joint positions in masonry are:

a) intersecting walls, piers, floors, etc.;

b) window and door openings;

c) changes in height or thickness of walls;

d) chases in walls;

e) movement joints in the building as a whole or in floor slabs.

Areas above doors and above or below windows may benefit from being reinforced to distribute tensile stresses (see 5.4.5).