Masonry Magazine March 1971 Page. 35
Where uniform loads are applied above the apex of the triangle and where strong mortar and units are used in the construction of the wall, it may be assumed that "arching action" will exist and the lintel may be designed to carry its own weight plus that of the triangle of masonry above it. Where loads are applied below the apex of the triangle, arching cannot take place and all uniform loads directly above the lintel are considered in the design. Concentrated loads may be assumed to be transferred to the lintel downward from the apex of a triangle whose sides slope at 60 degrees from the horizontal. The load is transferred as a uniform load over the base of the triangle, as indicated in Figure 4b.
Ordinarily, loads from the floor joists and rafters in residences and other light-load structures may be considered as uniformly distributed when the height of the masonry between the lintel and the bottom of the hoists or rafters is more than one-third the joist or rafter spacing. Where the members bear directly on the lintel or are relatively heavy, they usually should be treated as concentrated loads. As a general rule, lintel loads in residential structures will range from about 200 to 300 lb. per lineal ft. with wall load only, and from 700 to 1000 lb. per lineal ft. where floor or roof and wall loads are transmitted. Heavier loads may be encountered in commercial buildings.
Table 1 can be used to determine the lintel size and amount of reinforcement required for the uniform loads mentioned above (300 lb/ft for wall loads: 1000 lb/ft for floor and roof loads) for residential structures. Where concentrated loads, or unusual loading occurs, Table 2 will be of value. It lists resisting moments (Mn) and shears (VR) for various 6-inch and 8-inch wide reinforced lintels made up with concrete masonry of four different prism strengths (f/1500, 2000, 2500, and 3000 psi). Use of Table 2 requires the calculation of total shear and moment loads on lintel as shown in the following example.
Example: A reinforced concrete masonry lintel is to be designed to carry a total uniform load (w) of 500 lb per foot and a concentrated load of 2000 lb at midspan. Clear span of the opening is six feet. Determine steel reinforcement required for 8-inch wide lintel if prism strength is 1500 psi.
Shear: Vwl+P
2
V=(500 x 6.671 +2000=2668 lb
2
Moment: Mwl+ PL
8
4
M = 500 x 6.67 x 12 + 2000 x 6.67 x 12
M
B
73,386 in. lb.
4
From Table 2, it may be seen that this shear of 2668 lb. and moment of 73,386 in-lb can be carried on an 8 x 16 inch lintel containing two No. 4 (½-in. dia.) steel reinforcing bars.
Standard 8 x 8 x 16 units with web cut-outs.
or heavy waterproof paper ing units to retain concrete.
ete masonry lintel
Floor Loads above Apex (no Load transmitted to Lintel). Lintel carries own Load plus Load of Masonry Triangle above it.
Length of Influence of Concentrated Load
Lintel
Effective Span
C. to. C. bearing
Loading due to Wall
Distribution of Concentrated Load
Concentrated Load
Floor Level-
FIGURE 4a. Lintel Loading
Effective Span
C. to C. bearing
FIGURE 4b.
Influence of Concentrated Load