In this month's issue of Concrete Masonry Designs Online:
Allowable Stress Design Of Concrete Masonry Lintels
TEK 17-1C Structural (2009)
Vertical loads carried by lintels typically include: (1) distributed loads from the dead weight of the lintel, the dead weight of the masonry above, and any floor and roof loads, dead and live loads supported by the masonry; and (2) concentrated loads from floor beams, roof joists, or other beams framing into the wall. Axial load carried by lintels is negligible.
Most of these loads can be separated into the four types illustrated in Figure 1 above: uniform load acting over the effective span; triangular load with apex at mid-span acting over the effective span; concentrated load; and uniform load acting over a portion of the effective span. The designer calculates the effects of each individual load and then combines them using superposition to determine the overall effect, typically by assuming the lintel is a simply supported beam.
For some confi gurations, the masonry will distribute applied loads in such a manner that they do not act on the lintel. This is called arching action of masonry. Arching action can be assumed when the following conditions are met (see also Figure 2 below):
The loads supported by a lintel depend on whether or not arching action can occur. When arching is not present, the lintel self-weight, the full weight of the wall section above the lintel and superimposed loads are considered. Self weight is a uniform load based on lintel weight (see Table 1).
When arching occurs, the wall weight supported by the lintel is taken as the wall weight within the triangular area below the apex (see Figure 2 above and Table 2). This triangular load has a base equal to the effective span length of the lintel and a height of half the effective span. Any superimposed roof and fl oor live and dead loads outside this triangle are neglected, since they are assumed to be distributed to the masonry on either side of the lintel. Loads applied within the triangle need to be considered, however.
Uniform loads that intersect the 45o triangle are typically distributed as follows. The portion of the load intersecting the 45o triangle is distributed over the lintel span, i.e., the load is assumed to spread out at 45o angles so that the uniformly distributed load on the lintel is reduced by the ratio of the width of load within the triangle to the lintel span. Distributed loads that do not extend entirely across the 45o triangle may be distributed to only a portion of the lintel.
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