LINTELS

A lintel is a horizontal structural member placed to span openings for doors, windows and corridors. Lintels are usually rectangular in shape. Lintels may be made out of several materials such as wood, stone, brick, reinforced brickwork, RCC or rolled steel sections embedded in cement concrete.

Reinforced cement concrete lintels are mostly used these days for all spans as other materials have some drawbacks. For example, wooden lintels are liable to decay, stone lintels are costly and not available for long lengths without defect, and steel sections become loose when exposed to sun due to the high rate of expansion.

Purpose of Lintels

The purpose of providing a lintel is just the same as that of an arch. Lintels take the load of the wall above the opening and transfer to the walls. The effective load causing on a lintel is shown in Fig. 19.7.

The lintels are easy and simple in construction and do not need special form work. The ends of lintels are built into the masonry and thus the load carried by lintel is transferred to the masonry in jambs. The bearing of lintel should be either 10 cm or 40 cm for every 30 cm of span, whichever is greater. As a general rule, the depth of the lintel can be adopted as 1/12th of the span or 15 cm, whichever is greater.

Figure 19.7 Effective load acting on lintel

Types of Lintels

Based on the type of material used, lintels are classified as follows:

1. Wooden lintels
2. Stone lintels
3. Brick lintels
4. Steel lintels
5. RCC lintels and
6. Reinforced brick lintels

1. Wooden Lintels

Wooden lintels are the oldest type of lintels. It is even now used in areas where timber is abundantly available. However, the risk of using wooden lintels is that they are susceptible to catch fire and liable to decay by rot or termite.

Single piece of timber is used for smaller spans. For larger spans, built-up sections are to be used. Built-up sections may be formed by using two or three timber pieces and bolted to serve as a lintel. Figure 19.8 shows a wooden lintel composed of two timber pieces and two distance or packing pieces. Bolts are provided through the packing pieces. Sometimes, in order to strengthen the wooden lintels, mild steel plates are placed at the top and bottom and such lintels are called as flitched lintels.

Figure 19.8 Wooden lintel

Following important aspects have to be given due consideration while designing a wooden lintel:

1. Good quality woods like teak, rosewood, etc., without any defects should be used.
2. Bearing of lintel should be adequate and it should rest on a mortar base to have a firm and uniform support.
3. Width of lintel should be equal to the thickness of the opening and the depth 1/12th of the span 8 cm, whichever is greater.
4. Relieving arches of brick or stone may be designed wherever heavy loads are anticipated.
5. Wooden lintels help in securing the heads of frames of wooden doors and windows.

2. Stone Lintels

Basically stone lintels are preferred in places where stone is available in abundance. Further, stone lintels are to be adopted in places where stone masonry is used and in buildings where stone facing is used.

Stone lintels consist of slabs of stones which may be one or more pieces. Stone lintels are not recommended for the following reasons:

1. As stone is weak in tension, it can not withstand the transverse stresses.
2. Only spans up to 1 m may be tried with stone slabs. For large spans, relieving stone arches may be used.
3. It is extremely difficult to get long stone slabs without defects.

3. Brick Lintels

Brick lintels are used for short spans up to 1 m with light loadings. Brick lintels generally consist of bricks laid on end points or edges (Fig. 19.9).

Figure 19.9 Brick lintel

Brick lintels are constructed over temporary wooden supports. Bricks with frogs are generally preferred as they form juggles when bounded. Such construction possesses more shear resistance at the end joints. Lintel formed by using frog bricks is termed as joggled brick lintel.

Following points are to be given due consideration while deciding brick lintels:

1. Bricks should be first class standard bricks with sharp and square edges
2. Depth of brick lintels should be multiples of brick courses.
3. Bricks with frogs should be bonded using cement mortar of 1 : 3 ratio.
4. Door and window frames should be fixed with a setback of 2.5 cm from the external face of the wall.
5. Steel angles or flats may be provided at the bottom of the lintel.
6. Brick lintels should not be recommended for supporting heavy loads.

4. Steel Lintels

Rolled Steel Joints (RSJs) are used individually or embedded in concrete. Particularly for large openings and to support heavy loads, embedded RJS lintels are preferred. These lintels are also called as bressummers (Fig. 19.10).

For light loadings and short spans steel angles or channel sections in concrete are used. For large spans and heavy loads, built-up sections are used. Built-up sections comprise of three RSJs or channel sections which are held together at proper distances by tube-separators or cross-bolts. In order to protect these built-up sections from corrosion and fire they are embedded in concrete. In order to increase the bond or grip between the sections, a strip of metal lathing or wire netting is placed at the bottom of the steel section before concreting. The whole system is cured like an RCC lintel.

Figure 19.10 Steel lintel

5. RCC Lintels

Reinforced cement concrete lintels have practically replaced all other materials for the lintels. The RCC lintels have plenty of advantages over other materials as stated below.

1. They are fire-proof, durable, strong, economical and simple in construction.
2. Practically there is no restriction on length of span and loading.
3. There is no need to go in for relieving arches.
4. Use of precast units shall increase the speed of construction.

A concrete mix of 1:2:4 with mild steel reinforcement is usually adopted. Precast RCC lintels are preferred for a span of 2 m. While laying precast RCC units, special care has to be taken to see that the marked face of the lintel is on the top or upper side.

For large spans, lintels should be cast in place. Construction of RCC lintels is similar to any other RCC construction. Figure 19.11 shows the position of RCC lintel over a door or a passage. Adequate bearing is given for the lintel to sit well on the supports and the width is equal to the width of the wall.

Lintels with sunshades have to be planned sometimes. As stated earlier, the bearing for individual lintels should be at least one brick on each side. Window sunshades are also to be extended of this length of 23 cm on either side of the external opening. Sunshades should be projected for a length not less than that required to protect the window or door leaf which may have to be opened out from rains. As sunshade is a shading device, it is taken as a minimum of 60 cm for the windows of residence. For entrance doors, the length may be up to 1 m (Fig. 19.12).

Figure 19.11 Details of RCC lintel over door or passage

Figure 19.12 Lintel over a window with sunshade

6. Reinforced Brick Lintels

Reinforced brick lintels are used in certain cases. In such cases, bars are provided longitudinally in between the vertical joints and extending from joint to joint. Further, to resist the vertical shear additional 6 mm diameter steel stirrups may be used.