Arches and Lintels: Part 2 - Lintels
A structural look at Arches and Lintels
Arches and lintels are found wherever it is necessary to support a wall etc. over an opening which is usually not more than about two metres wide. They provide support in completely different ways and an understanding of this is fundamental to selecting, specifying and assessing them.
Both arches and lintels occur naturally to some extent. The natural arch at Durdle Dor in Dorset is famous. It was many centuries before mankind understood the mechanics of the arch and devised the means to successfully construct them. However, the lintel was in a sense easier to use, if not understand. There are now so many different types of arches and lintels that a complete listing would be outside the scope of this Paper.
Lintels are much simpler than arches, and the forces involved are completely within the lintel. They provide support by resisting the bending moments and shear forces resulting from the loads that are applied to them. The first examples were probably trees falling across a stream, and mankind quickly learnt how to make use of lintels. There are examples of stone being used as lintels as far back as prehistoric times. The earliest lintels were probably of stone, as used in The Great Pyramid of Giza, built about 2560 BC, and Stonehenge, built about 2300BC. The use of timber for lintels probably developed in parallel with stone and was greatly assisted by the development of Bronze Age tools in the Middle East before 3000BC. Much later, the introduction of iron tools in the Iron Age after 1200BC greatly increased the ability to work both stone and timber.
There are many different shapes of lintels, determined by the properties of the material and the purpose for which the lintel is required. Architects and Structural Engineers regularly incorporate them into their designs. Stone and timber lintels are generally rectangular in cross section. Concrete lintels are usually either rectangular, ‘L’ shaped (known as ‘boot’ lintels), or rectangular but with one vertical face sloping (used in cavity walls).
The development of concrete and steel lintels, together with the fashion for larger windows introduced in the 1950’s and extended further in the1960’s, required the use of stronger lintels to carry the increased loads. Windows up to 2.4m wide were not uncommon in the 1960’s. These larger spans in turn required more attention to be paid to the bearings at the ends of these lintels due to the greater loads from them onto the masonry below.
Further developments in building introduced corner lintels, which act as cantilevers, box lintels, and eaves lintels. As steel lintels were developed designs for solid walls, and internal walls became available. To improve the thermal insulation properties and provide easier fixings for items such as curtain rails open back lintels were introduced.
As timber frame houses grew in popularity with developers timber frame lintels made of steel appeared. In recent years the increased thermal insulation requirements have precluded the continued use of some types of lintels for the external walls of buildings e.g. concrete ‘boot’ lintels. Others, such as steel lintels, have had their designs revised by the inclusion of insulation within the cavity space of the lintel.
The design and manufacture of lintels are covered by various British Standards. BS 5977 covers the Specification for Prefabricated Lintels, and BS 7543 The Durability of Buildings, Building Elements, Products and Components their design life.
There are also material specific Standards for the main materials used for lintels. BS EN ISO 1461 covers the Hot Dipped Galvanising of steel lintels. BS 7973 covers the requirements for spacers that can be used in reinforced concrete lintels.
Stone lintels can be found made from a variety of, usually local, stone. Hard durable stone, such as granite, resists natural deteoriation well, but softer stone, such as limestone, can be significantly weathered away over time. In theory the stone has no tensile strength and thus cannot be proven by calculation to be adequate to support the load. Stone is also vulnerable to defects within it, and to fractures occurring either naturally, or as a result of the quarrying, transporting and placing of the stone, or as a result of loading or thermal movement. For layered stone, such as some limestones, the direction of the layering in the lintel can significantly affect its weathering and strength.
Once the stone has fractured its actual strength can fall to zero. It would then require careful removal and replacement, with adequate temporary support for the masonry above, in order to restore the structural strength and support to the wall.
Lintels can also be made from reconstructed stone, sometimes called cast stone. These are common in areas where the external face of the wall is built of a local stone e.g. the Purbeck area. Reconstructed stone lintels are usually reinforced with steel bar reinforcement of the type used for reinforced concrete lintels. However, the stone and the manufacturing process by casting can result in a porous texture in the lintel and this can allow moisture to reach and then corrode the steel reinforcement. The lintels then require replacement which can be an expensive process as they need to be specially manufactured to the same size and with the same or matching stone as the original ones. The use of stainless steel reinforcement in these lintels is very cost effective and preferred to plain steel reinforcement. The reinforcement should always be positioned using spacers complying with BS 7973.
Timber lintels probably developed in parallel with stone ones. Timber is much easier to work, given suitable tools, and has good thermal properties. If it is kept dry and free from wood boring insect and fungal decay it can last indefinitely. However, it is sometimes used on the external face of a wall and can decay much sooner than the wall it is supporting. Timber is also subject to initial drying and shrinkage after the tree has been cut down, and this can result in the timber ceasing to provide support for the masonry above it after the wall has been built. For this reason only well seasoned or kiln dried timber should be used for lintels.
In many Victorian buildings the openings in the outer surface of the wall are supported by arches. The inner face is often supported by a timber lintel, commonly about 70mm deep and 100mm wide. When these decay and require replacement pre-stressed concrete lintels are sometimes used. This is a mistake because the stresses in the concrete and its behaviour are completely different from the original lime mortar brickwork of the wall. Cracks can appear between the two different forms of construction. The commonly used 65mm deep pre-stressed concrete plank lintel requires composite action with structural brickwork above in order to support the design load. This is unlikely to be provided by the Victorian brickwork. Providing the original timber lintel was of adequate size it is preferable to replace it with a new piece of timber. If the original size timber was too small then it is usually possible to replace it with a 140mm deep by 100mm wide (or larger) timber by removing one (or more) course(s) of brickwork from above the position of the original timber lintel.
Timber lintels are light in weight, and easy to cut and fix. They also provide easy fixings for items such as curtain rails, and have a relatively good thermal performance compared to stone or concrete lintels.
Two types of concrete lintels, reinforced or pre-stressed, are commonly found in buildings. The reinforced ones are reinforced with steel reinforcing bars (or rods) and can be made either on site or in a factory. Common problems with this type of lintel are:-
Poor quality concrete found in site made lintels. This is typically poor compaction and the resultant honeycombing of the concrete.
Misplaced reinforcement. If the reinforcement is not in the correct position within the concrete then the structural strength, durability, and fire resistance of the lintel can be greatly reduced. Lintels with the reinforcement close to the surface of the concrete i.e. with less than the specified cover to the reinforcement, can fail within a few years of installation. Misplaced reinforcement not complying with British Standard 7973:2001 can affect both site and factory made lintels.
Pre-stressed lintels have been in general use since the 1950’s. They were widely used across openings in both internal and external walls until the increasingly onerous requirements for thermal insulation under The Building Regulations resulted in them being no longer suitable for external walls.
The lintels are factory made by stretching steel wire along the length of a mould and then casting the concrete around it. When the concrete has gained sufficient strength the ends of the wires are released and the shrinkage in the length of the wire induces a compressive stress into the concrete. The sooner the concrete can gain sufficient strength the sooner the transfer can take place, the lintel removed and the mould re-used. This led to the use of High Alumina Cement for the concrete. This cement gained strength so quickly that by steam curing the concrete the transfer could be made 24 hours after casting. This led to a one day cycle for the production of lintels, and also the floor and roof beams widely used in buildings, e.g. blocks of flats. Later, when failures of the beams occurred, it became clear that the high temperatures used for steam curing the concrete caused the High Alumina Cement to chemically convert to a different form, with a large reduction in strength. The lintels (and beams) can be recognised because the High Alumina Cement concrete is usually dark grey in colour, whereas Ordinary Portland Cement concrete is light grey. High Alumina Cement which has undergone chemical conversion is often light brown in colour.
Pre-stressed lintels come in many shapes. Some, such as the ‘L’ shaped lintels were designed to be self supporting when carrying the load from above, but others, such as the plank lintels depend upon several courses of structural brickwork to provide the compressive element of their total strength. Up to seven courses of brickwork or three courses of blockwork may be required. Lintels requiring brickwork or blockwork to achieve their design loadings are known as composite lintels. It is essential that the brickwork or blockwork has no openings or holes within the depth of the required courses. This can be a particular problem when a building is subsequently altered and the builder does not realise that the brickwork or blockwork above the concrete forms a structural part of the lintel. Composite lintels require propping at midspan during construction of the wall above. If the span exceeds about 1.5m then additional propping at quarter points of the span is also required. The lintels must be propped for at least seven days after the structural masonry above them is completed, and this period will need to be longer if cold weather is present at the time. British Standard 5977 says “Composite lintels should adequately propped along their span prior to, during, and after the construction of the infill. The props should not be removed until the masonry element has reached the design strength and in no case earlier than 7 days”. Sometimes the lintels have not been adequately propped and can be seen to sag at midspan. This is due to the weight of the wet masonry imposing too great a load on the unpropped lintel. Once the masonry sets the lintel is permanently set in the sagged shape. Sometimes a d.p.c. is placed over the concrete lintel before the structural masonry is built. This destroys the necessary bond between the concrete lintel and the structural brickwork above it. The concrete lintel is likely to sag under the weight of the wet masonry.
Over time the compressive stresses in the concrete cause it to shorten in length. This can result in cracks appearing at the ends of the lintel either above or below the lintel, or in both locations. Cracks below the lintel usually start at the end of the lintel and extend in a diagonally downwards direction towards the centre of the lintel through the masonry on which the lintel is bearing. Cracks extending upwards usually start at the ends of the lintel and extend diagonally upwards and away from the centre of the lintel. Cracks in the inner leaf of a cavity wall are more common than those in the outer leaf because the inner leaf is generally formed of blockwork with a lower strength than the brick outer leaf.
The ends of the pre-stressing wires are cut off flush with the ends of the concrete lintel after the load transfer has taken place. The ends of the wires are exposed at each end of the lintel. Historically, the ends of the lintels were painted with a bituminous paint to protect the steel wires from corrosion. More recently this practice has been discontinued and the exposed ends of the wires are now unprotected from corrosion. This also applies to pre-stressed concrete floor beams used to construct suspended floors.
Concrete ‘L’ shaped ‘boot’ lintels were popular at one period in time. They could either bear on both leaves of a cavity wall or the front edge could be shortened so that the concrete was only visible on the outer face of the wall for the width of the opening below. This arrangement introduced torsional stresses into the lintel and the greater weight of the brickwork forming the outer skin of the wall compared with the lighter blockwork forming the inner skin could cause the lintel to tend to rotate forwards and the front edge to drop vertically downwards.
Metal lintels are made of mild steel or, more recently, of stainless steel as well. Mild steel lintels can be hot dipped galvanised after manufacture (the best option) or made from pre-galvanised steel which has a much thinner galvanised coating. Some lintels have an applied finish to the steel, such as paint.
Historically steel lintels were often rolled structural steel angle sections. In more recent decades the proprietary formed steel lintel was introduced and there are many different shapes and types. Large openings may be spanned with a steel channel or joist / beam section supporting the inner skin or face of a wall with a steel plate welded onto the bottom flange of the section. The plate extends out to form the support for the outer part of the wall.
The corrosion protection of the lintel is particularly important if it is to last for the design life of the building, normally 60 years, without the need for replacement. Hot dipped galvanised lintels to BS EN ISO 1461 and its predecessors have proved to be durable in many locations, but the Zinc Millennium Map should be consulted to determine the Annual Average Atmospheric Corrosion rate for zinc in the UK and the Republic of Ireland. Lintels made from pre-galvanised sheet may last as little as 15 years in some areas unless additional protection is provided to the zinc. Additional protection may also be required if, for example, the mortar contains substances which are aggressive to the zinc coating.
Generally the simpler the lintel shape the less likelihood there is of corrosion starting in laps or crevices. An example of a ‘Top Hat’ design of lintel is shown in Figure 1 below. These are formed from one piece of metal without welding or the crevices that result from two pieces of metal being overlapped and spot welded. They maintain the continuity of the brickwork and blockwork across the opening, (which makes the fixing of curtain rails etc. much easier), have good thermal properties, and are less likely to cause cracks in the plaster due to the differential expansion or shrinkage found with other types of lintels. However, they have a continuous metal path from the inside of the wall to the outside and this is not good for meeting the increasingly onerous thermal performance required by changes in legislation.
In recent years the availability of stainless steel lintels at economic prices has greatly improved the ability of the lintel to achieve the design life of the building with ease. They are available in a wide variety of shapes and sizes. Non-standard lintels can be made within a few days by some manufacturers.
Lintels can be formed from brickwork, either reinforced or post-tensioned. Reinforced brick lintels are made by including metal reinforcement within the bed joints of the brickwork. The reinforcement is invisible from the outside of the finished lintel. The bed joint reinforcement can either be expanded metal, welded lattice, (such as ‘Murfor’), twisted wire (such as ‘BrickTor’), individual twisted strips (such as ‘Spiro-Bars’), or rectangular welded strips. The materials can be either galvanised steel or more commonly in recent years, stainless steel. The absence of the front edge of a conventional lintel on the face of the brickwork does not necessarily mean that there is no lintel in the wall. Reinforced brickwork is not suitable for bricks laid on end (i.e. a soldier course) over an opening because of the absence of bed joints in which to place the reinforcement. Proprietary reinforcement systems are also available.
Brickwork lintels can be formed for soldier course brickwork by using bricks with holes in them, commonly three hole bricks, and threading a bar or threaded rod through the holes. At each end a thick spreader plate washer is placed on the bar and then the nuts. After the mortar has set sufficiently the nuts are tightened and the lintel can take the load from above. This type of lintel needs full support across the opening when the bricks are laid and this is usually done by providing a timber or scaffold board. The lintel provides the soldier course appearance over the opening, with no visible support. However, most soldier courses are formed of plain brickwork and do not have reinforcement in them. Therefore they need a separate lintel underneath to provide structural support. This would usually be a steel lintel, the front edge of which should be visible below the soldier course.
For some openings combination lintels are used. These comprise one type of lintel or material for supporting the outer surface or skin of the wall and another for the inner surface or skin. These can be stainless steel for the outer lintel and galvanised steel for the inner lintel to give greater corrosion protection to the outer lintel in, for example, coastal locations. The two parts of the lintel are separated by a plastic strip to provide a thermal break and to isolate the different types of metal from bi-metallic corrosion.
Another combination is a galvanised steel ‘Z’ shape outer lintel with a rectangular timber inner lintel. This gives the long term durability of galvanised steel on the outer surface of the wall and good thermal performance and easy fixing for curtain rails on the inner surface.
A similar combination is a galvanised steel ‘Z’ shape outer lintel with a reinforced or pre-stressed concrete inner lintel. This was sometimes done by developers for cheapness. However, with the increasing thermal requirements under The Building Regulations its use has ceased.
Other combinations of materials may occasionally be found in buildings.
The bearing of the ends of the lintel on the masonry below is important as it is the means by which the loads from the lintel are transferred into the wall and thence to the foundations. Historically the spans of openings were usually small, often not exceeding 1.2m, and a bearing of about 4 ½” (one half brick) (110mm) was usually sufficient for stone or timber lintels. With the introduction of wider spans, and sometimes heavyweight floor loading on the lintels, the end bearing was required to be increased to at least 150mm. However, although this is usually possible when the lintel is in the same plane as the wall it is frequently not possible when the lintel is at right angles to, say, a 100mm thick blockwork wall. It is important that the actual bearing stress on the wall is checked to ensure that it is within the acceptable limits for the masonry and that the ‘150mm minimum bearing’ specified by lintel manufacturers is not followed blindly. The masonry in, for example, old lime mortar buildings or new lightweight thermal blockwork may require a larger bearing area or the use of padstones to distribute the bearing loads into the wall at an acceptable stress.
Openings with no lintels
Some openings do not have a lintel across them. For example, from the 1950’s to the 1970’s and earlier it was common practice on housing developments for the outer leaf of a cavity wall to have the brickwork built across the top of the door or window frame without using a lintel. The timber door or window frame was of sufficient size and strength to be regarded as providing support for the masonry above. With the introduction of replacement windows (and doors), especially ones made of PVCu, the masonry no longer had any structural support from the frame below. Typically PVCu windows are designed to have a gap of about 5mm all round the perimeter of the frame to allow for fitting in the existing opening, and thermal movement, so they provide no loadbearing support at all. As a result the masonry can crack and sag, and lintels have to be fitted retrospectively to correct a problem that should have been rectified by the window installer before the replacement windows were fitted. This is a commonly found problem.
Bay windows with no lintels
Bay windows are usually either of a rectangular or segmental shape in plan. There were popular in the 1930’s and the first floor joists and the roof structure were often supported on loadbearing timber window frames. When these are replaced with non-loadbearing replacement windows, such as PVCu, the installer rarely takes account of the fact that the timber window frames are loadbearing and there are no lintels to carry the load from the first floor joists and the roof structure. As a result the area of wall between the ground floor and first floor windows is deprived of its previous support. It can then settle and cracks appear at the junctions of the bay window wall and the main wall of the house. Similarly, the roof structure can also settle. Previous attempts by some replacement window installers to include loadbearing mullions between the individual window frames were unsuccessful because the top and bottom ends of the mullions stopped short of the loadbearing structure of the house. Very recently a new product called a baypole jack has appeared on the market from various manufacturers. It is a small screw jack which is fitted at the bottom end of the baypole and tightened to provide a loadbearing path for the loads from above. As this is an additional cost to the window installer there is the temptation to use the cheapest baypole jack available, which may not provide the required support. All replacement bay windows from their inception until very recently are likely to have no structural support for the loadbearing mullions.
Sustainable Construction - Current Best Practice.
The increasingly onerous thermal insulation requirements of The Building Regulations have significantly influenced the design and materials used for lintels. To achieve the highest levels of thermal insulation and sustainable construction it is necessary to use separate lintels to support the inner and outer leaves of cavity walls in order to avoid the thermal bridging that is inherent in combination lintels. The use of combined concrete lintels such as the ‘L’ shaped ‘boot’ lintel became obsolete some time ago.
The use of two separate ‘L’ shaped steel lintels, spaced apart by nearly the width of the cavity and positioned back to back with almost full width cavity insulation between them will avoid the thermal bridging and provide a ‘U’ value that is nearly as good as the wall itself. A d.p.c is required from above the top of the inner lintel down to the lower part of the outer lintel in order to deflect any moisture in the cavity to the outside of the wall. A typical example of current best practice sustainable construction is shown in Diagram 3. Where the inner leaf of the wall supports a floor larger ‘L’ shaped standard lintels are available, and for greater loads a standard 225mm high rectangular ‘C’ shaped lintel can be used without adversely affecting the thermal performance of the wall. For even greater loads such as those from a concrete floor a standard parallel flange channel (PFC) can be used to support the floor and inner leaf of the wall. An insulated cavity closer is required around the perimeter of the opening, between the inner and outer skins of the wall.
Lintels provide a useful method of providing support over openings. There are many different types, and some are more structurally stable than others. Being perceived as ‘simple’ building components they are sometimes not treated with the respect they deserve. In specifying or assessing lintels the implications of the type and condition should be carefully considered and where necessary, e.g. for a house purchase report, brought to the attention of the present or future owner of the building. When selected and installed correctly lintels should last for at least the 60 years design life of domestic dwellings, without the need for structural repairs.
Chris Shaw CEng FICE FIET MIStructE MCMI
15 Mareschal Road
Tel: / Ans: / Fax: 01483 536577
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