The Use Of Precast Concrete Lintels

 

Loads from building work, whether it is brickwork, a roof or a concrete slab, above

An opening in a wall, need to be carried by horizontal structural members known

as lintels. Precast concrete lintels are being used on almost every brick building

erected in the world today.

 

The use of precast concrete lintels replaced the combination of steel angles and

brickwork, supporting the loads over openings, of years gone by.

 

It is often seen that in many instances lintels are being installed in applications they are not designed for. Many lintels are also installed incorrectly mainly because many builders are not aware of their limited, correct application. Another use of precast lintels, is in the rib and block slab industry where the lintels act as pre-stressed beams, supporting the hollow block void formers which in turn provide depth to the slab. These differ in size, design and manufacturing specifications.

 

Design and application of lintels

Precast concrete lintels should be professionally designed and laboratory tested in order to ensure that it will comply with minimum standards. They consist of concrete and reinforcing steel wires pre-stressed to a set minimum (in relation to the strength of the wires used and the strength of the concrete).

 

Where the lintels alone cannot resist the forces it has to carry, one can use steel bars to reinforce the brickwork above the lintels, forming a “brick beam” to protect the lintel against abnormal stresses, such as high roof or slab loads transferred to them. Good practice on site in addition to this, is to place a non-corroding metal plate under the ends of lintels wherever there are control joints. This permits lintels to move slightly and helps the control joints to function properly.

 

Advantages of using precast lintels

Precast concrete lintels offer time and financial advantages over cast-in-place reinforced concrete beams. They are designed to support high gravity loads and they expedite clay or concrete masonry construction for both the commercial and residential contractor. They also require less skill to install than conventional concrete beams.

 

It is faster to install pre-cast lintels than steel or in-situ cast concrete beams. There is no need to fabricate steel angles to span openings and rust-proofing is not required. One or two workers can easily install these lintels without effort or injury. Precast lintels save time and money by eliminating unnecessary man hours in the preparation of shuttering, cutting rebar, pouring concrete, and curing.  Not only do these lintels offer the mason contractor savings in labour costs, due to less workers required for installation, they also save the cost of material such as concrete, steel reinforcing, etc., which would be much more expensive.

 

In the unfortunate event of fire in a building, precast lintels maintain a high fire rating, surpassing that of untreated structural steel or timber products.

 

The exterior façade of a building can easily integrate cast stone veneer and precast concrete lintels with brick veneer, providing a modern variation of traditional commercial and residential architectural design. The extensive use of masonry materials on the exterior conveys stability, provides a warm and inviting image for employees and visitors, and can complement a residential or commercial building significantly.

 

Pre-cast lintels are easy to order and install and are always readily available.  

 

Correct application of precast concrete lintels

A precast concrete lintel can only work the way it is meant to do, if it has a sufficient number of reinforced brick courses built on top of it forming a reinforced brick beam. Comparing a reinforced brick beam with a single span, simply supported reinforced concrete beam, the lintel with its pre-stressed wires, acts as the reinforcement of the beam. Just as a reinforced concrete beam needs to have a minimum depth in relation to its span length, (other factors are also involved in the case of a concrete beam), the same way a lintel needs to have the correct number of brick courses over it in order to do the work it is intended to do.

 

Problems often encountered on building sites

One major problem arising from incorrect installation, is sagging of long lintels i.e. over garage doors or wide sliding doors and openings. This can also result in the lintel detaching from the brickwork which means that the brickwork alone then needs to act as the support of the roof or building work above it. If the brickwork is not adequately reinforced, the structure can collapse.

 

Lintels are sometimes, although built in correctly, subjected to loads far higher than they can safely carry. It is sometimes found that lintels are placed over openings without any brickwork above, expecting it to carry a high load from say, roof trusses, a concrete slab or the like.

 

Other problems often encountered on building sites, are that the lintels do not have sufficient supports at the built-in ends or that lintels are used to support cantilevered brickwork, which it is not designed for at all.

 

The NHBRC (National Home Building Research Council) has laid down minimum standards that must be adhered to where precast concrete lintels are being used.

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