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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