Masonry Magazine March 1971 Page. 34
Recommendations and Construction Details
Lintels should have a minimum bearing of at least 8 in. at each end. Larger bearing areas are required for lintels having long spans or carrying heavy loads. A rough rule of thumb is to provide 1 in. of bearing for every foot of clear span. An adequate bearing area is necessary to distribute the high compressive stresses induced by the end reactions. In the case of long spans or heavily loaded areas above openings, the designer should investigate bearing stresses to ensure that the allowable compressive stress is not exceeded in the supporting masonry. High stresses may necessitate the use of solid or solid-top units, or of standard units with cores filled with concrete or grout for one or more courses of masonry under the lintel bearing.
Lintels are sometimes constructed as a portion of a continuous bond beam course. This helps distribute shrinkage and temperature stresses in the masonry above openings. Also, experience has shown that this type of installation is more satisfactory in areas where earthquake activities or high winds may occur. Installation of a typical two-course built-in-place reinforced concrete masonry lintel is shown in Figure 3.
Mortar used for bedding precast lintels or for laying units in reinforced masonry lintels should be of the same quality as used in laying the wall, except that such mortar should be at least equal in strength to Type N mortar, ASTM Specification C 270.
Where reinforced concrete masonry lintels are used, the first course of masonry above the lintel should be laid with full mortar bedding so that the cross ribs as well as the face shells of the units will bear on the lintel and thus reduce the shear stress between the core fill concrete and the face shell of the unit.
Concrete, or grout, used to fill the cores of reinforced masonry lintels should conform to the requirements of ASTM C 476, "Standard Specification for Mortar and Grout for Reinforced Masonry". Suggested proportions of materials by volume are
FIGURE 2. Reinforcing Details
TRANSVERSE
SECTION
SIDE
ELEVATION
TRANSVERSE
SECTION
TRANSVERSE
SECTION
SIDE ELEVATION
Std. 8 x 8 x 16 unit (2 or 3 core).
Shear ties (No. 3 bars at 16" o.c.)
8 x 8 x 8 Lintel unit
1 part Portland Cement: 3 parts sand: 2 parts coarse aggregate (pea-gravel), with sufficient water added to produce a consistency as fluid as possible without segregation of the materials.
Design
Selection of the proper lintel size and reinforcement depends upon the load to be supported by the lintel. In general, two types of loading are encountered: (1) uniformly distributed loads, or those that may be assumed to be uniformly distributed, such as the dead weight of the lintel and the masonry above the lintel, and floor and roof loads, where the floor and the roof consist of slab construction supported directly by the masonry; and (2) concentrated loads, such as loads from roof trusses, rafters, floor beams, or joists bearing directly on the lintel. Where a considerable height of running bond masonry is structurally continuous over a lintel, it is assumed that the masonry will arch
SIDE ELEVATION
Rectangular Ties-16" on centers
Pla
over
FIGURE 3. Typical two course rein.