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FREESTANDING WALLS

Freestanding walls are attractive, robust and enduring, but they need careful attention to design and specification.

Proper design is critical to freestanding walls, because their relative slenderness affects their stability in relation to horizontal forces, particularly wind. Design involves more than just deciding on the length and height required and the arbitrary incorporation of a few piers.

A typical construction can be seen in figure 1. A wall of only half-brick (or single skin) thickness has very limited potential and is not considered stable above 725mm high, even in a very sheltered location. A wall on such a site should be no higher than 1925mm even if it is one brick thick (215mm), and in a very exposed location this drops to 1075mm. A wall one and a half bricks thick (327mm) has a maxi-wall on sites in the UK. It gives maximum height and minimum foundation width for walls of half-brick, one-brick and one and a half brick thickness in sheltered and exposed locations.

A very sheltered location is typically one shielded by neighbouring buildings in a town or city in a region that does not normally experience excessively high wind. By contrast, unsheltered locations in regions that are liable to very high winds will be subject to strong horizontal wind forces. Between these two extremes conditions vary considerably. Designing for a worst case scenario would be uneconomic in most instances, but designing for an average exposure would be dangerous. The BRE Good Building Guide 14, Building simple plan brick or blockwork freestanding walls, is a prescriptive guide for the design of freestanding boundary walls on sites in the UK. It gives maximum height and minimum foundation width for walls of half-brick, one-brick and one and a half brick thickness in sheltered and exposed locations.

For information for those about to design or build a free standing wall order our Design of Free Standing Walls publication.

ADDING STRENGTH

Piers are recommended to strengthen the ends of walls of half-brick and one-brick thickness as shown in figure 2. Piers would be suitable to support a light gate, say 1200mm high by 800mm wide and weighing a maximum of 10kg. Specialist advice should be sought for heavier gates.

Increasing the thickness of a plain wall will improve its wind resistance, but the plan shape of a straight solid wall is not very efficient because of its narrow base. Curved or zigzag walls have greater effective width and so better resistance to horizontal force; figure 3 shows typical examples: staggered, echelon, serpentine and diaphragm plan forms.
The diaphragm wall is particularly interesting. It appears to be a thick wall, but it is not solid. Cross ribs (usually at five stretcher intervals) are bonded into both half-brick leaves so the whole structure acts as one in resisting wind forces.

A diaphragm wall of one and a half bricks overall thickness uses less than 5% more material than a one-brick thick solid wall, but can resist wind loads on walls over 2m high in most locations. Increasing the overall width of the wall, and hence the length of the ribs, creates a very strong structure.

Diaphragm walls of 440mm overall (two bricks) and 3m high are stable in exposed locations. Tall walls of this type are sometimes found enclosing the gardens of Victorian country houses. Piers can also be used to stiffen a straight freestanding wall, but must project both sides and need to be quite large to be very effective. Their efficiency can be greatly improved by reinforcement with built-in vertical steel rods anchored into the foundations. Half-brick thick walls with reinforced piers 1800mm apart can be stable up to 2.5 m high. BRE Good Building Guide 19, Building reinforced, diaphragm and wide plan freestanding walls, provides detailed guidance on the design and construction of these more efficient forms. Values for wall height, foundation width and diameter of steel reinforcement for piers are given for the various levels of exposure across the UK.

MOVEMENT CONTROL

Freestanding walls longer than about 10m require movement joints to control expansion caused by changes in moisture content and temperature. These vertical joints break the wall into discontinuous lengths to prevent cumulative stress that could cause cracking, movement or instability.

It is recommended that movement joints be located no further than 6m from a corner or end, and no more than 12m apart. If the wall changes level it is preferable to locate movement joints at those positions. Short returns in brickwork can be rigid, and if they interrupt long straight runs they can concentrate stress and cause cracking. Such returns should be treated as corners even if they are only a half brick offset.

Some manufacturers recommend more frequent movement joints in new freestanding walls. This is because although the differences are not great, some clay bricks have greater potential for moisture expansion than others.

A manufacturer’s advice should be obtained for specific proposals, but as a general guide the standard recommendations can be used for soft-mud, stock and fletton bricks; however, when low water absorption, engineering types of brick are used, the distance of joints from ends and corners should be reduced to 4m and the spacing between joints to 8m.
The joints should be continuous for the full height of the wall, including any coping or capping. They should be 15mm wide and filled with a soft, compressible polyethylene or polyurethane cellular foam to keep stones or mortar out and allow the gap to close partially over time. Fibreboard or polystyrene boards are not compressible enough, a flexible sealant can give the joint a neat finish – especially in copings or cappings – but it must not be pointed with mortar.

To maintain alignment of the wall across a movement joint slip ties can be incorporated, spaced at third points between ground level with a topmost tie four or five courses below the top of the wall. Ties should be of stainless steel, shaped at one end to bond with mortar in a bed joint on one side of the movement joint. The other end should be plain and enclosed in a plastic sleeve with a gap at the end at least equal to the width of the movement joint; it is set in the corresponding mortar joint in the adjoining length of wall. The ties allow the movement gap to close, but resist any other movement.

DURABILITY

Because they are exposed to the weather on both faces, freestanding walls get wetter and colder than the external walls of buildings and remain so for longer, so durability is a prime consideration. For guidance on selection of bricks and mortars see the BDA Design Note 7 ‘Brickwork durability‘.