Concrete lintels.

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Since Portland cement was first mass produced towards the end of the nineteenth century it has been practical and economic to cast and use concrete lintels to support brickwork over openings.

Concrete is made from reasonably cheap materials, it can easily be moulded or cast when wet and when it hardens it has very good strength in resisting crushing and does not lose strength or otherwise deteriorate when exposed to the weather. The one desirable quality that concrete lacks, if it is to be used as a lintel, is tensile strength, that is strength to resist being pulled apart. To provide the necessary tensile strength to concrete steel reinforcement is east into concrete.

For a simple explanation for the need and placing of reinforcement in concrete lintels suppose that a piece of india rubber were used as a lintel. Under load any material supported at its ends will deflect, bend, under its own weight and loads that it supports. India rubber has very poor compressive and tensile strength so that under load it will bend very noticeably, as illustrated in Fig. 90. The top surface of the rubber becomes squeezed, indicating compression, and the lower surface stretched, indicating tension. A close examination of the india rubber shows that it is most squeezed at its top surface and progressively less to the centre, and conversely most stretched and progressively less up from its bottom surface to the centre of depth.

A concrete lintel will not bend so obviously as india rubber, but it will bend and its top surface will be compressed and its bottom surface stretched or in tension under load. Concrete is strong in resisting compression but weak in resisting tension, and to give the concrete lintel the strength required to resist the tension which is maximum at its lower surface, steel is added, because steel is strong in resisting tension, This is the reason why rods of steel are cast into the bottom of a concrete lintel when it is being moulded in its wet state.

Lengths of steel rod are cast into the bottom of concrete lintels to give them strength in resisting tensile or stretching forces. As the tension is greatest at the underside of the lintel it would seem sensible to cast the steel rods in the lowest surface. In fact the steel rods are cast in some 15mm or more above the bottom surface. 

The reason for this is that steel very soon rusts when exposed to air and if the steel rods were in the lower surface of the lintel they would rust, expand and rupture the concrete around them, and in time give way and the lintel might collapse. Also if a fire occurs in the building the steel rods would, if cast in the surface, expand and come away from the concrete and the lintel collapse. The rods are cast at least 15 mm up from the bottom of the lintel and 15 mm or more of concrete below them is called the concrete cover.

Fig. 90 Bending under load.

Head of openings in solid walls and Timber lintels.

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Head of openings in solid walls.
Solid brickwork over the head of openings has to be supported by either a lintel or an arch. The brickwork which the lintel or arch has to support is an isosceles triangle with 600 angles, formed by the bonding of bricks, as illustrated in Fig. 89. The triangle is formed by the vertical joints between bricks which overlap 1/4 B. In a bonded wall if the solid brickwork inside the triangle were taken out the load of the wall above the triangle would be transferred to the bricks of each side of the opening in what is termed ‘the arching effect’.

Lintel is the name given to any single solid length of timber, stone, steel or concrete built in over an opening to support the wall over it, as shown in Fig. 89. The ends of the lintel must be built into the brick or blockwork over the jambs to convey the weight carried by the lintel to the jambs. The area of wall on which the end of a lintel bears is termed its bearing at ends. The wider the opening the more weight the lintel has to support and the greater its bearing at ends must be to transmit the load it carries to an area capable of supporting it. For convenience its depth is usually made a multiple of brick course height, that is about 75 mm, and the lintels are not usually less than 150 mm deep. 

 Fig. 89 Head of openings.

Timber lintels.

Up to the beginning of the twentieth century it was common practice to support the brickwork over openings on a timber lintel. Wood lintels are less used today because wood may be damaged during a fire and because timber is liable to rot in conditions of persistent damp.

Bonding of bricks at rebated jambs - Walls.

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 Just as at an angle or quoin in brickwork, bricks specially cut have to be used to complete, or close, the 1/4 B overlap caused by bonding, so at jambs special closer bricks B wide on face have to be used.

Provided that the outer reveal is 1/4 B wide, the following basic rules will apply irrespective of the sort of bond used or the thickness of the wall. If the rebate is B deep the bonding at one jamb will be arranged as illustrated in Fig. 88. In every other course of bricks a header face and then a closer of 1/4 B wide face must appear at the jamb or angle of the opening. To do this and at the same time to form the 1/4 B deep rebate and to avoid vertical joints continuously up the wall, two cut bricks have to be used.

These are a bevelled bat (a ‘bat’ is any cut part of a brick), which is shaped as shown in Fig. 88, and a king closer, which is illustrated in Fig. 88. Neither of these bricks is made specially to the shape and size shown, but is cut from whole bricks on the site.

In the course above and below, two other cut bricks, called bevelled closers, should be used behind the stretcher brick. These two bricks are used so as to avoid a vertical joint. Figure 88 shows a view of a bevelled closer. 

Fig. 88 Bonding at rebated jambs.

Where the rebate is 1/2 B deep the bonding is less complicated. An arrangement of half bats as quoin header and two bevelled closers in alternate courses for English bond and half bats and king closers in alternate courses for Flemish bond is used.

Jambs of openings and Rebated jambs - Walls.

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Jambs of openings.
The jambs of openings for windows and doors in solid walls may be plain (square) or rebated.

Plain or square jambs are used for small section window or door frames of steel and also for larger section frames where the whole of the external face of frames is to be exposed externally. The bonding of brickwork at square jambs is the same as for stop ends and angles with a closer next to a header in alternate courses to complete the bond.

Rebated jambs.

Window and door frames made of soft wood have to be painted for protection from rain, for if wood becomes saturated it swells and in time may decay. With some styles of architecture it is thought best to hide as much of the window frame as possible. So either as a partial protection against rain or for appearance sake, or for both reasons, the jambs of openings are rebated.

Figure 87 is a diagram of one rebated jamb on which the terms used are noted.

As one of the purposes of a rebated jamb is to protect the frame from rain the rebate faces into the building and the frame of the window or door is fixed behind the rebate.

The thickness of brickwork that shows at the jamb of openings is described as the reveal. With rebated jambs there is an inner reveal and an outer reveal separated by the rebate.

The outer reveal is usually 1/2 B wide for ease of bonding bricks and may be 1 B wide in thick solid walling. The width of the inner reveal is determined by the relative width of the outer reveal and wall thickness.

The depth of the rebate is either 1/4 B (about 51 mm) or 1/2 B (102.5mm). A 1/4 B rebate is used to protect and mask solid wood frames and the 1/2 B deep rebate to protect and mask the box frames to vertically sliding wood sash windows. The 1/2 B deep rebate virtually covers the external face of cased wood frames  to the extent that a window opening appears to be glass with a narrow surround of wood.

Fig. 87 Rebated jamb.

Openings in solid walls.

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For the strength and stability of walling the size of openings in walls is limited by regulations for both solid and cavity walls.

-  Jambs of openings and Rebated jambs - Walls.

-  Bonding of bricks at rebated jambs - Walls.

-  Head of openings in solid walls and Timber lintels.

-  Concrete lintels.

-  Reinforcing rods and Casting lintels - Walls.

-  Prestressed concrete lintels and Composite and non-composite lintels- Walls.

-  Boot lintels - Walls.

-  Pressed steel lintels - walls.

-  Brick lintels - walls.

-  Brick Arches – Semi-cirular arch – Rough and axed arches.

-  Brick arches: Gauged bricks and Two ring arch.

-   Brick Arches – Segmental arch, Centering, Flat camber arch, Flat gauged camber arch.

-  Solid walls: Thermal insulation. Internal insulation.

Slate and tile hanging - Walls.

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In positions of very severe exposure to wind driven rain, as on high open ground facing the prevailing wind and on the coast facing open sea, it is necessary to protect both solid and cavity walls with an external cladding. The traditional wall cladding is slate or tile hanging in the form of slates or tiles hung double lap on timber battens nailed to counter battens. Slate hanging has generally been used in the north and tile in the south of Great Britain. Either natural or manufactured slates and tiles can be used.

As a fixing for slating or tiling battens, 50 x 25 mm timber counter battens are nailed at 300 mm centres up the face of the wall to which timber slating or tiling battens are nailed at centres suited to the gauge (centres) necessary for double lap slates or tiles, as illustrated in Fig. 85.

As protection against decay, pressure impregnated softwood timber battens should be used and secured with non-ferrous fixings to avoid the deterioration and failure of steel fixings by rusting.

Where slate or tile hanging is used as cladding to a solid wall of buildings normally heated, then the necessary insulation can be fixed to the wall behind the counter battens. Rigid insulation boards of organic or inorganic insulation are fixed with a mechanically operated hammer gun that drives nails through both the counter battens, a breather paper and the insulation boards into the wall.

The continuous layer of breather paper, that is fixed between the counter battens and the insulation, is resistant to the penetration of water in liquid form but will allow water vapour to pass through it. Its purpose is to protect the outer surface of the insulation from cold air and any rain that might penetrate the hanging and to allow movement of vapour through it. 

Fig. 85 Slate hanging.

For vertically hung slating it is usual to use one of the smaller slates such as 405 x 205 mm slate which is headnailed to 50 x 25 mm battens and is less likely to be lifted and dislodged in high wind than longer slates would be. Each slate is nailed with non-ferrous nails to overlap two slates below, as illustrated in Fig. 85, and double lapped by overlapping the head of slates two courses below.

At angles and the sides of openings a slate one and a half the width of slates is used to complete the overlap. This width of slate is specifically used to avoid the use of a half width slate that might easily be displaced in wind.

Internal and external angles are weathered by lead soakers — hung over the head of slates — to overlap and make the joint weathertight. Slate hanging is fixed either to overlap or butt to the side of window and door frames with exposed edges of slates pointed with cement mortar or weathered with lead flashings.
At lower edges of slate hanging a projection is formed on or in the wall face by means of blocks, battens or brick corbel courses on to which the lower courses of slates and tiles bell outwards slightly to throw water clear of the wall below.

Tile hanging is hung and nailed to 40 x 20 mm tiling battens fixed at centres to counter battens to suit the gauge of plain tiles. Each tile is hung to battens and also nailed, as security against wind, as illustrated in Fig. 86.

At internal and external angles special angle tiles may be used to continue the bond around the corner, as illustrated in Fig. 86. As an alternative and also at the sides of openings tile and a half width tiles may be used with lead soakers to angles and pointing to exposed edges or weathering to the sides of the openings.

As weather protection to the solid walls of buildings with low or little heat requirements the hanging is fixed directly to walling and to those buildings that are heated the hanging may be fixed to external or internal insulation for solid walling and directly to cavity walling with cavity insulation.

Fig. 86 Tile hanging.

External weathering to walls of brick and block and Rendering.

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External weathering to walls of brick and block.
In exposed positions such as high ground, on the coast and where there is little shelter from trees, high ground or surrounding buildings it may well be advisable to employ a system of weathering on the outer face of both solid and cavity walling to provide protection against wind driven rain. The two systems used are external rendering and slate or tile hanging. 

Rendering.
The word rendering is used in the sense of rendering the coarse texture of a brick or block wall smooth by the application of a wet mix of lime, cement and sand over the face of the wall, to alter the appearance of the wall or improve its resistance to rain penetration, or both. The wet mix is spread over the external wall face in one, two or three coats and finished with either a smooth, coarse or textured finish while wet. The rendering dries and hardens to a decorative or protective coating that varies from dense and smooth to a coarse and open texture.

Stucco is a term, less used than it was, for external plaster or rendering that was applied as a wet mix of lime and sand, in one or two coats, and finished with a fine mix of lime or lime and sand, generally in the form imitating stone joints and mouldings formed around projecting brick courses as a background for imitation cornices and other architectural decorations. To protect the comparatively porous lime and sand coating, the surface was usually painted.

The materials of an external rendering should have roughly the same density and therefore permeability to water as the material of the wall to which it is applied. There are many instances of the application of a dense rendering to the outside face of a wall that is permeable to water, in the anticipation of protecting the wall from rain penetration. The result is usually a disaster.

A dense sand and cement rendering, for example, applied to the face of a wall of porous bricks, will, on drying, shrink fiercely, pull away from the brick face or tear off the face of the soft bricks, and the rendering will craze with many fine hair cracks over its surface. Wind driven rain will then penetrate the many hair cracks through which water will be unable to evaporate to outside air during dry spells and the consequence is that the wall behind will become more water logged than before and the rendering will have a far from agreeable appearance.