A lintel is a horizontal structural member which spans across the supports of an opening such as a door or window in a building. Depending on the span of the opening, a lintel may be structurally significant or insignificant – considering whether it requires serious structural consideration or not. Lintels can be made of materials such as timber, concrete, or steel. Concrete lintels are usually provided with nominal reinforcement, due to the inherent low tensile strength of concrete. When a lintel is continuous in a building and connects all the structural members together, it is called a chained lintel.
In Nigeria, it is common to see residential buildings chained at the lintel level. What is meant by chaining is that all the structural members are joined monolithically at the lintel level with reinforced concrete. Chained lintels can also be called continuous lintels. This process is usually done when the blockwork is built alongside the columns of the building. It is rarely done in a purely framed building where the columns, beams, and slab are done before the blockwork panels are built.
Eye brows can be raised over this ‘wasteful approach’ of using excessive concrete and reinforcement on elements that are not ‘structurally significant’. This can lead to questions such as;
(1) Is it possible that the amount of money spent on constructing blockwork simultaneously with the frame and adopting a chained lintel offsets the cost of constructing a purely framed building?
(2) Can chained lintels be entirely avoided irrespective of the approach used?
(3) Are there special advantages of doing chained lintel in a building?
This issue is pertinent because the structural frame of a building is usually designed to stand alone without considering the effect of such lintels. Also, such arrangement is not usually captured in the structural drawings of a building, but it is done on site for low scale residential buildings. Why the extra cost? The lintels are usually 230 x 230 mm in dimension.
Advantages of chained lintels
The perceived advantages of chained lintels are as follows;
(1) It can improves the rigidity of the building. This is a no-brainer since more redundants are being added to the frame of the building. As a result, the building can behave more as a unit due to the increased linking members.
(2) It improves the robustness of the building.
(3) It can improve the lateral stability of the building, even though it is rarely critical in simple residential buildings such as duplexes in a region of no seismic event.
(4) It can cover up the inherent dimensional inaccuracies associated with building the blockworks and the columns at the same time, even though strict quality control can improve that.
(5) Introducing chained lintels reduces the buckling length of columns.
Disadvantages of chained lintels
(1) All the advantages highlighted above are usually catered for when designing the building as a pure frame. What is then the need of the extra advantages in terms of cost?
(2) The reinforcements provided in the building are the same as when no consideration for chained lintels is made.
(3) The construction speed is reduced.
A Structural Perspective
To look at this issue from a structural perspective, let us investigate the effect of chained lintels on the structural response of a simple two storey building under the effect of normal actions. In order to achieve this, let us model a simple two storey residential building and investigate its structural behaviour with and without chained lintels. The building data is shown below;
Dimension of all columns = 230 x 230 mm
Dimension of floor beams = 450 x 230 mm
Dimension of roof beams = 300 x 230 mm
Thickness of floor slab = 150 mm
Dimension of lintels (model 2) = 230 x 230 mm
ULS action on floor slabs = 12 kN/m2
ULS action on floor beams (blockwork + rendering + selfweight) = 15 kN/m
ULS action on roof beams = 5 kN/m
ULS action on lintels = 3.5 kN/m
(N/B): All loads were assumed.
From the figures above, the maximum hogging moment on the internal and external beams are 45.5 kNm and 68.4 kNm respectively, while the sagging moments are 28 kNm and 41.3 kNm respectively.
The results below show the building model with lintels and the internal forces acting on the building.
Comparison of results
Under the effect of gravity actions, the following results were observed in the structural members;
In the building with chained lintel, the bending moment on the intermediate support of the external beam reduced by about 6.8%. However, the end support moment increased by about 43.8% (from 7.47 kNm to 13.3 kNm). A reduction in span moment was also observed for floor beams in buildings with chained lintel. This same behaviour was observed in the internals beams.
The bending moment in the corner columns increased from 5.69 kNm to 12.338 kNm when chained lintels were introduced. This is due to the fact that lintels are subjected to the load from the blockwork courses that get to the sofit of the beams. Axial load in the columns also increased from 101 kN to 143 kN. This same increase in internal forces was observed for the intermediate columns.
Therefore, introducing chained lintels appears to worsen action effects on the columns, but favours the floor beams, except for end support moments.
Limitation of the study: The loadings on the lintels appeared to be exaggerated, and some considerations were not made to reduce the wall load on the floor beams since the chained lintel will carry some of the loads. The author therefore welcomes discussions and further investigations in the study.
Thank you for reading.