How to design reinforced masonry lintels Use engineering formulas or tables provided by BIA, NCMA, or other sources By David C. Gastgeb ince people first began putting windows in their shelters, they have needed a way to support the weight of the wall above the window. The solution was the lintel, a beam that spans from one side of the window to the other. Wood, stone, steel, concrete, brick, and block all have been used as lintels in masonry walls. Of these, though, the only material that matches the appearance of masonry is masonry itself. Masonry lintels must be reinforced. They can be constructed of standard clay masonry units (solid or hollow), glazed brick and tile, specially formed concrete masonry lintel units, bond beam concrete masonry units, or standard concrete masonry units with grooved, depressed, or cutout webs. These units are laid in a masonry wall in a way that S creates a channel in which steel reinforcing and grout are placed. Masonry lintels are not difficult to construct, whether built in place or prefabricated onsite or at a plant and then delivered. Shoring is used if the lintel is built in place. Prefabricated masonry lintels eliminate shoring and can be loaded immediately after they’re placed in the wall. Though steel angles are the simplest way to make lintels in masonry walls, reinforced brick and block lintels have several advantages. Though they must be reinforced, masonry lintels require much less steel (and thus material cost) to carry loads. They cost less to maintain because they don’t need periodic painting as exposed steel does. Their built-in fireproofing provides additional safety. If built in place, they require no special lift- Lintels are designed to support a triangular section of wall above the opening that is half as high as the opening is wide. Outside this triangle, arching action of the masonry is assumed to support the wall. ing equipment, as precast concrete lintels do. And they eliminate cracking that can be caused by differential movement between steel lintels and masonry. Determining the loads To design a masonry lintel, you must first determine the load to be supported. The Brick Institute of America (BIA), National Concrete Masonry Association (NCMA), and authors Schneider and Dickey in their book, Reinforced Masonry Design (Ref. 1), all use a graphic load diagram to do this (see drawing). In this method, the lintel is designed to support a triangular section of masonry above it. The height of this triangle (triangle ABC in the drawing) is one-half the clear span of the opening (L/2); its sides are at 45° to the lintel. Outside this triangle, arching action of the masonry is assumed to support the weight of the wall and any uniform loads, such as roof trusses or floor joists. But for this arching to occur, sufficient masonry must be above the apex of the triangle. For normal wall thicknesses and loads, a height of 8 to 16 inches of masonry above the apex allows arching. This arching action produces a horizontal thrust at each end of the lintel. If a lintel over a long span supports heavy loads and one end is near a wall corner, another opening, or an expansion joint, arching action may have to be ignored. The lintel must then be designed to support all the loads above it. Concentrated loads from roof trusses or floor joists, whether above or below the apex, are transferred down as a uniform load along the base of a triangle with sides at 60° to the horizontal. Schneider and Dickey calculate the length of influence of the concentrated load (distance AE in drawing). No arch action should be assumed for walls laid in stack bond. All loads above the lintel are considered to act directly down on the lintel unless distributed by bond beams or other structural members. Design methods You can design masonry lintels in two ways. The most efficient design is obtained by using formulas. Using the working-stress theory of elastic design and the straight-line theory of stress distribution, authors Schneider and Dickey have developed basic design formulas for reinforced masonry lintels. These formulas and design examples are given in BIA Technical Notes 17A Revised, “Reinforced Brick Masonry, Flexural Design” (Ref. 2), and in Schneider and Dickey’s book, Reinforced Masonry Design. This design procedure is similar to that of ACI 530/ASCE 5 Building Code Requirements for Masonry Structures (Ref. 3). Instead of performing the calculations and the required stress checks for flexure, bearing, shear, and deflection, you can use several available design aids. BIA in Technical Notes 17H, “Reinforced Brick and Tile Lintels” (Ref. 4), lists resisting moments and shears for reinforced brick lintels with various areas of reinforcement. Once you determine the loading, use it to calculate the moment and shear to be resisted, then choose the correct reinforced brick lintel section. The tables assume a compressive strength of masonry of 2000 psi. Results for a higher compressive strength can be obtained by increasing the values in the tables in proportion to the desired masonry strength. NCMA, in its technical notes 25 (Ref. 5), 25A (Ref. 6) and 81 (Ref. 7), includes easy-to-use design tables for reinforced concrete ma- Shoring holds a reinforced masonry lintel in place until the grout cures and gains enough strength to support the wall above. Masonry lintels match wall appearance better than precast concrete lintels and require less steel than steel angle lintels. sonry lintels. One table gives the moment and shear produced by triangular loading. The Reinforced Masonry Engineering Handbook (Ref. 8), by James E. Amrhein, also contains tables for use in flexural design, with examples on how to use them. Regardless of which method you use—calculations or design tables—don’t base lintel design on rules of thumb. To efficiently Reinforced concrete masonry lintels are made from special lintel units, bond beam units, or standard units with cutout webs. design reinforced masonry lintels, you must analyze loads and resulting stresses carefully. David C. Gastgeb is the Midwest region engineer for the Acme Brick Co., Oklahoma City. References 1. Robert R. Schneider and Walter L. Dickey, Reinforced Masonry Design, Second Edition, 1987, Prentice-Hall Inc., Englewood Cliffs, NJ 07632. 2. “Reinforced Brick Masonry, Flexural Design,” BIA Technical Notes on Brick Construction, Number 17A, Brick Institute of America, 11490 Commerce Park Dr., Reston, VA 22091. 3. ACI 530/ASCE 5 Building Code Requirements for Masonry Structures, American Concrete Institute, P.O. Box 19150, Detroit, MI 48219. 4. “Reinforced Brick and Tile Lintels,” BIA Technical Notes on Brick Construction, Number 17H, BIA. 5. “Concrete Masonry Lintels,” NCMA-TEK 25, National Concrete Masonry Association, P.O. Box 781, Herndon, VA 22070. 6. “Concrete Masonry Lintels,” NCMA-TEK 25A, NCMA. 7. “Lintels for Concrete Masonry Walls,” NCMA-TEK 81, NCMA. 8. James E. Amrhein, Reinforced Masonry Engineering Handbook, Fourth Edition, 1983, Masonry Institute of America, 2550 Beverly Blvd., Los Angeles, CA 90057. PUBLICATION #M910108 Copyright 1991 The Aberdeen Group All rights reserved