Various types of loads are subjected on masonry lintel which need to be identified and calculated for design of masonry lintels. Lintel is defined as a transverse load beams which are 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 dead load of masonry lintel
- 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 Lintel
All solid masonry and concrete lintels must be adequately grouted and the dead load of the lintel can be computed if the enough information about the dimension of the lintel is available. For the design purposes, initial or preliminary cross sectional dimensions for the lintel are considered.
Since, lintels are an important part of the wall so the width of the lintel 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. Not only does the computation of masonry self-weight depend on masonry unit types for example light weight, medium weight, or normal weight but also on the unit weight of the grout that is 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 are offered by NCMA could be utilized and considerable errors can be avoided.
The values provide by the NCMA for specific size of lintels are provided in Table-1, and the values are based on grout unit weight of 145 pound per foot3, normal unit weight of 145 and light weight of 100.
Table-1: Lintel Weights in Kg/ 30cm
Dead Load of the Wall Above the Lintel
The dead load masonry above the lintel is the weight of masonry contained in 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 design for consists of masonry dead load in 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 degree.
The dead load for case where triangle is formed due to 60o over lintel effective span is greater than dead load of masonry contained in triangle created by 45o over the effective span of the lintel.
It is advised to employ the triangular formed by 45o for the calculation of 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-Weight
Furthermore, the dead load of the wall above masonry lintel may be computed depend on tributary area basis when there is uncertainty about the occurrence of arching action as shown in Figure 3.
Fig.3: No Arching Action
When the height of the wall above the masonry lintel is equal or smaller than half of lintel effective span plus 20 cm, the arching action is neglected. In the case where arching action is neglected, the dead load is computed depend on complete rectangular area of the wall above the masonry lintel.
The height of the wall 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, minimum allowable concrete lintel depth of 20 cm can be considered for masonry lintel depth for small span length and 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 opening
Live and dead Loads exerted on masonry lintel by roof or floors could be either concentrated or uniform loads.
With regards to concentrated loads, depend on results of tests, a method for specifying concentrated load distribution on wall is suggested by NCMA.
As per results of tests carried out for concrete block, brick masonry and ACC masonry, it is assumed that concentrated loads may be dispersed at slope of 2:1. Figure-4 shows the distribution of concentrated loads on masonry lintel.
Fig.4: Distributions of Concentrated Loads on Masonry Lintel
As far as uniformly distributed loads are concern, there are two cases which should be considered.
Firstly, when the line of roofs or floors are located above a specified distance which is equal to half the effective span measured from the top of the lintel plus 20 mm, then it is considered that arching action is occur and the uniformly distributed live or dead load is not supported by the lintel but rather by adjacent masonry.
Secondly, when the line of roofs or floors is within the specified distance, then 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 roof or floor sheathing is nailed to the ledger beam that is connected to the wall through bolts and the will is supported by an opening. Consequently, the ledger beam transfers the load of the roof or floors to the wall that is located over an opening.