A significant opening within masonry walls will always require a lintel of some sort to bridge over it. In contrast to popular opinion, lintels are structural elements though classified as secondary structural elements whose primary function is to support the masonry that arches over the opening. This is achieved by spreading all the vertical actions applied on the lintel to either side of the opening.
This article offers advice on the different types of lintels that are available, the way in which they are designed and the detailing requirements of lintels.
Lintels are beam elements designed to support a portion of a wall having an opening within it. The actions on the lintel can be a triangular formation of the masonry units above the opening. Bonding of the wall causes it to arch over the opening. Figure 1 indicates how vertical actions are applied on lintels.
The arching action of masonry over a lintel can be seriously impacted by the presence of other significant openings. This prevents the load from being distributed uniformly across the masonry. Additionally, the presence of floor joists and principal beam elements within the interaction zone will result in the lintels supporting more loads. Figure 2 illustrates a perfect scenario.
Note that in instances where stack bonding is used to construct the masonry walls, arching actions do not apply. Thus, all the masonry above the lintel must be assumed to be fully supported by it.
As rightly pointed out in the introductory aspect of this post, lintels are secondary structural elements acting as direct support to masonry walls. Their design is typically governed by deflection and is treated in the same way as rectangular concrete beams. Excessive movement of a lintel will result in cracking above the opening. Hence it is generally recommended that the max deflection at midspan be limited to span/500 or 5mm whichever is lesser¹.
For a lintel sandwiched between masonry walls, careful consideration must be given to the way the applied vertical actions are distributed on the lintel. The arch actions within the masonry depend on the type of bonding within the wall. If a stretcher type of bond is used, no further modification is required. However, if a stack bonding is used, arching action is impossible except where bed reinforcement is present.
Concentrated and uniformly distributed actions from the floors above must be taken into account when designing a lintel. The load distribution angle for concentrated actions is 60° whereas uniformly distributed actions are transmitted directly into the lintel with no spread. This is illustrated in (Figure 3).
The design of a lintel to be installed within a loadbearing wall must allow for the self-weight of the masonry it is supporting, any point loads whose spread encroaches upon the lintel, and the actions from any floor immediately above the lintel.
Lintels are sometimes sold as pre-cast concrete units, and on the manufacturer technical details, contain very vital information necessary for selecting a lintel. Typically the technical details contain load carrying capacity for each specification of lintel, coupled with span requirements. This are all compared with the applied actions the lintel is expected to support during it design life as well as the geometry of the opening. Thus where pre-cast unit are bought from a local manufacturer, it is imperative to understand how to read the tables and what they should be compared with. For instance are the load specified at service or ultimate limit state?. This should be addressed by the guidance notes of the local manufacturer.
Where the design of a lintel relies on arching action, temporary supports are required until the mortar reaches sufficient strength for the arching action to take place. This usually take 24hours or longer depending on the type of mortar used.
Lintels are predominantly designed for bending and shear only. However it is possible for them to be subjected to torsion forces. Where arching action is guaranteed, the magnitude of these forces will be negligible. However if no arching action can be guaranteed within the wall, then the design of the lintel should incorporate torsion. These requires the use of hollow steel sections or pre-cast concrete lintels, as they are basically resistance to torsional forces.
Note where significant torsional forces are applied to the lintel, it is recommended that the rotational displacement should not exceed 1.5mm from the horizontal supports¹.
Two materials are commonly employed in making lintels, pre-cast reinforced concrete and structural steel sections. However, it is not uncommon to see cast in place concrete lintels.
Concrete lintels are typically heavy and can support long spans and very durable. But very visible when placed in walls with no render finish. Steel lintels in contrast are lighter and can be made into various complex shapes such as arches which allows for unique opening in walls. They can also incorporate a cavity tray. Steel lintels are easier to conceal giving the impression as though they are not present. However they deflect more than their concrete counterparts, which limits their viability for long span.
Figure 4 illustrates examples of concrete lintels in various configurations. The support conditions of lintels require careful and adequate detailing, poor detailing is a major cause of failure. The minimum bearing length for a lintel is 150mm. However, wherever a lintel span less than 1.2m, the bearing length may be reduced to 100mm². This only applies if the wall in question is non-load bearing and has full arching action above the lintel.
Bonding is equally a very important parameter that can impact on the performance of a lintel in bearing. No lintel should rest on a half masonry unit, as this will not be sufficiently fixed with the rest of the wall (figure 5).
A tension crack will develop between the half unit and the adjacent masonry unit as the bearing stress is transmitted, potentially resulting in localised failure. Instead, the lintel should be made to rest on a monolithic masonry unit (i.e. without a joint between it and the adjacent masonry unit at the point where the lintel bears onto the wall).
1. The Institution of Structural Engineers (2018) Manual for the design of plain masonry in building structures to Eurocode 6 (2nd ed.), London: IStructE
2. BS EN 1996-1-1:2005+A1:2012 Eurocode 6: Design of masonry structures. General rules for reinforced and unreinforced masonry structure
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