Various loads are subjected to masonry lintel, which needs to be identified and calculated to design masonry lintels. The lintel is defined as a transverse load beam positioned over an opening in a wall to support the imposed loads above the lintel, as shown in Figure-1.
In this article, various design loads exerted on masonry lintel will be discussed and explained.
Types of Design Loads for Masonry Lintel
- Self-weight or a dead load of masonry lintel
- The dead load of the wall above the opening
- Dead load and live load transferred from the roof or the floor supported by the wall over the opening
Fig.1: Masonry Lintel Over Window Opening in a Masonry Wall
Self-Weight or Dead Load of Masonry LintelAll solid masonry and concrete lintels must be adequately grouted. The dead load of the lintel can be computed if enough information about the dimension of the lintel is available. For design purposes, initial or preliminary cross-sectional dimensions for the lintel are considered. Since lintels are an important part of the wall, the lintel's width is the same as the wall, and the only unknown that is required to estimate the self-weight of the lintel is its depth. Depth of approximately 20 mm per linear of 300 mm of the span can be considered for preliminary design. The computation of masonry self-weight depends on masonry unit types, for example, lightweight, medium weight, or normal weight, and the unit weight of the grout employed for the wall, and it may be taken as 1651.94 Kg/m3 or 2242.59 Kg/m3. As an alternative for the method of dead load calculation mentioned in the above section, the dead load of the lintels that have specific height and breadth which NCMA offers could be utilized, and considerable errors can be avoided. The values provide by the NCMA for a specific size of lintels are provided in Table-1, and the values are based on grout unit weight of 145 pounds per foot3, normal unit weight of 145, and lightweight of 100. Table-1: Lintel Weights in Kg/ 30cm
Dead Load of the Wall Above the LintelThe dead load masonry above the lintel is the weight of masonry contained in a forty-five-degree triangular area above the lintel if arching action is considered to occur. Consequently, the dead load for which the masonry lintel must be designed consists of masonry dead load in the triangular area plus self-weight of masonry lintel. It may be claimed that the degree which made the triangular area above masonry lintel vary from 45 to 60 degrees. The dead load for the case where the triangle is formed due to 60o over the lintel effective span is greater than a dead load of masonry contained in the triangle created by 45o over the effective span of the lintel. It is advised to employ the triangular formed by 45o to calculate wall dead load over the lintel, as illustrated in Figure-2.
Fig.2: Dead Load of Masonry in case of Arching Action and Masonry Self-WeightFurthermore, the wall above masonry lintel's dead load may be computed depending on tributary area basis when there is uncertainty about the occurrence of arching action, as shown in Figure 3.
Fig.3: No Arching ActionWhen the wall height above the masonry lintel is equal to or smaller than half of the lintel effective span plus 20 cm, the arching action is neglected. When arching action is neglected, the dead load is computed depending on the complete rectangular area of the wall above the masonry lintel. The wall height cannot be specified until lintel depth is determined, and the latter should be assumed for design purposes which can be revised later if required. For the design, a minimum allowable concrete lintel depth of 20 cm can be considered for masonry lintel depth for small span length, and a greater depth of 40 or 60 cm may be utilized for longer spans.
Dead load and live load transferred from the roof or the floor supported by the wall over the openingLive and dead Loads exerted on masonry lintel by roof or floors could be either concentrated or uniform loads. Regarding concentrated loads, depending on the results of tests, a method for specifying concentrated load distribution on walls is suggested by NCMA. As per the tests carried out for concrete block, brick masonry, and ACC masonry, it is assumed that concentrated loads may be dispersed at the slope of 2:1. Figure-4 shows the distribution of concentrated loads on masonry lintel.
Fig.4: Distributions of Concentrated Loads on Masonry LintelAs far as uniformly distributed loads are a concern, two cases should be considered. Firstly, when the line of roofs or floors is located above a specified distance equal to half the effective span measured from the top of the lintel plus 20 mm, it is considered that arching action occurs. This is because the uniformly distributed live or dead load is not supported by the lintel but by adjacent masonry. Secondly, when the line of roofs or floors is within the specified distance, the uniformly distributed live or dead loads are supported by the masonry lintel. Additionally, part of masonry lintel may be subjected to uniformly distributed loads. Finally, an example for uniformly distributed loads is when the roof or floor sheathing is nailed to the ledger beam connected to the wall through bolts, and an opening supports the will. Consequently, the ledger beam transfers the roof or floors to the wall that is located over an opening. Read More: Different Types of Lintels and their Uses in Building Construction Types of Openings in Walls, its Parts and Types of Lintels and Arches for Openings